110 Specifica Tion Notes

The designer should be alert to the tendency for specifications to escalate. When a flyback converter is to be considered and potential requirements are large, costs are often particularly sensitive. The designer should establish with the customer the real limitations of the application. It may well be that a typical performance of 6 regulation on the auxiliary outputs of a multiple-output unit would be acceptable. This allows a semiregulated flyback system to be used. To guarantee a result of...

1101

EES. 1.12.1 Characteristics of a (ypical undervoltage transient protection circuit. (a) Load current transient. (b) Typical undervoltage transient excursion without protection circuit fitted, (c) Undervoltage transient excursion with protection circuit fitted. EES. 1.12.1 Characteristics of a (ypical undervoltage transient protection circuit. (a) Load current transient. (b) Typical undervoltage transient excursion without protection circuit fitted, (c) Undervoltage transient excursion with...

1112 Power Circuit

The above specification requirements can be met using a single-ended flyback system without secondary regulators (see Fig. 2.1.1). To meet the need for dual input voltage by a link change, voltage doubling techniques can be employed for the input line rec'it rs when they are set for llfl-V operation. Consequently, the r V c_ c_ _ _ c_j c rectified DC line will be approximately 300 V for either 110-V or 220-V nominal inputs. A voltage analogue of the primary current for primary power limiting is...

1113 Transformer Design

The design of the transformer for this power supply is shown in Part 2, Chap. 2. 1.72 PROBLEMS 1. From what family of converters is the flyback converter derived 2. During which phase of operation is the energy transferred to the secondary in a flyback converter . 3. Describe the major advantages of the flyback technique. 4. Describe the major disadvantages of the flyback technique. 5. Why is the transformer utilization factor of a flyback converter often much lower than that of a push-pull...

114 Crowbar Performance

More precise crowbar protection circuits are shown in Fig. 1.11.1b and c. The type of circuit selected depends on the performance required. In the simple crowbar, there is always a compromise choice to be made between ideal fast protection (with its tendency toward nuisance operation) and delayed operation (with its potential for voltage overshoot during the delay period). For optimum protection, a fast-acting, nondelayed overvoltage crowbar is required. This should have an actuation voltage...

115 Limitations Of Simple Crowbar Circuits

The well-known simple crowbar circuit shown in Fig. 1.11. la is popular for many noncritical applications. Although this circuit has the advantages of low cost and circuit simplicity, it has an illdefined operating voltage, which can cause large operating spreads. It is sensitive to component parameters, such as temperature coefficient and tolerance spreads in the zener diode, and variations in the gate-cathode operating voltage of the SCR. Furthermore, the delay time provided by Cl is also...

118 Selecting Fuses For Scr Crowbar Overvoltage Protection Circuits

In the event of an overvoltage stress condition caused by the failure of the series regulator in a linear power supply, the crowbar SCR will be required to conduct and clear the stress condition by blowing the series protection fuse. Hence, the designer must be confident that the fuse will open and clear the ulty circuit before the SCR is destroyed by the fault current. If a large amount of energy is dissipated in the junction of the SCR within a short period, the resultant heat cannot be...

119 Type 3 Overvoltage Protection By Voltage Limiting Techniques

In switchmode power supplies, the crowbar or clamp voltage protection techniques tend to be somewhat less favored because of their relatively large size and dissipation. By its nature, the off-line switchmode power supply tends to fail safeM that is, to a zero or low-voltage condition. Most failure modes tend to result in zero output voltage. Since the high-frequency transformer provides galvanic isolation between the input supply and the output lines, the need for crowbar-type overvoltage...

121 Output Ripple and Noise

Where very low levels of output ripple are required, the addition of a small LC noise filter near the output terminals will often eliminate the need for expensive low-ESR capacitors in the main secondary reservoir positions. For example, a typical 5-V 10-A supply may use the highest-quality low-ESR capacitors in positions Ci, C2, and C3 of the single-stage filter shown in Fig. 2.1.1, but this will rarely give a ripple figure of less than 100 mV. However, it is relatively easy to keep ripple...

1210 Optimum Flux Density

The choice of optimum flux density 5opt will be a matter for careful consideration. Unlike with the flyback converter, both quadrants of the BIH loop will be used, and the available induction excursion is more than double that of the flyback case. Consequently, core losses are to be considered more carefully as these may exceed the copper losses if the full induction excursion is used. For the most efficient design, the copper and core losses should be approximately equal. Figure 2.12.2 shows...

1215 Control And Drive Circuits

The control and drive circuits used for this type of converter are legion. They range from fully integrated control circuits, available from a number or manufacturers, to the fully discrete designs favored by many power supply engineers. A discussion of suitable drie circuits will be found in Part 1, Chaps. 15 and 16. For reliable operation, the drive and control circuits must provide the following basic functions 1. Soft start. This reduces inrush current and turn-on stress, and helps to...

1216 Flux Doubling Effect

The difference in the operating mode for'the single-ended transformer and the push-pull balanced transformer is not always fully appreciated. For the single-ended forward or flyback converter, only one quadrant of the BIH loop is used, there is a remnant flux B and the remaining range of induction is often quite small. (Figure 2.9,2a and b shows the effect well.) in the push-pull transformer, it is normally assumed that the full BIH loop may be used and that B will be incremented from Bmto each...

122 Operating Principles

Figure 2.12,1 a shows the general arrangement of power sections for the half-bridge push-pull converter. The switching transistors Ql and Q2 form only one side of the bridge-connected circuit, the remaining half being formed by the two capacitors Cl and C2. The major difference between this and the full bridge is that the primary of the transformer will see only half the supply voltage, and hence the current in the winding and switching transistors will be twice that in the full-bridge case....

124 Practical Circuit Description

Figure 1.12.4 shows a practical implementation of this technique. In this circuit, switch SW1 or Q1 is replaced by Darlington-connected transistors Q3 and Q4. These transistors operate as a switch and linear regulator. FIG. 1.12.4 Example of an undervoltageprotection circuit. FIG. 1.12.4 Example of an undervoltageprotection circuit. Although Q3 and Q4 are now shown positioned between the two capacitors CI and C2, it has been demonstrated above that since they still form a series network, their...

131 Transfer Function

The small-signal transfer functions for these two operating modes are quite different, and they are dealt with separately in this section. In practice, when a wide range of input voltages, output voltages, and load currents is required, the flyback converter will be required to operate (and be stable) in both complete and incomplete energy transfer modes, since both modes will be encountered at some point in the operating range. As a result of the change in transfer function at the point where...

1321 General Conditions

Figure 2.13.1 shows the power section of a typical off-line bridge converter. Diagonal pairs of switching devices are operated simultaneously and in alternate sequence. For example, Ql and Q3 would both be on at the same time, followed by Q2 and Q4. In a pulse-width-controlledsystem, there will be a period when all four devices will be off. It should be noted that when Q2 and Q4 are on, the voltage across the primary winding has been reversed from that when Ql and Q3 were on. In this example, a...

133 Type 1 Overpower Limiting

The first type is a power-limiting protection method, often used in flyback units or supplies with a single output. It is primarily a power supply short-circuit protection technique. This and the methods used in types 2 and 4 are electronic, and depend on the power supply remaining in a serviceable condition. The supply may be designed to shut down or self-reset if the overload is removed. In this type of protection, the power (usually in the primary side of the con- verter transformer) is...

134 Type 1 Forma Primary Overpower Limiting

In this form of power limiting, the primary power is constantly monitored. If the load tries to exceed a defined maximum, the input power is limited to prevent any further increase. Usually, the shape of the output current shutdown characteristic is poorly defined when primary power limiting is used on its own. However, because of its low cost, primary power limiting has become generally accepted in lower-power, low-cost units (particularly in multi-output flyback power supplies). It should be...

136 Type 1 Form C Pulsebypulse Overpowercurrent Limiting

This is a particularly useful protection technique that will often be used in addition to any secondary current limit protection. * The input current in the primary switching devices is monitored on a real-time basis. If the current exceeds a defined limit, the on pulse is terminated. With discontinuous flyback units, the peak primary current defines the power, and hence this type of protection becomes a true power limit for such units. With the forward converter, the input power is a function...

142 Foldback Principle

Figure 1.14.1 shows a typical reentrant characteristic, as would be developed measured at the output terminals of a foldback-limited power supply. A purely resistive load will develop a straight load line (for example, the 5-fl load line shown in Fig. 1.14.1). A resistive load line has its point of origin at zero, and the current is proportional to voltage. As a resistive load changes, the straight line (which vill start vertically at zero load i.e., infinite resistance) will swing clockwise...

143 Foldback Circuit Principles As Applied To A Linear Supply

FOLDBACK output CURRENT LIMITING 1.115 14. FOLDBACK output CURRENT LIMITING 1.115 REGULATOR TRANSISTOR AND CURRENT LIMIT CIRCUIT REGULATOR TRANSISTOR AND CURRENT LIMIT CIRCUIT ED3. 1.14.2 (a) Foldback current limit circuit, (b) Regulator dissipation with reentrant protection. ED3. 1.14.2 (a) Foldback current limit circuit, (b) Regulator dissipation with reentrant protection. put voltage is zero (output short circuit). At short circuit, the current in R1 is very small, and the voltage across...

144 Lockout In Foldback Currentlimited Suppues

With the resistive load (the straight-line loads depicted in Figs. 1.14.1 and 1.14.3), there can only be one stable point of operation, defined by the intersection of the NON LINEAR LOAD LINE (LOCK OUT AT 'P2') EK3. 1.14.3 Overload and start-up characteristics of a foldback, current-limited supply, showing performance for linear and nonlinear load lines. NON LINEAR LOAD LINE (LOCK OUT AT 'P2') EK3. 1.14.3 Overload and start-up characteristics of a foldback, current-limited supply, showing...

145 Reentrant Lockout With Crossconnected Loads

Lockout problems can occur even with linear resistive loads when two or more foldback-limited power supplies are connected in series. (This series connection is often used to provide a positive and negative output voltage with respect to a common line.) In some cases series power supplies are used to provide higher output voltages. Figure 1.14,5a shows a series arrangement of foldback-limited supplies. Here, positive and negative 12-V outputs are provided. The normal resistive loads R1 and R2...

15 Energy Storage Phase

The energy storage phase is best understood by considering the action of the basic single-output flyback converter shown in Fig. 2.L2. When transistor Ql is turned on, the start of all windings on the transformer FIG. 2.1.2 Simplified power section of a flyback (buck-boost) converter. will go positive. The output rectifier diode D1 will be reverse-biased and will not conduct therefore current will not flow in the secondary while Q1 is conducting. During this energy storage phase only the...

154 Correct Turnoff Waveform

If the base current is reduced more slowly during the turn-off edge, the base-emitter diode will not be reverse-biased, and transistor action will be maintained throughout turn-off. The emitter will continue to conduct, and earners will continue to be removed from the complete surface of the chip. As a result, all parts of the chip discontinue conducting at the same instant. This gives a much faster turn-off collector-current edge, gives lower dissipation, and eliminates hot spots. However, the...

155 Correct Turnon Waveform

During the turn-on edge, the reverse of the above turn-off action occurs. It is necessary to get as much of the high-resistance region of the collector conducting as quickly as possible. To achieve this, the base current should be large, with a fast-rising edge thus earners are injected into the high-resistance region of the collector as quickly as possible. The turn-on current at the beginning of the on period should be consider ably highe than that necessary to maintain saturation during the...

16 Energy Transfer Modes Flyback Phase

When Q1 turns off, the primary current must drop to zero. The transformer ampere-turns cannot change without a corresponding change in the flux density LB. As the change in the flux density is now negative-going, the voltages will reverse on all windings (flyback action). The secondary rectifier diode Dl will conduct, and the magnetizing current will now transfer to the secondary. It will continue to flow from start to finish in the secondary winding. Hence, the set-ondary (flyback) current...

166 Widerange Proportional Drive Circuits

Where the range of input voltage and load are very wide, the circuit shown in Fig. 1.16.1 will have some limitations, as follows. When the input voltage is low, the duty-cycle will be large, and Ql may be on for periods considerably exceeding 50 of the total period. Further, if the minimum load is small, LI will be large to maintain continuous conduction in the output filter. Under these conditions, the collector current is small, but the on period is long. During the long on period, a...

168 Turnon Action

When Q2 is turned off, the starts of all windings will go positive by flyback action, and Q1 will be turned on. Regenerative drive from PI and Si maintains the drive, holding Q1 and Q3 on apd rapidly recharging Cl. This action is maintained until Q2 is turned on again to complete the cycle. The advantage of this arrangement is that the core can be reset rapidly by using a high auxiliary supply voltage without excessive dissipation in R1 and Q2. Hence, in this circuit the conflict between...

169 Proportional Drive With Highvoltage Transistors

IfQl is a high-voltage transistor, it is probable that some shaping of the base drive current will be required for reliable and efficient operation, as shown in Sec, 15.1 of Part 1. Figure 1.16.3 shows a suitable modification to the drive circuit in Figure 1.16.2 for high-voltage transistors base drive shaping has been provided by R4, D3, C2, R3, and Lb. EDS. 1.16.3 Push-pull-type proportional drive circuit with special drive current shap ing for high-voltage transistors. EDS. 1.16.3...

171 Complete Energy Transfer

If the flyback current reaches zero before the next on period of Ql, as shown in Fig. 2.1.5a, the system is operating in a complete energy transfer mode. That EE3. 2.1.5 (a) Primary current waveform I and secondary current waveforms I, (discontinuous-mode) operation, ib) Primary and secondary waveforms for incomplete energy transfer (continuous-mode) operation. EE3. 2.1.5 (a) Primary current waveform I and secondary current waveforms I, (discontinuous-mode) operation, ib) Primary and secondary...

172 I ncom plete Energy Transfer

If, in the circuit example shown in Fig. 2.1.2, the on period is increased and the off' period correspondingly decreased, more energy is stored in he transformer during the on period. For steady-state operation, this extra energy must be extracted during the off' period. If the input and output voltages are to be maintained constant, it will be shown that the load current must be increased to remove the extra energy. The slope of the input and output current characteristics cannot change,...

18 Transfer Function Anomaly

Ve effective volume of core and air gap 1. MULTIPLE OUTPUTFLYBACK SWITCHMODE POWER SUPPLIES 2.13 This power is proportional to the shaded area to the left of the BIH curve in Fig. 2.1.6 it is clearly larger for the example in Fig. 2.1.66 (the incomplete energy transfer case). Much of the extra energy is stored in the air gap consequently, the size of the air gap will have a considerable effect upon the transmissible power. Because of the very high reluctance of the air gap, it is quite usual to...

1810 The Wea Ving Lowloss Snubber Diode

As shown above, to reduce secondary breakdown stress during the turn-off of high-voltage bipolar transistors, it is normal practice to use a snubber network. Unfortunately, in normal snubber circuits, a compromise choice must be made between a high-resistance snubber (to ensure a low turn-on current) and a low-resistance snubber (to prevent a race condition at light loads where narrow pulse widths require a low CR time constant). This paradox often results in a barely satisfactory compromise....

182 Snubber Circuit With Load Line Shaping

Figure 1.18. la shows the primary of a conventional single-ended flyback converter circuit PI, Q1 with a leakage inductance energy recovery winding and diode P2, D3. Snubber components Dl, Cl, and R1 are fitted from the collector to the emitter of Ql. Figure 1.18,16 shows the voltage and current waveforms to be expected in this circuit. If load line shaping is required, then the main function of the snubber components is to provide an alternative path for the inductively maintained primary...

185 Establishing Snubber Component Values By Calculation

Figure 1.18.1b shows typical turn-off waveforms when the snubber network Dl, CI, R1 shown in Fig. 1.18.1 is fitted. In this example, CI was chosen such that the voltage on the collector Vce will be 70 of the VceQ rating of Ql when the collector current has dropped to zero at time f2. Assuming that the primary inductance maintains the primary current constant during the turn-off edge, and assuming a linear decay of collector current in Ql from t, to t2, the snubber current I, will increase...

188 Dissipation In Snubber Resistor

The energy dissipated in the snubber resistor during each cycle is the same as the energy stored in Cl at the end of the off' period. However, the voltage across Cl depends on the type of converter circuit. With complete energy transfer, the voltage on Cl will be the supply voltage Vcc, as all flyback voltages will have fallen to zero before the next on period. With continuous-mode operation, the voltage will be the supply voltage plus the reflected secondary voltage. Having established the...

189 Miller Current Effects

When measuring the turn-off current, the designer should consider the inevitable Miller current that will flow into the collector capacitance during the turn-off edge. This effect is often neglected in discussions of high-voltage transistor ac tion. It results in an apparent collector-current conduction, even when Ql is fully turned off. Its magnitude depends on the rate of change of collector voltage (dVcldt) and collector-to-base depletion capacitance. Further, if the switching transistor Ql...

2022 Type 2 Boost Regulators

Figure 2,20.2a shows the general arrangement of the power sections of a boost regulator. The operation is as follows. When Ql turns on, the supply voltage will be impressed across the series inductor LI. Under steady-state conditions, the current in LI will increase linearly in the forward direction. Rectifier D1 will be reverse-biased and not conducting. At the same time (under steady-state conditions), current will be flowing from the output capacitor CI into the load. Hence, CI will be...

203 Parasitic Effects In Switchmode Output Filters

Figure 1.20.1a shows a single-stage LC output filter (such as might be found in a typical forward converter. It includes the parasitic elements Cc, R ESL, and ESR. The series inductor arm LI shows an ideal inductor L in series with the inevitable winding resistance R. The parasitic distributed interwinding capacitance is included as lumped equivalent capacitor Cc, The shunt capacitor CI includes the effective series inductance ESL and the effective series resistance ESR. The equivalent circuit...

205 Highfrequency Choke Example

To get the best performance from the high-frequency choke L2, the interwinding capacitance should be minimized. Figure 1.20.3a shows a 1-in-long ferrite rod choke with a yi6-in diameter, wound with 15 turns of closely packed 17 AWG wire. Figure 1.20,36 shows a plot of phase shift and impedance as a function of frequency for this choke. The phase shift is zero at the self-resonant frequency, which in this case is 4.5 MHz. The impedance plot in Fig. 1.20.3c shows the improvement obtained by...

205 Inductor Design Example

Calculate the inductance required for a 10-A, 5-V type 1 buck regulator operating at 40 kHz with an input voltage from 10 to 30 V, when the ripple current is not to exceed 20 of IDC (2 A). Procedure Maximum ripple current will occur when the input voltage is maximum that is, when the voltage applied across the inductor is maximum. 1. Calculate the on time when the input is 30 V. where tp total period (ton + foff) 2. Select the peak-to-peak ripple current. This is by choice 20 of DC, or 2 A in...

206 Resonant Filters

By selecting capacitors such that their self-resonantfrequency is near the switching frequency, the best performance will be obtained. Many of the small, low-ESR electrolytic capacitors have a series self-resonant frequency near the typical operating frequencies of switchmode converters. At the self-resonantfrequency, the parasitic internal inductance of the capacitor resonates with the effective capacitance to form a series resonant circuit. At this frequency, the capacitor impedance tends to...

207 Resonant Filter Example

Figure 1.20.5 shows a typical output stage of a small 30-kHz, 5-V, 10-A flyback converter with a two-stage output filter. (In flyback converters, the transformer inductance and CI form the first stage of the J.C power filter.) A second stage high-frequency filter L2, C2 has been added. For this example, the same 1 in, 5 i6-in-diameter ferrite rod inductor used to obtain plot c in Fig. 1.20.3 is used for L2. The 15 spaced turns on this rod give an inductance of 10 fi.H and a low interwinding...

207 The Ripple Regula

A control technique which tends to be reserved for the buck-type switching regulator is the so-called ripple regulator.17 This is worthy of consideration here, as it provides excellent performance at very low cost. The ripple regulator is best understood by considering the circuit of the buck regulator shown in Fig. 2.20,5a. A high-gain comparator amplifier A1 compares a fraction of the output voltage Vou, with the reference VR when the output fraction is higher than the reference, the series...

21 Introduction

Because the flyback converter transformer combines so many functions (energy storage, galvanic isolation, current-limiting inductance), and also because it is often required to support a considerable DC current component, it can be rather more difficult to design than the more straightforward push-pull transformer. For this reason, the oliowin section is entirely devoted to the design of such transformers. To satisfy the design requirement, many engineers prefer to use an entirely mathematical...

2112 The Case For Constantvoltage Or Constantcurrent Reset Highfrequency Instability Considerations

At high frequencies the area of the BIH loop increases, giving an increased core loss and a general degradation of the desirable magnetic properties. In particular, some materials show a modification of the BIH loop to a pronounced S-shaped characteristic. This S shape can lead to instability if constant-current resetting is used in the control circuit. This effect is best understood by considering Fig. 2.21.10. If constant-current reset is used, then the magnetizing force H is the controlled...

21142 Step 2 Calculate the Minimum Secondary Voltage Required from the Converter Transformer

The maximum on time is 50 of the total period, or 14.3 jjls at 35 kHz. When the SR is fitted, there will be an unavoidable minimum delay on the leading edge of the on pulse, as a result of the time required to take the core from B, to 5sat, even when the reset current is zero. Previous experience with the 6025 material using fast diodes indicates that this delay will typically be 1.3 (as. (The actual value can be calculated when the turns, core size, and secondary voltage have been...

212 Operating Principles

In simple terms, the saturable reactor is. used in high-frequency switchmode supplies as a flux-saturation-controlled power switch, providing regulation by secondary pulse-width control techniques. The method of operation is best explained by considering the conventional buck regul'*''- circuit shown in Fig. 2.21.1. This figure shows the output LC filter and rectif such as would be found on the secondary of a typical single-ended FIG. 2.21.1 . Typical secondary output rectifier and filter cir-...

213 Simple Power Failure Warning Circuits

Figure 1.21.1 shows a simple optically coupled circuit typical of those often used for power failure warning. However, it will be shown that this type of circuit is suitableonly for type 1 failures, that is, totallinefailureconditions.lt operates as follows. The ac line input is applied to the network R1 and bridge rectifier D1 such that unidirectional current pulses flow in the optical coupler diode. This maintains a pulsating conduction of the optical coupler transistor Ql. While this...

214 Dynamic Power Failure Warning Circuits

The more complex dynamic power failure warning circuits are able to respond to brownout conditions. Many types of circuit are in use, and it may be useful to examine some of the advantages and disadvantages of some of the more common techniques. Figures 1.21.2 and 1.21.3 show two circuits that will ensure that sufficient warning of failure is given for all conditions. In the first example, a fraction of the DC voltage on the power converter reservoir capacitors CI and C2 is compared with a...

215 Independent Power Failure Warning Module

The previous two power failure circuits must be part of the power supply, as they depend on the internal DC header voltage for their operation. Figure 1.21.4 shows a circuit that will operate directly from the line input and is independent of Ihis circuit has its own bridge rectifier D1-D4, which again provides a unidirectional half-sine-wave input to the feed resistor Rl, ZD1, and the optical cou- EE3. 1.21.4 Independent power failure module for direct operation from ac line inputs. EE3....

215 Saturable Reactor Quality Factors

The effectiveness of the saturable reactor as a power switch will be determined by several factors as follows The magnetization current can be considered a leakage current in the off' state of the switch. The reactor's quality as an off' switch that is, its maximum impedance_ will be defined by its maximum inductance. This in turn depends on the permeability of the core in the unsaturated state and the number of turns. Increasing the number of turns will, of course, increase the inductance and...

216 Selecting Suitable Core Ma Terials

The ideal core material would match the ideal BIH characteristic shown in Fig. 2.21.3 as closely as possible. That is, it would exhibit high permeability in the nonsaturated state (values from 10,000 to 200,000 are possible), and the saturated permeability and hysteresis losses would be very low. To minimize the turn-on delay, the squareness ratio BJBS should be as high as possible (values between 0.85 and 0.95 are realizable). The hysteresis and eddy-current losses should be small to minimize...

217 Controlling The Saturable Reactor

As explained in Sec. 21.2, to control the saturable reactor in switching regulator applications, it is necessary to reset the core during the off' period to a defined position on the BIH characteristic prior to the next forward power pulse. The reset (volt-seconds), may be applied by a separate control winding (transductor or magnetic amplifier operation see Fig. 2.21.7) or by using the same primary power winding and applying a reset voltage in the opposite direction to the previous power pulse...

217 Fast Power Failure Warning Circuits

The previous systems shown in this section respond quite slowly to brownout conditions, because they are sensing peak or mean voltages. The filter capacitor in the warning circuit introduces a delay. Its value is a compromise, being low enough to prevent a race between the holdup time of the power supply and the time constant of the filter capacitor, but large enough to give acceptable ripple voltage reduction. It is possible to detect the imminent failure of the line before this has fully...

218 Current Limiting The Saturable Reactor Regulator

Figure 2.21.8 shows a simple current-limiting circuit which operates as follows. When the output current is such that the voltage across RI exceeds 0.6 V -< n- FIG. 2.21.8 Saturable reactor buck regulator with current-limiting circuit R1 and Q2. FIG. 2.21.8 Saturable reactor buck regulator with current-limiting circuit R1 and Q2. sistor Q2 will conduct and provide a reset current via D2 when the secondary voltage goes negative, thus limiting the maximum current. When this type of current...

219 Pushpull Saturable Reactor Secondary Power Control Circuit

The discussion so far has been limited to single-ended systems. In such systems, the same time (volt-seconds) is required to reset the core during the off period as was applied to the core to set it during the on period. Therefore, if control is to be maintained under short-circuit conditions, the duty ratio cannot exceed 50 unless a high-voltage reset circuit is provided or a reset tapping point is provided on the SR winding. In the push-pull system shown in Fig. 2.21.9, two saturable reactors...

221 AC Polarization

It is clear that the flux density in the core must change at a rate and amplitude such that the induced (back) emf in the winding is equal to the applied emf (losses are assumed to be negligible). Hence, to support the ac voltage applied to the primary (more correctly, the applied volt-seconds), a change in flux density kBaa is required. (This is shown on the vertical axis in Fig. 2.2. lb. The amplitude of AB, is therefore proportional to the applied voltage and the on period of the switching...

222 Example

Consider the triple-output forward-converter secondary circuit shown in Fig. 1.22.1. Assume that the 5-V output is a closed-loop regulated output, fully stabilized and adjusted. There are two auxiliary 12-V outputs, positive and negative, which are now semiregulated as a result of the closed-loop,control on the 5-V line. Assume that the regulation performance required from the 12-V outputs is such that additional series regulators would aot normally be required (say, 6 ), Further assume that to...

222 The Effect of an Air Gap on the AC Conditions

It is clear from Fig. 2.2.1 b that increasing the core gap results in a decrease in the slope of thpfBi characteristic but does not change the required AEi Hence there is an increase in the magnetizing current iHt This corresponds to an effective reduction in the permeability of the core and a reduced primary inductance. Hence, a core gap does not change the ac flux density requirements or otherwise improve the ac performance of the core. A common misconception is to assume that a core which is...

223 Constantcurrentsuppues

The constant-current supply is not so well known, and therefore the concept can be a little more difficult to grasp. In the constant-current supply, the previous FIG. 2.22.1 Output characteristics of a constant-voltage power supply, showing constant-current and reentrant-current protection locus. FIG. 2.22.1 Output characteristics of a constant-voltage power supply, showing constant-current and reentrant-current protection locus. constant-voltage characteristics are reversed. Figure 2.22.2...

223 The Effect of an Air Gap on the DC Conditions

A DC current component in the windings gives rise to a DC magnetizing force Hjx on the horizontal H axis of the BIHloop, ( dc is proportional to the mean DC ampere-turns.) For a defined secondary current loading, the value of DC is defined. Hence, for the DC conditions, B may be considered the dependent variable. It should be noted that the gapped core can support a much larger value of H (DC current) without saturation. Clearly, the higher value of H, HDC2, would be sufficient to saturate the...

232 60hz Line Transformers

Very often small 60-Hz transformers will be used to develop the required auxiliary power. Although this may be convenient, as it allows the auxiliary circuits to be energized before the main converter, the 60-Hz transformer tends to be rather large, as it must be designed to meet the insulation and creepage requirements of the various safety specifications. Hence, the size, cost, and weight of a 60-Hz auxiliary supply transformer tends to make it less attractive for the smaller switchmode...

2321 Preregulator Operation

The nominal 70-V DC nonregulated header voltage VH is applied to a network of resistor R1 in parallel with transistor Q2, in series with the normal linear regulator transistor Ql, as shown in Fig. 2.23.1. When the power supply has been set to give a low output voltage (for exam- pie, 0 V at 2 A), transistor Q2 is turned off, and the majority of the applied header voltage will appear across Rl. Hence, the maximum dissipation will appear in the resistor, and 01 is relieved of the high-stress...

233 Drive Circuit

Figure 2.23.2 shows the basic elements of the drive circuit for the cascaded power sections. VH*+70Y Rl 'C Ql +Y OUT 0-60V Figure 2.23.2 shows the basic elements of the drive circuit for the cascaded power sections. VH*+70Y Rl 'C Ql +Y OUT 0-60V The operation is best understood by considering four extreme operating conditions. These will be 1. Low-output-voltage, high-current conditions 2. High-output-voltage, high-current conditions 3. Intermediate-voltage, high-current conditions (say, 30 V,...

235 Distribution Of Power Losses

Figure 2.23.3 shows how the power losses are distributed between the two power transistors Ql and Q2 and the series resistor Rl over the output voltage range for the maximum output current of 2 A. Note that the peak power conditions for transistors Ql and Q2 occur at different voltages and that both devices can be mounted on the same heat sink. This need be rated only for the worst-case combination, which never exceeds 41 W. This is considerably lower than the 140 W that would have been...

235 Stabilized A Uxiliary Converters

Many variations of this basic self-oscillating converter are possible. By using a high-voltage zener on the input side, it is possible to provide stabilized auxiliary outputs and also maintain a constant operating frequency. 110 220 ''NRUSH LINE LIHITING U CIRCUIT 110 220 ''NRUSH LINE LIHITING U CIRCUIT ED3. 1.23.2 Stabilized auxiliary power converter of the self-oscillating flyback type, with energy recovery winding P3 and synchronization input Q2. ED3. 1.23.2 Stabilized auxiliary power...

236 Highefficiency A Uxiuary Suppues

Figure 1.23.3 shows a more efficient version of the previous circuit, in which the loss incurred in the feed resistors R1 and R2 has been eliminated by using a sep- big. 1.23.3 Aiudliarypower converter of the self-oscillating flyback type with a j iq_v ac cooling fan supply, suitable for use with 110-v ac fans in dual input voltage applications. big. 1.23.3 Aiudliarypower converter of the self-oscillating flyback type with a j iq_v ac cooling fan supply, suitable for use with 110-v ac fans in...

236 Voltage Control And Current Limit Circuit

Laboratory variable supplies are usually designed to provide constant-voltage or constant-current performance with automatic crossover between the two modes. FIG. 2.23.3 Distribution of power loss in piggyback linear power supply. FIG. 2.23.3 Distribution of power loss in piggyback linear power supply. Figure 2.23.4 shows a typical output characteristic with the supply set for 30 V and 1 A. Load lines for 60 il, 15 tl, and the critical value 30 (i are shown. It will be seen that the mode of...

237 Control Circuit

A suitable control circuit is shown in Fig. 2.23.5. Amplifier A1 provides the voltage control, and amplifier A2, the constant-current control. The method of operation is best explained by considering the conditions when a 60-il load is applied to the output with the controls set as specified above. Under these conditions, the supply is in the constant-voltage mode, and amplifier A1 will compare the internal reference voltage (TL 431) with the voltage developed across the divider network R15,...

24 1 Dividual Block Functions

. us elements of the block schematics in Figs. 2.24.4, 2.24.5, and 2.24.6. re considered in more detail. 24. SWnCHMODE VARIABLE PCWER SUPPLIES Figure 2.24.4 shows the internal circuit for the input power section, block 1 the auxiliary supply, block 2 and the power converter, block 3. Block 1, Input Filter. In block 1 the ac line input is taken via the supply switch SW1 and fuse FSI to the input filter inductor LI. Inrush current limiting is provided by thermistor THI in series with the...

241 Step 1 Select Core Size

If a typical secondary efficiency of 85 is assumed (output diode and transformer losses only), then the power transmitted by the transformer would be 130 W. We do not have a simple fundamental equation linking transformer size and power rating. A large number of factors must be considered when making this selection. Of major importance will be the properties of the core material, the shape of the transformer (that is, its ratio of surface area to volume), the...

242 Masterslave Operation

In this method of parallel operation, a designated master is selected, and this is arranged to provide the voltage control and drive to the power sections of the remainder of the parallel units. Figure 1.24.1 shows the general arrangement of the master-slave connection. Two power supplies are connected in parallel. (They could be switching or linear supplies.) Both supplies deliver current to a common load. An interconnection is made between the two units via a link (this is normally referred...

243 Special Properties Of Flyback Converters

An investigation of the flyback technique (see Chap. 1) reveals a very useful property of flyback converters The energy storage cycle and the energy transfer cycle may be considered entirely independent operations. Consider the simple diagonal half-bridge flyback power section shdfcvn in Fig. 124.2. During the period when both FETs are on, energy is being stored in the transformer magnetic field. Since the secondary is not conducting when the FETs ire on, the transformer may be considered a...

243 Voltagecontrolled Current Sources

This method of parallel operation relies on a principle similar to that of the master-slave, except that the current-sharing P-terminal connection is made at a much earlier signal level in the control circuit. The control circuit is configured as a voltage-controlled current source. The voltage applied to the P terminal will define the current from each unit, the total current being the sum of all the parallel units. The voltage on the P terminal, and hence the total current, is adjusted to...

244 Forced Current Sharing

Current Sharing Power Supply

This method of parallel operation uses a method of automatic output voltage adjustments on each power supply to maintain current sharing in any number of parallel units. This automatic adjustment is obtained in the following way. Because the output resistance in a constant-voltage supply is so low (a few milliohms or less), only a very small output voltage change is required to make large changes in the output current of any unit. With forced current sharing, in principle any number of units...

244 Operating Principles

Consider once again the flyback power section shown in Fig. 2.24.2. Assume energy has been stored in the main transformer T1 during the on period. It will now be shown that this energy may be transferred to the load as any combination of current and voltage, provided that the power conservation criteria are satisfied. The first requirement that must be satisfied (if the core is to continue to operate at a defined flux density under steady-state conditions) is that the equality of the forward...

245 Parallel Redundant Operation

The purpose of parallel redundant operation is to ensure maintenance of power even in the event of one power supply failure. In principle, n supplies (where n is two or more) are connected in parallel to supply a load that has a maximum demand that is n 1 of the total combination rating. Hence, if a ttipply fails, the remainder of the units will take up the load without an interruption in the service. In practice, the failed supply may short-circuit (for example, the SCR overvoltage crowbar may...

245 Practical Limiting Factors

As previously mentioned, with the circuit shown in Fig. 2.24.2, the flyback voltage as referred to the primary cannot exceed the input voltage Vcc, as diodes Dl and D2 would then conduct the flyback energy back to the supply and it would not be transferred to the output. Assuming that Vcc is 300 V, the secondary turns for maximum 60 V output must be at least 20 of the primary turns. 2. Maximum On Period. If the lower-voltage operating point P5 is now considered, the...

248 Step 8 Establishing Core Gap Size

Figure 2.2.1a shows the full hysteresis loop for a typical ferrite material with and without an air gap. It should be noted that the gapped core requires a much larger value of magnetizing force H to cause core saturation hence, it will withstand a much larger DC current component. Furthermore, the residual flux density B, is much lower, giving a larger usable working range for the core flux density, AB. However, the permeability is lower, resulting in a smaller...

249 Block Schematic Diagram General Description

Figure 2.24.3 is a block schematic of the basic functional elements of the complete variable switchmode supply. The major functions are described below. Block . Block 1 is the input filter, voltage doubler, and rectification circuit. It converts the 1151230-V ac input to a nominal 300-V DC output to the converter section, block 3. Block 1 also contains current limiting to prevent excessive inrush current when the system is first switched on, and the voltage doubler option for 115-V operation....

2511 General Considerations

In the flyback converter design in Chap. 24, the flyback voltage (as referred to the primary winding) cannot exceed the supply voltage, as diodes D2 and D3 would then return the flyback energy to the primary circuit, and it would not be transmitted to the output. The need to satisfy the above requirement defines the turns ratio that must be used at the maximum output voltage of 60 V. Further, since the output voltage covers a wide range (from 0 to 60 V), and the secondary turns cannot be...

274 The Saturable Reactor Power Regulator Application

Consider a reactor wound on a core of ideal square-loop material and fitted in series with output rectifier diode D1 (position A in Fig. 2.21.1). This gives the circuit shown in Fig. 2.21.4. EEG. 2.21.4 Single-winding saturable reactor regulator with simple voltage-controlled reset transistor Ql. EEG. 2.21.4 Single-winding saturable reactor regulator with simple voltage-controlled reset transistor Ql. In the circuit shown in Fig. 2.21.4, assume that the core is unsaturated at a point S3 on the...

32 Selftracking Voltage Clamp

When a transistor in a circuit with an inductive or transformer load is tuned off, the collector will tend to fly to a high voltage as a result of the energy stored in the magnetic field of the inductor or leakage inductance of the transformer. In the flyback converter, the majority of the energy stored in the transformer will be transferred to the secondary during the flyback period. However, because of the leakage inductance, there will still be a tendency for the collector voltage to...

33 Flyback Converter Snubber Networks

The turn-off secondary breakdown stress problem is usually dealt with by snubber networks a typical circuit is shown in Fig. 2.3.2. The design of the snubber network is more fully covered in Part 1, Chap. 18. Snubber networks will be required across the switching transistor in off-line FIG. 2.3.2 Dissipative snubber circuit applied to the collector of an off-line flyback converter. flyback converters to reduce secondary breakdown stress. Also, it is often necessary to snub rectifier diodes to...

39 Line Filter Design

The design approach used in Sees. 3.4 through 3.8 was to consider the line filter as an attenuating voltage divider network for common-mode RF noise. This approach is used in preference to normal filter design techniques, asj he source and load impedances are not definable in the powerline environment. The interference noise generator, in switchmode supplies, is very often a high-voltage source in series with a high impedance this tends to a constant-current source. To give good attenuation,...

42 Primary Components

4.2.1 Input Rectifiers and Capacitors There are no special requirements imposed on the input rectifiers and storage capacitors in the flyback converter. Hence these will be similar to those used in other converter types and will be selected to meet the power rating and hold-up requirements. (See Part 1, Chap. 6.) 4.2.2 Primary Switching Transistors The switching transistor in a flyback supply is very highly stressed. The current rating depends on the maximum load, efficiency, input voltage,...

53 Types Of Fuses

A time-delay fuse will have a relatively massive fuse element, usually of low-melting-point alloy. As a result, these fuses can provide large currents for relatively long periods without rupture. They are widely used for circuits with large inrush currents, such as motors, solenoids, and transformers. These fuses are low-cost and generally of more conventional construction, using copper elements, often in clear glass enclosures. They can handle short-term high-current-transients, and because of...

612 Dc Output Voltage And Regulation For Rectifier Capacitor Input Filters

It has been shown 26, 83 that provided that the product to X Cg X RL > 50, the DC output voltage of the rectifier capacitor input filter (with a resistive load) will be defined mainly by the effective series resistance Rs and load power. However, when the ripple voltage is low, this criterion also holds for the nonlinear converter-type load. Figures 1.6.7 and 1.6.8 show the mean DC output voltage of the rectifier capacitive input filter as a function of load power and input m s voltage up to...

614 Selecting Reservoir Andor Filter Capacitor Size

In the above example, the reservoir and or filter capacitor values were chosen to meet the rather simplistic Ce 1.5 aF W criterion indicated in Sec. 6.12. In practice, one or more of the following five major factors may control the selection RMS ripple current rating Ripple voltage Voltage rating Size and cost Holdup time This rating must be satisfied to prevent excessive temperature rise in the capacitor and possible premature failure. (See Part 3, Chap. 12.) The problem at this stage is to...

62 Classes Of Operation

There are three classes of operation Type A, fured on time, variable off' time Type B, fured off' time, variable on time Type C, variable on time, off time, and repetition rate (frequency) The major differences in the performances of these classes are as follows Type A will operate at an extremely low frequency when the load is light. Type B will have a low frequency when the load is maximum. Type C has a more desirable characteristic, as the frequency remains reasonably constant from full load...

63 General Operating Principles

In the self-oscillating converters considered here, the switching action is maintained by positive feedback from a winding on the main transformer. The frequency is controlled by a drive clamping action which responds to the increase in magnetization current during the on period. The amplitude at which the primary current is cut off, and hence the input energy, is controlled to maintain the output voltage constant. The frequency is subject to variations caused by changes in the magnetic...

64 Isolated Selfoscillating Flyback Converters

A more practical implementation of the self-oscillating technique is shown in Fig. 2.6.4. In this example, the input and output circuits are isolated, and feedback is provided by an optical coupler OC1. Components D3, C4, and R8form a self-trackingvoltage clamp (see Sec. 3.2). This clamp circuit prevents excessive collector voltage overshoot (which would have been generated by the primary leakage inductance) during the turn-off action of Ql. Components D1 and C3 are the rectifier and storage...

65 Control Circuit Briefdescription

A very simple control circuit is used. The diode of the optical coupler OC1 is in series with a limiting resistor R9 and a shunt regulator U1 (Texas Instruments TL430). When the reference terminal of the shunt regulator VI is taken to 2.5 V, current will start to flow into the cathode of VI via the optocoupler diode, and control action is initiated. The ratio of R12 and Rll is selected for the required output, in this case 12 V. The optocoupler transistor responds to the output control circuit...

66 Rectifier And Capacitor Waveforms

Figure 1.6,3a shows the familiar full-wave rectifier waveforms that would be obtained from the circuit shown in Fig. 1.6.2. The dashed waveform is the half EIG. 1.6.3 Rectifier and capacitor voltage and current waveforms in a full-wave capacitor input filter, (a) Capacitor voltage waveform ( rectifier diode current waveform (c) capacitor current waveform. EIG. 1.6.3 Rectifier and capacitor voltage and current waveforms in a full-wave capacitor input filter, (a) Capacitor voltage waveform (...

676 Power Factor And Efficiency Measurements

From Fig. 1.6.3, it can be seen that the input voltage is only slightly distorted by the very nonlinear load presented by the capacitor input filter. The sinusoidal input is maintained because the line input resistance is very low. The input current, however, is very distorted and discontinuous, but superficially would appear to be a part sine wave in phase with the voltage. This leads to a common error The product Vin(rms) x in(rms) is assumed to give input power. This is not so This product...

68 Effective Input Current Ie And Power Factor

In Figs. 1.6.4, 1.6.5, and 1.6.6, the rms input, peak, and ripple currents are all given as a ratio to a calculated effective input current I, FIG. 1.6.4 RMS input current as a function of loading, with source resistance factor R as a parameter. Rsfa son -W I Hsf*1Son-W V BRIDGE Rsf soon -w I Rsf ' 50 il W Rsf 150 -W I VOLTAGE Rsf 500 12 -W f DOUBLER Rsfa son -W I Hsf*1Son-W V BRIDGE Rsf soon -w I Rsf ' 50 il W Rsf 150 -W I VOLTAGE Rsf 500 12 -W f DOUBLER FIG. 1.6.5 RMS filter...

72 Power Limiting And Currentmode Control As Appued To The Selfoscillating Flyback Converter

The self-oscillating complete energy transfer flyback converter responds particularly well to the application of current-mode control. This will be explained by reference to the circuit shown in Fig. 2.6. . The voltage across R4 (which sets the maximum collector current) cannot ex- ceed 0.6 V under any conditions, because at this point Q2 will turn on, turning off the power device Ql. This will occur irrespective of the condition of the voltage control circuit, because the control circuit...

725 Operating Principles Practical Circuit

A bias voltage is set up on the base of Ql by the current in Rl, Dl, and D2. Q1 conducts to develop a second bias voltage across R2 of approximately one diode drop (0.6 V). The current flow in R3 is similar to that in R2, and a third bias voltage is set up across R3 which is slightly less than that across R2, since the resistance of R3 is lower than that cf R2, Hence, under quiescent conditions, transistor Q2 is close to conducting. At the same time, capacitor C3 will charge through R4,R2,...