111 Specification Example For A 110w Directoffline Flyback Power Supply

For the following example, a fixed-frequency single-ended bipolar flyback unit with three outputs and a power of 110 W is to be considered. It will be shown later that the same design approach is applicable to variable-frequency self-oscillating units. Although most classical design approaches assume that the mode of operation will be either entirely complete energy transfer (discontinuous mode) or entirely incomplete energy transfer (continuous mode), in practice a system is unlikely to remain...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

208 Commonmode Noise Filters

The discussion so far has been confined to series-mode conducted noise. The f j. ter described will not be effective for common-mode noise, that is, noise voltages appearing between the output lines and the ground plane. The common-mode noise component is caused by capacitive or inductive coupling between the power circuits and the ground plane within the p & supply. Initially this must be reduced to a minimum by correct screening and layout at the design stage. Further reduction of the...

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...

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...

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...

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...

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 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...

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 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...

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...

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...

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...

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....

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...

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...

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...

73 Voltage Control Loop

Figure 2.7.1 shows the collector and emitter current waveforms of Ql under steady-state voltage-controlled conditions. The emitter cunrent waveform shows a DC offset as a result of the base drive current component Ib. Afi analogue voltage of the emitter current will be developed across R4. The voltage across R5 (Vrj) shows the effect of the snubber current in R5 imposed on the voltage across R4, resulting in a rapid increase in voltage toward the end of the conduction period. The voltage...

739 Type 2 Output Constant Current Limiting

Power supplies and loads can be very effectively protected by limiting the maximum current allowed to flow under fault conditions. Two types of current limiting are in common use, constant current and foldback current limiting. The first type, constant current limiting, as the name implies, limits the output current to a constant value if the load current tries to exceed a defined maximum. A typical characteristic is shown in Fig. 1.13.1. From this diagram, it can be seen that as the load...

746 Foldback Current Limits In Switchmode Supplies

The previous limitations would also apply to the application of foldback protection in switchmode supplies. However, in switchmode units, the dissipation in the control element is no longer a function of the output voltage and current, and the need for foldback current protection is eliminated. Consequently, foldback protection should not be specified for switching supplies. It is not necessary for protection of the supply and is prone to serious application problems, such as lockout. For this...

83 Transistor Active Start Circuit

Figure 1.8.2shows the basic circuit of a more powerful and fast-acting start system, incorporating a high-voltage transistor Ql. In this arrangemiflt, the resistance of R1 and R2 and the gain of Ql are chosen such that transistor Ql will be biased into a fully saturated on state soon after initial switch-on of the supply. As CI and C2 charge, current flows in R1 and R2 to the base of Ql, turning Ql fully on. Zener diode ZD1 will not be conducting initially, as the voltage on C3 and the base of...

84 Impulse Start Circuits

Figure 1.8.3 shows a typical impulse start circuit which operates as follows. Resistors R1 and R2 (normally the discharge resistors for the reservoir capacitors CI and C2) feed current into capacitor C3 after switch-on. The auxiliary supply capacitor C4 will be discharged at this time. The voltage on C3 will increase as it charges until the firing voltage of the diac is reached. The diac will now fire and transfer part of the charge from C3 into C4, the transfer current being limited by...

922 Step 2 Select Optimum Induction

The optimum induction Bopt is chosen so as to make the core and copper losses approximately equal. This gives minimum overall loss and maximum efficiency, provided that core saturation is avoided. From Fig. 2.9.1, at 100 W and a frequency of 30 kHz, an optimum peak flux density Bopt of approximately 150 mT is recommended for push-pull operation. Remember, in the push-pull case, the differential excitation (AB) will be twice this peak value, giving a flux density swing of 300 mT p-p. (See Fig....

925 Multiple Output Applications

Figure 2.9.4 shows a typical multiple-output forward converter secondary, in which all outputs share a common return line. The negative output is developed by reversing D5 and D6. Note that the phasing of the secondary is such that D3 and D5 conduct at the same time during the on period of Ql. Figure 2.9.4 shows a typical multiple-output forward converter secondary, in which all outputs share a common return line. The negative output is developed by reversing D5 and D6. Note that the phasing of...

Ac Powerline Surge Protection

With the advent of direct-off-line switchmode power supplies using sensitive electronic primary control circuits, the need for input AC powerline transient surge protection has become more universally recognized. Measurements carried out by the IEEE over a number of years have demonstrated, on a statistical basis, the likely frequency of occurrence, typical amplitudes, and waveshapes to be expected in various locations as a result of artificial and naturally occurring electrical phenomena....

Dctodc Switching Regulators

The following range of DC-to-DC converters, in which the input and output share a common return line, are often referred to as three-terminal switching regulators. Functionally, switching regulators have much in common with three-terminal linear regulators, taking unregulated DC inputs and providing regulated DC outputs. They will often replace linear regulators when higher efficiencies are required. Switching regulators are characterized by the use of a choke rather than a transformer between...

Dutyratiocontrolled Pushpull Converters

The push-pull converter is not generally favored for off-line applications, because the power switches operate at collector stress voltages of at least twice the supply voltage. Also, the primary utility factor for the main transformer is not as good as in the half-bridge or full-bridge converter, because a center-tapped primary is used, and only half of the winding is active at each half cycle. However, at low input voltages, the push-pull technique has some advantages over the half or full...

F

Recommended, as the distributed capacitance can degrade the transient per- In power supplies where provision is made to interlink one or more units in a parallel forced current sharing mode, current sharing communication between supplies is required. This link is normally referred to as the P-terminal link. In master-slave applications this link allows the master to control the output regulators of the slave units. In forced current sharing applications this link provides communication between...

Forward Converters

The transformer for the converter shown in Fig. 2.10.1 follows the same general principles used for the single-ended forward converter. The major difference is that an energy recovery winding will not be required. Unfortunately, there are no fundamental equations for the selection of core sizes. The choice depends on a number of variables, including the type of material, the shape and design of the core, the location of the core, the type of cooling provided, and the allowable temperature rise....

J

ED3. 2.13.2 Primary voltage and current waveforms for full-bridge converter. Consider a cycle of operation under steady-state conditions in Fig. 2.13.1. Assume that the drive circuit initiates a turn-on pulse for Ql and Q3. These two devices will start to turn on. Collector current will now flow via the primary winding of TIP through thp primary of the proportional drive transformers T2A and T2B. By positive regenerative feedback, the turn-on action of the two transistors is enhanced, and this...

Kn Vout x ton

When Ql is off, the diode is conducting, and the volt-seconds applied to LI is Hence, to meet the volt-seconds equality on LI (neglecting losses), ( in Vout) X on Vout X i0ff Vout x (d *) output voltage off' time, Ql iiput voltage defined as the - -uty ratio D, the preceding equation will simplify to Since D caniv- greater than unity, it will be clear from this equation that accounting for K-vis, the output voltage must always be less than the applied input voltage in the > -> * regulator....

Multipleoutput Compound Regulating Systems

It has been shown9-10-14'15'16,17 that it is possible to combine DC-to-DC transformers (nonregulated DC-to-DC converter circuits) with buck- or boost-derived regulators to produce regulated single- or multiple-output converter combinations. Using integrated magnetics and various modeling techniques,16 a wide range of possible combinations have been demonstrated. In many cases, these combinations are not clearly distinguishable as combinations of converter . Various advantages and disadvantages...

Np y AB X Ae

Change in flux density from Br to 5sat, T In this example, the secondary voltage V, applied to the core at the start of the on period may be calculated from the duty ratio and the output voltage as follows where V t required output voltage, V ton on period, (j,s 'off off' period, p.s

The Dctodc Transformer Concept

Throughout the previous designs, the high-frequency transformer has been considered an integral part of the converter topology. However, it can be shown that the transformer does not change the fundamental form of the converter. The basic building blocks remain the buck and boost regulators. In all the forward converter topologies, the main function of the transformer was to provide input-to-output galvanic isolation and voltage transformation. It would be useful at this stage to introduce the...

Twotransformer Selfoscillating Converters

In the two-transformer self-oscillating converter, the switching action is controlled by saturation of a small drive transformer rather than the main power transformer, resulting in improved performance and better power transformer efficiency. As the main power transformer is no longer driven to saturation, more optimum transformer design is possible, and the shape of the B H loop is not so critical. Further, better transistor switching action is obtained, because the collector termination...

Vjb 2 x 362 10 x 362 760 V

With the correct drive waveform and snubber networks, the collect& current will have dropped to zero before this high-voltage condition is reached therefore, the transistor will be chosen for a Vcex rating of 760 V minimum. To prevent secondary breakdown, the designer must ensure that the collector current has reached zero before a collector voltage of Veex is approached. This is achieved by using a suitable snubber network. (See Part 1, Chaps. 17 and 18.)

117 Overvoltage Clamping With Scr Crowbar Backup

It is possible to combine the advantages of the fast-acting voltage clamp with the more powerful SCR crowbar. With this combination, the delay required to prevent spurious operation of the SCR will not compromise the protection of the load, as the clamp circuit will provide protection during this delay period. For lower-power applications, the simple expedient of combining a delayed crowbar as shown in Fig. 1.11.1 a with a parallel zener clamp diode (Fig. 1.11.2a) will suffice. In more critical...

Common Requirements An Overview

The direct-off-line switchmode supply is so called because it takes its power input directly from the ac power lines, without using the rather large low-frequency (60 to 50 Hz) isolation transformer normally found in linear power supplies. Although the various switchmode conversion techniques are often very different in terms of circuit design, they have, over many years, developed very similar basic functional characteristics which have become generally accepted industry standards. Further,...

Jrj u g

Bus and feeder systems in industrial plants c. Heavy appliance outlets with short connections to the service entrance d. Lighting systems in commercial buildings Note Category B locations are closer to the service entrance. The stress voltages may be similar to those for category A, but currents up to 3000 A may be expected. 3. Category C, Outside and Service Entrance. This location is outside the building. Very high stress conditions can occur, since the line and insulator spacing is large...

52 Fuse Parameters

Let Through Energy Arcing Pre Arcing I2r

From an electrical standpoint, fuses are categorized by three major parameters current rating, voltage.rating, and, most important, let-through current, or Z2t rating. It is common knowledge that a fuse has a current rating and that this must exceed the maximum DC or rms current demanded by the protected circuit. However, there are two other ratings that are equally importantfor the selection of the correct fuse. The voltage rating of a fuse is not necessarily linked to the supply voltage....

723 Basic Principles

Figure 1,12.3a shows the method of energy storage and delivery, hen SW1 is open, capacitors CI and C2 are charged in parallel from the supply lines via resistors R1 and R2. They will eventually charge to the supply voltage vs. If this circuit is now removed from the supply lines and SW1 is closed, CI and C2 will be connected in series, and a voltage of 2 X Vs will be provided at the terminals. In Figure 1.12.36, this circuit in its charged state is shown connected to a linear regulator circuit...

62 Typical Dualvoltage Capacitor Input Filter Circuit

Figure 1.6.1 shows a typical dual-voltage rectifier capacitor input filter circuit. A link option LK1 is provided which allows the rectifier capacitor circuit to be configured as a voltage doubler for 120-V operation or as a bridge rectifier for 240-V operation. The basic rectifier capacitor input filter and energy storage circuit C5, C6, and D1 through D4 has been supplemented with an input fuse FS1, an inrush-limiting thermistor NTC1, and a high-frequency noise filter LI, L2, L3, CI, C2, C3,...

22 Core Parameters And The Effect Of An Air

Hysteresis Loop With Air Gap

Figure 2.2.1a shows a typical BIH hysteresis loop for a transformer-grade ferrite core, with and without an air gap. It should be noted that although the permeability slope of the BIH loop changes with the length of the air gap, the saturation flux density at the combined core and gap remains the same. Further, the magnetic field intensity H is much larger, and the residual flux density Br much lower, in the gapped case. These changes are vety useful for flyback transformers, which use only the...

1 T 1 Wbrn2

Fx3730 Core

A second consideration for the selection of peak flux density is the possibility of core saturation under transient loading conditions at maximum input voltage. FIG. 2.12.3 Hysteresis and eddy-current loss in a pair of FX 3730 cores as a function of total flux at 100 C, with frequency as a parameter. Courtesy of Mallard Lid. FIG. 2.12.3 Hysteresis and eddy-current loss in a pair of FX 3730 cores as a function of total flux at 100 C, with frequency as a parameter. Courtesy of Mallard Lid.

613 Example Of Rectifier Capacitor Input Filter Dc Output Voltage Calculation

Consider the previous example for a 250-W unit. The input power is 357 ind a voltage doubler circuit is to be used at 110 V input. The total series re - ce S . UtpUt V0ltaBe 0f a ful -wave bridge-rectified capacitor input filter as a function of greater 6 S0UrCC res,stance as a P meter. Valid for capacitor values of 1.5 VV or FIG. 1.6.8 Mean DC output voltage of a voltage doubler capacitor input filter as a function of load power, with effective source resistance as a parameter. Valid for...

57 Snubber Components

Because power FET devices are not subject to the same secondary breakdown mechanisms that occur with bipolar devices, from a reliability standpoint, it is often considered that snubber components are not essential. However, in most FET applications, a small RC snubber network will still be fitted across the FETs to reduce RF radiation and meet the dv df limitations of the FET. With very high dvldt, some power FETs display a failure mode resulting from conduction of the internal parasitic...

1811 Ideal Drive Circuits For Highvoltage Bipolar Transistors

Figure 1.18.4 shows a combination of the snubber diode and Baker clamp circuits, with a push-pull base drive to Ql. This arrangement is particularly suitable for high-voltage flyback converters where the collector voltage may be of the order of 800 V or more during the flyback period. It operates as follows. When the drive voltage goes high, Q2 is turned on and Q3 off. Current flows via R3, Q2, C2, and D7 to the base of the power transistor Ql. The overdrive provided by the low-impedance R3, C2...

173 Type 7 Scr Crowbar Overvoltage Protection

As the name implies, crowbar overvoltage protection requires the short-circuiting of the offending power supply output in response to an overvoltage condition on that output. The short-circuiting device, usually an SCR, is activated when the overvoltage stress exceeds a preset limit for a defined time period. When the SCR is activated, it short-circuits the output of the power supply to the common return line, thus collapsing the output voltage. A typical simple SCR crowbar overvoltage...

74 Active Limiting Circuits Triac Start Circuit

For high-power converters, the limiting device is better shorted out to reduce losses when the unit is fully operating. EES. 1.7.2 Resistive inrush-limiting circuit with triac bypass for improved efficiency. Note Higher inrush current for bridge operation. EES. 1.7.2 Resistive inrush-limiting circuit with triac bypass for improved efficiency. Note Higher inrush current for bridge operation. Position R1 will normally be selected for the start resistor so that a single triac or relay may be used....

Lowpower Selfoscillating Auxiliary Converters

Many of the larger power converters require a small amount of auxiliary power for the supply of the control and drive circuits. Often the auxiliary requirements are derived from 60-Hz line transformers. This is not always very efficient, as the size of the transformer will often be determined by the need to meet VDE and UL creepage distance specifications rather than by the power needs. As a result, the transformer will often be larger in size than is required to meet the power requirements...

102 Typical Causes Of Turnon Voltage Overshoot In Switchmode Supplies

In most switchmode power supplies, a controlled start-up sequence is initiated at switch-on. Shoiild the turn-on be from a line input switch, the first action will be inrush limiting, where a resistive element in series with the line input reduces the peak inrush currents for a few cycles while the input capacitors are charged Following this inrush limiting, there will be a soft-start action. For soft start, the pulse width to the power switching devices is progressively increased to establish...

52 Operating Principle

In the circuit shown in Fig. 2.5.1, the high-voltage DC line is switched to the primary of a transformer by two power FET transistors, FT1 and FT2. These switches are driven by the control circuitry such that they will both be either on or off' together. Flyback action takes place during the off state, as in the previous flyback examples. The control, isolation, and drive circuitry will be very similar to that previously used for single-ended flyback converters. A small drive transformer is...

60hz To 50hz Converter

60hz 50hz Converter Schematic

ELECTROMAGNETIC INTERFERENCE EMI IN SWITCHMODE POWER SUPPLIES Electromagnetic interference EMI , otherwise referred to as radio-frequency interference RFI , the unintentional generation of conducted or radiated energy, is indefatigable in all switchmode power supplies. The fast rectangular switching action required for good efficiency also produces a wide interference spectrum which can be a major problem. Further, for proper operation of any electronic system, it is imjftlrtant that all the...

43 Transformer Faraday Screens And Safety Screens

To prevent circulation of RF currents between the primary and secondary windings or between the primary and the grounded safety screen, the main switching transformer will usually have at least one RFI Faraday screen in the primary winding. In some applications, an additional safety screen will be required between the primary and secondary windings. There are major differences in construction, location, and connection between the Faraday RFI screens and the safety screens. Safety regulations...

45 Reducing Radiated Emi In Gapped Transformer Cores

Femte flyback transformers and high-frequency inductors will usually have a relatively large air gap in the magnetic path, to define the inductance or to prevent saturation. Considerable energy can be stored in the magnetic field associated with this air gap. Unless the transformer or choke is screened, an electromagnetic field EMI will be radiated from the gap, and this can cause interference to the supply itself or to local equipment. Further, this radiated field may exceed the radiated EMI...

38 Line Impedance Stabilization Network Lisn

Line Impedance Stabilization Network

Figure 1.3.6 shows the standard LISN, used for the measurement of line-conducted interference, as specified by CSA C108.8-M1983 Amendment 5, 1983. Similar networks are specified by the FCC and VDE. In principle the wideband line chokes Ll and L2 divert any interference noise currents from the supply into the 50-R test receiver via the 0.1-p.F capacitors C3 or C4. The line not under test is terminated in 0.1 jiF and 50 I. It is normal to test both supply lines independently for common-mode...

Acknowledgements

We progress not only by our own efforts, but also by utilizing the work of those around us and by building on the foundations of those who went before. The reference section is an attempt to acknowledge this, and I have no doubt that many more works should have been mentioned. I sincerely apologize for any omissions it is often difficult to remember the original source. I am grateful to the many who have contributed to this work, but worthy of special mention is my former...

Power Supply Problems Tektronix 2213

Units, Symbols, Dimensions, and Abbreviations Used in this Book xix PART 1 FUNCTIONS AND REQUIREMENTS COMMON TO MOST DIRECT-OFF-LINE SWITCHMODE POWER SUPPLIES 1. COMMON REQUIREMENTS AN OVERVIEW 1.3 LI Introduction. 12 Input Transient Voltage Protection. 1.3 Electromagnetic Compatibility. 1A Differential-ModeNoise. L5 Common-Mode Noise. 1.6 Faraday Screens. L7 Input Fuse Selection. L8 Line Rectification and Capacitor Input Filters. 1.9 Inrush Limiting. 1.10 Start-Up Methods. 1.1 1 Soft Start....