0075

The remaining height in bobbin must accommodate primary-to-secondary insulation plus the filament wires (about one turn of no. 22 wire at 0.0281-in diameter). Obviously the E625 core is marginal of the other two, the ETD44 appears preferable. 16.6.7 Toroidal core transformer for current-fed topology The circumference of the inside diameter (ID) of a toroid (it X ID) is much greater than the bobbin width of an EE core of roughly equal Ae. Hence the toroid permits more turns per layer than the EE...

01

Figure 4.8 Circuit during Q1 and Q2 off time. Current Ix, stored in Lm during Ql, Q2 on time, also flows through leakage inductance L,. During the off time, energy stored in Lm must be delivered to the secondary load as reflected into the primary across Lm. But also flows through Lh and during the off time, the energy it represents VzL , I2) is returned to the input source Vdc through diodes Di, D2. This robs energy which should have been delivered to the output load and continues to rob energy...

015 011

Figure 4.2 Inductance per 1000 turns (Alg) for various ferrite cores with various air gaps. Also the point in ampere turns cliff' point where saturation commences. (Courtesy Ferroxcube Corporation.) Figure 4.2 Inductance per 1000 turns (Alg) for various ferrite cores with various air gaps. Also the point in ampere turns cliff' point where saturation commences. (Courtesy Ferroxcube Corporation.) available for all cores at various air gaps, Eq. 4.14 would give the number of turns for any selected...

02

2.0 Figure 13.10 Gain curve for the zero-voltage-switching circuit of Fig. 13.(Courtesy Jovanovic, Tabisz, Lee) no current industry-wide consensus on the future or even the present value of resonant power supplies. It is of interest to consider the pros and cons on the subject. First, it is interesting to note that to the author's knowledge, resonant power supplies are not listed in any of the major manufacturers' catalogs. This may indicate that resonant supplies are presently not...

025r1

Since the doubled-frequency forward converter has twice the rms current, it will have twice the wire area of the original converter of half the output power. And since it has half the number of primary turns, its resistance is one-fourth the resistance of the original forward converter. With twice the rms current, its I2R losses are equal to that of the original forward converter of half the output power. 7.3.3 Derivation of output power relations for half-bridge topology The half bridge is...

How To Repair Tektronix Switch Mode Powersupply

Thus the Baker clamp has satisfactorily solved two significant problems. It prevents a sufficient forward bias on the base-to-collector junction to cause appreciable storage time. It also permits the circuit to work equally well with large changes in load current and over a large production spread in transistor beta because of the redistribution of input currents between D2 and D3 as base current demands change. However, it is still desired to provide reverse base current to at the instant of...

074

If the 3019 ferrite core is selected, its bobbin width is 0.459 in and height is 0.198 in. For a no. 26 wire diameter of 0.0182 in, the number of turns per width is 0.459 0.0182 25. The number of layers per bobbin height is 0.198 0.0182 10. Thus the 138 turns could be accommodated within six layers. If any of the above toroidal cores were selected, the 138 turns could easily be accommodated in three layers. The skin effect is no problem as the AC amplitude is small, and Table 7.6 shows...

1 ka und i VNArtL

The first integrated-circuit pulse-width-modulating control chip. (Courtesy Silicon General Corp.) Figure 5.2 (b) PWM UC1846. The first integrated-circuit current-mode control chip. (Courtesy Unitrode Corp.) Figure 5.2 (b) PWM UC1846. The first integrated-circuit current-mode control chip. (Courtesy Unitrode Corp.)

10

Figure 10.12 Toshiba amorphous MB cores. Temperature rise versus core losses. (Courtesy Toshiba Corp.) 406 Magnetics and Circuits Design . N 406 Magnetics and Circuits Design . N Figure 10.13 Design example of magnetic-amplifier postregulator. Magnetic amplifier blocks Vs< ) for a time tb and is a zero-impedance short circuit for a time tr. Then Vos Veo (UT). Time tf s controlled in a negative-feedback loop by the current that Q2 forces into MA via Dm. That current is controlled by error...

100

Note Skin depths are taken from Table 7.5 RJRic from Eq. 7.21. Note Skin depths are taken from Table 7.5 RJRic from Eq. 7.21. However, Table 7.6 should not be misinterpreted. Although Fig. 7.6 shows that Rac Rdc increases as wire diameter increases (d S increases), Rac actually decreases as wire diameter increases and larger wire sizes will yield lesser core losses. This, of course, is because Rdc is inversely proportional to d2 and decreases more rapidly than Rac decreases as a result of...

1000

Note Data are for bipolar magnetic circuits (first- and third-quadrant operation). For unipolar circuits (forward converter, flyback), divide by 2. Association (MMPA)5'6 and IEC publications from the American National Standards Institute.7 The various core geometries shown in Fig. 7.2 are pot or cup cores, RM cores, EE cores, PQ cores, UU or UI cores. The pot core is shown in Fig. 7.2e. It is used mostly at low power levels up to 125 W and usually in DC DC converters. Its major advantage is...

102 Linear and Buck Regulator Postregulators

A linear regulating postregulator is the best approach for output currents up to 1.5 A. The 1.5-A limit comes about because of low cost and low internal dissipation. Linear regulators with up to 1.5 A of output current are available as integrated circuits in plastic T0220 packages at a cost of about 500. They require no additional external components other than a small filter capacitor. They are usually specified as requiring 2 V (typically, 3 V for the worst case) minimum input-output...

1034 Slave output voltage shutdown with magnetic amplifiers

Heretofore, the magnetic amplifier was presented only as a means of voltage regulating the slave output voltage. That was done by controlling the flux level to which the core is reset at the end of the power transistor on time. The further down Bx was pushed, the longer the blocking time tb, the shorter the firing time tf, and hence the lower the DC output voltage. The magnetic amplifier can also be used to shut down the DC output voltage completely. This is done by pushing the initial flux...

1036 Core loss and temperature rise calculations

Toshiba provides curves useful in calculating core temperature rise for each of its cores. Figures 10.9, 10.10, and 10.11 show core loss versus toted flux change in maxwells for its three largest MB cores. Recall that flux change in maxwells equals flux density change in gauss multiplied by core area in square centimeters. Thus, dividing the maxwells shown in the curves by the core area gives the flux density change in gauss. The maximum maxwells shown on the curves then correspond to the total...

1037 Design examplemagneticamplifier postregulator

Design a magnetic-amplifier postregulator for the output of the forward converter shown in Fig. 10.13a. Specifications are Forward converter switching frequency 100 kHz Slave output voltage 15 V The main output voltage is Vom Vdc(Nsm Np)(tOQ T). The main feedback loop, in keeping Vom constant, then must keep the product Vdcton constant. Thus ton is a maximum when Vdc is a minimum. In the usual case, the number of turns on the T1 reset winding Nr is set equal to the turns on the power winding...

1038 Magneticamplifier gain

When the MA has fired, it has close to zero impedance and the DC current through it is determined only by the DC output impedance and the slave output voltage. That is simply the specified DC output current. But to bring the MA to its fired state, a current equal to twice the coercive current Ic is required to force the core from the left to the right side of the hysteresis loop (Fig. 10.2). That current comes from the transformer secondary Vsp. Similarly, when the core is reset to the left...

104 Magnetic Amplifier Pulse Width Modulator and Error Amplifier

Thus far, in this chapter, magnetic amplifiers as postregulators only have been considered. In this section, an interesting example of the use of a magnetic amplifier used simultaneously as a pulse-width modulator and error amplifier is described.9 It may be puzzling why, with the current enormous proliferation of inexpensive semiconductor pulse-width modulating chips with their built-in width modulators and error amplifiers, there is interest in magnetic elements to perform these functions....

113 RCD Turnoff Snubber Operation

In Fig. 11.1a, when the Q1 base receives its turnoff command, the transformer leakage inductance attempts to maintain the peak on current which had been flowing just before the turnoff command. That peak current divides in some way between the off-turning collector and CI through diode D1 which has latched in. The amount of current IC1 flowing into CI slows up the collector voltage rise time, and by making CI large enough, the rising collector voltage and falling collector current intersect so...

115 Design ExampleRCD Snubber

Design the RCD snubber for the forward converter of Sec. 11.2. Recall that the peak current Ip just before turnoff was 3.45 A, transistor fall time was 0.3 xs, and transistor dissipation without a snubber was 19 W. From Eq. 11.3 _ (3.45 2)(0.3 x 106) 2 x 184 0.0014 (juF But recall that a forward converter transformer is designed so that maximum transistor on time is at minimum DC voltage and will be forced to be 0.8772. For a switching frequency of 100 kHz, this is 4 xs. But in Sec. 11.2,...

1151 rcd snubber returned to positive supply rail

The RCD snubber is often (and preferably) returned to the positive supply rail as shown in Fig. 11.3. It works exactly in the same way as when it is returned to ground as in Fig. 11.1. At turnoff, D1 latches in and CI slows up collector voltage rise time with its charging current flowing into Vdc. At the following turnon, CI is discharged through Ql and the supply source Vdc. The advantage of returning i l, D1 to Vdc instead of to ground is Figure 11.3 When snubber is returned to positive rail,...

117 Snubber Reduction of Leakage Inductance Spike to Avoid Second Breakdown

The snubber offers a second very important advantage in addition to slowing up voltage rise time and thus decreasing average transistor dissipation. It prevents second breakdown, which occurs if the instantaneous voltage and current cross the reverse-bias safe operating area (RBSOA) boundary given in the manufacturer's data sheets (Fig. 11.5). This boundary can be crossed by the omnipresent leakage inductance spike (Fig. 2.10) which occurs at the instant of turnoff. Transistor manufacturers...

121 Introduction

Before going into the details of stabilizing a feedback loop, it is of interest to consider in a semiquantitative way, why a feedback loop may oscillate. Consider the negative-feedback loop for a typical forward converter in Fig. 12.1. The essential error-amplifier and PWM functions are contained in all pulse-width-modulating chip. The chip also provide many other functions, but for understanding the stability problem, only the error amplifier and pulse-width modulator need be considered. For...

1210 Type 3 Error Amplifier When Used and Transfer Function

In Sec. 2.3.11.2, it was pointed out that the output ripple Vor Ra dl where Ra is the ESR of the filter output capacitor Ca and dl is twice the minimum DC current. Now most aluminum electrolytic capacitors do have an ESR. Study of many capacitor manufacturers' catalogs indicates that for such capacitors, R0C0 is constant and equal to an average value of 65 x 10 6. Thus, using conventional aluminum electrolytic capacitors, the only way to reduce output ripple is to decrease Ra, which can be done...

1213 Design Example Stabilizing a Forward Converter Feedback Loop with a Type 3 Error Amplifier

Design the feedback loop for a forward converter having the following specifications Switching frequency 50 kHz Output ripple (peak to peak) < 20 mV Assume that the output capacitor is zero ESR. First the output LC filter and its Refer to Fig. 12.15. From Eq. 2.47 of the type advertised as having corner frequency are calculated. Now it was assumed that the output capacitor had zero ESR so that ripple due to ESR should be zero. But there is a small capacitive ripple component (Sec. 1.2.7)....

1215 Conditional Stability in Feedback Loops

A feedback loop may be stable under normal operating conditions when it is up and running, but can be shocked into continuous oscillation at turnon or by a line input transient. This odd situation, called conditional stability, can be understood from Fig. 12.17a and 12.176. Figure 12.17aand 12.176 contains plots of total open-loop phase shift and total open-loop gain versus frequency, respectively. Conditional stability may arise if there are two frequencies (points A and C) at which the total...

124 Error Amplifier Transfer Function Poles and Zeros

The circuit of an operational amplifier with a complex impedance Zx input arm and a complex impedance Z2 feedback arm is shown in Fig. 12.9. Its gain is Z2jZ1. If Zx is a pure resistor i l and Z2 is a pure re- Figure 12.8 Where to locate break frequencies Fz and Fp. The farther apart Fz and Fp are spread, the greater the phase margin. But spreading them further apart reduces low-frequency gain, which reduces the degeneration of low-frequency line ripple. It also increases high gain, which...

125 Rules for Gain Slope Changes Due to Zero and Pole Frequencies

The zero and pole frequencies represent points where the error-amplifier gain slope changes. A zero represents a +1 change in gain slope. Thus (Fig. 12.10a), if a zero appears at a point in frequency where the gain slope is zero, it turns the gain into a +1 slope. If it appears where the original gain slope is -1 (Fig. 12.106), it turns the gain slope to zero. Or if there are two zeros at the same frequency (two factors in the numerator of Eq. 12.3 having the same RC product) where the original...

133 Resonant Converter Operating Modes

13.3.1 Discontinuous and continuous above resonance and below resonance operating modes Operating modes can be discontinuous as in Fig. 13.1. In the discontinuous mode (DCM), as noted, output voltage regulation is accomplished by varying the switching frequency. Power is delivered to the load as sequence of discrete current or power pulses separated by times long compared to their duration. If the output voltage must be raised because Vdc has gone down or DC load current has been increased, the...

14

MPP cores OD 0.8 in, ID 0.5 in, height 0.25 in, lm 5.09 cm. All inductances in microhenries. Note Magnetics Inc. MPP cores OD 0.8 in, ID 0.5 in, height 0.25 in, lm 5.09 cm. All inductances in microhenries. table 4.3 Maximum Number of Turns and Maximum Inductance at Those Turns for Various Peak Currents lp at a Maximum Inductance Falloff of 10 Percent from Zero Current Level

142 Forward Converter Waveshapes

The circuit schematic for these waveshapes is shown in Fig. 14.1. It is a 125-kHz forward converter designed for 100 W, and waveshapes are shown at 80 and 40 percent of full load. Full-load outputs are 5 V at 10 A and 13 V at 3.8 A. Waveshapes are shown for nominal input volt- C, 160 HF, 100 V C2 3200 HF, 16 V C3 1500 (IF, 25 V C4. Cs 1.0 HF. 50 V D2, D3 MBR 1045 D4, 06 MBR 415 D,. 1N 4937 Q,. RFP 12N18 Vde 38-60 V C, 1.0 (iF Tl core-782E272-3F3 (Ferroxcube) Np-13 Turns, 2 18 in parallel N5-5...

16

2.3.9.1 First-quadrant operation only. The transformer core in the forward converter operates in the first quadrant of the hysteresis loop only. This can be seen in Fig. 2.10. When Q1 is on, the dot end of T1 is positive with respect to the no-dot end and the core is driven, say, in a positive direction on the hysteresis loop and the magnetizing current ramps up linearly in the magnetizing inductance. When Q1 turns off, stored current in the magnetizing inductance reverses the polarity of...

172

Devices similar to Linear Technology regulators from the other major supplier of such IC regulators requiring only a minimum number of components external to the package. Control Package Temp. 1000-up Number_(VI_IV)_max ttyp) (mAtyp) Scheme Options* EV Kit Ranges** Features_( )

1a

Ground, and RA and D2 in series charge the positive end of C2 up to one diode drop below V0. Thus if the drops in Dl and D2 are almost equal, C2 is charged up to a voltage closely equal to V. Now the voltage across C2 moves up and down with the negative end of the internal reference voltage (GND pin), and it is that voltage which is regulated. Regulators like those in Fig. 17.16 are designed as buck regulators with the internal power transistor emitter connected to the Vsw pin so that it can...

2 4 6 810

Figure 16.2 Fluorescent lamp light output in lumens per watt. For T12(1.5-in lamp diameter) and T17(1.88-in lamp diameter) versus frequency. (From High-Frequency Fluorescent Lamps, Campbell, Schultz, Kershaw, Illumination Engineering, Feb. 1953.) smaller and lighter, had no audible noise, and was less expensive. And at this high frequency, the lamp showed no flicker and conducted and radiated EMI was easier to suppress. These advantages of high-frequency operation, though significant, could not...

200 X 1q6

Sprague 673D, 674D aluminum electrolytics Now assume a regulator boosting from +5 to +15 V with 25-W (1.66-A) output. Assume a Mallory VPR 200 mF capacitor rated at 25 V. Then from the above relation Then from the above relation, the peak-to-peak ripple is V r 4 dco (ESR) 4 X 1.66 X 0.145 0.963 V. Modern tantalum capacitors may have lower ESR and yield lower ripple. Ripple may be reduced by increasing capacitance, using capacitors with higher voltage ratings or paralleling capacitors. All these...

2045

Note From Eq. 7.7, P 0.00050Bmtu AeAiyDcmB, where P is in watts, Bmax in gauss, Ae and Ab in square centimeters, f in hertz, Z> cmtl in circular mils per rms ampere, bobbin winding space factor 40 percent. For Bm< UI 1600 G. For other Bm , multiply by Bm 1600. For ),, , 500 circular mils rms ampere. For other > , , multiply by 500 Dom . For push-pull topology, multiply powers by a factor of 2. Note From Eq. 7.7, P 0.00050Bmtu AeAiyDcmB, where P is in watts, Bmax in gauss, Ae and Ab in...

24

MPP cores OD 1.84 in, ID 0.95 in, height 0.71 in, lm 10.74 in. All inductances in microhenries. Note Magnetics Inc. MPP cores OD 1.84 in, ID 0.95 in, height 0.71 in, lm 10.74 in. All inductances in microhenries. the on time, that primary peak current, multiplied by the turns ratio Np Ns is driven into the secondary where it decays linearly as shown in Fig. 4.1c. In most cases, output voltages are low and input voltages are higher, resulting in a large Np Ns ratio and a...

2500

Figure 16.10 Fluorescent lamp operating voltages and currents. The source impedance and ballast impedance determine the ballast operating voltage at the manufacturer's specified operating current. Operating at currents lower than the manufacturer's specified value results in less input and less light power output. At higher currents than specified, power input and light power output increase but lamp lifetime is decreased. (From Fluorescent Lamp Light Sources, Illumination Engineering...

3 40

Figure 12.2 (a) An RC integrator has a gain dVa dVin of -20 dB decade beyond Fp VvnRlCl. If the scales are such that 20 dB is the same linear distance as 1 decade in frequency, a gain slope of - 20 dB decade has a -1 slope. Such a circuit is referred to as a -1 slope circuit. (6) An RC differentiator has a gain of +20 dB decade. At Fz H2-R2C2, where XC2 R2, gain asymptotically approaches 0 dB. If scales are such that 20 dB is the same linear distance as 1 decade in frequency, a gain slope of +...

3 6 91215

Figure 2.3 Hysteresis loop of a typical ferrite core material (Ferroxcube 3C8). Flux excursions are generally limited to 2000 G up to about 30 kHz by requirement to stay on the linear part of the loop. At frequencies of 100 to 300 kHz, peak flux excursions must be reduced to about 1200 or 800 G because of core losses at these higher frequencies. of the push-pull topology or to applications where simple and inexpensive fixes could avoid the problem. This subtle failure mode in push-pull...

38 V 49 V

Lp (calculated in Sec. 4.3.2.7) 56.6 p,H Recall from Sec. 4.3.2.7 that Ca was calculated as 2000 i,F. But it was pointed out there that at the instant of turnoff, the peak secondary current of 66 A would cause a thin spike of 66 x 0.03 2 V across the anticipated ESR of 0.03 V for a 2000- xF capacitor. It was noted that either this thin spike could be integrated away with a small LC circuit or CD could be increased to lower its ESR. Here, both will be done. Capacitance Ca will be increased to...

4

Figure 5.7 Slope compensation in the UC1846 current-mode control chip. A positive ramp voltage is taken off the top of the timing capacitor, scaled by resistors i x, R2 and added to the voltage at the top of the current resistor R,. By choosing Rlt R2 to make the slope of the voltage added to V, equal to half the downslope of the output inductor current, reflected into the primary and multiplied by R the output inductor average current is rendered independent of power transistor on times....

401

Skin depth S 2837 Vf-, S in mils for F in hertz. *From Eq. 7.19. Skin depth S 2837 Vf-, S in mils for F in hertz. Thus Eq. 7.21 indicates that the wire's AC-to-DC resistance RaJ Rdc (1 + F) is dependent only on the ratio of wire diameter to skin depth. Figure 7.6 plots RaJRdc against the ratio d S from Eq. 7.21. 7.5.4 AC DC resistance ratio for various wire sizes at various frequencies Because of skin effect, the AC-to-DC resistance of round wire is dependent on the ratio of the...

46

Figure 16.11 (a) American National Standards Institute (ANSI) specifications for various fluorescent lamps (6) volt ampere characteristics at different operating currents for various hot cold cathode lamps. Figure 16.12 Block diagram of a modern fluorescent lamp light source. Output frequency of the DC AC inverter is set by a series or parallel self-resonant oscillator in the range of 20 to 50 kHz. The ballast is usually a capacitor or the controlled source impedance of a series LC resonant...

50

100 200 500 1000 2000 5000 10,000 pk - Peak AC Flux Density (Gauss) NOTE Telsa Gauss x 10 4 Figure 16.16 Characteristics of Micrometal powdered iron cores, a more lossy but far less expensive core than KoolMu or MPP material. High-Frequericy Power Sources for Fluorescent Lamps TABLE 16.2 Possible KoolMu Cores for Current Feed Inductor

710

It thus can be seen that at higher DC output voltages, the efficiency is significantly higher than at lower voltages. When realistic input line ripple voltages are assumed, efficiency for 5-V output for input line tolerances of 15 percent are in the range 32 to 35 percent. 1.2.4 Linear regulator efficiency versus output voltage When ripple is taken into account, the minimum headroom of 2.5 V must be guaranteed at the bottom of the ripple triangle at the low tolerance limit of the input AC...

727

The transistor currents shown in Fig. 6.24 show no sign of an end of on-time spike. The numerical data of Fig. 6.24 are summarized in Table 6.2. It is seen from Table 6.2 that efficiency has averaged about 71 percent with a constant load over the 38- to 60-V range of telephone industry specifications for power supplies. This compares to the 50.6 percent efficiency for the same Royer with the same 49.8-il load resistor without the series input inductor (Fig. 6.23). The voltage drop down to zero...

747

Figure 14.20 Significant waveforms in 50-kHz flyback supply of Fig. 14.18. Figure 14.20 Significant waveforms in 50-kHz flyback supply of Fig. 14.18. Voltage and current into 15 V rectifier (03, Fig. 14.18) Voltage and current into 5 V rectifier (D2, Fig. 14.18) Voltage and current into 15 V rectifier (03, Fig. 14.18) V15 current 0.39 A, voltage 19.39 V V5 current 2.08 A, voltage 5.00 V Voltage and current into 5 V rectifier (D2, Fig. 14.18) V,5 current 0.39 A, voltage 19.39 V V5 current 2.08...

756 Proximity effect

Proximity effect11-15 is caused by alternating magnetic fields arising from currents in adjacent wires or, more seriously, from currents in adjacent winding layers in a multilayer coil. It is more serious than skin effect because the latter increases copper losses only by restricting the conducting area of the wire to a thin skin on its surface. But it does not change the magnitude of the currents flowing only the current density at the wire surfaces. In contrast, in proximity effect, eddy...

780

Figure 14.13 Transformer center tap current and drain-to-source voltage (Q2i at minimum (photo PP13), nominal (photo PP14), and maximum (photo PP15) input voltage for one-fifth of maximum output currents. ing current or for a given maximum magnetizing current, from too low a total DC output current. The magnetizing current can become larger than originally specified if the two transformer halves inadvertently separate slightly, thus decreasing the magnetizing inductance and increasing the...

819

Figure 14.10 Significant waveforms in 200-kHz 85-W converter of Fig. 14.8. The assumption that the amplitude measured with a current probe in the drain as for photo PP6 is more valid than the measurement with the same probe in the transformer center tap (as photos PP1 to PP3) is verified by measuring voltage drop across a small current-monitoring resistor in series in the transistor source. Measurement with a current probe in series in the drain gives exactly the same absolute currents as does...

823 A spike of high reverse base current a at the instant of turnoff Fig 82a

If base input current is simply dropped to zero when it is desired to turn off, collector current will remain unchanged for a certain time (storage time ts). Collector voltage will remain at its low Vce(sat) value of about 0.5 V and when it finally rises, will have a relatively slow rise time. This comes about because the base-to-emitter circuit acts like a charged capacitor. Collector current keeps flowing until the stored base charges drain away through the external base-to-emitter resistor....

831 Baker clamp operation

In Fig. 8.6, a large current Ix of the desired pulse width is provided at the anode of D2. The current is large enough and has a sufficient overdrive to turn on the maximum current in Ql with the desired speed when Ql is a minimum beta transistor. As Ql commences turning on, D3 is reverse-biased, draws no current, and is effectively out of the circuit. All the 71 flows through D2 into the base, yielding very fast rise time. However, when the collector voltage has fallen low enough to...

832 Transformer coupling into a Baker clamp

8.3.2.1 Transformer supply voltage, turns ratio selection, and primary and secondary current limiting. The circuit of Fig. 8.7 provides all the required drive characteristics for the Baker clamp high forward and reverse base drive for Q2 at relatively low primary current drawn from the housekeeping supply Vh. It also provides the reverse Q2 base voltage which permits it to tolerate its Vcov rating. It works as follows. First, the T1 turns ratio Np Ns is chosen as large as conveniently possible...

8354 Base drive transformer primary inductance and core selection At

The start of the Q1 on time, Vc is at Vh and at the end of the on time, it is desired that Vc collapse to ground so as to store a current in Np. Now assume that Vc falls linearly from Vh to ground in the minimum Q1 on time ion(min). Then at the end of the on time, Np must be carrying a current Figure 8.13 Fast CI recharge circuit for proportional base drive. In Fig. 8.12, if CI cannot be recharged to Vh in the minimum Q2 on time, emitter-follower Q3 is interposed between R1 and CI for fast...

836 Miscellaneous base drive schemes

A wide variety of specialized bipolar base drive schemes have evolved through the years. They are more often used at lower power levels and, by various circuit tricks, seek to achieve two common goals. (1) a low-parts-count scheme to obtain substantial reverse base voltage, reverse base current, or a base-emitter short circuit at turnoff and at turnon and (2) forward base current adequate to drive lowest beta transistors at maximum current without long storage times for high beta transistors at...

838

Figure 14.16 Significant waveforms in 200-kHz converter of Fig. 14.8. Figure 14.16 Significant waveforms in 200-kHz converter of Fig. 14.8. drains down to ground. However, A1 and SI are fictitious turnons. At those times, the drain is driven down to ground by the positive drain bump after turnoff of the opposite transistor. This phenomenon is an extreme example of the situation seen in photo PP13 obtained by increasing the primary magnetizing current by increasing the transformer gap at low DC...

91

Figure 14.3 The 125-kHz 100-W forward converter of Fig. 14.1 at 40 percent full load. they come at a high repetition rate, their average dissipation can be high and can exceed the conduction dissipation of VdsIdston T. The overlap dissipation at turnon is not as serious as at turnoff. At turnon, the power transformer leakage inductance presents an infinite impedance for a short time and causes a very fast drain-to-source voltage fall time. The same leakage inductance does not permit a very fast...

91 Introduction

Over about the past 10 years, the power MOSFET has revolutionized the power supply industry and in an enormous way changed the rest of the electronics industry. The faster switching speed of MOSFETs has permitted increasing power supply switching frequencies from about 50 to 200 or 400 kHz. It has thus made power supplies smaller and made possible a host of new products which were feasible only with smaller power supplies. The increasingly smaller size of personal computers is a prime example....

918

P - type MOSFET showing inherent body diode Figure 9.18 Inherent body diodes in N- and P-type MOSFETs. In the N-channel MOSFET, the diode prevents a negative drain-to-source voltage. In the P-channel MOSFET, the diode prevents a positive drain-to-source voltage. P - type MOSFET showing inherent body diode Figure 9.18 Inherent body diodes in N- and P-type MOSFETs. In the N-channel MOSFET, the diode prevents a negative drain-to-source voltage. In the P-channel MOSFET, the diode prevents a...

92

Its length and resistance in ohms per foot as read from the wire tables for the selected wire size. It was also assumed that 7rms is the rms current as calculated from its waveshape (Sees. 2.2.10.2, 2.3.10.4). There are two effects, skin and proximity effects, which can cause the winding losses to be significantly greater than (Irms)2Rdc Both skin and proximity effects arise from eddy currents which are induced by varying magnetic fields in the coil. Skin effect is caused by eddy currents...

9212 Mosfet draintosource body diode

Inherently in solid-state structure of a MOSFET, a parasitic body diode is located across the drain-source terminals as shown in Fig. The diode polarity is such as to prevent reverse voltage across the MOSFET. The forward current handling capability and reverse voltage rating of the diode are identical to those of the MOSFET itself. Its reverse recovery time is faster than a conventional AC power rectifier diode, but not as fast as discrete fast-recovery types. Manufacturers' data sheets show...

923 Mosfet gate rise and fall times for desired drain current rise and fall times

Very rapid drain current rise and fall times are undesirable as they cause large L di dt spikes on ground buses, supply rails, and large C dV dt capacitatively coupled spikes into adjacent wires or nodes. The question thus arises as to what gate voltage rise time is required to yield a desired drain current rise time. This can be seen from the transfer characteristics shown in Fig. 9.36 and 9.3d. In a MOSFET, switching time between zero and a drain current Id is only the times required for the...

925 Mosfet flda temperature characteristics and safe operating area limits45

The most common failure mode in bipolar transistors second breakdown comes about because their on-voltage drop Vce(sat) decreases with temperature. This imposes limits (RBSOA curve of Fig. 8.4) on the Ic - Vce trajectory the transistor may not cross during the turnoff transition. Manufacturers state that only a single crossing of this limit curve may cause the bipolar to fail in the second-breakdown mode. However, MOSFETs, because their on-voltage drop or r(jg increases with temperature, do not...

927 Mosfet switching speed and temperature characteristics

MOSFET switching speed is significantly independent of temperature. Drain current rise and fall times depend only on the time required for the gate voltage to cross the narrow boundary between the gate threshold voltage (Vgsth) and VgI in Fig. 9.36. This depends on the output resistance of the source-sink driver and the effective gate input capacity. The source-sink output resistance is usually a discrete exter- nal resistance which has a low-temperature coefficient. Further, since gate input...

929 Paralleling MOSFETs7

In paralleling MOSFETs, two situations must be considered (1) whether the paralleled devices share current equally in the static case when they are fully on and (2) whether they share current equally during the dynamic turnon-to-turnoff transition. With paralleled MOSFETs, in either the static or dynamic case, the concern is that if one MOSFET hogs a disproportionate part of the current, it will run hotter, and long-term reliability will decrease or, in the short run, will fail. Unequal static...

A

Where h effective round wire height 0.866 (wire diameter d) 0.866d Ft copper layer factor Ntd w (where Nl number of turns per layer, w layer width, d wire diameter note Ft 1 for foil) The ratio is given for a number of different values of a variable p, which is the number of coil layers per portion. A portion is defined as a region where the low-frequency magnetomotive force H dl OA-aNI) ranges from zero to a peak. This portion often misinterpreted is clarified thus. Consider that the primary...

Ampere Turns

Figure 8.9 Inductance per 1000 turns At for Ferroxcube 1408PA3C8 pot core. A small core suitable for the transformer T1 of Fig. 8.7. It can be a 2N2222A an 800-mA 40-V device whose rise and fall times are under 60 ns. It comes in a small T018 package and cost is under 25 By changing the circuit of Fig. 8.7 to the simpler one of Fig. 8.10, greatly improved performance with all the advantages of Baker clamping is achieved. Current gain in T1 can be doubled without increasing Vh, and better...

C

Transformer secondary plus output rectif iers and filter Figure 2.6 Output rectifiers serve as free-wheeling diodes in a push-pull circuit. Secondaries carry the normal free-wheeling ledge current during the 20 percent dead time. This should be considered in estimating secondary copper losses. in the buck regulator of Fig. 1.4. In the buck, the free-wheeling diode was essential as a return path for inductor current when the transistor turned off. When the transistor turned off, the polarity...

Ci R2

Figure 12.15 A Type 3 error amplifier. It has a pole at the origin, two zeros, and two poles. Its transfer function is s.Rl(Cl + C2)(l + si 3C3) l + si 2(ClC2 (Cl + C2) (b) A first zero at a frequency of This is the frequency where the impedance of R2 equals that of CI. (c) A second zero at a frequency of This is the frequency where the impedance of (R1 + R3) equals that of C3. (d) A first pole at a frequency of This is the frequency where the impedance of R2 equals that of the series...

D wv v514bl

5.6.7.10 Turns ratio selection in overlapping mode. Equation 5.14a gives the relation between output input voltage and on time for the overlapping mode for a preselected choice of 7 turns ratio Nv As good a choice for Nx is the value calculated from Eq. 5.14a, which makes D 0.5 at the nominal value of input voltage Vdcn. Then for all DC input voltages less than Vdcn, there will be overlapping on times (D > 0.5) and the output voltage-on-time relation is given by Eq. 5.14a for that calculated...

D2

Figure 6.22 (a) Basic Royer oscillator. (b) Square hysteresis loop of T1 core, (c) Characteristic high current spikes at end of on time. These spikes are major drawbacks in Royer oscillators. As long as the core is on the vertical part of its hysteresis loop, the positive feedback from Np to Nb widings keeps a transistor on and in saturation. When the core has moved to either the top or the bottom of its hysteresis loop, coupling between the collector and base windings immediately drops to zero...

E2

Figure 5.1 A basic voltage-mode PWM controller. Output voltage only is directly sensed by the error amplifier. Regulation against load current changes occurs only after the current changes cause small output voltage changes. The current-limit amplifier operates only to shut down the supply when a maximum current limit is exceeded. Transistor on time is from start of sawtooth until the sawtooth crosses V . amplifier that the fraction of the output KVa is fed to the inverting input so that when...

Flyback Converter Topologies

All topologies discussed thus far, with the exception of the boost regulator (Sec. 1.4) and the polarity inverter (Sec. 1.5) deliver power to their loads during the time when the power transistor is turned on. Flyback topologies described in this chapter operate in a fundamentally different way. During their power transistor on time, they store energy in their power transformer while load current is supplied from an output filter capacitor. When the power transistor turns off, the energy stored...

Fundamental Switching Regulators Buck Boost and Inverter Topologies

There are about 14 basic topologies (basic block diagrams) commonly used to implement a switching power supply. Each topology has unique properties which make it best suited for certain applications. Some are best used for AC DC off-line converters at low (< 200 W) output power, some at higher output power. Some are a better choice for high AC input voltages (> 220 V AC) some are better for an AC input of 120 V or less. Some have advantages for higher DC output voltages (> 200 V) or in...

High Frequenc for

High-frequency power sources for enormously high-volume market for supplies. This may be appreciated the mid-1980s, shipment of fluores million annually.1 From the introduction of the fluor 1970s, fluorescent lamps were drivi line via a series inductor or via a 60 combination. The inductor or tran ferred to as magnetic ballast (Fig. K The series inductor is essential e ment because the lamp's volt ampe tive input impedance. The signific input current increases, its voltage from a...

T

From Eq. 4.11, primary circular mils requirement is (primary circular mils) 500 X 2.7 1350 circular mils This calls for No. 19 wire of 1290 circular mils, which is close enough. From Eq. 4.12, secondary rms current is and from Eq. 4.12, the required number of circular mils is 500 X 21 10,500. This calls for No. 10 wire, which, of course, is unpractically large in diameter. A foil winding or a number of smaller diameter wires in parallel with an equal total circular-mil area would be used....

Info

Adjustable output voltages up to 6.2V can be obtained with the adjustable versions If LT1173. LT1111, LT1107. LT1108, or LT1110. inductor capacitor and resistor values Figure 5 inductor capacitor and resistor values Figure 5 LBout Vou, IpGM SW LTC1174 LTV1574 Gnd LBout Vou, IpGM SW LTC1174 LTV1574 Gnd Figure 17.14 (6) Linear Technology micropower buck regulators. Positive-to-Negative Voltage Conversion Positive-to-Negative Voltage Conversion

IT tt

Where pp is the peak primary current (Fig. 6.156). This is equal to the average or DC output current at minimum line input and maximum current output. The output voltage at minimum output load resistor Ra is Now the transformer turns ratio must be chosen and the magnitudes of the resonant LC components selected to yield the peak resonant currents given above. Choose, as in Sec. 6.4.2 as per the suggestion in the Chambers paper,9 that the T1 primary voltage is taken as 60 percent of the voltage...

J

Ns2 4 turns of 2 paralleled No. 24, wind bipolar, layer 3 Figure 14.8 A 200-kHz 85-W DC DC converter +5 V at 8.0 A, +23 V at 1.9 A. core and copper losses increase more sharply. Thus, above 200 kHz, it is questionable whether the advantage of a small decrease in size is worth the penalty of increased dissipation and transformer temperature rise. Here, as for the forward converter of Fig. 14.1, the feedback loop was not closed. At each DC input voltage, the pulse width was manually set (as...

Hwp

Regulator output with large C, small R tor transient response to the step is observed by the oscilloscope through the filter shown to keep switching frequencies out of the display. The series RC network used to stabilize the supply is connected from the output of the internal voltage error amplifier (V pin) to ground. Initially, a large C2 (greater than 2 mF) and relatively small R3 (about 1 kii) is used. This almost always yields a stable DC loop. But with C2 so large, the supply responds to...

K

Minimum Ia Switching frequency Minimum output ripple (peak to peak) It is assumed that the filter output capacitor has an ESR and Fm will occur on the -1 slope of the LC filter. This permits the use of a Type 2 error amplifier with the gain characteristics of Fig. 12.6. The circuit is shown in Fig. 12.12. First La, CQ will be calculated and the gain characteristic of the output filter will be drawn. From Eq. 2.47 where dl is twice the minimum output current 2 x 1 2 A and Vor is the output...

Low InputVoltage Regulators for Laptop Computers and Portable Electronics

The explosion in the use of laptop computers and portable electronics in recent years has led to the formation of a new sector of the power conversion industry. This sector consists of low-input-voltage, battery-fed boost, buck, and polarity-inverting configurations (Sees. 1.3 to 1.5). They are almost entirely contained in one integrated-circuit (IC) package, and most require externally only a single inductor, capacitor, and diode plus about three to five small resistors. Since they operate at...

Mmmtmmmm mmmwmmwm mmmmmm

V R0 l P l,n Pln Efficiency 15.05 49.8 0.302 4.55 0.127 6.37 71.4 Top V (Q1)50V, 5 us cm V E V R0 l P l,n Pln Efficiency 15.05 49.8 0.302 4.55 0.127 6.37 71.4 v Ro lo po l n Pin Efficiency 18.08 49.8 0.363 6.56 0.151 9.03 72.7 v Ro lo po l n Pin Efficiency 18.08 49.8 0.363 6.56 0.151 9.03 72.7 Figure 6.24 Waveform in a current-fed Royer oscillator. By adding an inductor in series between Vcc and the transformer center tap, the high-current spikes at the start and end of the transistor on time...

Mp

. 4.19 A. . 2.31 A. . 0.95 A. . 0.49 A. . 0.20 A. Note step at start of ramp Note smaller step at start of ramp Note no step L0 has run dry Note much lower on time Note still lower on time (2) 1(01) 2 A, 10 p-S cm ldc- 4.4 A (3) l(Q1) -- 2 A, 10 (xs cm ldc - 2.30 A (4) l(Q1)-2 A, 10 (iS cm ldc- 0.95 A Figure 1.6 A 25-kHz buck regulator, showing the effect of the output inductor La running dry or moving from the continuous mode to the discontinuous mode. In panel (a), note that on time remains...

Magnetics and Circuits Designs

Transformer and Magnetics Design 267 7.2 Transformer Core Materials and Geometries and Peak Flux 7.2.1 Ferrite core losses versus frequency and flux density for widely used core materials 268 7.2.2 Ferrite core geometries 271 7.2.3 Peak flux density selection 275 7.3 Maximum Transformer Core Output Power, Peal Flux Density, Core and Bobbin Areas, and Coil Current Density 277 7.3.1 Derivation of output power relations for forward 7.3.2 Derivation of output power relations for...

Power Factor Power Factor Correction

15.1 Power Factor What Is It and Why Must It Be Corrected The term power factor is borrowed from elementary AC circuit theory. When a sinusoidal AC power source feeds either an inductive or a capacitive load, the load current is also sinusoidal but lags or leads the input voltage by some angle x. For an rms input voltage V and rms input current , the apparent power taken from the line is VIr But the actual power delivered to the load is only ViIi cos x. It is only the component of input current...

Push Pull and Forward Converter Topologies

The three earliest switching regulator topologies discussed in the previous chapter had the significant drawbacks that the output returns were DC-common with the input returns and that multiple outputs were not possible (except for the restricted case discussed in Sec. 1.3.8). In this chapter, the most widely used switching regulator topologies are discussed. These topologies the push-pull, single-ended forward converter, and two modifications of the latter, the double-ended and interleaved...

R1

Figure 2.11 Critical secondary currents in forward converter. Each secondary has the characteristic ramp-on-a-step waveshape because of the fixed voltage across the output inductor and its constant inductance. Inductor current is the sum of the secondary plus the free-wheeling diode current. It ramps up and down about the DC output current. Primary current is the sum of all the ramp-on-a-step secondary currents reflected by their turns ratios into the primary. Primary current is then also at...

R2

The first 2.5 V of gate rise time until the threshold is reached yields a delay of only 58 ns. Although this slows up maximum switching frequency, it does not, of course, cause any switching losses. The gate voltage rise time of 231 ns within the 0- to 10-V level is thus fast enough. But in the circuit of Fig. 9.66, gate voltage fall time is determined only by the emitter resistor Re and the gate input capacity as there is no constant current discharging that capacity. In Fig. 9.66,...

Ri

Figure 6.11 SCR resonant full bridge. This can deliver twice the output power of the half bridge. half those of the half bridge for equal output power. Hence only the half bridge will be discussed here in detail. The half bridge can be operated series-loaded as in Fig. 6.10 with the secondary load reflected via transformer T1 in series with a series resonant (C3 resonating with the series combination of L3 and LI when Ql is on or with the series combination of L3 and L2 when Q2 is on). In the...

S

Figure 10.9 Core loss versus total flux change. Toshiba MB 21 x 14 x 4.5 core. Core area 0.118 cm2 AB A4> (maxwells) 0.118. (Courtesy Toshiba Corp.) Figure 10.9 Core loss versus total flux change. Toshiba MB 21 x 14 x 4.5 core. Core area 0.118 cm2 AB A4> (maxwells) 0.118. (Courtesy Toshiba Corp.)

S16

** Half-bridge driver requires external control circuit Battery-Powered DC DC Conversion Solutions Selection Guide The following tables are a short form component selection guide for a collection of commonly used battery-powered DC DC conversion applications. No design is required since inductor, capacitor and resistor values are completely specified. Choose the appropriate LTC DC DC converter for your application from the following tables. The LT1073, LT1107, LT1108, LT1110, LT1111, LT1173,...

S9b

Vin 120V, Vo 3.56 V fs S56klh, lo 4.2A 1st waveform Vgs(20VjDiv) 2nd waveform Vcr(20VjDiv) 3rd waveform Ip (1A Div) 4ih waveform Vds(100V Divj Figure 13.2 Measured waveforms on an actual circuit as Fig. 13.1. (Courtesy F. Lee and K. Liu) started rising shortly after the negative diode current (D2) has come back up again to zero as discussed above. It is also seen that the voltage across the secondary capacitor in waveform 2 (which is a replica of the voltage across the primary magnetizing...

Transformer and Magnetics Design

In Part 1, characteristics of most of the frequently used topologies were discussed in sufficient depth to permit the choice of a topology most suited to the power supply specifications. Most frequently, the topology will be selected to minimize the power transistor's off-voltage stress at high line and peak current stress at maximum output power. Other considerations are to minimize parts count, cost, and required volume of the complete supply. Minimizing potential RFI problems is also a...

Turnon Time

Figure 8.3 (a) Typical turnoff transition falling current and rising collector-to-emitter voltage for a power transistor no snubber at collector. (6) Typical switching times for a typical high-current high-voltage transistor 2N6836, 15 A, 850 V (Vcev). (Courtesy Motorola Inc.) Figure 8.3 (a) Typical turnoff transition falling current and rising collector-to-emitter voltage for a power transistor no snubber at collector. (6) Typical switching times for a typical high-current high-voltage...

Ilt

Figure 6.16 Basic Cuk converter with input and output not isolated significant voltage and currents. with the left-hand end of L2 at ground and its right-hand end at -V , the stored energy or current in L2 flows down through Dl, up through Rot and back into the right-hand end of L2. When Ql is on, current in LI ramps up at a rate dl dt Vic L1 (Fig. 6.16e). Since V2 has gone down by the same amount as VI (Vp), the left-hand end of L2 is at - Vp and current in L2 ramps up at a rate dlldt (Va -...

T T T V T

V dc-'o 1 cr 1o y dc vf on) O O r7i and for Tcr Tvi Ts, P(Ton) VdcIQ(TJT). At turnoff as seen in Fig. 1.56, it is assumed that current hangs on at its maximum value Ia until the voltage has risen to its maximum value of Vdc in a time Tvr. Then current starts falling and reaches zero in a time Tcf. Total turnoff dissipation is then and for Tvr Tc T , P(Tott) VdcI0 TJT). Total AC losses (the sum of turnon plus turnoff losses) are then Total losses (the sum of DC plus AC losses) are P, Pdc + Pac...

Vac lf

_Here again, Pa is the minimum specified value of output power and onis the maximum on time calculated from Eq. 4.17 at the minimum specified DC input voltage Vdc. 4.4.2.4 Discontinuous and continuous-mode flyback design example. It is instructive to compare a discontinuous- and continuous-mode flyback design at the same output power levels and input voltages. The magnitudes of the currents and primary inductances will be revealing. Assume a 50-W, 5-V output flyback converter operating at 50...

Vd vdcmin

To avoid problems arising from T1 leakage inductance spikes, N2 is usually selected twice this minimum value.14 Thus 5.6.7.11 Output input voltage-on-time relation for overlapping-mode design at high DC input voltages where operation has been forced nonover-lapping. Now with selected from Eq. 5.15 and N2 from 5.16b, when Vdc is less than its nominal value, the relation between output voltage and on time is given by Eq. 5.14a. At nominal input Vdcn, D Ton T is 0.5 and at DC input voltages...