Constant-Frequency Soft-Switching Converters Introduction and a brief survey Active-clamp (auxiliary-switch) soft-switching converters, Active-clamp forward converter Textbook 20.4.2 and on-line notes The zero-voltage transition full-bridge converter Textbook Section 20.4.1 and on-line notes DC Transformer 1 Soft-switching converters with constant switching frequency With two or more active switches, we can obtain zero-voltage switching in converters operating at constant switching frequency The second switch may be one that is already in the PWM parent converter (synchronous rectifier, or part of a half or full bridge). In other cases, the second switch is a (hopefully small) additional auxiliary switch Examples: Two-switch quasi-square wave (with synchronous rectifier) Two-switch multiresonant (with synchronous rectifier) Active-clamp switch (forward, flyback, other converters) Phase-shifted bridge with zero voltage transitions These converters can exhibit stresses and characteristics that approach those of the parent hard-switched PWM converters, but with zero-voltage switching over a range of operating points 2
Two-switch ZVS-QSW converters: already studied Original one-switch version Add synchronous rectifier Q2 can be viewed as a synchronous rectifier Additional degree of control is possible: let Q2 conduct longer than D2 would otherwise conduct Constant switching frequency control is possible, with behavior similar to conventional PWM Can obtain µ < 0.5 3 The multiresonant switch Basic single-transistor version 2-switch (synchronous rectifier) version 4
Multiresonant switch characteristics Single transistor version Analysis via state plane in supplementary course notes 5 Multiresonant switch characteristics Two-transistor version with constant frequency Favorable characteristics and wide ZVS range in constant-frequency operation Voltage and current stresses are 2-3 higher than in the PWM parent 6
ZVS active clamp circuits The auxiliary switch approach Forward converter implementation Flyback converter implementation Active-clamp clamp circuit can be added to any single switch in a PWM converter Main switch plus auxiliary switch behave as an (unloaded) ZVS-QSW converter resulting in zero-voltage transitions Improved transformer reset, improved transistor utilization Beware of various patents (e.g. Vinciarelli (1982) for use in forward converter) 7 Zero-voltage transition converters The phase-shifted full bridge converter Buck-derived full-bridge converter Zero-voltage switching of each halfbridge section Each half-bridge produces a square wave voltage. Phase-shifted control of converter output A popular converter for server frontend power systems Efficiencies of 90% to 95% regularly attained Controller chips available 8
Active-clamp (auxiliary-switch) soft-switching converters Can be viewed as a lossless voltage-clamp snubber that employs a auxiliary current-bidirectional switch Operation (resonant transitions) similar to ZVS-QSW operation Can be added to the transistor in any PWM converter Not only adds ZVS to forward converter, but also resets transformer better, leading to better transistor utilization than conventional reset circuit 9 The conventional forward converter Max v ds = 2V g + ringing Limited to D < 0.5 On-state transistor current is P/DV g Magnetizing current must operate in DCM Peak transistor voltage occurs during transformer reset Could reset the transformer with less voltage if interval 3 were reduced 10
The active-clamp forward converter Better transistor/transformer utilization ZVS Not limited to D < 0.5 Transistors are driven in usual half-bridge manner, similar to 2-switch ZVS-QSW: 11 Approximate analysis: ignore resonant transitions, dead times, and resonant elements 12
Charge balance V b can be viewed as a flyback converter output. By use of a currentbidirectional switch, there is no DCM, and L M operates in CCM Similar to an unloaded two-switch ZVS-QSW converter 13 Peak transistor voltage Max v ds = V g + V b = V g /D which is less than the conventional value of 2 V g when D > 0.5 This can be used to considerable advantage: improved transistor and transformer utilization Design example: 270 V V g 350 V max P load = P = 200 W Compare designs using conventional 1:1 reset winding and using active clamp circuit 14
Conventional case Peak v ds = 2V g + ringing = 700 V + ringing Let s let max D = 0.5 (at V g = 270 V), which is optimistic Then min D (at V g = 350 V) is (0.5)(270)/(350) = 0.3857 The on-state transistor current, neglecting ripple, is given by i g = DnI = Di d-on with P = 200 W = V g i g = DV g i d-on So i d-on = P/DV g = (200W) / (0.5)(270 V) = 1.5 A 15 Active clamp case: scenario #1 Suppose we choose the same turns ratio as in the conventional design. Then the converter operates with the same range of duty cycles, and the on-state transistor current is the same. But the transistor voltage is equal to V g /D, and is reduced: At V g = 270 V: D = 0.5 peak v ds = 540 V At V g = 350 V: D = 0.3857 peak v ds = 570 V which is considerably less than 700 V 16
Active clamp case: scenario #2 Suppose we operate at a higher duty cycle, say, D = 0.5 at V g = 350 V. Then the transistor voltage is equal to V g /D, and is similar to the conventional design under worst-case conditions: At V g = 270 V: D = 0.648 peak v ds = 767 V At V g = 350 V: D = 0.5 peak v ds = 700 V But we can now use a lower turns ratio that leads to lower reflected current in Q1: i d-on = P/DV g = (200W) / (0.5)(350 V) = 1.15 A Conclusion: the active clamp circuit resets the forward converter transformer better. The designer can use this fact to better optimize the converter, by reducing the transistor blocking voltage or on-state current. 17 Active clamp forward converter analysis of operating waveforms and characteristics D 3 18
Waveforms (including L l ) D 3 19 Discussion 20
Details: different modes Interval 3 can end either when D3 becomes reverse-biased when i l reaches zero, or by D2 becoming forward-biased when v ds reaches V g +V b In either case, both end by the end of interval 4 Similar discussion (in reverse) applies to intervals 7 and 8 21 Simplified waveforms (neglecting L l ) D 3 Secondary-side D3/D4 switching is ideal instantaneous Primary side ZVS predicted well (pessimistic ZVS boundary) 22
State-plane analysis (neglecting L l ) D 3 23 State-plane analysis (neglecting L l ) D 3 24
State-plane analysis (neglecting L l ) D 3 25 State-plane analysis (neglecting L l ) D 3 26
State plane trajectory including intervals 5 and 6 27 D 3 Averaging 28
D 3 Averaging 29 D 3 Averaging 30
Average output voltage 31 The system of equations that describes this converter page 1 32
The equations that describe this converter page 2 33 Results 34
Active clamp converters: other examples Basic switch network reduces to: (if the blocking capacitor is an ac short circuit, then we obtain alternately switching transistors original MOSFET plus the auxiliary transistor, in parallel. The tank L and C ring only during the resonant transitions) 35 Example: addition of active clamp circuit to the boost converter The upper transistor, capacitor C b, and tank inductor are added to the hard-switched PWM boost converter. Semiconductor output capacitances C ds are explicitly included in the basic operation. 36
Active clamp circuit on the primary side of the flyback converter 37