Constant-Frequency Soft-Switching Converters• Introduction and a brief survey•Activeclamp (auxiliaryswitch) softswitching converters•Active-clamp (auxiliary-switch) soft-switching converters, • Active-clamp forward converter•Textbook 2042andon-line notesTextbook 20.4.2 and online notes• The zero-voltage transition full-bridge converter• Textbook Section 20.4.1 and on-line notes•“DC Transformer”ECEN5817 Lecture 38ECEN 58171The multiresonant switchBasic single-transistor version2switch 2-switch (synchronous rectifier) versionECEN 58172Multiresonant switch characteristicsSingle transistor versionAnalysis via state plane in supplementary course notesECEN 58173Multiresonant switch characteristicsTwo-transistor version with constant frequencyFavorable characteristics and wide ZVS range in constant-frequency operationVoltage and current stresses are 2-3 higher than in the PWM parentECEN 58174o tage a d cu e t st esses a e3geta te paetZVS active clamp circuitsThe auxiliary switch approachForward converter implementation Flyback converter implementation• Active-clamp circuit can be added to any single switch in a PWM converterM i it h l ili it h b h ( l d d) ZVSQSW •Main switch plus auxiliary switch behave as an (unloaded) ZVS-QSW converter resulting in zero-voltage transitions• Improved transformer reset, improved transistor utilizationECEN 58175• Note: beware of various patents (e.g. Vinciarelli (1982) for use in forward converter)Zero-voltage transition convertersThe phase-shifted full bridge converterBuck-derived full-bridge converterZero-voltage switching of each half-bridge sectionA popular converter for server front-end power systemsEfficiencies of 90% to 95% regularly bridge sectionEach half-bridge produces a square wave voltage. Phase-shifted control of converter outputEfficiencies of 90% to 95% regularly attainedController chips availableECEN 58176co e te outputActive-clamp (auxiliary-switch) soft-switching converters• Can be viewed as a lossless voltage-clamp snubber that employs a auxiliary c rrentbidirectional s itchcurrent-bidirectional switch• Operation (resonant transitions) similar to ZVS-QSW operation•Can be added to the transistor in any PWM converter•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 circuitgECEN 58177The conventional forward converter•Max vds= 2Vg+ ringingLi it d t D 05•Limited to D< 0.5• On-state transistor current is P/DVg• Magnetizing current must operate in DCM• Peak transistor voltage occurs during transformer resetECEN 58178• Could reset the transformer with less voltage if interval 3 were reducedThe active-clamp forward converter• Better transistor/transformer tili tiutilization•ZVS• Not limited to D < 0.505Transistors are driven in usual half-bridge manner, similar to 2-switch ZVS-QSW:ECEN 58179Approximate analysis:ignore resonant transitions, dead times, and resonant elementsECEN 581710Charge balanceVbcan be viewed as a flyback converter output. By use of a current-bidirectional switch, there is no DCM, and LMoperates in CCMECEN 581711Similar to an unloaded two-switch ZVS-QSW converterPeak transistor voltage• Max vds= Vg+ Vb= Vg /D’ which is less than the conventional value of 2 Vgwhen D > 0.5• This can be used to considerable advantage: improved transistor gpand transformer utilization• Design example:270 V ≤ Vg≤ 350 Vmax Pload= P = 200 WCompare designs using conventional 1:1 reset winding and Compare designs using conventional 1:1 reset winding and using active clamp circuitECEN 581712Conventional casePeak v= 2V+ ringingPeak vds= 2Vg+ ringing= 700 V + ringingLet’s let max D = 0.5 (at Vg= 270 V), which is optimisticThen min D (at Vg= 350 V) is(0 )(2 0)/(3 0) 0 38 (0.5)(270)/(350) = 0.3857 The on-state transistor current, neglecting ripple, is given byig= DnI = Diq-ongq-onwith P = 200 W = Vg ig = DVgiq-onSo iq-on= P/DVg= (200W) / (0.5)(270 V) = 1.5 AECEN 581713Active clamp case:scenario #1Suppose 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 ih h i l i l/’di d dcurrent is the same. But the transistor voltage is equal to Vg /D’, and is reduced:At Vg= 270 V: D = 0.5 peak vds= 540 VAt Vg= 350 V: D = 0.3857peak vds= 570 Vwhich is considerably less than 700 VECEN 581714Active clamp case:scenario #2Suppose we operate at a higher duty cycle, say, D = 0.5 at Vg= 350 V. Then the transistor voltage is equal to Vg /D’, and is similar to the conventional design ddi iunder worst-case conditions:At Vg= 270 V: D = 0.648 peak vds= 767 VAt Vg= 350 V: D = 0.5peak vds= 700 VBut we can now use a lower turns ratio that leads to lower reflected current in Q1:iq-on= P/DVg= (200W) / (0.5)(350 V) = 1.15 AConclusion:the active clamp circuit resets the forward converter transformerConclusion: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.ECEN 581715Active clamp forward converteranalysis of operating waveforms and characteristicsD3D4DD2ECEN 581716Waveforms(including Ll)D3D4iD4D2iD3ECEN