Constant-Frequency Soft-Switching Converters

• Introduction and a brief survey

• Active clamp (auxiliary switch) soft switching converters• Active-clamp (auxiliary-switch) soft-switching converters,

• Active-clamp forward converter

• Textbook 20 4 2 and on-line notesTextbook 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”

ECEN5817 Lecture 38

ECEN 58171

The multiresonant switch

Basic single-transistor version

2 switch 2-switch (synchronous rectifier) version

ECEN 58172

Multiresonant switch characteristicsSingle transistor version

Analysis via state plane in supplementary course notes

ECEN 58173

Multiresonant switch characteristicsTwo-transistor version with constant frequency

Favorable characteristics and wide ZVS range in constant-frequency operationVoltage and current stresses are 2-3 higher than in the PWM parent

ECEN 58174

o tage a d cu e t st esses a e 3 g e t a t e pa e t

ZVS active clamp circuitsThe auxiliary switch approach

Forward converter implementation Flyback converter implementation

• Active-clamp circuit can be added to any single switch in a PWM converter

M i it h l ili it h b h ( l d d) ZVS QSW • Main switch plus auxiliary switch behave as an (unloaded) ZVS-QSW converter resulting in zero-voltage transitions

• Improved transformer reset, improved transistor utilization

ECEN 58175

• Note: beware of various patents (e.g. Vinciarelli (1982) for use in forward converter)

Zero-voltage transition convertersThe phase-shifted full bridge converter

Buck-derived full-bridge converter

Zero-voltage switching of each half-bridge section

A popular converter for server front-end power systems

Efficiencies of 90% to 95% regularly bridge section

Each half-bridge produces a square wave voltage. Phase-shifted control of converter output

Efficiencies of 90% to 95% regularly attained

Controller chips available

ECEN 58176

co e te output

Active-clamp (auxiliary-switch) soft-switching converters

• Can be viewed as a lossless voltage-clamp snubber that employs a auxiliary

c rrent bidirectional 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 circuitg

ECEN 58177

The conventional forward converter

• Max vds = 2Vg + ringing

Li it d t D 0 5• Limited to D < 0.5

• On-state transistor current is P/DVg

• Magnetizing current must operate in DCM

• Peak transistor voltage occurs during transformer reset

ECEN 58178

• Could reset the transformer with less voltage if interval 3 were reduced

The active-clamp forward converter

• Better transistor/transformer

tili tiutilization

• ZVS

• Not limited to D < 0.50 5

Transistors are driven in usual half-bridge manner, similar to 2-switch ZVS-QSW:

ECEN 58179

Approximate analysis:ignore resonant transitions, dead times, and resonant elements

ECEN 581710

Charge balance

Vb can be viewed as a flyback converter output. By use of a current-bidirectional switch, there is no DCM, and LM operates in CCM

ECEN 581711

Similar to an unloaded two-switch ZVS-QSW converter

Peak transistor voltage

• Max vds = Vg + Vb = Vg /D’

which is less than the conventional value of 2 Vg when D > 0.5

• This can be used to considerable advantage: improved transistor g p

and transformer utilization

• Design example:270 V ≤ Vg ≤ 350 Vmax Pload = P = 200 W

Compare designs using conventional 1:1 reset winding and Compare designs using conventional 1:1 reset winding and using active clamp circuit

ECEN 581712

Conventional case

Peak v = 2V + ringingPeak vds = 2Vg + ringing= 700 V + ringing

Let’s let max D = 0.5 (at Vg = 270 V), which is optimistic

Then 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-on

with P = 200 W = Vg ig = DVg iq-on

So iq-on = P/DVg = (200W) / (0.5)(270 V) = 1.5 A

ECEN 581713

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

i h h i l i l / ’ d i 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 = 0.3857 peak vds = 570 V

which is considerably less than 700 V

ECEN 581714

Active clamp case:scenario #2

Suppose 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

d di iunder worst-case conditions:

At Vg = 270 V: D = 0.648 peak vds = 767 VAt Vg = 350 V: D = 0.5 peak vds = 700 V

But 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 A

Conclusion: 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 581715

Active clamp forward converteranalysis of operating waveforms and characteristics

D3

D4

DD2

ECEN 581716

Waveforms(including Ll)

D3

D4 iD4

D2

iD3

ECEN 581717