soft-switching converters with constant switching...

ECEN5817, ECEE Department, University of Colorado at Boulder

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

ECEN 58171

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(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)

ECEN 58172

• 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


Two-switch ZVS-QSW converters: already studied

• Q2 can be viewed as a synchronous rectifier

Original one-switch version

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

Add synchronous rectifier

ECEN 58173

similar to conventional PWM

• Can obtain µ < 0.5

The multiresonant switch

Basic single-transistor version

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2-switch (synchronous rectifier) version


Multiresonant switch characteristicsSingle transistor version

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Analysis via state plane in supplementary course notes

Multiresonant switch characteristicsTwo-transistor version with constant frequency

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


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

ECEN 58177

Active 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)

Zero-voltage transition convertersThe phase-shifted full bridge converter

Buck-derived full-bridge converter

Zero-voltage switching of each half-

A popular converter for server front-end power systems

ECEN 58178

Zero-voltage switching of each half-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


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

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The conventional forward converter

• Max vds = 2Vg + ringing

• Limited to D < 0.5

• On-state transistor current is P/DVg

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• 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


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:

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

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Charge balance

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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

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 V when D > 0 5which is less than the conventional value of 2 Vg when D > 0.5

• This can be used to considerable advantage: improved transistor

and transformer utilization

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

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


Conventional case

Peak vds = 2Vg + ringing= 700 V + 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(0.5)(270)/(350) = 0.3857

The on-state transistor current neglecting ripple is given by

ECEN 581715

The on state transistor current, neglecting ripple, is given by ig = DnI = Did-on

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

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

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 Vg / D’, and is reduced:

At Vg = 270 V: D = 0.5 peak vds = 540 V

At Vg = 350 V: D = 0.3857 peak vds = 570 V

which is considerably less than 700 V

ECEN 581716


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 under worst-case conditions:

At Vg = 270 V: D = 0.648 peak vds = 767 V

At 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:

id-on = P/DVg = (200W) / (0.5)(350 V) = 1.15 A

ECEN 581717

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.

Active clamp forward converteranalysis of operating waveforms and characteristics

D3

D4

D2

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Waveforms(including Ll)

D3

D4

D2

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Discussion

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Details: different modes

• Interval 3 can end either when D3 becomes reverse-biased when il reaches

zero or by D2 becoming forward biased when v reaches V +Vzero, or by D2 becoming forward-biased when vds reaches Vg+Vb

• In either case, both end by the end of interval 4

• Similar discussion (in reverse) applies to intervals 7 and 8

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Simplified waveforms(neglecting Ll)

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D3

D4

D2 • Secondary-side D3/D4 switching is ideal instantaneous• Primary side ZVS predicted well (pessimistic ZVS boundary)


State-plane analysis (neglecting Ll)D3

D4

D2

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D4

D2

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D4

D2

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D4

D2

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State plane trajectoryincluding intervals 5 and 6

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Averaging

D3

D4

D2

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Averaging

D3

D4

D2

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Averaging

D3

D4

D2

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Averageoutput voltage

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The system of equationsthat describes this converter

page 1

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The equations that describe this converterpage 2

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Results

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Active clamp converters: other examples

Basic switch network reduces to:

(if the blocking capacitor (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

ECEN 581735

C ring only during the resonant transitions)

Example: addition of active clamp circuit to the boost converter

ECEN 581736

The upper transistor, capacitor Cb, and tank inductor are added to the hard-switched PWM boost converter. Semiconductor output capacitances Cds are explicitly included in the basic operation.


Active clamp circuit on the primary sideof the flyback converter

ECEN 581737

soft-switching converters with constant switching...

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