ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lectures 39-40 Zero-voltage transition converters The phase-shifted full bridge

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

1 Lectures 39-40

Zero-voltage transition convertersThe phase-shifted full bridge converter

Buck-derived full-bridge converter

Zero-voltage switching of each half-bridge section

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

A popular converter for server front-end power systems

Efficiencies of 90% to 95% regularly attained

Controller chips available


2 Lectures 39-40

Actual waveforms, including resonant transitions


3 Lectures 39-40

Issues with this converter

It’s a good converter for many applications requiring isolation. But…

1. Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown

2. The resonant transitions reduce the effective duty cycle and conversion ratio. To compensate, the transformer turns ratio must be increased, leading to increased reflected load current in the primary-side elements

3. During the D’Ts interval when both output diodes conduct, inductor Lc stores energy (needed for ZVS to initiate the next DTs interval) and its current circulates around the primary-side elements—causing conduction loss


4 Lectures 39-40

Result of analysisBasic configuration: full bridge ZVT

• Phase shift assumes the role of duty cycle d in converter equations

• Effective duty cycle is reduced by the resonant transition intervals

• Reduction in effective duty cycle can be expressed as a function of the form FPZVT(J), where PZVT(J) is a negative number similar in magnitude to 1. F is generally pretty small, so that the resonant transitions do not require a substantial fraction of the switching period

• Circuit looks symmetrical, but the control, and hence the operation, isn’t. One side of bridge loses ZVS before the other.


5 Lectures 39-40

Effect of ZVT: reduction of effective duty cycle


6 Lectures 39-40

Phase-shifted control

Approximate waveforms and results

(as predicted by analysis of the parent hard-switched converter)


7 Lectures 39-40

Diode switching analysis


8 Lectures 39-40

Diode commutation: intervals 3 and 4


9 Lectures 39-40

Waveforms: ZCS of D6


10 Lectures 39-40

Approaches to snub the diode ringing(a) conventional diode snubber


11 Lectures 39-40

Approaches to snub the diode ringing(b) conventional passive voltage-clamp snubber


12 Lectures 39-40

Approaches to snub the diode ringing(c) simplify to one passive voltage-clamp snubber


13 Lectures 39-40

Approaches to snub the diode ringing(d) improvement of efficiency in voltage-clamp snubber


14 Lectures 39-40

Approaches to snub the diode ringing(e) active clamp lossless snubber


15 Lectures 39-40

Approaches to snub the diode ringing(f) primary-side lossless voltage clamp


16 Lectures 39-40

Another application of the ZVT: DC Transformer

Operate at a fixed conversion ratio with high duty cycle, leading to high efficiency—avoids the problems of circulating currents

Use other elements in the system to regulate voltage

PFCAC line

ZVT DC-DC Load

DC-DC Load

DC-DC Load

isolation

350 V 5 V 1 V


17 Lectures 39-40

Active clamp circuits

Can be viewed as a lossless voltage-clamp snubber that employs a current-bidirectional switch

See Vinciarelli patent (1982) for use in forward converter

Related to other half-bridge ZVS circuitsCan be added to the transistor in any PWM converterNot only adds ZVS to forward converter, but also resets

transformer better, leading to better transistor utilization than conventional reset circuit


18 Lectures 39-40

The conventional forward converter

• Max vds = 2Vg + ringing

• 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

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


19 Lectures 39-40

The active-clamp forward converter

• Better transistor/transformer utilization

• ZVS

• Not limited to D < 0.5

Transistors are driven in usual half-bridge manner:


20 Lectures 39-40

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


21 Lectures 39-40

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.


22 Lectures 39-40

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 in practical applications where there is a specified range of Vg


23 Lectures 39-40

Design example

270 V ≤ Vg ≤ 350 V

max Pload = P = 200 W

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


24 Lectures 39-40

Conventional case

Peak 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(0.5)(270)/(350) = 0.3857

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


25 Lectures 39-40

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


26 Lectures 39-40

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 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

Documents

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lectures 39-40 Zero-voltage transition converters The phase-shifted full bridge