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Zero-voltage transition converters The 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.

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zero voltage transition converters the phase shifted full bridge converter
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

issues with this converter
Issues with this converter

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

  • Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown
  • 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
  • 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
issues with this converter5
Issues with this converter

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

  • Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown
  • 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
  • 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
approaches to snub the diode ringing d improvement of efficiency in voltage clamp snubber
Approaches to snub the diode ringing(d) improvement of efficiency in voltage-clamp snubber
another application of the zvt dc transformer

PFC

DC-DC

Load

Load

Load

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

5 V

1 V

350 V

ZVT

AC line

DC-DC

isolation

DC-DC

active clamp circuits
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 circuits
  • 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
the conventional forward converter
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
the active clamp forward converter
The active-clamp forward converter
  • Better transistor/transformer utilization
  • ZVS
  • Not limited to D < 0.5

Transistors are driven in usual half-bridge manner:

charge balance
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.

peak transistor voltage
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

design example
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
conventional case
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

active clamp case scenario 1
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
active clamp case scenario 2
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