the conventional forward converter n.
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The conventional forward converter. Max v ds = 2 V g + ringing Limited to D < 0.5 On-state transistor current is P / DV g Magnetizing current must operate in DCM Peak transistor voltage occurs during transformer reset

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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
  • 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 circuits some examples
Active clamp circuits: some examples

Basic switch network reduces to:

(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 resonant transitions)

example addition of active clamp circuit to the boost converter
Example: addition of active clamp circuit to the boost converter

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.