The conventional forward converter
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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:


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


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.


Active clamp circuit on the primary side of the flyback converter
Active clamp circuit on the primary converter sideof the flyback converter



Active clamp forward converter
Active clamp phase-shifted full bridge converterforward converter


Waveforms including l l
Waveforms phase-shifted full bridge converter(including Ll)


Details different modes
Details: different modes phase-shifted full bridge converter


About l l
About phase-shifted full bridge converterLl


Definitions
Definitions phase-shifted full bridge converter


Subinterval 1
Subinterval 1 phase-shifted full bridge converter


Subinterval 2
Subinterval 2 phase-shifted full bridge converter


Subinterval 21
Subinterval 2 phase-shifted full bridge converter


State plane subinterval 2
State plane, subinterval 2 phase-shifted full bridge converter


Subinterval 3
Subinterval 3 phase-shifted full bridge converter


Subinterval 3 state plane trajectory
Subinterval 3: state plane trajectory phase-shifted full bridge converter


Subinterval 4
Subinterval 4 phase-shifted full bridge converter


Subinterval 5
Subinterval 5 phase-shifted full bridge converter


Subinterval 6
Subinterval 6 phase-shifted full bridge converter


State plane trajectory including intervals 5 and 6
State plane trajectory phase-shifted full bridge converterincluding intervals 5 and 6


Averaging
Averaging phase-shifted full bridge converter


Averaging1
Averaging phase-shifted full bridge converter


Averaging2
Averaging phase-shifted full bridge converter


Average output voltage
Average phase-shifted full bridge converteroutput voltage


The system of equations that describes this converter page 1
The system of equations phase-shifted full bridge converterthat describes this converterpage 1


The equations that describe this converter page 2
The equations that describe this converter phase-shifted full bridge converterpage 2


Results
Results phase-shifted full bridge converter


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