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I am designing a DC/DC converter. I chose the fullbridge topology. The requirements are as follows:

  • Vin = 75 - 125 Vdc
  • Vout = 200V
  • P = 600W
  • Fs = 100 Khz

The basic schematic is as follows: DC/DC schematic

I use ZVS for soft switching. The problem is the big spike in the Vds on the MOSFETs.

Vds M2 and M4 The image above shows the Vds on the down MOSFETs and the Vgs for all MOSFETs.

  • signal A is the up - left mosfet
  • signal B is up - right mosfet
  • signal C is down - left mosfet
  • the signal D is down - right mosfet

I see that the voltage spike is generated in right leg transition (when D switch is falling). The same spike occurs on the primary current. Why does it occur? Is the problem in the primary or secondary?

I provide a large time for discharging the parasitic capacitances (over 400ns). Which components oscillate in my design?

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    \$\begingroup\$ How well is the earth loop formed by your scope probe earthing wire minimized? \$\endgroup\$
    – Andy aka
    Apr 28 '16 at 22:12
  • \$\begingroup\$ If I am not wrong, the ZVS is obtained using phase shift PWM with LC tank. In your case I dont see any C, also the two L seems more to rapresent an equvalent transformer circuit, rather to real inductors. Your describtion is too vague and without details to get a help for such complex problem. \$\endgroup\$ Apr 29 '16 at 7:51
  • \$\begingroup\$ The C is the Coss of the mosfets. The value for inductor L8 is a leackeage inductance of my transformer in series with shim inductor and L9 is the Magnetizing inductance. Anyway the simulate don't work. The second image above show the voltages in the real circuit. the current in the primary side as follows [IMG]i64.tinypic.com/5p03ts.jpg[/IMG] \$\endgroup\$
    – yelstor
    Apr 29 '16 at 14:26
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From your waveforms, it seems that the current through M4 M8 pair (right lower MOSFETs) is not zero at the time they stop conducting. This current (and the energy related to it) excites some oscillations. It is not important where they happen; it is the non-zero current that causes problem.

You can use current transformers (ferrite rings with ~100 turns winding, the latest loaded to few Ohms resistor, the ring is placed over the high current wire) to see what currents do you have.

And my last question is: why do you expect ZVS conditions for this circuit? I did not simulate it, but it is not obvious it works in ZVS mode.

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  • \$\begingroup\$ the circuit is not for simulating zvs. I use this circuit for simulating hard switching but I forget place snubber circuit in the primary mosfets and this generates a really big voltage spike that spoling the mosfets. but this I decide to use a ZVS that no needed snubbers. \$\endgroup\$
    – yelstor
    Apr 29 '16 at 13:54
  • \$\begingroup\$ The circuit is not for simulating zvs. I use this circuit for simulating hard switching but I forgot place snubber circuit in the primary mosfets and this generates a really big voltage spike that damage the mosfets. for this reason I decide to use a ZVS that no needed snubbers. the current is this [IMG]i64.tinypic.com/5p03ts.jpg[/IMG] \$\endgroup\$
    – yelstor
    Apr 29 '16 at 14:12
  • \$\begingroup\$ I suggest you to study resonant LC converter with serial connection of Primary winding of transformer, resonant inductor and resonant capacitor. This design is good for your specs. You do not need snubbers. \$\endgroup\$
    – Master
    Apr 29 '16 at 15:19

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