I need to build an isolated power supply for an IGBT gate driver. +15V, -8V.

What I'm trying: I'm using a 1:1.3 transformer and driving it with a 15V square wave. The output voltage is split 1/3, 2/3, using capacitors. I'm a little uncertain on the theory of this circuit though.

I simulated this circuit with LTSPICE and it seems to be working except I am not getting the voltages that I expect on the output. Spice shows +36V and -18V. I must be doing something wrong but I'm not sure what. The right side is just a resistor/ cap acting as crude model of an igbt gate. And the bottom part generates a pwm with deadtime to drive the gate signals.

What do I change to get +15V and -8V instead of 36/-18?

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  • \$\begingroup\$ The transformer switching should be 100KHz not 10Khz but it didn't make a big difference in the simulation. \$\endgroup\$
    – Thomas C.
    Mar 9 at 20:31
  • \$\begingroup\$ Can you zoom in a little? It's not clear if the primary is 34 or 84uH. \$\endgroup\$ Mar 9 at 20:32
  • \$\begingroup\$ Should be 94u on primary and 158u on secondary. I got the 94u value from the datasheet of the tx, I calculated the 158u \$\endgroup\$
    – Thomas C.
    Mar 9 at 20:34
  • \$\begingroup\$ Key observation: DC current flow G-E is zero. Why not use a simple regulator (e.g. zener shunt) to set that voltage (plus generous bypass caps), and a total supply of ~23V? Then the driver can be common-ground e.g. IXDD614CI. \$\endgroup\$ Mar 9 at 20:34
  • \$\begingroup\$ I considered using a shunt regulator. But this IGBT needs a lot of gate drive current. All the shunt resistors I say have a max of 100mA. I'm not sure if that's enough for this IGBT. FF1400R12IP4 is the IGBT. \$\endgroup\$
    – Thomas C.
    Mar 9 at 20:36

1 Answer 1


I think I figured it out. I needed to drive the one leg of the TX with +15/0 and the other leg stays at Vcc/2 using a cap divider. This gets me +18/-9V

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