I built a single phase inverter, as shown below. However, for PWM gate control signal of maximum 9V peak, i noticed that the output PWM voltage across the a 10k resistive load does not increase above 6V no matter how much I increase the Vdc, above .

I suspect the mosfet is in satuation, but it does not make sense as according to the datasheet states that the saturation current at 9V Vgs is above 10A.

Can someone explain to me why the peak load voltage is constant and how may I increase it with increasing Vdc?

For your information, I am using STP75NF75 N channel mosfet. I get similar results even if i change the switch to IRF740 N channel mosfet.

Apologies for the confusion due to the incorrect image!

enter image description here

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    \$\begingroup\$ because without a proper high-side driver for your upper MOSFETs (which you've drawn as IGBTs btw) they're operating as simple source-followers and will never produce an output any higher than (Vdc - Vgth). \$\endgroup\$ – brhans Dec 5 '16 at 18:04
  • \$\begingroup\$ You show IGBTs, but in the text mention MOSFETs. This is causes confusion, and wastes the time of of the people you seek help from. \$\endgroup\$ – Olin Lathrop Dec 5 '16 at 18:22
  • \$\begingroup\$ Sorry the the wrong image, i have editted it. Thanks for the notice \$\endgroup\$ – Alex Anderson Dec 6 '16 at 2:41
  • \$\begingroup\$ Does this mean that I would have to drive the high side mosfet gate higher than the lowside mosfet gate? \$\endgroup\$ – Alex Anderson Dec 6 '16 at 3:01

The information you give is conflicting. You show IGBTs, but then say you are using N channel MOSFETs.

However, the reason seems to be that you are using NPN or N channel devices for the high side switch, but not driving their gates/bases high. NPN or N channel devices will always have their emitter/source below the base/gate when on.

Either you need to use a special high side driver intended for N channel devices on the high side, or P-type high side switches. The inputs of the P-type devices will still need high voltage, but can be easier to drive. At least you won't need a voltage above the positive power rail to drive them.


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