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Considering a 3 phase inverter using MOSFETs without body diode or GAN as transistors. How can the FETs safely emulate diodes?

I mean I understand the transistors in question must be able to conduct in both direction in their on state. But is this all synchronized? Take the scenario below where M1 is ON conducting current to the motor.

schematic

simulate this circuit – Schematic created using CircuitLab

Now when M1 is OFF, I take the basic idea is to have M2 go on immediately. But before this can happen, I presume one still require some deadtime to avoid a shortcircuit. But would not this in theory produce "infinite" negative voltage across the inductor during this deadtime?

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    \$\begingroup\$ As WRB says - all MOSFETs you'll meet in this application will have parasitic body diodes. When M2 conducts it has reverse polarity applied to DS so the body diode is forward biased. The MOSFET may be delayed in turn on very slightly to avoid "shoot through". I have seen products with an external Shottky added across M2 to conduct in place of the body diode to reduce losses. A Schottky used in this way can be of lower rating than when only a diode and no MOSFET is used as the diode is used only very briefly at the start of the cycle. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jun 27, 2016 at 9:32

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Power MOSFETs have a built-in diode, called a body diode. Every MOSFET can be modelled as

schematic

simulate this circuit – Schematic created using CircuitLab

While it's not ideal for the purpose, it can be used as a flyback diode.

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  • \$\begingroup\$ I thought there existed power mosfets without body diodes. But at least the GAN FETs does not have them, and appears to be meant to be used without diodes at all for lower losses. \$\endgroup\$
    – Imbrondir
    Commented Jun 26, 2016 at 22:56
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    \$\begingroup\$ @Imbrondir while power GaN FETs don't have a true reverse body diode, they will still act like they have one, they will conduct in the reverse direction just fine but with a voltage drop more like 2V, EPC has some explanations in some of their datasheets. The reason for not using external diodes is that GaN FETs are so blazingly fast (think sub nS switching) that their dead time can be made really short, an external diode would never be able to keep up with unless it's series ESL was in the pH range, and the efficiency change would be bugger all anyway. \$\endgroup\$
    – Sam
    Commented Jun 26, 2016 at 23:46

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