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I'm not sure if I really should include flyback diodes or if the body diodes of the Mosfets is enough for a three-phase inverter, used for motor control. The Mosfets I planned to use are the following:

https://www.infineon.com/dgdl/irfh3707pbf.pdf?fileId=5546d462533600a40153561a2e0b1e78

The Mosfet is specified for 29A, but I will limit the current to 7A. The PWM frequency will be 25kHz. As far as I can see, the recovery time is fast enough such that it can act as a flyback diode and also the continous current seems with 3.5A to be enough for 7A continous, since the diode will not be always on and the current will drop fast.

Do you think the body diode is enough for this application or should I include schottky diodes as flyback diodes or even take Mosfets with body diodes with higher current ratings?

Bodydiode

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BOdy diode must conduct forward current for time constant T=L/DCR for DC resistance of coil.

Since your input Imax is 7A and spec Idc max is 3.5A with 96A pulse, I must presume your motor T=L/R is ~ <1ms so energy is low enough to handle. (E=0.5LI^2 * T) at first glance.

However deadtime in commutation must exceed reverse recovery time to prevent shootthru due to layout inductance and diode capacitance.

EDIT

The deadtime requirement here is due to inductive load L/R =T and thus VI*T energy in body diode. Where with a resistive load, it is the diode recover time which is much lower.

While effects of reverse recovery can be minimized by choosing a MOSFET with a body diode that has a low reverse recovery charge (QRR) and fast reverse recovery time (tRR) or by using an external Schottky diode with low QRR.

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  • \$\begingroup\$ Hi Tony, I use the following gate driver for the three-phase inverter: ti.com/lit/ds/symlink/drv8304.pdf, so the dead time is 120ns (version H), which is enough for the body diode then. About the motor I cannot say much, since it should be able to drive different motors in the power range of 1-200W. Probably such motors have a lower time constant, as I saw in some datasheets. \$\endgroup\$
    – HansPeterLoft
    Commented Sep 17, 2018 at 16:59
  • \$\begingroup\$ How can you estimate how much energy the body diode can handle? Is this the "Single Pulse Avalanche Energy"? And can I calculate the maximum inductivity of the motor to be: L=2*E/(T*I^2), where E is this Single Pulse Avalanche Energy, I the 7A max current and T the diode recovery time? \$\endgroup\$
    – HansPeterLoft
    Commented Sep 17, 2018 at 17:12
  • \$\begingroup\$ Assuming 1% dutycycle for body diode, it has the same thermal response as a single pulse on fig 11, so take 1% of PWM frequency and compute back from 150'C max junction temp ( Note 1 on p10) to determine Rjc with 3.5A@1V=3.5W Trr is 30ns affects SOA while L/R =T is much longer during current discharge affects diode dissipation. so if 1us 1% that reads 5'C/W whereas 7A*Vf~10W so 50'C rise in body diode. is my quick read. Then deadtime must be > 1us \$\endgroup\$
    – D.A.S.
    Commented Sep 17, 2018 at 18:19
  • \$\begingroup\$ Very smart answer. Covers all the crucial issues. No funny pictures. +1 \$\endgroup\$
    – user105652
    Commented Sep 17, 2018 at 18:46
  • \$\begingroup\$ haha body conducting switch \$\endgroup\$
    – D.A.S.
    Commented Sep 17, 2018 at 19:25

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