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I'm making a H-bridge for a 12V DC-motor. I have to choose a flyback diode but I have a hard time determining the motor's back EMF voltage. I'm assuming this voltage won't be more than double the supply voltage of 12.6V. The maximum current through the motor is 6A. I'm also assuming that my transistor Q5 should be on when changing the direction of the motor leaving a path for the current made from the back EMF to flow in. Is this correct?

I was also wondering if there is a need of some sort of decoupling when a high current is rushing to V_Batt?

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    \$\begingroup\$ Why switching with Q5 when you have already paid for and accepted the losses of Q1 and Q2? \$\endgroup\$
    – winny
    Commented Nov 17, 2021 at 10:47
  • \$\begingroup\$ @winny using Q5 overcomes shoot-through if done correctly... However, it's a bit like the old song; there was an old woman who swallowed a fly... I don't know if you have that in Sweden? \$\endgroup\$
    – Andy aka
    Commented Nov 17, 2021 at 10:53
  • \$\begingroup\$ why have you drawn an inductor parallel with the motor? \$\endgroup\$ Commented Nov 17, 2021 at 11:03
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    \$\begingroup\$ @Jasen just found out it isn't. I was wrong it is a DC series motor. \$\endgroup\$
    – Nego24
    Commented Nov 17, 2021 at 21:13
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    \$\begingroup\$ for series or parallel. you need to connect the stator and the brushes separately if you want to be able to reverse the motor using the h bridge. \$\endgroup\$ Commented Nov 18, 2021 at 2:41

2 Answers 2

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typical back EMF is slightly less than the supply voltage.

worst case is slightly more if there an external force driving the motor.

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External diodes are not necessary as the MOSFETs include body diodes, intrinsically (notice in the symbol, the triangle pointing at/away from the line segment (channel) denoting a diode junction).

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