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How do I calculate the size of the diodes used in an H-bridge for a DC motor? What is the relationship between the motor parameters and the diode parameters?

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    \$\begingroup\$ I like this question. I've answered with the ad-hoc heuristics others have told me. However, someone at EE.SE must be able to answer with the proper maths, science and engineering. \$\endgroup\$
    – gbulmer
    Commented May 7, 2016 at 18:16

2 Answers 2

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This is not a proper answer. The post is to give some schematics to help generate some comments based on @gbulmer's answer.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. At t = 0 the H-bridge and motor is in conduction.

schematic

simulate this circuit

Figure 2. At t = 0+ all H-bridge switches are open. Note the voltage reversal on the motor.

  • At the instant before the H-bridge opens a current I is flowing in the motor.
  • At the instant after the H-bridge opens the same current is flowing in the motor due to its inductance. Diodes D2 and D3 provide the path for the current.
  • To forward bias the diodes we must get a voltage reversal on the motor coil.
  • This circuit relies on the PSU being able to sink the current from the motor somehow.

As discussed in gbulmer's answer comments I suspect that the diodes should only have to be rated for supply voltage (plus safety margin) and maximum motor current on the basis of Figure 2.

You may treat this "answer" as a wiki post and feel free to improve it.

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  • \$\begingroup\$ I would agree that the voltage shouldn't change but if the motor is stalled and then switched off, wouldn't that be the current the diodes would need to protect against? \$\endgroup\$ Commented May 11, 2016 at 13:44
  • \$\begingroup\$ Agreed. The stall current has to be taken into account as per "the diodes should only have to be rated for ... maximum motor current on the basis of Figure 2." \$\endgroup\$
    – Transistor
    Commented May 11, 2016 at 17:34
  • \$\begingroup\$ I guess as gbulmer mentioned, it's more of a loose process when picking diodes as opposed to a mathematical formula. I'm not personally okay with that, but I need to buy some diodes :) So is there any concern with over estimating? For example, say you have a motor with a stall of 3A, instead of a 3A diode, is there any harm going with a 6A diode just to be safe? \$\endgroup\$ Commented May 12, 2016 at 4:01
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    \$\begingroup\$ Having a positive safety margin is good engineering practice. Just watch the diode turn-on time. Slow diodes may allow the voltage to go sufficiently negative for long enough to do some damage. Can anyone else help with this? \$\endgroup\$
    – Transistor
    Commented May 12, 2016 at 6:01
  • \$\begingroup\$ What is turn-on time referenced by in a datasheet? \$\endgroup\$ Commented May 12, 2016 at 6:25
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I haven't seen a definitive, engineering answer with explanation to this. So this is just a 'Strawman' or 'Aunt Sally' to be knocked down or improved.

The 'folklore' agglomerated answer I derived from people with more experience than me was:

  • the diodes must handle more than the 'normal operating current', but
  • don't need to handle as much as the stall current,
  • should have a reverse voltage about 5x or more higher than the operating voltage
  • should switch much faster than the PWM frequency, but
  • the body diodes of a MOSFET H-bridge switch might be enough anyway,

That all sounds reasonable, but feels unsatisfyingly ad-hoc. So I'd like a better answer with the maths, science and engineering too.

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    \$\begingroup\$ "The diodes must handle more than the 'normal operating current', but don't need to handle as much as the stall current." I've never read up on it but it seems to me that the worst current the diode will ever see is the current in the coil at the time of interruption. That would be the stall current. "... should have a reverse voltage about 5x or more higher than the operating voltage." I'm wondering why? When reverse biased they should only ever see the supply voltage. "... should switch much faster than the PWM frequency." Agreed. \$\endgroup\$
    – Transistor
    Commented May 7, 2016 at 17:53
  • \$\begingroup\$ @transistor My point is the ideas sound plausible, but had 'hand-waving'. IIRC, the reverse voltage was on the basis that the inductance of the motor could create a relative high voltage, which would be applied to the diodes if all the H-bridge switches were opened. AFAICT their was an argument that the current through the motor could never be as big as stall if it were moving, and if it were stalled the reverse currents could never approach the stall current, as it was purely inductive with no 'motor as a generator' energy. But I wrote, I'd like the engineering reasoning and maths. \$\endgroup\$
    – gbulmer
    Commented May 7, 2016 at 18:04
  • \$\begingroup\$ Sorry if it wasn't clear but I'm not arguing with you at all. Your answers are very plausible. Your answer prompted me to think the subject through some more and now you have me thinking about what happens when all H-bridge switches are opened at the same time. I'll post a schematic as a half-baked answer to generate some comments. \$\endgroup\$
    – Transistor
    Commented May 7, 2016 at 18:36
  • \$\begingroup\$ @transistor - I didn't take your comments as being negative in any way, I hope I didn't come across as too 'bullish'. I think the OP posted a good question, which deserves to have a good answer, but I haven't found a proper, convincing engineering model on the web; that is astonishing. I posted because those are the heuristics I've gathered, but they are not really science and engineering. I'm happy to have them knocked down or properly explained. I decided not to put the ad-hoc explanations into the bullet points to keep it as a terse 'Strawman' or 'Aunt Sally'. \$\endgroup\$
    – gbulmer
    Commented May 7, 2016 at 18:46
  • \$\begingroup\$ No problem. See my "answer". \$\endgroup\$
    – Transistor
    Commented May 7, 2016 at 18:53

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