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In a BLDC motor drive circuit:

enter image description here

What is the point of the bottom three flyback diodes on each of these MOSFETs?

This is how I imagine the current is flowing when the MOSFETs are on:

enter image description here

Once the inductors inside the motor are charged up, and all the MOSFETs switch to the off state I imagine the current flows like this back to the positive terminal because of the inductive voltage spikes being a higher voltage than the voltage of the positive terminal:

enter image description here

Since the current continues to flow in the same direction after the MOSFETs are switched off due to the inductors inside the motor, the bottom diodes would be in reversed biased mode and wouldn't allow any current to flow. Is my drawing of the current flowing correct? Or am I missing something? What's the point to the bottom diodes?

Trev347
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  • _"when I am powering my motor and I switch phases, the current in my circuit increases which means the motor isn't properly discharging the phases."_ - perhaps you have shoot-through or you aren't synchronizing properly. Power MOSFETs have an inherent diode which provides the recirculation path for inductive back-emf. – Bruce Abbott Sep 26 '22 at 01:17
  • @BruceAbbott in order to solve the shoot through problem I turn off all MOSFETS and I put a 1ms delay between switching MOSFET phases which should work because they have a turn on / turn off time within the nano second range (using an IRFZ44n). Probably not the best solution, but that's the best I can do for now haha. – Trev347 Sep 26 '22 at 01:55
  • It could possibly be the synchronization problem, but I have an interrupt on my MCU that immediately switches phases when the motor rotates so I don't think it would be that either. You have any other ideas on why that would be happening? – Trev347 Sep 26 '22 at 01:58
  • If the current goes to the positive supply, where does it come *from*? – user253751 Sep 26 '22 at 02:13
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    @Trev347 Do you have enough dead-time to avoid cross-conduction? – Hearth Sep 26 '22 at 02:19
  • @Hearth in the datasheet for an IRFZ44N it shows the Turn-On Delay Time is 12 ns, Rise Time is 60 ns, Turn-Off Delay Time is 44 ns, and Fall Time is 45 ns which means a total of 161 ns. Delaying 1ms should be plenty as far as i'm aware. – Trev347 Sep 26 '22 at 02:33
  • "*don't allow any current to flow to the negative terminal because they are reverse biased*" What? Trace out how you think the current should flow and post. It almost sounds like you don't know that current flows in a loop, and that you think current flows out from one end of the motor coil through the top diode and into the + terminal of the power supply and that you think current wants to flow out of the other end of the same motor coil through the bottom diode and into the negative terminal of the battery. If so, this is COMPLETELY wrong. – DKNguyen Sep 26 '22 at 03:46
  • The purple trace here is how flyback current flows in an H-bridge and a three-phase BLDC driver is no different: https://electronics.stackexchange.com/questions/386798/why-dont-flyback-diodes-in-h-bridge-damage-power-supply Note that it flows in a loop. If you do not understand this, then you need to take several steps back before trying to build a brushless motor driver and cover some fundamentals. – DKNguyen Sep 26 '22 at 03:52
  • @BruceAbbott you were actually correct about my phases not being in sync. That was why the current would continue to increase when I switched phases. – Trev347 Sep 27 '22 at 06:46
  • @TooTea Alright that made a lot of sense thank you. The only question I have about that is "to the topmost terminal of the motor". What do you mean by this? – Trev347 Sep 27 '22 at 07:32
  • @TooTea Got it I see what you mean now. Alright that comment made a lot of sense and answered my question. Thanks! – Trev347 Sep 27 '22 at 07:37

3 Answers3

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If you turn both top and bottom MOSFETs off with current running through a winding (in the circuit shown) then one top diode and one bottom diode will conduct.

If you leave out the bottom Schottky diode then the body diode of the MOSFET will conduct rather than the missing diode, and it generally has inferior performance compared to a Schottky diode. Not that it matters much with that horrible drive circuit.

Spehro Pefhany
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  • Why does the bottom diode conduct if it is reverse biased? Since the bottom one is reverse biased it shouldn't be conducting right? – Trev347 Sep 26 '22 at 01:44
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    And I realized a long time ago that the drive circuit was pretty bad, so I switched it out for a FAN7382N IC – Trev347 Sep 26 '22 at 01:59
  • Imagine the current is flowing from + switch to - switch and you open the minus switch. It will continue to flow in the same direction with the lower diode conducting- the drain will go a volt or so below ground. – Spehro Pefhany Sep 26 '22 at 03:50
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Suppose that one of the motor coils - (say the one between terminals 1 and 2 label them any way you want) has a current flowing in it and then you switch off all the MOSFETS, where will the current flow now?

One of the top diodes will be forwards biased, and one of the bottom diodes will be too. energy will briefly flow back into the supply capacitor (not shown in the schematic)

The current flowing out of terminal 2 flows through the upper diode connected to that terminal anbd the currwent flowing into terminal 1 flows theough the lower diode, connected to that terminal the inductance will make sure that this current flows, and the flow will make sure that the diodes are forwards biased.

Jasen Слава Україні
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The whole idea about inductors is that they try hard to keep current going the same way it went before, so if in your #1 picture current enters the topmost (green phase) terminal of the motor and exits the middle terminal, #2 should look exactly the same as far as the motor is concerned.

Just complete the circuit in #2 to get a closed loop for the current. When it comes to the plus of the battery, it then flows through the battery (charging it a little) and from the minus of the battery through the bottom diodes (/MOSFET body diodes) to the topmost terminal of the motor.

TooTea
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