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Discussion Starter · #1 · (Edited)
I have a scooter from 2008. I'm pulling off a lot of the old parts (controller) to be replaced with something new and FOC. I took apart the controller and was surprised to see it had 30 mosfets inside.

30 mosfets/3 = 10 per phase for each H bridge. 10 does not divide by 4 evenly so the H bridge will have 2 "legs" with 2 mosfets and 2 "legs" with 3 mosfets in them. This makes for an unbalanced H bridge.

1. Why would anyone build a controller like this?
2. The problems with cumulative gate capacitance would be higher on the gates of one current path through the H bridge than the other. It looks like all 4 legs are driven from the exact same gate drivers. Isn't this bad for mosfet control?
3. The mosfets are rated for 80a each. This would ideally make one current path capable of 160 phase amps and the other 240 phase amps. Doesn't this imbalance produce sub-optimal motor drive results?
4. On aliexpress, I found a 15 mosfet controller. Isn't this equally problematic as a 30 mosfet controller?
5. Wouldn't this make the motor "surge" and "lag" depending on which current path through the H bridge was active?
6. Any one phase in delta is "seeing" a current source and a current sink make of mosfets in 2 different H bridges. In WYE, it's 2 phases at a time, but all the rest is the same. I don't see how the odd number of mosfets per bridge can work. It seems that half at most of the "current flows" through any given motor phase would see the advantage of the imbalanced H bridges. Did I get that right? I think it might be even lower...like 1/4th?

This seems really weird to me! Why would anyone build a controller like this?
 

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... I took apart the controller and was surprised to see it had 30 mosfets inside.

30 mosfets/3 = 10 per phase for each H bridge. 10 does not divide by 4 evenly so the H bridge will have 2 "legs" with 2 mosfets and 2 "legs" with 3 mosfets in them. This makes for an unbalanced H bridge.
Right, but why would you have multiple H bridges? As the diagram from cricketo shows, you need two "legs" per phase, and 10 divides by 2. For each "SW" in cricketo's diagram, you would have five MOSFETs in parallel.

If you take out one pair of drivers from the typical three-phase driver (such as SW5 and SW6 in the diagram), you have an H-bridge driving a single-phase motor (which has two terminals, two drivers per terminal to drive that terminal positive or negative).


A delta-wired three-phase motor has three terminals, so three pairs of drivers, as in the diagram; I don't know of a letter which would describe this. ;) On the other hand, "H pattern" is used to describe gearshifts with more than four gears, so maybe it's still one big H bridge? HH bridge?
H-pattern:


Still all called H-pattern:
 

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Discussion Starter · #4 ·
Thanks for posting folks. I appreciate the feedback!

I see my confusion: I was thinking in terms of an H bridge used with a brushed motor. In that scenario, the 10 mosfets would make an imbalanced H bridge. In a 3 phase controller (such as BLDC) a single phase in the controller is half of any given H bridge. Once I saw that 6 mosfet layout posted by Cricketo, I realized where my mental process was wrong. And then of course, "Oh duh, I knew that!"

Add-on question: This works for 30 mosfets, but not for 15 as each phase now has 5 mosfets which doesn't divide in half evenly. I did find a 15 mosfet controller on ebay. I have to wonder how that works.

 

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Add-on question: This works for 30 mosfets, but not for 15 as each phase now has 5 mosfets which doesn't divide in half evenly. I did find a 15 mosfet controller on ebay. I have to wonder how that works.
I could be mistaken (in which case I am sure brian_ will correct me) but I think the diagram I posted is just one of the ways to do it. Specifically that arrangement of mosfets allows you to fire each phase with either polarity, so like for forward vs reverse. If you had no need for change of directions, or had ability to switch the polarity of the power source (mosfets don't care for polarity on source/drain, right?), you could get away with as few as 3 mosfets total.
 

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Discussion Starter · #6 ·
I could be mistaken (in which case I am sure brian_ will correct me) but I think the diagram I posted is just one of the ways to do it. Specifically that arrangement of mosfets allows you to fire each phase with either polarity, so like for forward vs reverse. If you had no need for change of directions, or had ability to switch the polarity of the power source (mosfets don't care for polarity on source/drain, right?), you could get away with as few as 3 mosfets total.
1. I think a 3 mosfet BLDC controller is theoretical. I used to mess with RC stuff a lot. I don't recal an ESC with 3 mosfets, but I have seen a zillion with 6 or more. Considering how China LOVES to cut corners, if they could make a 3 mosfet controller instead of one with 6, you have to know they would do it! This is a whole other topic in itself.
2. Your drawing above is the only way I think I have ever seen a 3 phase controller power section drawn. You can go look at VESC stuff since it is all open source and they are all that way. An odd number of mosfets making a balanced half a bridge doesn't seem likely to me. I could be wrong. There's probably really good reasons why 15 fet controllers are not common.
3. I understand the purpose of an H bridge to reverse the direction of current flow. I don't think most people are in question about this. I have wonder how else you would reverse current flow with mosfets so that 5 mosfets balances out in both directions of flow. I'm not seeing it.
 

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I could be mistaken (in which case I am sure brian_ will correct me) but I think the diagram I posted is just one of the ways to do it. Specifically that arrangement of mosfets allows you to fire each phase with either polarity, so like for forward vs reverse. If you had no need for change of directions, or had ability to switch the polarity of the power source (mosfets don't care for polarity on source/drain, right?), you could get away with as few as 3 mosfets total.
No. In a three-phase motor, the direction of motor rotation is determined by the order in which the phases are driven, not the polarity of the supplied power.

If you used only three drivers, connected to the three motor phases, the controller (which wouldn't be an inverter) could only connect each phase to one battery polarity (let's assume positive), or nothing. When both ends of a winding phase are connected to positive, there is no voltage across the phase and so no current. When either end is connected to nothing, there's no current. You need "push-pull" ability to drive the motor... and thus an inverter.
 
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