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Searching Amazon for a PWM controller board I found this:

https://www.amazon.com/dp/B07MH43SXR

Picture of the board

Apparently, they are 6 MOSFETs. I understand that an H bridge is built with 4 MOSFETs. Is there any other common drive circuit that would use 6 MOSFETs?

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  • \$\begingroup\$ I’m voting to close this question because reverse engineering a photo of something on a sales site without a link to a datasheet is not engineering, it's clairvoyance, or guesswork, and a waste of our time. \$\endgroup\$
    – Neil_UK
    Commented Apr 10, 2022 at 15:59
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    \$\begingroup\$ @Neil_UK In general I would agree, but this is a very simple circuit, and not much clairvoyance is involved. We're not asked to reverse-engineer a Cray here. As an engineer I like a puzzle just like the next person, and this looks like an OK one. \$\endgroup\$ Commented Apr 10, 2022 at 16:07
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    \$\begingroup\$ @All - Please be nice and comply with the site's Code of Conduct (CoC) in everything you write. Nothing snarky / sarcastic / condescending (or worse). Some comments here were deleted after being flagged. Although polite constructive criticism is allowed in comments - with the emphasis on polite & constructive - snarky / mocking etc. is NOT. Although reverse-engineering of whole PCBs from photos usually doesn't work here, and is often voted as off-topic, we can see what community voting decides. Remember: Be nice or say nothing. \$\endgroup\$
    – SamGibson
    Commented Apr 10, 2022 at 16:13
  • \$\begingroup\$ 4 connections means 2 for power in, 2 for motor legs. without anything else, we don't even know if it is bidirectional control. going forward and reverse is one of the main points of a full h-bridge. it claims 2kW max which is a lot so wouldn't be surprising if all those heat sinks were just to support the heat generated from inefficiencies in swinging that power. \$\endgroup\$
    – Abel
    Commented Apr 10, 2022 at 16:20

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I would not assume that's an H-bridge. Nothing in your link says it's an H-bridge or that it is capable of bidirectional control and the crude potentiometer control would also not be suited for an H-bridge since the zero point between forward and reverse is poorly defined.

To control a brushed DC motor unidirectionally is very easy...you just throw a MOSFET in series with the motor and put as many in parallel as required for the current. With a 40A rating, I could easily see this being the case.

I would expect an H-bridge that supports 40A involves considerably more components than what I am seeing here, even though the underside of the PCB is hidden.

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  • \$\begingroup\$ there's a 12V h bridge built on two bts7960 that's not that big and can handle over 20A in reasonable airflow (I didn't need more so I can't speak to whether it meets the full amperage claims) but yes swinging 50V at 40A can require beefier/more components. \$\endgroup\$
    – Abel
    Commented Apr 10, 2022 at 16:31
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The driver could be a single-direction "chopper", with all 6 mosfets in parallel.

A symmetric H bridge would need multiples of 4 mosfets, but for this application a symmetric H-bridge is unnecessary.

It could be an H-bridge with just one half controlled by PWM, and the other half acting as static direction switches. The direction switches can be optimized for low on-resistance, but can be slow - a typical tradeoff in MOSFETs. The other half of the bridge can be paralleled higher resistance but faster devices.

For example:

schematic

simulate this circuit – Schematic created using CircuitLab

Even if all the P/N mosfets are identical, M1 and M2 won't experience switching and gate drive losses, so may run cool enough that single devices are sufficient. Then M3+M4 and M5+M6 are driven with PWM, and dissipate more power, and thus are paralleled.

The same thing could be accomplished in a top-bottom split, as noted by nkgyuen:

schematic

simulate this circuit

The worse-performing PMOS switches M1,M2 could be selected for sufficiently low RDS(ON) without concern for gate capacitance. The faster, lower-resistance NMOS devices M3-M6 would have no trouble dealing with all the switching.

If that's how indeed this device is designed, I'd consider it at least somewhat clever.

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    \$\begingroup\$ And the flip side is maybe the PMOS you can find are so bad that you need to parallel them on top since the RDson for the same die size is about double that of an NMOS, all other things equal. \$\endgroup\$
    – DKNguyen
    Commented Apr 10, 2022 at 19:19
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    \$\begingroup\$ Yep, that’s what you’d expect based on theoretical concerns. Then there’s what’s cheap that week and it’s likely it drives the ultimate decisions. That stuff is made with razor-thin margins. \$\endgroup\$ Commented Apr 11, 2022 at 3:41

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