18v15 + external mosfet?

Just a question have anyone tried to “hack” a 18v15 or it’s sibling by adding external mosfet such as IRF1324PBF?
I’m considering a hack where I use a external H-bridge with the above mentioned MOSFet’s on a large heatsink to get a “super” Motor Controller that can handle 50A/12V or similar.

I’m considering if the external MOSFet’s should be connected in parallel to the existing ones, or if I have to desolder the IRLR8743PbF’s.

I just love the motor controllers, but need “some more” power.
Just to give a hint, I’m considering replacing the steering units on a dissected http://www.tent-tents.com/overzichtpagina3.asp?id=221&arid=4968 which have motors rated at 50A/12V
The original controller are relay based, with no smooth operation or computer control…



Changing the MOSFETs is possible but probably not very easy, and your success will likely depend on the details of your implementation. The MOSFETs will need some kind of heat sinking to get the kind of current you need, yet you need to keep the wires as short as possible and as thick as possible. If I were to try it, I would remove the existing MOSFETs, make the big-MOSFET H-bridge in some separate way that does not require the high currents to go through the motor controller PCB, and then use the shortest wires possible to hook up power, ground, and the four MOSFET gates to the motor controller.

I’m somewhat concerned about shoot-through issues since the gate charge on your MOSFET is more than double the biggest one we’ve used. You can check for that by initially setting your PWM frequency low and not having much of a load; if your motor controller current goes up a lot with frequency without changing the load, you probably have shoot-through problems. If you post some pictures of a really-nice external H-bridge that works except for a likely shoot-through problem, we could probably be persuaded to send you some custom firmware with modified gate drive timings.

- Jan

Sounds really interesting.
My idea at the moment is to do exactly what you say:

  • Move the MOSFET’s to a external heat sink
  • Less than 2" / 5cm away
  • Move the “current” away from the controller-board as much as possible

The IRF1324PBF where just an example, I searched the local electronic-store for the “biggest”/minimum resistance TO-220 packaged MOSFET I could find in stock :open_mouth:
I prefer to burn a couple of mosfet’s that’s “off-board” then get a black smoke poof on the main controller :laughing:
I haven’t tinkered with MOSFET for some 15 years, so the shoot-through and gate-charge reminder where good.

What’s the maximum gate-charge you have tried thus far?
I just have to find some good TO-220’s that satisfies your suggestion and have insulated thermal connections, then I can stick it on a big heat sink.

I guess this project is going to be one of the heavier projects with your controllers, a minimum of 1000Kg, I haven’t decided if it’s going to drive the caravan directly, or be stuck on a caravan-trolly with a big water/sand tank to get traction :sunglasses:

I think the biggest gate charge we’ve used in this design is around 80 nC, though we might have used some 110 nC in some prototypes (though that’s worst-case on the datasheet, so the units we had might not have been that high).

- Jan