DC Drive for 4.8-HP 24-VDC Motor

What is catching on fire? The motor, the drive MOSFETS, or the freewheeling diodes?

The power mosfet.

In that case, does one device burn up, or do all burn?

You may have a problem with load sharing, which can be taken care of by designing the circuits for each transistor to limit its current to the device's maximum.

Additionally, devices are rated for maximum currents with proper heat sinking or heat dissipation.

I feel you may be right.

He needs something like a 0.25 ohm resistor (of a suitably high wattage and also cooled on a heatsink) in line with each MOSFET (also cooled)......

All the MOSFETs may be burnt, but one at a time as the one with the least resistance burns through first, then the next, and so on............

If I may say so, its a typical "Beginners Mistake", if it is what we both think it is.....

As Andy says... back in the olden days... If you were designing a power supply and needed more power, you could add additional pass transistors (in parallel) to handle the additional current... assuming that you had ample unregulated supply to feed it... until the cows came home.

All transistors are NOT alike... even from the same batch... so we add "current sharing" resistors on the emitters (as described by Andy), to equalize the load handled by each transistor or (FET).

Bill

I suspect the wiring is seriously wrong.

He is using 10 of these MOSFETs which if wired correctly has the capability of supplying 1700 amp which is 11 times approx more than his motor would draw on full load this is way overkill, which also suggests his circuit is incorrect.

One of the good things with power MOSFETs is there ability to current balance due to resistance increase with temperature increase so in a well designed circuit with equalised wiring if the temperature increases in one MOSFETs it will shed some load to the other MOSFETs thus negating the need for current balancing resistors, but in this instance it sounds like some short circuit is taking place or other problems as previously suggested.

Garth

Bonjour Francis,

there are several ways to solve this problem. one way would be to replace your MOSFETs with "intelligent" MOSFETs which automatically limit the current to a safe value. see for example http://www.farnell.com/datasheets/40545.pdf. These are not the only ones available. they will solve your problem with the least amount of work.

Secondly, (and good to do even if you upgrade your MOSFETs) you should make sure that the resistance from supply to each MOSFETs and from each MOSFETs to load is equal. So each path from common neg through the mosfet to the motor should have the same section (mm2) and the same length.

Other points to look at are the heat sinks as already mentioned and the rise time of the switching signal. If this rise is slow it increases the power dissipation in the mosfet.

I hope this helps

Chas.

You are aware these MOSFETs are rated 120 Amp continuous drain current at 100 Degree C . pretty serious expensive burn up. Motor max running Amps about 150 allow 250Amp oveload and start up. 460 Amps from the mosfets at 100 Deg and more if running cooler.

I am suprised that in PWM mode they would overheat, on resistance is only0.009 Ohm I am asuuming the heat sink is large enough to handle sytem reqirements.

I would check that full gate voltage is being supplied to turn them fully to avoid heat build up in the mosfets. Otherwise has the motor being checked for shorted windings on top of other suggestions.