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Mysterious FET destruction on high-power H bridge

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Mechatrommer:
fair enough, the end result you end up with less than maximum torque that a motor can give. for traction system (in fact everythings that need motor power is "traction" system ;) ) thats fine as you can even beefed up the motor even more. and be ready to get mocked by another Mr Wiseman for those small smd shunts running at 100A+. its reasonable to put current limit on something that can hit a living thing we dont want to hit a cat and cut it into half by the hoverboard. safety, power consumption/efficiency and overall system costs are another aspects to consider, the OP may decide which is more economical between paralleling mosfets or current limit. but without full clue on what his system is doing and the why braking need to be done by reversing h-bridge (i guess there's another better way to do that in the bridge control) saying other options as utter bullshit is a way of Mr Wiseman saying...

Siwastaja:
Extra torque per ampere you can get out of a high-efficiency motor running near stall currents is marginal. The reason is, the iron saturates, the efficiency plummets and any extra power you put in just turns to heat. The motor iron just can't produce any arbitrary torque out of any arbitrary current, the current-vs-torque curve is fairly linear only up to a point.

Think about it this way, a motor which is 90% efficient at nominal current may be only 70% efficient at 2*nominal current, which still makes sense if you need that torque and cool the motor (or use it for short peaks only), but then at, say 5*nominal current, the efficiency may be, say, 30%, and torque only slightly more than at 2*nominal current. Diminishing returns.

Stall current is just defined by the total ESR of the system, nothing else. High efficiency motors minimize this ESR, so the more efficient the motor, the larger the ratio between stall current and sane (productive) currents.

For a small few-watt toy motor, the efficiency is crap anyway. It makes very little sense trying to current-limit such motor, just run it off a simplest bridge directly.

For a 95% efficient EV motor, the stall current can be 20 times the nominal current.

It's all about power level, and the title says: high-power.

Current sensing costs about 50 cents. FET cost is easy to substantiate. Development time cost trying to find out why the FETs are blowing up may be priceless.

The most complexity and price in a motor controller goes in the power stage FETs, gate drivers, link capacitors, PCB itself, heatsinking, layout design time, controller MCU, and control code. All of this architecture already supports current control very well, it's just a few percents of the total complexity to add one.

In my experience, the turning point after which current sensing is almost always A) cheaper, B) easier, C) better in every imaginable way, is somewhere around maybe 100W of motor power.

You make it sound like it's not generally needed, even in power levels we are discussing here. You are horribly wrong in this. You are not just offering "other options", you are offering extremely bad and false advice. You just have so little idea what you are talking about that a fruitful discussion is very difficult, because almost every assumption you make is fundamentally wrong. One more example: you assume running 100A through a shunt resistor would be some kind of weak or expensive link. No it's not, at 100A, every other part needs to sustain such currents as well, 100A shunts are still relatively a small part; and what's best, when you do sense current, you don't need 100A parts to handle normally 30A torque-producing currents! You only need them if you are really going to drive motors which still produce actual torque at sensible efficiency at 100A! In which case, you have designed a much more capable and powerful motor drive, at practically the same cost.

Given the OP's numbers (like actually measuring 100A current, with MOSFETs rated to 80A abs. max at forced roomtemp infinite heatsink), it's a total no-brainer, and questioning the need of current sense in this case is simply utter bullshit, which is why I'm calling you out on it.

Siwastaja:
For reference, here's my latest one, Vdc = 25.2V, Iout=25A continuous, 35A peak.

You can see how the current shunt resistor occupies about 1/8th of the space occupied by the two FETs. Power loss in the 1mOhm shunt is 0.625W, whereas it's approx. 8W in two FETs total (the current always runs through two fets), so it's less than 10% of the loss budget - or if you think in the terms of efficiency, maybe a drop from 98% to 97.8%. These ratios stay even if you scale the system to larger FETs and larger shunts.

This shunt power loss percentage (now 10% of total converter losses) could be lower if I didn't need accuracy in current measurement, but I wanted to have it.

Also note how I placed the DC link MLCCs. Probably would got away with less, but a low-inductance DC link does reduce ringing energy (up to a limit), and EMC.

adrianza:
I don't know if it's still a topic of interest, but at first glance, this schematic can't work properly, which can be seen, as Mechatrommer said, from the capture of the oscilloscope where it can be seen that the charging pump is not working properly.
If we read the MC33883 datasheet completely, we will see that there is an entire chapter dedicated to the choice of capacitors depending on the frequency and the MOSFETs used. Also in the datasheet we will notice that the capacitor in CP_OUT must be connected to VCC, not to GND.
I built this bridge by reading the datasheet very carefully and it works perfectly with both inductive load and resistive load (28V, 90A, 45KHz).

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