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Mysterious FET destruction on high-power H bridge

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Mechatrommer:

--- Quote from: rschlaikjer on June 21, 2019, 10:11:03 pm ---
--- Quote ---that is surprising because from your capture.. your high side mosfet should not be properly ON when its Vs reaches near Vdc, and indicating charge pump is not working as well...
--- End quote ---
The voltages on that timing trace should be taken with a grain of salt; I was using some lower quality probes that have convenient micrograbbers but are otherwise not the best.

--- End quote ---
you cant be sloppy when measuring / finding fault on this kind of thing at that power level, otherwise you'll experience what you already experienced, alot of burnt mosfets. and esp when you ask advice from other people. Vg, Vds are important parameters if you want to know exactly whats going on, otherwise you can just simply follow the advice of that Mr Wiseman above and jump to current limit protect circuit. sure it will be the "solution for all" problems your circuit probably gone through, linear operation, shorted output, back emf etc... but i cant guarantee you on that (esp if Vds polarity changes), he probably can. if this one is for one-off, it will be fun to add plethora of protection circuits, thermal protection is another fancy term. but if this one is for mass production to sell to the world, good luck competing with China, even if you have to follow that 10X safety factor margin avionic or toyota grade adviced by that Mr Wiseman. if you work with boeing or toyota (who have firm sale statistics and customers) and have plenty of experience in this then thats fine. but please measure properly to avoid wasting people's time.


--- Quote from: rschlaikjer on June 21, 2019, 10:11:03 pm ---Here is a capture with the actual Rigol probes on Q1 (low side, teal) and Q2 (high side, yellow) gates when driving in one direction with low PWM. The ~23V reading here is a lot more reasonable; I believe that the charge pump is working fine (input voltage rail is 12V)

--- End quote ---
well even that capture is weird as mosfet spent 60% of time at 10Vg. back emf / inductive load is weird and i dont claim full expertise, we can wait what Mr Wiseman the knowing all who have the right to give "not utter bullshit" advice will say about this.

Siwastaja:
Dear Mechatrommer,

No one can guarantee that adding the current limit solves the problem which happened to blow the FETs. (It's very likely, though.) In most beginner circuits, there are more than one critical problem to fix. Fix the most obvious ones first.

However, the lack of current limit in this circuit is a 100% certain culprit that will blow more MOSFETs, even if it wasn't the exact cause for this case. This is the reason to fix this thing as a very first step, before doing anything else, to minimize time wasted. This is also why I didn't spend much time to look at the scope traces, and didn't and won't comment about them. You can pay me for consultation to take a closer look at them if you are that interested in seeing my comments.

After all, we have seen data of 100A actually running, which is multiple times more than sustainable, exceeding even the wildly optimistic MOSFET front page Id specs. At manageable currents, everything else changes as well, let's see the scope traces again then (with better probing techniques).

Talking about avionic or Toyota grade is 100% bullshit. Even the crappiest and cheapest Chinese motor controllers (think about hoverboards and ebikes) ALL have proper current sense and limiting, because it's the right thing to do to get even a barely working product; and it's A LOT cheaper than not having one. Another reason is that these products are used in traction, where the user really wants torque control, and by having a current sense, it comes for free.

Fake devices designed to be bricks are an exception. These do exist on Ebay market to some extent. But then again, for this, you don't have to use beefy MOSFETs, or MOSFETs at all. Any devices soldered to any board will do.

The fact that you picked up some mysterious "10x safety factor" supposedly claimed by me, shows you didn't understand a word of what I was saying. With an accurate, active current limit, any safety factor can be minimized, lowering the cost. But your way of working with the problem necessitates calculating for worst case current limited by ESRs in the system, which will inevitably be at least 10x.

You know, you don't need to reply if you have nothing to say and only want to confuse people? Stop wasting our time, thanks.

Mechatrommer:

--- Quote from: Siwastaja on June 22, 2019, 07:07:12 am ---Even the crappiest and cheapest Chinese motor controllers (think about hoverboards and ebikes) ALL have proper current sense and limiting

--- End quote ---
the proof is in the pictures. (no picture no proof not justify to be believable) the sharpest eye may find me which one is current limiting. first 4 pictures is recently replaced with beefier mosfet. the last 2 pictures is a working 60A ESC that i dismantled about 4 years ago. at 100A of shunt resistor current sense, thats a "not so heavyweight" shunt. ymmv.


--- Quote from: Siwastaja on June 22, 2019, 07:07:12 am ---The fact that you picked up some mysterious "10x safety factor" supposedly claimed by me, shows you didn't understand a word of what I was saying.

--- End quote ---
140A+ stall current. 1000A rated mosfet... lets do simple 1st grade math rather than talking nonsense.... safety margin 1000 / 140 = 7X yeah well... not close enough to 10X. but whos confusing who? or simply an exageration? ;D

--- Quote from: Siwastaja on June 21, 2019, 10:19:40 pm ---Mechatrommer's "proper" MOSFETs without active current limit would be something rated to 1000A continuous, with like 0.5ohm or less Rds(on), bare minimum, and I'm being really conservative here.

--- End quote ---

Siwastaja:
For the cheap Chinese hoverboard PCB, take a look at
https://d3nevzfk7ii3be.cloudfront.net/igi/G1ulQv3mmIFBAUcv

The current sense resistors are below the elcap bank on the left side. Measuring DC link current, after bulk capacitance. Not perfectly optimal (the Kelvin sensing is done wrong as well), but probably does the job well enough despite the flaws.

Yes, if you go back in time enough, you'll find more and more cheap designs without current sense. And they are notorious for blowing up sometimes. Or being more expensive, BOM-wise, than they need to be. But the Chinese have learned, as well.

Because, duh, without current limiting, you need to dimension everything for the stall current. And with high-efficiency motors, stall current is many times more than the desired running current. Which again is something a bit more than (like 2x) the rated nominal current.

By the way, current sense doesn't necessarily need a resistor. Integrated gate drivers (for example, in TI's DRV series I have used) which sense Vds during on time to implement a current-limit based on Rds(on) exist and are very handy. You can't see that on the layout, it works with zero extra components!

Yes, if your particular motor has a stall current of 140A at the full voltage differential (which may be twice the DC bus voltage, if you reverse the motor while running!), then no need to go for 1000A rated MOSFETs. Do proper thermal calculations for 140A, and you'll likely end up with a FET with a front page Id rating of around 300-500A. (By the way, choosing a FET with a bigger front page current rating than your actual current has nothing to do with "safety margins". The front page number is just a wrong number for the job. Use the right numbers and calculate correctly - THEN add safety margin. The better your calculation, the less safety margin you need. Often just 20-30% is enough!)

But since you never ever want to run a motor at its stall current - after all, the iron is likely saturated and it's not providing torque for the current - why spend to create a controller capable of supplying 140A, when you can do, for example, 30-40A much cheaper, while still creating the same actual torque, and protect the motor at the same time. I'm baffled at your massive brute force strategy.

Siwastaja:
Sorry I'm still going on about this,

There is an actual engineering reason why you don't see current measurement in some cheap Chinese RC helicopter inverters ("ESC"), such as one posted by Mechatrommer.

The use case is quite specific, and they have taken the "sidestep edge cases one by one" approach I warned about earlier. It works, because in RC heli motor control, there are fewer edge cases, and most of them are rare occurrences (and people accept that cheap controllers blow up in such rare cases.)

Basically, you can just ramp up the PWM in a hard-coded ramp speed to limit initial current to lower than the stall current. Then, the mechanical load on a heli blade is very predictable, it's rotating in free air and the mechanical torque is some fixed exponential function of RPM (which is sensed from back-EMF by the ESC). (Ask aerodynamic / fan design experts to get the exact exponent.)

Initial ramp-up taken care of, a sudden mechanical load increase remains an edge case. But if you hit the rotor blade (plastic or carbon fiber) somewhere, it tends to bend or shatter and snap off very quickly - not getting stuck for more than a few milliseconds, maybe.

And indeed, I have heard of people using RC heli ESCs to power eBike conversions, with blown controllers resulting. The workaround tip has been, use much larger controllers [so that they are dimensioned for the worst case stall current].

Traction on wheels is completely different: stalling with a massive mechnical load for unpredictable time (thing about running steep uphill!) is not some special edge case, it's part of everyday normal operation. Hence you see the current sense even in the cheapest Chinese controllers, if they are meant to drive traction systems (ebikes, hoverboards)...

You need understanding and experience to see what is an acceptable engineering trade-off, and when to do them. Don't force application-specific optimizations to general case when it's unlikely they apply.

But if you have a mocking attitude against knowledge, you are bound to fail to build knowledge yourself. Don't do that to yourself.

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