Why controller mosfet blow often just after a stop?

I was just watching the oen of the old video of Icecube57 about controlelr mosfet blow when used with the high kv 5303 ( well known to blow controller easy and when i saw the conditins that made the fet to blow this remind me exactly teh same situation many times!

Here is that famous perticular situation:
You ride your new setup or higher voltage setup..
It look to go well for let say 5 minutes.. so you think. GREAT this controler is powerfull and can take that current well !
then there is a street intersection or a stop and you need to slow down or stop... and... then you want to re-accelerate to return to your cruising speed... and suddenly BRRRRRRRRRRRRR... your motor vibrate and you know you have some fets blown

It also sometime happen when you decelerate to very low speed before to stop...

I'm asking because i'm trying to figure out why this particular situation make the fet to blow...

For all the dozen times i had controller that blow in my life i would say 90% happened i that situation.

And strangely this rarely happened when accelerating!! and never happened during sustained high power!! witch is what we could believe due to the increase of heat due to limited dissipation...

but why??

-Fet Thermal stress that change?

-Fet junction are accumulating little internal damage but still work but have more and more chance to fail over the time...? and the fate is the same every time.. the max dammage limit occur strangely when you stop your ebike before restarting... ? lol

-Restarting from a stop require higher phase current with PWM very low making ultra high frequency peak making ultra high overshoot voltage shooting your controller fets the death punch?

Opinions?

Watch the Icecube57 vid showing a great exemple:

https://youtu.be/pMuoEh3PHUs

Doc

Two things to remember

Back EMF is only generated when the motor moves (draws less current running than stopped)

Back EMF is reduced when the magnets get hot (motor will pull more amps)

---
After a hard run and you stop the motor heat soaks and stator heat is transferred to the magnets

Now The second time you accelerate from a stop you draw more amps with a typical controller

flathill said:

Two things to remember

Back EMF is only generated when the motor moves (draws less current running than stopped)

Back EMF is reduced when the magnets get hot (motor will pull more amps)

---
After a hard run and you stop the motor heat soaks and stator heat is transferred to the magnets

Now The second time you accelerate from a stop you draw more amps with a typical controller

Click to expand...

That's an interesting thoery Flathill.

I know BEMF is responsible for the voltage diff between the motor and battery and witch let more or less current.. well... phase current as well..

So you say hot magnet make the stator to draw more current?..

Hot magnet have less mag field? making less inductance on the stator ( more eddy current)

I thought magnet could loose magnetic field but that this effect is permanent and not temporary usually?...

or... maybe when you say that:

Back EMF is reduced when the magnets get hot (motor will pull more amps)

Click to expand...

you mean that when heat is transfered to the magnet this make the stator resistance to get lower so it draw more current... ?

Doc

when you come to a total stop the controller is no longer cycling the mosfet driver circuits because motor phases are stopped.

during this period, the charge stored in the charge pump capacitor on the high side mosfet drains down through the 510 ohm resistor on the hiside gate drive transistor bias resistor. there is no charge stored in the capacitor any more because there are no cycles for it to push the current through the hiside zener and store it above the battery on the hiside cap.

so gate voltage that is available when that gate is turned on has dropped as the bias voltage drops and then, when the gate is then enabled, it has such low voltage that the current carrying electrons are not present in the channel so the rush of current through the channel will overheat it rapidly, and if it is warmer then it will get hot fast so within one or two cycles it should overheat. jmho.

i think this is why most lose the hiside mosfet when the motor stalls as well.

maybe it has to do with 'block time', where the first second or so the phase current is allowed to be much higher ?

Right after a stop:

Warm or Hot FETs,
Block Time not limiting current,
No back EMF,
Accelerating,
Etc.

That's what happened to my 24 FET controller several times. Never got warm, just occasionally blew on startup.

I think the root problem is reducing the shunt so far the instantaneous current limiting in the hardware is no longer able to protect the FETs, combined with a low resistance motor and wiring, and low inductance makes it all worse.

did this happen with kelly controllers, too?
They accelerate smoothly, so they probably don't have block time.

Not, they have proper phace current limiting. Infineons monitor only battery current.