Do Mosfets Share Current?

[rhitee05]

LFP, you keep this sort of thing up and you'll need to start growing your beard out and dying it grey yourself...

i didn't notice but there is another identical mosfet on the other side of the board in parallel, so it seems to be there to stop the ringing of the paralleled fets. Nice to know for those paralleing fets.

I've read this advice elsewhere. The bead presents a high impedance and high frequencies, so it helps to damp out oscillations between the FETs.

Also, reducing the ringing that LFP referred to is a major EMI benefit. In a commercial product (that has to pass FCC testing), I would expect this to be more likely than squeezing a couple more amps out. If the designer is competent enough to put in this feature, I expect him/her to be competent enough to allow proper margin for operation as well.

 

[Electroglide]

its probably come up before but here is a pic of a mosfet with an iron bead around one high amp leg, from a quick search it seems this plays a similar role to capacitors to stop ringing and get rid of transient voltages. Can anyone elaborate further and do you guys think it would be useful to be using these?
It occurs to me that its a bit similar to an inductor.
Leg with iron bead has different hole location on the board to allow for the bead
this fet is off an mppt

I would be curious to see if they use a separate gate resistor for each device or whether they tie the devices to a common gate resistor. I could see the ferrite bead being useful for controlling the ringing of the gates when there is no impedance separating them, and less effect when there is a separating gate resistance on parallel devices. Depending on the frequency, these ferrites would appear like there is a resistive element in series with the gate. Probably >100MHz at a guess. One of the reasons that you see individual gate resistors(10R to 15R) on individual devices is to control the ringing between parallel devices, but it causes higher miller induced voltage gate blips because of the higher impedance between the gate driver and devices which reduce how fast you can switch the devices opposite the devices that are freewheeling the current. Opps ... run on sentence lol. I have seen SMD ferrites used in drive circuits before, and it did remove the high frequency ringing, but it did introduce other artifacts that were less desirable. I was not total convinced that they were necessary and that a better layout would have given the same reduction in ringing if separate gate resistors were used. Sounds like a research project.

 

[whatever]

sorry re; my error calling it a high amp leg, I didn't realise it was the gate.......memory not so good
I have had a look and it appears they both use same resistor to the gate, interesting also there is a resistor and small cap in series joining the two high amp legs. I will take some closeup pics and post when I can.

 

[scriewy]

If you hooked your controller to a colossus motor with 2mOhm of phase resistance and just a couple uH of inductance, then even at like 10battery amps, your phase amps can be in the hundreds.

what what what whaaaat, so can i just measure phase resistance with DMM ?
1st time i stumbled upon any 1 mentioning this, till now most said rule of thumb phase current 3 times batt current, now you scare me , so is phase resistance simply dependent on total wire lenght after it's wound around coils ?

is a 3 phase motor divided to 3 sets of coils, and each phase is wound around 1/3 of motor coils ?

so high wind motor that has longer 1 strand phase wire around 1/3 of coils then low wind motor with 3 strands phase wire but shorter around 1/3 coils will have higher resistance than 3 strands shorter wire ?

if i'l put wattmeter on phase to record peak amps, while limiting controller to 20 bat amps will it be ok or something will FK up this way ? (i'l power wattmerer with external power so it wont shut down every puls)

will the result i get (if i get) stay linear after i increase bat amps ?

i'v shunted my clyte 72v 40a controller to peak 80A with clyte 405, i thought FETs will never see above 240A, maybe they do

this morning i powered controller with 56v as usualy just to see if controller draws it's usual 0.03A and after a second heard poooof then motor twiched, i turned bat off then back on controller drew the usual but no response to throttle, i know some FET blew for sure cos wheel cogs when all powered off, but why it blew, maybe cos of standing all night in 10c, or maybe i wore down the junctions inside the FETs with my 80A bat peaks that maybe pulsed more than triple inside FETs.
i rode in new config maybe 2 weeks, when i peaked 80A it was for brief seconds 5 tops, and mostly i dont go WOT from stop, usualy draw 10-40A

 

[Arlo1]

, or a circuit designed by an EE master with a very poofy and fuzzy grey beard.

Speaking about a person in particular?

 

[Alan B]

Phase current is AC (flows both directions).

Rules of thumb don't apply in all cases.

You are starting to understand. Enjoy your journey of discovery.

 

[scriewy]

after few hours i remembered some note, controllers that can't be programmed usually preset to a certain phase current, so i am guessing it's relative to bat current otherwise i wouldn't feel more torque after shunting, but what happens in next case.

same controller in it's original state limited to 40a bat current and preset to 3x phase current with motor number 1, then swap motor 1 with motor 4 that has much lesser inductance, is controller still gona output only 3x current on phases ?

is the outcome different if it's clyte analogue or infenion that we set to 3x phase current ?

if it's AC then wattmeter wont see it bum.

got 4 dead FETs 3 on blue and 1 on yellow, have only 2 spare MUhahahaaa, i also red some side of FETs is under less stress ? like in 9 FET controllers there are 2 FETs on some side and only 1 on the other, which side is it ?
cos looks that's the only way i can fix it now, i'l pull 1 FET off the lesser stressed sides.
jhon mentioned some FETs directly connected to battery + (this evening looked for it, connected with drain to +), are those what's called high side ?

could be the FETs blew cos of incident i had 3 weeks ago when thumb throttle broke inside and magnet got stuck and i didnt notice it while braking on red street light, later i noticed motor still pushes me into the junction to get roadkilled i hit the brakes more but not for full stop, zigzaged a bit not to stall the motor, in few seconds controller popped and motor started jerking, turned power off. so could be that other FETs got weakened a bit on that same blue phase and it was a matter of time till they pop.

the journey is maaaaaadnes

 

[ctirad]

Kelly controllers read across the RdsOn value of the shunts, which then means they auto-de-rate themselves as temp increases and the RdsOn goes up.
The down side is, that means you can have 2.5x LESS torque on a hot controller than a cold one... Which is also a bit wonky.

It looks like a proper and safe regulation to me. The more warm is the controller, the less current flows and thus it limits further increase of the temperatre.
The usage of the Rds(on) as a shunt is quite clever anyway.

What method does use the Infineon for the phase current measure, BTW?

 

[liveforphysics]

Kelly controllers read across the RdsOn value of the shunts, which then means they auto-de-rate themselves as temp increases and the RdsOn goes up.
The down side is, that means you can have 2.5x LESS torque on a hot controller than a cold one... Which is also a bit wonky.

It looks like a proper and safe regulation to me. The more warm is the controller, the less current flows and thus it limits further increase of the temperatre.
The usage of the Rds(on) as a shunt is quite clever anyway.

What method does use the Infineon for the phase current measure, BTW?

It takes a W.A.G. (wild ass guess)

If you know battery current is 25amps, and you're at a 50% duty cycle, then phase current will be roughly 50amps, but you actually have no idea what the spikes and peaks and overshoot may happen to be.