I know I don't have the flyback diode on the motor. And the MIC4422 is flipped in the schematic.
I want to control the high power motor with this high power mosfet while using PWM.
To drive the mosfet(AUIRFS8409-7P), I got a MIC4422 to increase the mosfet switching speed, but I still get some extreme heat after only a few second of running and there is no load on the motor. For sure no heatsink can dissipate this heat.
I didn't put any resistor at the gate of the mosfet to increase the switching speed.
I try lowering down the pwm frequency. At about 50 hz, the heat is OK with heat sink, but the frequency is too low for the motor to run smoothly and it's vibrating a lot. Maybe I could find a equilibrium point between the ''vibration'' and the heat output.
Do you know anything to reduce the switching time even more, when the motor will have load, it will be even worst. Is my circuit ok?
BTW, when the mosfet is not under pwm, the mosfet is not even a little bit warm. The problem is really with the switching time.
The battery supply 12v, so the gate get 12v .
Where is the heavy decoupling on the MIC4422? It definitely needs this. 0.1uF + 1uF + 10uF
ceramic or more with that beast of a MOSFET, within mm of the chip pins. The wiring from MIC4422
to the MOSFET needs to be low inductance (short, wide traces right next to each other for source+gate)
You have no freewheel diode across the motor, so it will be causing high voltage breakdown everytime
the MOSFET switches off - probably the cause of your heating.
I know I don't have the flyback diode on the motor. And the MIC4422 is flipped in the schematic.
You have no freewheel diode across the motor, so it will be causing high voltage breakdown everytime
the MOSFET switches off - probably the cause of your heating.
So if you know you are breaking the rules why are you asking us what is your problem ?
think about it...
MarkT:
Where is the heavy decoupling on the MIC4422? It definitely needs this. 0.1uF + 1uF + 10uF
ceramic or more with that beast of a MOSFET, within mm of the chip pins. The wiring from MIC4422
to the MOSFET needs to be low inductance (short, wide traces right next to each other for source+gate)
You have no freewheel diode across the motor, so it will be causing high voltage breakdown everytime
the MOSFET switches off - probably the cause of your heating.
ok thanks.
Just to be sure, I put one of each of those capacitor in parallel, from VS to GND?
And should I put a big capacitor(ex. 500uf) for the motor? Can this help in anyway for anything or will mess up the pwm.
I think I told you to use heavy decoupling on the driver chip.
Several Low ESR caps, maybe 1000uF, close to the driver chip.
I think the chip had 9 Amp drive capacity, and the mosfet's input capacitance was 14nF.
I also told you to never use the mosfet without heatsink.
And I think you have been told to use a 30Amp kickback diode for your 100Amp motor.
Leo..
You can't get 1000uF ceramic decoupling capacitors!! The driver needs 0.1 + 1 + 10, ideally.
Yes the motor supply will benefit from big electrolytic decoupling too, but the priority is fix
the lack of free-wheel diode.
Personally I'd always use a half-H-bridge so each MOSFET can act as diode for the other and
you get a choice of modes (active braking needs this for instance).
I'm curious as to just what function a "decoupling capacitor" is to provide in a PWM setup. What location would it be placed in the circuit?
Learn electronics. Decoupling caps are a standard component for digital circuits. The purpose is to reduce
noise and negative spikes generated by the switching of the chip and the sudden inrush current at the time of the pulse. This tends to pull down the voltage on the power pin. It is specific to digital chips in general.
A mosfet driver has other considerations such as the one raised by Wawa about the charge/discharge the gate source capacitance.
On the power pins of the chip (Vs & GND, From pin-1 to GND and from pin-8 to GND))(where do decoupling caps normally go ?)
go in this PWM circuit?
the fact that it needs decoupling has nothing to do with the fact it is a PWM circuit.
It has to do with the fact that it is switching , and possibly switching rapidly, generating noise on the power bus.
Decoupling caps ACROSS Vs and GND . (as close to the chip as possible)
The below definition you referenced does not seem to fit the apparent function of the capacitors between Vs and GND. It seems these capacitors perform the typical function of stabilizing the supply voltage to the chip in case the supply wiring is a long run or or routed such that inductive/reactive elements are introduced (same components often recommended for regulating chips such as the 7805). The small capacitor on the output probably does provide some decoupling function for chip protection. Add the capacitors and I doubt the heating issue will improve. I think another issue is the problem.
"A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect it has on the rest of the circuit. An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit."
The output cap is just simulating a capacitive load in a test circuit.
Best to stick with the decoupling recommendation of the datasheet.
I mentioned 1000uF Low ESR caps not for the value, but for the ESR.
I think this is not a circuit for breadboarding or thin long wires.
I hope OP realises that.
Leo..
Minus any voltage drops. This could be a problem as some N MOSFETS may not fully turn on until the gate voltage is somewhat higher than the drain voltage.
I add a diode (MBRF30100CT-LJ) across the motor and now everything work fine. For decoupling capacitor, I only had on hand some 0.22uf ciramic capacitor, so I add 19 of them in parallel across GND and VS.
I dont know their ERS value or whateverm but it's working
Now the mosfet is always a room temperature, even without a heat-sink, it's a success for now.
I will mow work on the H-bridge with my HIP4081A. I wonder if I could use again about 18 of 0.22uf for decoupling until a get some better one. Or is it a stupid idea? What decoupling caps do you suggest, I can't find it in the datasheet.
What is this 10,000 pf polycarbonate from output pins 6 & 7 to GND? Why do I need it?
You could remove them and see if the issues return.
And how, might I ask, do you explain the relevance of that ? (I'd love to hear that)
How does capacitance on the supply bus explain overheating of the Mosfet ? (which now seems to be fine after he followed my and others recommendation to add a flyback diode)
I don't know where you got the idea the decoupling caps (or lack thereof ) was in any way related to the overheating. I commented on them in reference to the OP's question about where they go . I answered his question by referencing the example circuits in the datasheet. I never suggested it was a solution to the overheating which more than likely was the diode , (which I commented on in my first post in this thread).
