Require help with Mosfet H-Bridge please

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
I am playing around with a Mosfet H-Bridge to switch 125kHz signal through a load. The circuit has P-Channel on high side and N channel on bottom of the arms. The Fets are controlled by Mosfet drivers. I also have some fets switching the series load resistance to change power electronically.

The issue is that the P-Channel devices are not switching off fast enough and therefore still on when N channel on same arm is switched on, causing excessive current drain.

Let me know if you have experience with MOSFETs used in this type of switching application and I can post the circuit and part numbers of devices used.

Thanks in advance.
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
Yes.
The circuit is being controlled by micro controller PWM with DeadBand Gap.

On the scope, the rising waveform for the P-Chan is steep but falling is more like capacitor discharge curve. Even if I remove the other switching devices and only leave resistance load, the fall waveform is sloppy. The min load is 1k. The waveform improves a little if I lower the resistive load.
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
ICS0089 = TC4426A MOSFET Driver
ICS0107 = DMP3036SSD-13 P-Channel MOSFET
ICS0106 = STS8DN3LLH5 N-Channel MOSFET
Q1 to Q8 are FDV301N Mosfets
Signal on IC5 pins7,8 and also on 5,6 have steep rise but slow turn off.
The signal at Gate drives for the IC106 are triangle shape and not pulling properly to ground. I thought maybe drain of FDV301 was too high capacitance but still looked same when they are removed.
The signals from all the ICS0089 are nice steep rise and fall square waves
 

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ElectricSpidey

Joined Dec 2, 2017
1,336
I don't see any obvious errors in the circuit.

I assume you do have all of your de-coupling caps at each driver.

I would double and triple check your processor signals.

Beyond that I doubt I can help any further...sorry.
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
Yes, decoupling right next to the drivers and Fets.
I also tried only switching One of the P-Channels with 1k load to ground and still get slow turn off.
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
With the driver (ICS0089) and the P-FET (ICS0107).
The driver is specified to drive quite high capacitive loads and signal to gate of P-Channel is nice and square.
On output, I isolated so that ONLY a 1k resistor to ground is attached (As well as the flyback diodes). And still see the falling edge does not get back to ground almost until next rising edge. I thought maybe the Schottky diode were capacitive and removed them but it did not help. I am baffled why I do not get nice square waveform out of the P-FET.

Below shows part of circuit I tested in isolation (with 1k resistor to ground on pins 7,8)
 

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Thread Starter

davidhuisman

Joined Oct 15, 2020
10
Which spec do you think? They have Vds = 30V, i cont = 10.6A, Vgs(th) is -1 to -3V. Listed as fast switching device with Rise time = 14ns, fall time = 31.6ns. My input waveform is 4us wide.
 

RIKRIK

Joined Oct 11, 2019
107
If your in a simulator, id just play around with things. Few things you could shuv between your driver and pfet. A zener diode going to ground, a pullup resistor.a diode pointing up or in series or inbetween. A 10n capacitor.

Just go wild ;).
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
I ran a simple simulation. Though I have not used the exact FET parameters or set up the drive signal properly, I can see a definite problem when the load resistance gets too high.
Is there a reason P Channel mosfet cannot drive a light load properly, or is there a workaround?
 

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ElectricSpidey

Joined Dec 2, 2017
1,336
Is there a reason P Channel mosfet cannot drive a light load properly, or is there a workaround?
I can't think of anything that wouldn't also apply to N-Channel.

In your sim the PMOS is upside down, also on your main schematic the diodes are reversed on the PMOS.
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
1,336
In that sim image the drain is connected to + and the source is connected to ground via a 1k resistor.

The connections in your main schematic are correct.
 

Thread Starter

davidhuisman

Joined Oct 15, 2020
10
Oh, you mean for the simulation. Yes that is wrong.

I think I understand what is happening in my circuit.

The load is a tuned circuit without DC path to ground (That is why I added the 1k). I believe the output capacitance discharging through the 1k is keeping the signal high when it should be right off.

Also, the series resistors to the N chan cause LPF with Cin of the Fets and causing input signal to be converted to triangle wave.

What I will do is place the control signal (use a hate) on input of the driver IC to N channel fets so they get full current required to turn then on and off properly.

The P Channel I will replace with Push Pull output so the in line N fet can discharge the output quickly.

Only issue left will be how to switch various load resistors in series with the load.
 
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