MOSFETS and H bridge

Unfortunately, the schematic is so small you can't read the component values. Some questions:

• What are th values on your series gate resistors?

• What is the load?

• Do you have problems driving a pure resistive load? Good time to see if you have cross conduction problems.

• Do you have a mechanism to prevent cross conduction?

• The HIP driver is being stressed by the 300V signal. I wouldn't expect it to live either.

• At some time, are all the FETs in an off condition? This is a no-no, except to deal with cross conduction issues.

• I hope you have more bypassing than the single 0.1uF cap on the H-bridge. Each half bridge needs bypassing and the layout is critical for each half-bridge, minimizing lead length. Little sliver spikes can kill your parts.

• What does the 12V power supply look like during switching?

I don't see how you can get 300V on the source of M1, Jess. There is a diode to ground there so it can't go negative very far. There is a substrate diode in the FET, so it can't go positive very far unless your

12V supply is also going to 300V (or your diodes are open or they are very slow).

Have you actually observed these voltages with an oscilloscope and, if so, have you taken precautions against probe pickup and "ground bounce"?

Cheers, John S

Hi,

I am using HIP4081A to drive an inductive load. I am controlling the HIP chip with a microprocessor using PWM. The power supply of the chip is +12 Volts. The HIP Chip is driving the H bridge made with four NMOSFETS (IRF540N)

HIP chip is driving the NMOSFETS which in turn injecting the current into the load and the voltage is appearing across the load as a sinusodial waveform of frequency 100KHz. The frequency of the PWM signal is 100KHz. The thing is that for example at 10% duty cycle, about 300 volts appeared across the inductor load. I suspect that the 300 volts appears at the source of the M1 transisotr as shown in the attached diagram. This source point is connected to the pin 19 of the HIP chip "BHS". According to the data sheet voltage on BHS should be 80 volts maximum. But in my case it is 300 volts peak to peak. Can this voltage make the HIP chip burnt and the transistors timing goes off?

Problems:

1. The transistors get hot after 10 to 20 minutes ( M1 and M4 ).
2. HIP chip also does not work right but it works more than 10 to 20 minutes sometimes more than that.

Questions: Is the high voltage on the BHS reason of getting the transistors hot? Because the Drain voltage is 12 volts and the source voltage is 300 volts when the inductor just started charging up?

HIP chip circuit link

any suggestions? Note: BH_HIP and BL_HIP are shorted together and AH_HIP and AL_HIP are shorted together.

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I'm not hip to the hip...

Troubleshooting tips:

1. The reason why mosfets get hot is too much current flowing through them. This is either due to cross conduction or to large of a load. Use a low resistive low(e.g., 1M ohm resistor) and see what gets hot.

1. The hip should only get hot due to the driving of the gates. This should not happen in general unless you are pushing the hip. You can increase the gate capacitance or resistance to slow down the charging of the gate and decrease current to the gates. This could increase cross-conduction.

2. Cross conduction should not happen when driving motors as only 2 mosfets are on at any one time. The point of an h-bridge is generally to prove reverse polarity to a motor for reversing it. But when you are either in the forward or reverse state only 1 mosfet will actually be "working"(toggling/PWM) and another will be fully on while the other 2 will be off.

1. A larger voltage across an inductive load than the supply voltage is due to back emf(the Faraday effect). This should not happen. You use freewheeling diodes that provide a relative short for this high voltage path to ground which should limit it to about 1.5V. Mosfets have these diodes built in but external ones can be added for safety.

2. The PWM Rate has to be low enough so that the mosfets actually can switch from fully on to fully off and vice versa. Too much gate capacitance or resistance will slow the transition time so much that the PWM rate will actually keep the mosfets in a medium resistive state.

As far as the pin 19 voltage issue. Yes, it is possible for 300V to damage the chip. It depends. If it has a maximum of 80V you should use a zener to cap it just in case. I suspect that your issues lay somewhere else. I do not believe 300V should ever appear across anything unless you have a rather large supply. Since you didn't give us the load(motor, inductor, or what?) and the application it's hard to say what should and shouldn't be. An H-bridge for a motor will behave quite a bit different than that of a switching regulator.

Work on simplifying the circuit and narrowing down the problem. It could be as simple as a mis-connected wire. You know how the h-bridge and driving circuitry should work so as long as you have an o-scope it shouldn't be hard to diagnose it.

Are those diodes 1N4002? If so, replace with high speed diodes. And also put high speed diodes to the +supply to bypass the FET substrate diodes.

Her first sentence is "I am using HIP4081A to drive an inductive load."

This is an excellent suggestion as it allows monitoring of the current as well as some protection against high current if there is cross conduction.

Her first sentence is "I am using HIP4081A to drive an inductive load."

Yes but there are generally two types of inductive loads that have distinct types of troubleshooting. SMPS and motors both used a similar topology but have different purposes. I've never heard of a true h-bridge used for SMPS but I suppose it could be used... but then why wouldn't she simply say "I'm driving a motor"?

This is an excellent suggestion as it allows monitoring of the current as well as some protection against high current if there is cross conduction.

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There should be no cross conduction if this is for a motor, which is why I made the distinction, unless she is used some advanced energy recovery methods or active breaking. If there is cross conduction, which is easy to measure, then there is definitely some big issue going on.

Maybe I didn't mean cross conduction. What I was thinking was shoot-through when I said cross conduction.

Ah! You mean driving the inductor of a buck converter?

Maybe, but I don't see that as relevant as regards an inductive load. How are they different?

I am having problems reading that schematic, it's too small.

But, I think I have enough of it in my head. It looks like you need to use some back EMF diodes on the inductive load to suppress the fly back energy that is released when you remove the current.

On the high side this is going to cause you problems due to the fact that it places the Mos fets into the linear region and maybe even in the off state. You must remember that that the difference between the source and gate voltage must exceed the threshold voltage of the Fet Vgs(th). If the voltage appearing at the source is higher than your driving signal can over come to exceed this threshold, you'll either place the mosfet in linear mode, which will make it operate hot if you are pushing them. Also, you can damage the Fets.

P.S.

Use High speed diodes when doing so and place them across the poles of the coils, polarized to only conduct on the reverse path.

Jamie

It has a charge pump to supply the gates of the upper FETs. The diode and capacitor in the upper left corner of the schematic serves that purpose. A high speed diode should be used there as well.

Ah! You mean driving the inductor of a buck converter?

Maybe, but I don't see that as relevant as regards an inductive load. How are they different?

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For a SMPS such as a buck rconverter that uses an active diode(e.g., mosfet) you will end up with cross conduction.

In the circuit it should be clear that if the diode is replaced with a switch and both switches are on then you get a short. With mosfets you'll get "shoot-through" or "cross-conduction".

In the SMPS design(or possibly other things) the "switches" are being toggled together and so every time they toggle there is a chance of shoot through.

In the case of a motor this should never happen since the purpose of the the

3 extra mosfets is to provide a polarity exchange and not to act as an active diode. i.e., only one mosfet will be toggling per pulse instead of two. It's not the number of mosfets toggling per se but the configuration that allows shoot through.

For a motor the h-bridge is really acting as a single mosfet that limits the average current to the motor except when polarity is being reversed(which generally doesn't happen very often). SMPS has a distinctly different behavior and generally require two mosfets.

Maybe a better way to put it, which I've already mentioned, is that an h-bridge for a motor should have shoot-through only during the change of polarity of the motor(again, which doesn?t occur very often