Help with H-Bridge problem

[JRoque]

Hello,

I'm racking my brains trying to figure out what's going on with my HIP4081A based H-bridge motor controller.

The problem: motor stops after I applied load to it.

The circuit consists of an Intersil HIP4081A driver and 4 Onsemi NTB125N02R FETs. I've taken the usual layout precautions keeping the switches close to the driver, etc. I changed the bootcaps to .22uF from what it shows in the schematic. I also added a couple of 15V TVS and a .1uF cap directly on the motor. I ordered a few zeners to clamp the gate voltage a 12 though the driver has internal clamps.

I've measured the dead time and it's 50ns as set by the 100K timing resistors. There's no heat on the FETs or the driver. The voltage on the driver does not go under threshold as far as I can measure in the scope. The motor runs fine otherwise.

You can see from the scope pic below the normal view of the motor moving forward with the upper switch on and the bottom one driven with a ~15.5Khz pulse. Here comes the puzzle. After I apply load to the motor, note on the second pic how both channels start pulsing with a weird curve on the upper switch. In fact, all of the outputs on the driver are pulsing under this condition. The only way to return it to normal operation is by cycling the DIS pin on the driver.

This is my first time with the HIP4081A and can use the help.

Thanks,
JR

[pminmo]

JR,
I'm having a difficult time reading your schematic, can you post a better one? One point is you don't show how you are driving ALI and BLI. The need to be from a low impedance drive close to the 4081. Have you looked at this app note:
http://www.intersil.com/data/an/an9325.pdf

[bdrmachine]

Please explain the points you are measuring with the scope, CH1 and Ch2. I have had good luck with 4081's in the past. Does the upper rail of the fet bridge droop significantly during motor load?

[bunalmis]

This is normal

ALI 1010101010 Turn right
BLI 0101010101

ALI 0101010101 Turn left
BLI 1010101010
-------------------------------------------

This is problem

ALI 1010101010 Motor stop (BHB voltage goes zero)
BLI 0000000000

ALI 0000000000 Motor stop (AHB voltage goes zero)
BLI 1010101010

[JRoque]

Hey guys, thanks for your quick replies.

Below's a new posting of the schematic; see if it's any better. This is the "High Voltage" board and it's driven separately by a uC through limiting resistors (3K). I read the appnotes and didn't find where it says the inputs need to be close, though it makes sense. I guess I can buffer those if need be.

The scope screens above are measuring the FET output signals right on the motor terminals. The one on the left is the normal run with AHO on the lower portion of the screen and BLO on the upper side. BLO has the PWM signal. The picture on the right shows the state during the fault.

I get similar curves from the driver output at the gates. In that 'undefined' state, the 4081 will switch both upper switches on and both lower off and then revert them. This is clearly why there's no output and the motor stops. Still to answer is why it gets in that state.

Good suggestion on the upper set rail sagging. I didn't notice significant drooping but will be looking at that more closely. Another thing to look at is the switches - they are fast. Turn on is 11ns typical!! Gate charge is 28 nC, input capacitance is 2710 pF. So they're easy to turn on, hard to turn off. That's why I added the reversed Schottkys on the gates.

Thanks again,
JR

[JRoque]

Ok fellas, thanks to your tips I solved the bug. I checked the bus voltage sag and it was brief but deep. I then downloaded all of the appnotes and found a reference to current loops in the layout. I guess 4081's pump is a little finicky with it's source. I found a possible current loop, cut and jumpered the trace and the problem is gone.

My lesson learned when sharing the bus voltage with the 4081 and the switches is to feed all of the 4081 needs first and then continue to the switches. My problem was that I fed part of the 4081, continued to the switches and then came back with a trace to power the chip. Bad dog, bad! I can now load it up to ~20A with no heat or breaks.

BTW, the project that I'm working on is to design and install a remote control on my son's toy car. This is the type that kids ride on so safety is priority #1. I'll be posting the project after I'm done.

Thanks again for your help.
JR