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I've developed an automotive H-Bridge circuit using the DRV8702-Q1. It's pretty close to the reference schematic in the datasheet but it doesn't work. I've been racking my brain for ideas as to what has gone wrong, but I can't find anything obvious. The host controller is a Raspberry Pi CM3, using 3.3V logic levels to the DRV8702-Q1, with VIN at 12V. The mode pin is pulled to ground to configure it as a full H-Bridge.

The nFault pin indicates that the H-Bridge driver is not in a fault state (constant at 3.3V), the SO pin hovers between 5.4V and 5.48V. VCP is the 9.87V, which matches the high voltage I see on the MOSFET gates.

The MOSFETs I'm using are FDD8447L-F085.

I think the layout appears similar to the datasheet as well, though not identical

enter image description here enter image description here

This is a scope of the low side gates (GL1, GL2) when EN is on. Changing the phase simply alternates which gate exhibits the waveform shown below. Looks nothing like what I'd expect. The high side gates seem to be stuck at the 9.87V (matching VCP) regardless of what signals are sent on PH/EN. The behavior seems wacky to me, maybe the DRV8702 was failed by ESD. enter image description here

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  • \$\begingroup\$ did you use the suggested PCB layout? \$\endgroup\$ Oct 23 '19 at 4:40
  • \$\begingroup\$ You may well have blown the gate's insulating oxide via ESD or overvoltage from misapplication or inductive kick. Do the FET's get hot if allowed to power the load during this self-turn-on? That's another typical clue. Or you could remove one and use an ohm meter. \$\endgroup\$ Oct 23 '19 at 4:46
  • \$\begingroup\$ @analogsystemsrf Added the layout \$\endgroup\$ Oct 23 '19 at 4:48
  • \$\begingroup\$ how does heat exit the FETs? \$\endgroup\$ Oct 23 '19 at 5:21
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    \$\begingroup\$ What's your load? What does Vcp measure (scope)? No ground plane? Show gate waveforms! Also, please follow the normal convention of ground down and Vcc up. Several of your transistors are upside down. \$\endgroup\$
    – winny
    Oct 23 '19 at 7:58
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The only explanation I can think for all the gates being high is that the gate driver has failed. Since all the mosfet gates are high, this opened the path directly to ground, overheating the current sense resistor and causing the resistance to change from 0.1 Ohms to 517 Ohms.

I also noticed that the current sense resistor is rated too low for the load I'm using (a 3W 0.1 ohm resistor is more appropriate).

Upon closer inspection I noticed some layout problems: the main current carrying traces of the H-Bridge are too narrow compared to the layout, and the VDrain should connect directly to the FET, whereas I had it connecting right to the VIN pin on the gate driver.

However, even with these problems I'm uncertain what caused the gate driver to fail. Perhaps ESD, or maybe somehow the layout problems above.

In the datasheet there's a note about limiting the current to VDrain with an 100Ohm resistor, perhaps this is the issue. However, the "typical application" schematic does not show any resistor on this trace, and the EVM module schematic just shows a jumper. Not entirely sure what to make of this.

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