I am using the three of the RT9624A half bridge driver as part of a 3-phase motor control circuit. Everything was working well in testing until I got to about 80% duty cycle and one of the drivers started getting hot. Probing it found that the low-side output had gotten internally shorted to VDD.
I looked back at the datasheet of the RT9624A and noticed that all the example schematics include a resistor in series with the VCC, apparently limiting current into the IC. Why is this necessary and is it always necessary? Could this have something to do with my failure? The datasheet does not seem to explain. I also notice that the datasheet warns that too small a bootstrap capacitance can lead to overcharging and damage the chip. They give a minimum of 0.1uF, and recommend 1.0uF. I have 0.47uF, which seems fine to me, but could I need a higher value?
This is my circuit, with GHA and GLA directly wired to 51nC total gate charge N-type mosfets. At 12V, this makes 4.25nF, and with a PWM frequency of 20kHz, switching losses should be less than 100mW. The part handles up to 800mW, so that shouldn't be the issue.
Edit: Although I haven't confirmed anything, I have several action items to prevent similar issues in the future.
Adding gate resistors, and also fattening and shortening the gate traces to limit potential harm from inductance loops.
Similarly reducing inductance of the SW/PHASE trace, and adding a schottky diode on the SW/PHASE node to prevent damage from negative voltage spikes. Currently my mosfet's body diode will be limiting those spikes to at worst -1.1V, but the RT9624A can only handle voltages below -0.3 for at most 100ns, so extra protection will help. If anything, based on my probing my circuit, I think it was low voltage produced by motor back EMF that might have caused the damage.
This different datasheet says "Pulling HO more than –0.3 V below HS can activate parasitic transistors resulting in excessive current flow from the HB supply, possibly resulting in damage to the IC. The same relationship is true with LO and VSS. If necessary, a Schottky diode can be placed externally between HO and HS or LO and GND to protect the IC from this type of transient."
The IR2301 specifically notes it is "Tolerant to negative transient voltage dV/dt immune" and says the VS/SW/PHASE pin can go as low as the bootstrapped voltage -25V, which in my case would be something like -8V. When dealing with a BLDC/3-phase controller, this is probably an additional feature that could be very useful. (and saved me this problem).
- This guide gives the motivation behind using a series resistor to not only bypass noise but also decouple the chip from the rest of the circuit. Accordingly, I will also be adding in the 2.2Ohm example resistance from the datasheet in between the battery power and VCC. I do find it interesting, though, that in looking at more than 10 other half bridge datasheets, not a single other included a series resistor on VCC.