I am working with a bootstrapped half bridge gate driver and I am wondering if anyone knows what function D24 is performing in the below image? I can't find this specific diode placement in any of the other circuits that I have found online. Thanks.

 

Where does the schematic come from? I as a hobbyist in this stuff don't see it actually working and not just because of D24. The reason I say the circuit is flawed is that there is no path to ground to recharge the bootstrap capacitor. There needs to be a time in the use of the half bridge driver when the HO(high output) is turned off and a low side to be turned on so the cap will recharge. Maybe someone smarter than me will chime in to agree or disagree.

 

This is coming from a customer and they want us to redesign their old board, I can promise you that this works when you PWM the input signal. When the motor is stopped the VS line is essentially grounded through the motor, when the motor is running VB just needs to be about 15V higher than VS in order to turn the MOSFETs back on. So basically you are counting on VB to hold its voltage value for longer than VS during the off cycle, otherwise you have to wait for VS to drop back to near ground for the bootstrap capacitor to charge back up.

 

Looks to me as though D24 could feed any voltage spikes from the motor directly back to the gates of the MOSFETs. Surely that's not the intention?

 

Maybe that is the intention? You don't want VS to be more than 30V higher than HO (max Vgs on the MOSFET is +/- 30V) so you add in a diode to not allow that to happen?

 

his is coming from a customer and they want us to redesign their old board,

Again just my opinion. If they want a redesign I'd bet they ARE having problems. Doing it the right way should take any problems out of the circuit/board. The charge on the boot cap doesn't last very long and limits the time the motor can run.

 

Is there any reason the customer is using high-side switching of the motor? That seems to be creating unnecessary complication.

 

That is a good question, the reasoning is that they have a circuit that is measuring motor back EMF off the positive terminal (I cropped that out from the image above). If you were to change to a low-side driver and try and measure the back EMF from the negative terminal it wouldn't work the same. But your question and shortbus' comments are all really irrelevant because they are changing to a full H-bridge in the new design. I just really want to know what D24 is supposed to be doing. Thanks.

 

That is a good question, the reasoning is that they have a circuit that is measuring motor back EMF off the positive terminal (I cropped that out from the image above). If you were to change to a low-side driver and try and measure the back EMF from the negative terminal it wouldn't work the same. But your question and shortbus' comments are all really irrelevant because they are changing to a full H-bridge in the new design. I just really want to know what D24 is supposed to be doing. Thanks.

Let's hope they do it right this time.

 

I just really want to know what D24 is supposed to be doing.

OK, another WAG. Probably to keep the HO signal from NOT turning on the mosfet gates. Without D24 HO would go straight to the motor.