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Question about gate drivers and bootstrapping

colhany , 03-30-2023, 06:49 PM
Hello everyone
I want to design an h-bridge DC motor driver IC using discrete MOSFETs and a gate driver.
Now, most motor drivers I have seen use a half-bridge gate driver like the IR2184, which utilizes a bootstrap circuit to use the NMOS as a high-side switch.
As far as I understand, bootstrap operation requires you to continuously alternate between the 2 MOSFETs in order for the bootstrap capacitor to have time to charge.
This is all good if, for example, I feed a 50% duty cycle PWM signal to the driver, in that case, the motors will keep running at 50% speed just fine. Now what if I run them at 100% (i.e. feed a fixed input voltage) for a long time? will the driver work for a little bit then the bootstrap capacitor gets discharged and the driver stops? and how can this be handled without limiting the duty cycle to, for example, a maximum of 90%?
thank you
qdrives , 03-31-2023, 02:39 PM
If you want to run a DC brush motor at 100% for an extended time, you need a separate bootstrap circuit.
I have seen such circuits in 'complete' drivers (most of them include the FETs too), but not in (half bridge) gate drivers (at least not for higher voltages > 100V).

From experience you can go up to 97% duty cycle.

If you want to create one of your own, look at L6205 (from ST) for inspiration.
colhany, 04-04-2023, 04:24 PM
Hi qdrives, thank you for the reply.I want to make a driver for low voltages, maybe up to 35 volts only.I have found a document from Infineon suggesting using one of their half-bridge drivers (which uses bootstrap) with an additional charge pump circuit using the 555 timer, I will try it out.https://www.irf.com/technical-info/designtp/dt92-4.pdfThis is the document I'm talking about.You can also look at it and tell me what you think, if you have time for this.Thank you
qdrives , 04-05-2023, 01:58 PM
Question is - when you are switching (so not 100%) will you damage the FET as you are feeding Vbus+12V and not Vs+12V?
If your gate resistor has a high(er) resistance it may be ok.
colhany , 04-08-2023, 03:00 PM
Thank you for the hint
I will try running a simulation to see what happens
qdrives , 04-08-2023, 04:16 PM
You will need to add the parasitics too.
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