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[SOLVED] Switch positive and negative (Choosing/Building an H-bridge)

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greywanderer012345:
I'm going to finally buy these things. These are what I'm going with:

http://www.digikey.com/product-detail/en/NTD2955-1G/NTD2955-1GOS-ND/1484753

http://www.digikey.com/product-detail/en/NTD5867NL-1G/NTD5867NL-1GOS-ND/2401422

 Please let me know if you see this in the next couple days and see that I'm about to order something that won't work for some reason I haven't accounted for.

jwatte:

--- Quote ---Shouldn't the gate to source current be small enough that the voltage drop across this resistor is less than threshold?
--- End quote ---

There are two currents to balance for the pull-up.

First is the current while the signal N channel pulls the gate down. This is wasted heat, and you want it small. 10 kOhm is great for this.

The internal switching resistance in the pull-down N-channel is likely 5 Ohms or less (BS170 value) so you don't really have to worry about being able to actually pull the gate down.

Finally, the switching speed when the pull-up turns the gate off is also decided by the pull-up resistor. You want this to be as fast as possible, both to reduce the amount of time the switch spends in the transition region (warming up) and to avoid cross-conduction with the low leg pulling the control down. A 1 kOhm resistor is actually kind-of too large to make it possible to drive both the N-channel leg and the P-channel leg with the same signal; similarly, if you're using PWM, 1 kOhm is not enough to switch a power P-channel fast enough. I've had to run with 100 Ohm pull-ups to make PWM H-bridges built this way robust before (driving a 5A RC car motor.) If you can find a way to make !MCU turn on later than MCU turns off, and turn off earlier than MCU turns on, for example by using two different comparator values on a PWM timer, then that's probably a good safeguard.

So what's an engineer to do? This is why MOSFET gate driver circuits are a thing! Start looking at IR2101 or IR2183 for examples. Note that those will lock-out if VCC is < 10 V, so not good for cases where the top-level voltage is small, but there are others that work for that case. Also note that those drivers generate a bootstrap voltage *above* the supply voltage, so that you can use an N-channel switch both for high leg and low leg. The trade-off is that you can't keep the high leg on forever; you have to have a PWM duty cycle to keep charging the bootstrap capacitor.

greywanderer012345:

--- Quote ---Finally, the switching speed when the pull-up turns the gate off is also decided by the pull-up resistor.
--- End quote ---
How does that work? Could you give a reference or explain why the resistance of the pull-up resistor affects switching speed? It shouldn't matter for this project, since it's not using PWM, and the direction is only going to be switched every 10 seconds or so. Thank you, though. This is vital information that would be frustrating to have to discover only after a future project doesn't work as expected.

jwatte:
So, after the lower-left N-channel turns off, the charge/voltage in the top P-channel gate needs to build up for it to turn off.
This charge is built up through the pull-up resistor.
The capacitance of the gate is actually substantial (can be up to 20 nF for a power MOSFET) and with a 1 kOhm resistor, it can take over a millisecond. With 10 kOhm, it will take 10x longer. And if you run PWM, you will be doing this hundreds or thousands of times in a second -- with a 10 kOhm resistor, it's possible you won't ever turn fully "on" or "off" and spend all the time in the transition zone. That way lies blown MOSFETs. You really do want to spend an absolute minimum of time in the transition zone if you switch any amount of current at all (say, more than 1/20th to 1/50th of rated current) to avoid switching out-heating the conduction heating and causing failure.

greywanderer012345:
Thanks for that reply. Quick, simple, and complete. I didn't know that FETs had capacitance and needed to charge to turn on/off. Definitely expands my understanding of the concept, and the importance of the resistor size. I'll make sure to get mosfet drivers or a motor controller for any project I plan on using PWM with.  :)

Update: Ah, after fully considering the implications of the resistor value on PWM, I realised why another suggestion I found makes sense. The suggestion was to keep the upper leg on and use PWM only on the lower leg. This would allow use of a higher value pull-up resistor, saving power, without the switching time problem. This requires 4 pins committed to operating the H-Bridge, though.

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