H-bridge - stall current

[ahmedragia21]

Hi,i've two questions , 1st one i've an H-bridge which stands total dc current up to 4A , and my motor's stall current above 6-8A , would this burn the circuit at the breaking of the motor ? . I'm sure i wont lock the shaft that time by hands , just automatic breaking of motor using a uC ...

also i posted a circuit of an h-bridge using MOSFETs IRF9540,IRF540, i would like you to review it whether it will work or not , because im going to send it today to the PCB company to make one of it .


Thanks !

 

[dknguyen]

THe stall current is also the motor starting current (since the motor is not spinning when you first apply power to it so there is no back EMF to oppose the voltage applied, just like the stall condition). It might be too much for your motor. So add a fuse and always very slowly and carefully ramp it up to speed.

Your gate drivers look okay. THe resistor values may be too high but those can always be changed after you get the PCB and start experimenting. I see you chose not to go with discrete schottkey diodes across every MOSFET and have decided to rely on the slower parasitic diode each MOSFET instead. I would add a LARGE decoupling capacitor across +12V and -12V. Also, the response might be faster for a given resistor value if you put a small NON-POLARIZED capacitor (like ceramic) in parallel with each GATE DRIVE resistor since this will provide AC transient currents a lower impedance path to flow when charging up the MOSFET's gate capacitance to let it turn on more quickly. If you do this, you can get away with larger resistor values and reduce the power consumption of your gate driver once the MOSFET has turned on. But however you change the gate resistors, remember that the optoisolator has to be able to handle the current.

Your opto-isolator LED current limiting resistors seem a bit large (5mA drive current), but that depends on your opto-isolator and it can always be changed after you get your board. From a quick look at your datasheet I would change those resistors to about 500R (470R) to get a current of 10mA in the LED.

THat's neat how you used the voltage divider to keep the gate voltage within the right limits, as well as having negative and positive gate voltages (relative to the source) for noise immunity. It should be okay as long as your motor voltage does not exceed 24V (otherwise you need to use MOSFETs that have higher maximum gate voltages). Your motor voltage also cannot fall below 20V (well it can, but not by too much) since that is what is needed to turn on your MOSFETs. THe lower you go, the less efficient the MOSFETs will be when turned on.

 

[ahmedragia21]

thanks , but you still didnt answer my question , would the circuti be burnt at that moment ? or it will be safe ?
also about my 2nd question , no comment ?

 

[dknguyen]

THe peak current when you brake the motor is the current the motor was drawing when you put on the brakes so if you are running it within the limits of your H-bridge and you brake it, it should also be within the limits of your H-bridge.

THat's why my concern lies with the stall/startup current. If you ramp up the speed slowly it should be okay I think.

 

[Hero999]

The motor starts within a couple of hundered ms so I don't see how it could be a problem unless your h-bridge is that weak.

How long are you planning to apply the brake for?

If it's just slowing down with the same load connected as when it started, it shouldn't be a problem.

 

[Hero999]

Yes, I've edited my post.