I'm just getting started in electronics. I'm designing a bipolar stepper motor driver. There are two H-bridges in this driver one for each of the motor coils. These H bridges are controlled by a microcontroller based on sensed current to bring the current in each coil to the desired level.
I want the H bridges to handle >3 amps at 24 volt. I considered using an integrated h-bridge driver but the low cost drivers seem to be made for <20V supply voltages and 24 volts brings the NCP5359's bootstrap pin to ~34volts, 1 volt under the absolute maximum. I don't like that.
So I decided to design a driver from discrete components. Here it is:
circuit
Mosfets- NTD4963N (Vds max>30V /Qgate <16nc @ Vgs=10V /Vgs max >20V)
PNP bjts- BC807 (Vec max > 45V)
NPN bjts- MMBT4401 (Vce max > 40V)
diode-SBAV99WT1G (Vr max >70V / Trecovery <6ns)
These were the cheapest parts at mouser that look like they'll do the job
The fets require 16nC of gate charge which will take roughly 100ns for the buffer to supply.
Their threshold voltage can be pretty low (min 1.45v) but that still gives 0.7v above 0.7 diode drop limit of the buffer to discharge, worst case. That could be a problem.
I've already found one problem in the design:
-if the driver is told to turn both fets off (input=high impedance) and the output is pulled high by whatever it is attached to it, current from the output is drained through the upper push pull buffer and into the attached transistor. Which will dissipate enough power to fry itself (>350mW).
-the boot capacitor drains under the same circumstances but I care less about that since it can be recharged.
I won't have to worry about those because of how I'm using it (one fet on at all times) but I'm new at this so there could be other problems I'm not aware of.
I want the H bridges to handle >3 amps at 24 volt. I considered using an integrated h-bridge driver but the low cost drivers seem to be made for <20V supply voltages and 24 volts brings the NCP5359's bootstrap pin to ~34volts, 1 volt under the absolute maximum. I don't like that.
So I decided to design a driver from discrete components. Here it is:
circuit
Mosfets- NTD4963N (Vds max>30V /Qgate <16nc @ Vgs=10V /Vgs max >20V)
PNP bjts- BC807 (Vec max > 45V)
NPN bjts- MMBT4401 (Vce max > 40V)
diode-SBAV99WT1G (Vr max >70V / Trecovery <6ns)
These were the cheapest parts at mouser that look like they'll do the job
The fets require 16nC of gate charge which will take roughly 100ns for the buffer to supply.
Their threshold voltage can be pretty low (min 1.45v) but that still gives 0.7v above 0.7 diode drop limit of the buffer to discharge, worst case. That could be a problem.
I've already found one problem in the design:
-if the driver is told to turn both fets off (input=high impedance) and the output is pulled high by whatever it is attached to it, current from the output is drained through the upper push pull buffer and into the attached transistor. Which will dissipate enough power to fry itself (>350mW).
-the boot capacitor drains under the same circumstances but I care less about that since it can be recharged.
I won't have to worry about those because of how I'm using it (one fet on at all times) but I'm new at this so there could be other problems I'm not aware of.
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