Hi,
I am trying to design a microstepping drive for my home-made CNC mill.
The design parameters:
1. Adjustable maximum curent 0.5-8A
2. Supply voltage 24-80V
3. Maximum step frequency 0.5-1 Mhz
4. 200-51200 steps per revolution (1.8deg motor)
So far I am using an 80V rated h-bridge driver to control 100V 25A power mosfets. This driver will go up to 1 MHz switching frequency according to specs. Current is sensed using a shunt resistor, and once it reaches a set point it shuts down mosfets into fast or slow decay (microcontroller selectable). The overall delay from the moment overcurrent occurs till mosfets are shut down is 200ns. So if I wait for another 800ns before I turn them on it should give me a 1 Mhz chopping
The issues I am having:
1.
I did a lot of research on existing drives and found many manufacturers advertise a step rate of 2-10MHz and PWM rate of ....20kHz.
I am hitting a brick wall with my head but can not understand how come you can go to the next step (i.e. adjust current) in .5 uS if your PWM cycle takes 40uS?
That absolutely doesnt make any sense to me. I would suppose pwm chopping frequency should be at least a few times higher than a maximum step rate. or do I miss something? I also understand that if I increase my chopping frequency to lets say 500kHz the heating will increase dramatically.
2.
As I mentioned above the max supply voltage is 80V. This is a limit for my h-bridge driver circuit. Can anybody advice a 12V voltage regulator which would take input up to 80 volts? So far I could find 60V max and its a switching reg.
Also I am putting 81V transient suppression diode on a power line. but datasheet says it will break down till 90-110V. Does it mean my circuits will be fried by the time it kicks on and does any difference?
I am not a EE major, so please, be gentle on me
And thanks tons for all your replies!
I am trying to design a microstepping drive for my home-made CNC mill.
The design parameters:
1. Adjustable maximum curent 0.5-8A
2. Supply voltage 24-80V
3. Maximum step frequency 0.5-1 Mhz
4. 200-51200 steps per revolution (1.8deg motor)
So far I am using an 80V rated h-bridge driver to control 100V 25A power mosfets. This driver will go up to 1 MHz switching frequency according to specs. Current is sensed using a shunt resistor, and once it reaches a set point it shuts down mosfets into fast or slow decay (microcontroller selectable). The overall delay from the moment overcurrent occurs till mosfets are shut down is 200ns. So if I wait for another 800ns before I turn them on it should give me a 1 Mhz chopping
The issues I am having:
1.
I did a lot of research on existing drives and found many manufacturers advertise a step rate of 2-10MHz and PWM rate of ....20kHz.
I am hitting a brick wall with my head but can not understand how come you can go to the next step (i.e. adjust current) in .5 uS if your PWM cycle takes 40uS?
That absolutely doesnt make any sense to me. I would suppose pwm chopping frequency should be at least a few times higher than a maximum step rate. or do I miss something? I also understand that if I increase my chopping frequency to lets say 500kHz the heating will increase dramatically.
2.
As I mentioned above the max supply voltage is 80V. This is a limit for my h-bridge driver circuit. Can anybody advice a 12V voltage regulator which would take input up to 80 volts? So far I could find 60V max and its a switching reg.
Also I am putting 81V transient suppression diode on a power line. but datasheet says it will break down till 90-110V. Does it mean my circuits will be fried by the time it kicks on and does any difference?
I am not a EE major, so please, be gentle on me
And thanks tons for all your replies!