Current Limiting Mosfet

Does the MOSFETS switch current to zero during the low part of the period? If so the motor inductance will limit the current to a few mA.

thanks 4 ur prompt reply....yes i think the MOSFET does switch current to zero!!! Could you please explain as to why this happens cause i would have thought that switching at such a high rate @ 63% duty cycle would allow me to get a mean voltage of 3.15v. In my present circuit i have the MOSFET's gate controlled by a PWM signal, drain connected to

Odd that it's working 'fine' in the LR mode. 10 ohms and 5V implies the motor coil is only seeing 1.2V and 0.4A. Motor coil power=0.5 Watt(16% of nominal). Dropper resistor only 1.5Watt. (even less shaft horsepower available when the motor starts to run up). For both chopper and LR mode, something like 20-30V would have been be better john

The interesting question here is whicih of the two L/R drive circuits on page of the data sheet - the top one with series blocking diodes, or the bottom one with parallel clamping diodes. I'd have gone for the bottom one myself, and then the average voltage across the coil isn't

If you gone for the top circuit, the situation is more interesting. Every time you turn on the MOSFET, you apply 5V across the coil, and the current builds up, limited by the inductance and and resistance of the coil. When you turn the MOSFET off after your 31.5usec, the current can only charge up the coil capacitance (which is considerable - try to measure the self-resonant frequency of your motor coil sometime), and for the next 18.5usec the current rhough the coil is is decreasing rapidly, while the voltage across the coil goes up well above 5V.

When you turn the MOSFET back on, the situation gets really interesting

- the coil capacitance is distributed along the winding, mostly between the successive layers of wire, and you are trying to discharge the capacitance rapidly, and in fact reversing the voltage across the coil, implying large currents within the coil, opposing the current you are trying to set up. and quite a lot of heating.

So you've effectively increased the resistive losees in your coils, and again, the 3.15V average voltage across the coil isn't going to generate the current you expected.

Setting up a PWM drive for a stepper motor is not as easy as it looks - the motor coils aren't resistors, but inductors, and in a motor set up for unipolar drive (as shown in the UCN5804 data sheet) you have to keep in mind that each pair of coils have almost as high a mutual inductance as their self-inductance. And don't forget the capacitance between the individual turns of each coil - you really do want to measure the self-resonant frequency of the coil if you want to find out what is actually going on.

Nav2u skrev:

what mosfet? how are you driving it? tried at 100% dutycycle and resistors?

-Lasse

ok...firstly im not using a 10ohm resistor its a 5ohm 10W resistor....my bad. I am using the second circuit on the page 5 of the datasheet, the MOSFET is IRF540

"how are you driving it?". I have the Gate connected to a PWM signal (20KHz....i tried some different frequencies from 400Hz to 20KHz as well no luck!!!) the duty cycle is 63% also tried 82% but changing that makes no difference. The source is then connected to the motor supply input and pin 2 & 7 of the chip.

"tried at 100% dutycycle and resistors?" No, but what i did try was got rid of the MOSFET and changed the supply voltage to 3.15v and this time around the stepper motor was hard locked not spinning but locked.

The easy solution would be to use resistors as in the application example!

Do you have access to an oscilloscope? I would think one is needed in order to understand the situation correctly. The inductance in the stepper is, I would guess, a few mH. So it will take about one ms, maybe more, to reach the current needed to move the motor, and at 63%@20KHz there is only some 30 us available. So some kind of "flywheel diode" is needed. If you want the motor to move at high speed, then 3.15 V is not good enough. Not only the inductance, but also the back-emf must be defeated. Typically steppers are driven by 30-50V, and current is regulated using a switch regulator. Some stepper driver chips include such regulators. Sorry, I did not read your data sheet carefully enough to say whether this chip has one or not.

The next question is whether you are turning on your MOSFET within the

Even with the typical part, you've got to supply at least 10nF of charge to get the gate to +4V - that's 0,25mA for 41usec - and anything up to 60nF to get the transistor fully on - which is 1.5mA over 41usec.

That mean a 680R pull-up resistor. If you want the MOSFET fully turned on for 35 of your 41usec, that resistor goes down to 100R - which dissipates 0.25W when the MOSFET is off.

What value of pull-up resistor are you actually using?

A logic-level MOSFET with a gate threshold closer to 2V would be a lot easier to turn on at 20kHz.

Turning off the MOSFET isn't going to be a problem - the 5804 can sink up to 1.5A, which would be enough to turn the IRF540 off in 40nsec if the 5804 were that quick.

Nav2u skrev:

as in using a N-channel fet as a high side switch? where does the pwm signal come from? whats the voltage? relative to ground?

if you are actually driving the fet properly 100% duty cycle should be fully on and give you the same result

-Lasse

Hey guys thanks for all your input.... I got hold of a signal generator and an oscilloscope and tried a couple of different things. The signal generator allows me to "DC Offset" my square wave....and this was it....offset the square wave +ve by a couple of volts and my motor started to rotate drawing about 0.8 to 0.9Amps. This obviously points out that the MOSFET is not being driven properly...exactly what you guys have been trying to explain me.

As suggested by Bill that the MOSFET is not a Logic Level MOSFET; i may be able to score a couple of MAX4427ESA

Really nice to see a bit of feedback from a poster. Way, way too often I've seen the guys supply some fascinating commentary and suggestions in response to a problem that's giving an OP some headache and yet see nothing by the OP in response. I can't imagine the guys want a pat on the head, more like confirmation that their comprehension skills are on form and that their advice was valid and of value. (Selfishly!); Over time, I've learnt a lot from specialists in the oddball threads that can turn up. If the OPs don't bother responding to give some kind of validation to the offered info then I lose out as well. Who of those contributors should I be listening to? :). john

ok Back 2 square 1! Unable to get the MOSFET driver chip....Yes its a N-channel FET...High Side = YES....PWM comes from a M16 micro, temp using a signal generator as i can easily change frequencies.....working with 5 volts and yes relative to ground.

If you guys were to design a stepper motor driver circuit using the UCN5804 chip and limit the current using a MOSFET how would you guys do it...suggestions will be highly appreciated.

Also I have looked around for other Stepper Motor Driver chips and they are so expensive and require a lot of external components...suggestions...

Using your cirquit I would probably use a P-channel FET, and a flywheel diode between drain and ground.

If you need to run at high speed, use higher voltage and a current regulator. That implies a comparator and a current sensing resistor (about 1 ohm) in the MOSFET source cirquit. The comparator should turn the MOSFET off at the right current and turn it on at some lower current (hysteresis).

One type I have used is PBL 3717. Current regulator included.