[DukerX]

1. The speed issue is rise and fall time, not frequency.

Only half-true, as increased PWM frequency leads to a higher rate of rise/fall instances, thus adding up the power dissipated while the transistor is not fully on.

Besides from the looks of the measurement results that the member posted, it seemed as though he had access to GOOD equipment and probably the parts and funds needed to solve the problem.....

For those measurements, I used MS Excel, AVR STK500, Mega48 and a multimeter.
The reason I'm hesitating to use this or that kind of component, is that I don't have it. I have a big variety of electronics scavenged from various equipment over the years.
THe only electronics I have actually bought, is the AVR microcontrollers, an STK500 MCU dev board and a breadboard. Almost forgot the 10MHz scope I bought used from my scool many years ago...

My budget goal is zero$, as I refuse to beleive I should be unable to make something from the variety of transistors, chips and other stuff I already have on hand.

For starters, I'll try to make a totem-pole driver from some of my PNP/NPN transistors.
Hope I can find 4 of each that has somewhat similar specs

[pminmo]

Only half-true, as increased PWM frequency leads to a higher rate of rise/fall instances, thus adding up the power dissipated while the transistor is not fully on.

I won't argue that point. But as you strive towards an ideal switch, that issue is less.

As to the open loop approach, I was speeking earlier more on a performance basis rather than cost basis. If you know all the parameters going in, motor characteristics, power supply voltage, switch characteristics you in theory could do what you want. I just wonder how the performance would be.............

[H500]

I learned by designing several stepper drivers. Nothing beats actual experimentation, so even though I say open loop PWM for steppers is a waste of time, I definitely encourage everyone to find out for themselves. :-)

Using a high voltage greatly improves the performance.....with a current sensed chopper design. An open loop pwm shuts off the mosfets at the wrong times. This does not allow the coil current to rise as fast as possible.

[Carel]

A totempole driver, whether it's discrete or on chip consists of a transistor for charging and a transistor for discharching, not with pullups. Discrete it consists basically of 2 transistors, 2 resistors and a diode. Discrete it has the flexibility to drive it through optocouplers and the controlling of the driving voltage. Component cost:
< 0.50 Euro. Pcb real estate < 1 * 1cm. Does it work at 20Khz? Cool at heavy load.
With an open loop PWM driven by a processor, you can reset the PWM when changing to a new position. It is obvious that when you step (open loop) at 19999 steps/sec with a PWM at 20000 that things are not good most of the time. I am not a stepper fan myself, in the end they are low power, low rev motors, but I responded to the problems with the driving of a mosfet.

Carel

Never argue with people with one book.

[DukerX]

The totem pole drive seems to be the proper thing to do...
I made this circuit to go from 5v to 12v just to try how it worked.

I managed to cut the switching time down to less then 0.2us for the IRFP250 driving a resistive load of 20ohms from +12V.
Significantly better then driving it from the 5V logic port.
I didn't have enough complementary pairs to make a full driver, so I'll try using some of my more common NPN/PNP transistors and see what difference that makes.

This is now starting to deviate so much from the original topic that I think I'll stop posting in this one, and try to find a thread that already discusses discrete driver design.

[Carel]

Congratulations! You're out of the business of alternative heating and you've beaten the chips by 100-200 nanoseconds. Try to find the variant which uses 2 NPN transistors (use BC547C) so you dont have to mess with complementary parts. I have measured the drivers build with these yesterday at 250 nanoseconds. Once you managed your drivers, you can revert to the original problem.

Carel.

[NC Cams]

RE: original problem.

The IRFP250 has a 2000 pf gate capacitance - hence you need a driver with a lot of balls to bring it into full enhanced mode at higher speeds . The logic level drive of a micro controller simply can't do that - you needed a buffer (IE: fet driver or totem pole or even an open collector TTL chip with a pull up) to do that.

Driving it with 12v instead of 5v did just that - got you full enhancement. For even better, faster enhancement, use 15 or 18v for Vgs (the rated Vgs for IRFP 250 is 20v) as per prior post #5.

If you still want to do logic level driving of the fets, try an IRLZ44 at 5 volts. The micro controller SHOULD drive it but the watch word here is "should".

I'd be more inclined to use an 8-10v Vgs with your same driver circuit and the IRLZ44 fet. If you have 12v, an LM2940T10 will give you a well regulated Vgs of 10v with PLENTY of current capability.

You'll have HALF the "on" resistance of the IRFP250 with the IRLZ44 and a bit more current capacity.

To improve device enhancement of the IRLZ44 as well as reduce conduction and swithing losses further if you use a TTL driver chip, add a pull up to your gate of the fet.

For 5v TTL, use a 680 ohm pull up to fet gate. If you want to use 8-10v make sure the TTL has an O/C output. Size the P/U to be compatible with the current capacity of the O/C output.

Keep in mind that International Rectifier "suggests" the use of a TTL buffer as the fet drive as opposed to direct micro contro drive - your totem pole should MORE than sufffice for this as compared to TTL w/pull up.

By the way, it is pretty easy to get 10v for Vgs from 5v via the doubler shown in IR's application note AN-969

While you're cruising around the IR website, you might want to check out AN-941 which has some ideas for discrete drivers...

Use the proper pull up/pull down logic on your totem pole to make sure that the fet stays OFF at power up when you don't have current going to T2.

EDIT: If you're using reclaimed parts, it would be a bit more helpful to provide a list of what you have early on so as to intergrate the best of what you have. It was easy to say "use this or that" but it wasn't your intent to design with a clean sheet of paper which we didn't learn until post 21.

Reminds me of the scene in "Apollo 13" movie where they show the engineers a pile of "Stuff" and tell them the task is to keep 3 astronauts alive and "thats ALL you have to work with".

Now THAT took creativity. I admire yours for doing what you're doing - wish we'd have know that was the objective sooner, however.