Best FET to use?

[Bern]

Thanks to all who helped on my last request, now another question. Getting ready to build a PWM circuit for a 2 ½ HP, 36 volt golf cart motor. Looking for the best deal in mos-fets and drivers. Is it best to use commercial drivers, or a couple of switching transistors to drive the gates? Which fets have you had good luck with? Minimum on R,- vs. I,- vs. gate drive power,- vs. price and availability, internal diode, etc. etc. This is for three or four motors, so need just a few, not in the OEM category any more. Retired gives more time but have lost most of my sources for information and parts I had. Thanks again, all.

[jwax]

Big project, Bern!
I bought a Curtis DC Controller for an electric go-kart I built. 48 volt @ 200 Amps, and that was for a 1 HP DC motor. It used 8 MOSFETS. You'll have thermal considerations, load-sharing, shut-down protection, to name a few.
If you still pursue building your own, one source is Advanced Power Technology in Bend, OR for power chips. Good luck!
John

[Bern]

Thanks John, but I think I can handle those types of design considerations. I am looking around at the surplus houses, and find devises like Intersil RFK70N06 for $2.50. Some of the other power FETs are available in this and lower price ranges. This is the area I am interested in, however I just have not had much experience in building these before.

[Joseph]

If the switching frequency is higher than a few hundred hertz, it may be best to use mosfet drivers. I like the IR2113. I haven't tried out the MIC4421 or MIC4422 yet, but their specs look good. You can use a 74C14 (CD40106) as the PWM controller if you like to understand what is going on more in your circuit. You simply couple a hysteresis oscillator stage to the PWM output stage by means of 33pF capacitors and the feedback signal changes the time constant of those 33pF caps by means of a small signal transistor collector with the feedback applied to the base, or an optocoupler if you want isolation. Then those PWM outputs go to the mosfet driver(s) after going though a paralleled resistor and diode to add needed dead time. This summarizes the construction of the heart of the UnivSPS circuit.<p>[ March 07, 2004: Message edited by: Joseph ]</p>

[toejam]

i have had good luck paralleling irfz44's. in order to keep them cool, a high current source to charge the gate caps is necessary to keep them cool.Dont forget to use a reverse diode to stop them from smoking-that is a bad habit.

[Bern]

Hi JOSEPH, sorry I didn’t mention frequency. I plan on using 20 KC. This sound like a good compromise between efficiency and audible noise. Haven’t put ideas on paper yet to see how they come together, but thinking about a 555 running at 20 KC, then taking the sawtooth signal and feed it to a comparator. The comparators' other input would come from the speed pot. By varying this voltage, I can get any pulse width I want. Then take this signal and drive the mosfets with a driver of some type. Actual there would be some other control circuits for speed ramp up/down, current feedback protection, etc., but this would be the basic PWM generator. Didn’t quite follow all the way through on your circuit, but I think I’m OK with my approach. Thanks TOEJAM, I think we are thinking along the same lines OK. Smoking, yeh, been there, done that, both my projects and me.

[Joseph]

Sounds good Bern. Your idea reminds me that I used the same principle in the PWM motor controller for my electric bicycle. The main difference is that I used the ubiquitous differential amplifier circuit operated with phase shifted feedback. The pot sets the voltage on the non-inverting input side while the motor drive pulse feeds back to the other input through a resistor voltage divider. A phase shifting capacitor connects that inverting input to ground to cause the self-oscillation. The slightly modified amp circuit performs the function of the 555 timer and the comparator. You could do the same thing with an integrated op amp as I did with the discrete components.

[Joseph]

Bern, I just thought of something to mention. The motor controller on the electric bicycle runs around 200hz. I guess that I decided that for an outdoor device where sound is not enclosed, the humming sound was not a bad thing at all, and actually provides a rather pleasant form of feedback to the user about motor loading and power output--over 30 amps at 12 volts.

[Bern]

Joseph, thanks for an idea. I could start with low frequency, and see how much the noise bothered. If not overly object able fine, if don’t like it, it would be a simple change to up it as needed. I could even put a pot on the 555, that way I could try different settings with no problem.

[Joseph]

Bern, you're welcome. I wish you a good time on your project. Let us know if you have any further questions.