Driving a mosfet directly?

[paul borgmeier]

passion1, May I suggest an IRL2505. It is a logic level fet. I have used them in the past with great success..

Dave Purola,
N8NTA

This is a great FET but it requires a large gate charge (130nC). If you are switching a lot, you would want to buffer the PIC pin as a minimum since the PIC pin cannot supply a lot of current. (i.e., it will take some time for the PIC to directly charge the gate, which means lots of FET heat if your load is high). One of the FETs recommended by Russ actually has an amazing low charge requirement and could be charge in less than a 1 uS directly by a PIC. Again, the proper answer to your question depends on what you are doing.

[RussMartin]

Attached is a circuit I've had repeated success with.

I suggested the 2N7000 because of its availability, general utility and very low cost (about 0.11 USD in single quantities). It's been around a long time. Both the MTP3055V and MTP3055VL are also readily available and moderately priced (about 1.16 USD in single quantities).

I have to echo Paul's questions:

What kind of load are you driving?
What current capacity do you actually need?
How often will you be switching?

We could probably be a lot more helpful if we knew these things!

[George]

Pauls question is quite important - especially the how often part, if you want yr fet to pwm - then you really have to use a driver or it will just end up as a load of smoke, if it's just occasional switching you should be fine - preferably with logic level fet.

[passion1]

Thanks guys.
I am learning a lot here and I appreciate your help.
Everything is starting to make sense and my own understanding of fet's is improving rapidly!
RussMartin,George
I am using pwm and is switching using frequencies up to 20kHz.
Although it is not needed right now, I would like to accomodate higher voltages at a later stage
and for that reason I have chosen the IRFP460 which can handle up to 500V and 20A.
Right now I am trying to determine what would be the best driver (if necessary) to use with the IRFP460 specifically
because I have already tested this mosfet to work well with my application while using lower voltages,
and at higher voltages (e.g. 200V) I should never exceed 3amps current.
Thanks again for all the suggestions, it really helped a lot!

[RussMartin]

Okay, you've given us voltage and current requirement (up to 200V, up to 3A), and the maximum switching rate (20kHz).

It would still be nice to know what kind of load you are switching--

[passion1]

RussMartin

Please excuse my ignorance but I am a programmer with little electrical background until now!
I do not know what more information I can provide apart from saying that the load is a LC circuit, with only an
inductor and capacitor. The capacitor determines the current usage which is no more than 3 amps.

Correct me if I am wrong, but since I won't be needing more than 3A and taking into consideration what Paul has said earlier,
it seems that I won't be needing a driver after all?
Thanks again for your willingness to help.

[Ioannis]

Hmm, an inductor could be very dangerous to the unprotected semiconductors! Now a new part should be added: a diode, reverse biased parallel to the load (L-C). If not, the first time the MOSFET will switch off, it will probably destroyed by the EMF of the coil.

Also an approximate value of the L and C would be helpful.

Ioannis

[RussMartin]

Please excuse my ignorance but I am a programmer with little electrical background until now! I do not know what more information I can provide apart from saying that the load is a LC circuit, with only an inductor and capacitor. The capacitor determines the current usage which is no more than 3 amps.

My apologies! Generically, what I and others are asking when we ask about the "load" is: "What kind of device or equipment are you turning on and off?" Is it a motor, a refrigerator, lighting equipment, or other device?

I'm sure you're not just turning an LC circuit on and off for the fun of it!