Driving a mosfet directly?

[passion1]

I would like to use a power mosfet IRFP460 to switch a 24V circuit.
The IRFP460 is a n-channel mosfet with the following rating:
Vdss = 500 V
Id(cont) = 20A
Rds(on) = 0.27 ohm
Vgs(th) = min 2V max 4V
Vgs continuous gate-to-source 20V

Can I drive this mosfet by connecting the gate directly to my PIC16F73 output port or do I need a mosfet driver?
Also, can I connect the mosfet source directly to the ground of the 24V supply, and drain goes to load with other side of load going to +24VDC?
If a driver is needed, can you recommend a suitable driver.
If a driver is not needed, is it then necessary to put a resistor inbetween I/O pin and gate and if so, what size?
Any help is appreciated.

[Charles Linquis]

You don't need a MOSFET driver, but connecting a small resistor (10 ohms or so) between the PIC output and the FET gate is always a good idea, this helps insure sharp switching with no ringing.

Since you are switching fairly high currents in the vicinity of your PIC, make certain you bypass the PIC well.

[Ioannis]

It depends on the current you need. More current, more drive. Check the Vgs to Id curves. They will reveal the actual drive you will need.

Ioannis

[passion1]

Charles / Ioannis

Thank you for the advice.
Can I connect the mosfet source directly to the ground of a 24-36V supply, and drain goes to load with other side of load going to +36VDC?

[Ioannis]

You should take care if the load is noisy. Otherwise there is no problem in that connection.

Ioannis

[paul borgmeier]

Also remember that the 16F73 output is VDD-0.7VDC. If you are you using 5.0V for the PIC VDD, your max Vgs will be 4.3VDC. (Your selected FET can have a Vgs threshold of 4.0 VDC !!). You won't be able to get much IDrain Current, possibly 3A max. If it were me (with what I know about your application and what you are driving), I would switch to a FET that was more logic-level friendly like the IRL520 (or something similar). Otherwise I would look at adding a driver near 10Vdc.

By the way, what are you driving and what amperage do you need?

[RussMartin]

With a maximum Vdss of 60V, it's only rated at 12A continuous (42A pulsed) but might the MTP3055VL (Fairchild, TO220 package) or similar critter be a possible solution? Vgs(th) is less than 2V and Rds(on) is about 0.1 ohm.

Alternatively, the 2N7000 (TO92 package) makes a pretty good little MOSFET driver.

[Ioannis]

I remind that for every ampere of the Drain pin there must be an amount of Gate drive. Don't get too excited byt the great amperage that datasheets state! Read carefully!

Ioannis

[passion1]

Paul, RussMartin, Ioannis

Thank you for the advice.

Paul, thank you for pointing out that I won't be able to get much IDrain current.
Any suggestion for a driver near 10Vdc, apart from the 2N7000 suggested by RussMartin?
I really appreciate your help.

[Ioannis]

Look at Microchip, MCP1401 is one example. Many others exist too.

Ioannis

[Dave]

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

[paul borgmeier]

Paul, RussMartin, Ioannis

Thank you for the advice.

Paul, thank you for pointing out that I won't be able to get much IDrain current.
Any suggestion for a driver near 10Vdc, apart from the 2N7000 suggested by RussMartin?
I really appreciate your help.


I didn't really point it out - Ioannis did in a post before me.

As far as a driver, it depends on what you are driving and how often. If you are switching a lot, you need something different than if you just want to turn something on once or once in a while. I'll ask again:

What you driving?
What Amperage do you need?
How often will you be switching?

[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

[passion1]

Ioannis
I forgot to mention that I have included a 1N4007 diode between the drain of the mosfet and +36VDC as protection.
The inductor is 2.6mH while the capacitor is 6600 pf.

[Ioannis]

Ok. I sure would recommend a faster diode like the 1N5819, MBR360 or similar.

I think with the suggestions found in this thread you will be OK!

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!