Well, switching the lamps should be easy enough. You're just switching them
on and off, yes?
In this case, I'd be tempted to go with BJT transistors instead of MOSFETs.
There's really no reason to use a MOSFET to switch the lamp, and the drive
circuit will be easier. You're not going to try dimming these with the FETs,
are you? Anyway, here is a sample schematic for driving your FET:

              +12V                  +Vdc
                |                     |
                C                     |
            +--B   (Q1 NPN)          LOAD
            |   E                     |
            |   |                     D
 Vin --RRR--+   +---R2R2R2---+-------G  (Q3 FET)
            |   |            |        S
            |   E            K        |
            +--B  (Q2 PNP)   A (ZENER)|
                C            |        |
                |            |        |
               GND          GND      GND

        Q1 and Q2 form what's called a totem-pole driver. something like the
old 2N3904 and 2N3906. Or 2N4401 and 2N4403. They help ensure the MOSFET
gate is turned on quickly by providing more gate drive than your PIC could.
size the resistor 'RRR' as normal for a PIC-to-transistor interface. You
might want to use a pulldown resistor on the bases of Q1 and Q2 to avoid
floating them if you tristate the output pins of the PIC. *NOTE* Q1 and Q2
are powered by +12V, not +5V. It doesn't have to be exactly +12V, but it
should be at least +10V. Since you are using it as a switch, you need to
ensure that the gate of the FET has enough voltage to fully turn on. I often
use the unregulated power from the +5V logic (eg before the regulator). The
supply voltage for Q1 should not exceed 20V, however. That would destroy the
FET. The resistor 'R2R2R2' and zener diode are there to prevent ringing at
turn-on and ensure the gate voltage does not go above 20V. This resistor is
usually 22 Ohm to 100 Ohm.

        I'll be out of the office all of next week (going to Europe), so i
won't be able to respond to any questions. I hope this gets you started.
Someone posted some links just a few days ago to International Rectifier's
appnotes on power FETs. I suggest you check those for further help.

-----Original Message-----
From: Edson Brusque [mailto:ebrusque@TERRA.COM.BR]
Sent: Thursday, March 01, 2001 4:46 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Power FET design application notes


Hello Phil,

> What is the load you are switching?

    basically lamps. 220V x 100W or 110V x 100W. AC or pulsating DC
(rectified AC, without the capacitor usually used).

>How fast do you need to switch it?

    I'm thinking in 20-30kHz

> are you switching the high side or the low side?  In other words, is the
fet
> between power and load, or between load and ground? or is it more
> complicated than that?

    I'm not switching yet because I don't know how to make it. :)

>We need some more information.

    Just ask.

    Thank you,

    Brusque

-----------------------------------
Edson Brusque
Research and Development
C.I.Tronics Lighting Designers Ltda
(47) 323-2685  /  (47) 9993-6453
Blumenau  -  SC  -  Brazil
www.citronics.com.br
-----------------------------------

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