At 03:20 PM 2/20/02 -0600, Dale Botkin wrote: >On Tue, 19 Feb 2002, Vasile Surducan wrote: > > > On Mon, 18 Feb 2002, Spehro Pefhany wrote: > > > > > > 0--------------------------0 > > > > > > > > FROM 12V TO LAMP > > > > XFORMER STRING > > > > > > > > 0------+ +-------0 > > > > | | > > > > | | > > > > | | > > > > | | > > > > D D > > > > Q1 G---+---G Q2 > > > > Nch S | S Nch > > > > | | | > > > > GATE DRV 0------------+ | > > > > | | > > > > | | > > > > PIC GND 0------+-----------+ > > > > > > > >Voila... > > > > > > Very nice, Dave! A couple of IRLZ544's maybe. They would have > > > a total drop of about 1.1V at 20A, so about as good as a triac. > > > At 10A, they'd beat it by a mile. > > > > > eh, eh you have too rested minds guys ! > > This will never work, take a look better ... > >Vasile, > >Can you tell me why you think it won't work? It looks to me like each >MOSFET's body diode will conduct "in reverse" during the half-cycle when >Vds < 0, and the opposing MOSFET will conduct if the gate is driven >high... I do see one thing less than ideal, though, which appears to be a >loss of the Vf of the body diode. For an IRLZ44 this looks like 2.5V max, >or around 1.7V for the 2SK2614. > >Looks like at my 7A or so load this would work out to... let me see... >1.7V * 7A = 11.9W dissiption, is that right? But only for half the cycle, >figure roughly 2.7W or so for the other half cycle (P=I^2R where I=7A and >R=.055 typ.) meaning an average of about 7.3W per package for the 2SK2614, >9.4W for the IRLZ44. That's 14+ to nearly 20W total... Sure wouldn't >have to worry about condensation in the box! Dale - this will work just fine. I've used it before. Remember that a conducting MOSFET looks like a resistor, NOT a saturated bipolar transistor. As such, it conducts BOTH directions when enhanced (on). In other words, the Vf of the body diode is swamped by the ON resistance of the FET. Because Dave drives both gates at the same time, both FETs are turned on. Remember Bob Pease's cute little reverse battery polarity protect circuit. I believe he even got a patent on it! There are only 2 components: a gate resistor and the MOSFET. The MOSFET is wired backwards: source and drain swapped with regard to normal polarity. As such, the body diode conducts when the battery is installed correctly. He then uses the gate to fully enhance the FET (also only when the battery is installed correctly) and the Vf of the body diode is completely swamped by the on-resistance of the FET. If you were protecting the positive lead of a battery, you would use a P channel MOSFET, with D tied to battery +, S to load, G to gnd via a 10K resistor. It works! dwayne Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu