| the 555 using the plans at: | | http://www.ledsales.com.au/kits/nixie_supply.pdf | calls for a IRF740. I noticed that the MAX1771 plans | always call for a IRF830 or a IRF644PBF.
The ABCs on power FETs: Three parameters, that you need to look at: 1) Vdss: or how high of a voltage it can handle. The max output of your switcher, without resorting to doublers or triplers. Higher usually better, but you usually pay a penalty, in ON resistance. 2) On Resistance (Ron): Lower the better. This times the current flowing thru it, determines how hot it gets. Remember, efficiency, is how much is going to the load, and NOT generating heat. Usually goes up with higher voltage rated FETs. 3) Gate Charge (Qg): The gate (to source) of a FET looks like a capacitor, though not a linear one, so over all charge, as opposed to capacitance, is more helpful. Whatever you use to drive the FET has to move this charge around. This is often the "Big Lie" in driving FETs. They have a high DC resistance, but if you have to switch them fast, then moving that charge around, makes the impedance (AC resistance) go down with switching frequency. See "Capacitive Reactance". A lower gate charge is better. Now look at those three FETs: IRF830: Vdss=500V, Ron=1.40ohm, Qg=24nC IRF740: Vdss=400V, Ron=0.55ohm, Qg=63nC IRF644: Vdss=250V, Ron=0.28ohm, Qg=63nC Any of the three can handle the voltage needed for your common nixie supply (180V-200V). So they're all okay in that respect. Ron: IRF644 rates the best, and the IRF840 is the worst. Qg: IRF830 is the best, but here we need a qualifier. The MAX1771 and MC34063 (MK1.5 & MKII) designs can easily move that gate charge around, so it really isn't an issue. If you use the MK1.0 (passive pulldown) version of the MC34063, then this parameter comes into play big time. In practice I'd use the IRF740 or IRF644 over the IRF830, for any circuit. Of course, I won't use the MK1.0 version of the MC34063 designs. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
