sci.electronics.design, comp.arch.embedded high voltage pnp's (250v)
Anyone know of a good pnp transistor spec'd up to 250 volts, a probable current limit of 1 amp and designed as a switching device. It's to serve as the switcher in a variable frequency (pulse width held constant) power supply. So far a 2n2907 has been tried, with a 100ma 50 volt limit, with impressive results. Zetex (now Diodes Inc) fzt957 & fzt958 with a 1 amp 300 volt limit were tried but they were inefective. The 2n2907 showed 47% efficiency with a 500ns pulse width; the fzt957 showed 38%. With 4 micro second widths, 2n2907 showed 61% & fxt957 showed 32% (). Apparantly, the higher collector/emitter voltage implies a greater effective restance and higher saturation voltage, which might explain why n channel fets with convoluted "high side" drive circuitry are used. I'm hoping there are some pnp devices in the relatively low 1 amp and 2 amp range that are more useful than the fzt957 units. Any suggestions?
Hul