I need to design a boost converter that outputs ~20 mA DC and an unloaded voltage of around 68-100 V. The inverter will be part of a Zener diode analyzer.
I built it on the idea of the Joule thief (blocking oscillator). During breadboard experiments at some point the base biasing diode burned.
I suspect it is due to transients from the base winding. Since then I modified the design to try and suppress the transients and not exceed the Veb voltage of the BD139 (is -5 V).
Also the shut-off transistor(BC548) was getting hot. From simulation it looks like it's also from transients(high voltage applied to collector).
My schematic values are different from the actual ones and component annotations.
- T1-main switch-----BD139-16
- R1-base bias resistor----10k--->~1mA base current for the transistor
- D1-base bias diode ----1N4148--->Vf~=0.65V
- C1-filter cap for the bias network(release some voltage stress from D1)
- R2-main switch base resistor(should help prevent high currents in D2,D1 and keep Vbe>-5V)----10nF
- D2-T2 protection diode ( a path for current to flow trough R2 away from the T1 base and T2's collector)----1N4148
- T2---switch off transistor(takes away the T1 base current when the output voltage is too high,the Zenner conducts in the reverse bias and turns on the optocoupler's led)
- R3---T2 base current and fototransitor's current limit resistor.(since BC548 has high hfe can be a larger value ~10k)
- D3----T2 base biasing diode--->Vbe T2~=0.65V
- D4----T1 collector protection diode(stop the high voltage spikes)
- D5----Output diode(any fast rectifier diode i'll use SF 28 G)
- Will this design will work and if not what improvements can be made?
- Will I need a higher base drive current if the collector inductor will have smaller values (~10-22 uH)?
I'm intrested about other other designs too, but my main focus is to keep the inductor's physical size low.