Hi All,
I'm trying to build a HV power supply to drive a scope CRT. I am using an arrangement very similar to this one:
R1, R2 = 330 ohm
R3, R4 = 68 ohm
Q1, Q2 = BD139
Transformer: Würth Elektronik 750871010
https://www.we-online.de/katalog/datasheet/750871010.pdfI need the symmetrical arrangement (as opposed to the single transistor joule-thief -style) because I have to drive a multiplier from the secondary to bring the voltage up to around 600V.
My problem is that even thought the circuit works, it's extremely inefficient and draws 0.5A at 5V (2.5W) even when the secondary is unloaded. The transistors are getting really hot. The 2.5W power consumption slightly increases as I'm loading down the circuit. With approximately 0.6W load (3mA at 200V) the circuit is still drawing approximately 3W, so the efficiency is around 20%.
I've got a hunch that since the transformer is unloaded, the flyback effect causes 60V+ voltage peaks on the primary side (collector), thus breaking down the C-B junction in the transistors and thereby keeping them open for a longer period.
I didn't try, but I'm assuming that with appropriate load the efficiency would eventually increase beyond 50%. However for my use case, I'd like this circuit to draw as little current as possible when IDLE-ing.
How can I achieve that?
Thanks!