two transistors voltage regulator using IRFBC30

[chungtat]

Hi,

I built a two transistors voltage regulator but it oscilated. Pass element is IRFBC30 and error amp. is BUH51. I chose them because the Vout is high, 405V. Vin is 450V. Operation current is about 40 mA.

It is a simple two transistor voltage regulator. Upper resistor is 81K and lower resistor is 120K for the voltage divider in the output. Voltage reference is 235V, using 3 zenor diodes in series connected to emittor of BUH51. hfe of BUH51 is about 27, based on measurement.

A 100 ohm resistor is connected between G of IRFBC30 and collector of BUH51. A 10K3 resistor is connected between Vin (that is D of IRFBC30) and collector of BUH51. This regulator is for a tube power amp. When I powered it up without preamp, it is stable, Vout is about 407V. But as I connected it with preamp, it oscillated and Vout is varied.

Could someone help me out how to make it stable?

 

[lineup]

Put one small cap between Base and Collector of BUH51. Can be 470pF or 1nF. Ceramic or small plastic does not matter. Make sure it have correct Voltage rating.

 

[chungtat]

Thanks a lot! The Vout is very stable after adding a small cap. Would you share the reason why? And how to calculate the suitable value of cap?

 

[lineup]

When we have loop it happens often we get oscillation. The first transistor increase the voltage, but the second then lowers the voltage. When it get lower the first try to increase. This way there will never be any stability. The compensation capacitor will slow down so making it more stable.