How to choice hFE of ballast transistor?

[aaronlee]

Hi,

We are using S912ZVCA for my project.

BCP53-10 hfe=63~160

BCP53-16 hfe=100~250

BTP2907SL3 hfe=180~390

Can we have the BTP2907SL3 instead of the BCP53 for ballast transistor?

You must avoid using a darlington transistor or transistors with high hFE.

What is the normal renge of the hFE?

Best Regards,

Aaron

[danielmartynek · Accepted solution]

Hi Aaron,
Yes, the PNP can be used. Nominally 100 – 200 hFE, but as long as it is below 400 at high temperature, there should be no problem.
Controlling the Ballast transistor must be delicate. With transistors with higher hFE, the regulation can be unstable at rapid input voltage change due to parasitic capacitance at base of the transistor that can generate false current that is then multiplied by the gain.
Also, transistors have low hFE at low temperatures and to deliver required current (Ic_required), their hFE (at low temperature) must be higher than:
hFE_min = Ic_required / Ibctlmax = Ic_required / 2.3mA (datasheet).

Regards,
Daniel