I have a circuit in which the C2001 (aka 2SC2001) transistor failed after many years. I Googled about, and tried to replace it with a BC337. The BC337 got kind of warm, and didn't work. B, C, and E pins were all in the correct PCB holes, I checked and double-checked, and triple-checked. The BC337 tested OK with my hFE meter.
So, is there something 'special' about the C2001 trannies? I have seen them referred to as 'switching' transistors somewhere, I don't know what that specially means. This is being used in a flyback circuit, which switches at approx ummm 140 kHz or so.
Thanks, any help appreciated!
My previous, sort-of related question is here: What type of voltage booster circuit is this?
Best Answer
What could cause transistor to overheat? Well, it's whether excessive base current, or excessive product of Ic x Vce.
This is your circuit:
Base current is not a problem, it is limited by R128 to 4.5mA. So, transistor could overheat by high Vce or Ic.
Say hFE=200. Then maximum Ic=900mA. But it will be lower, say it is 500mA. Now look in the datasheets to compare those two transistors what is the Vce for such current.
This is from 2SC2001 datasheet:
Vce=0.3V
So power dissipation would be 150mW, well within specification.
This is from BC337 datasheet:
It is off the chart, it is a bad sign for this situation. Vce would be much higher than 1.0V, say it is somewhere around 1.5V. So power dissipation would be 750mW, which is above maximum allowed power dissipation.
I do not say your circuit is drawing 500mA, peaks could be higher or lower. But you can see that in some situations there are differences between BC337 and 2SC2001.
Another relevant difference could be in hFE. It depends on your batch. There are huge differences:
2SC2001 (hFE from 90 to 400)
BC337 (hFE from 100 to 630)
So you could compare BC337 and 2SC2001 with very different hFE (you maybe compare BC337 with very low hFE, and 2SC2001 with higher hFE).
And the difference can be even bigger in hFE because hFE changes with Ic:
