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Can a D667A NPN transistor be replaced with a BD139 NPN transistor in this PSU?
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I have a PSU that had a D667A NPN transistor that took some damage. I am not 100% sure what its goal is in the circuit, but I have the impression it is used to drive the power transistor mounted on the heatsink you see in the picture.

enter image description here

The psu had a fuse that got blown violently, and there were some shorted components in the PSU (shorted power transistor, shorted electrolytic cap, shorted schottky diode)

I replaced the shorted components, and replaced the D667A NPN transistor this particular transistor with a BD139 NPN transistor I had lying around.

The original D667A NPN transistor datasheet mentions a transition frequency of 140Mhz.

In the BD139 NPN transistor datasheet I could not find this frequency, although other sites mention it as being 190Mhz. The other parameters were the closest I had compared to the original one, making me think (perhaps wrongly) that this is a suitable replacement.

Is my thinking correct, or should I replace it with a more suitable transistor. And if so, what would that replacement be ?

Although the fuse is kept intact now, I am not seeing any DC output voltage, so I am wondering if this BD139 might be the cause of that. (it not being able to turn on the power transition properly).

switch-mode-power-supply npn
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asked 4 hours ago

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Ah, I know a switching mode power supply unit frequency is usually only a couple of kHz. So your new BD139 should have no problem handling it. WARNING: I am only a friendly hobbyist. No guarantee no nothing won’t melt down or blow up. Good luck and cheers. PS – Your photo looks impressively very high standard, compared with your electronics. – tlfong01 4 hours ago

@tlfong01 it’s highly likely that the switching frequency is many tens of kHz and can be hundreds of kHz so your message is not really useful. Additionally, without some idea what voltages are involved, you should not state that the BD139 should have no problems given that the original transistor is rated for 100 volts operation whilst the BD139 is only 80 volt rating. There may be other things also that make it unsuitable. I don’t think your comment brings anything useful to the party. – Andy aka 3 hours ago

Even if you do find the right replacement, how do you know anything else did not take any damage ? Otherwise why not replace it with the original such as advertised here : bdent.com/nsearch.html?query=2SD667A ? – citizen 3 hours ago
@Andy aka, my apologies for my casual misleading comment that SMPS freq is a couple of kHz. I confess that my SMPS knowledge is out of date. I do remember some 20 years ago when I repaired ATX12 or earlier PSU, I did hear the buzzing nose, which means the frequency should be less than 20kHz. Some 10 years ago, I actually bought inductor coils and designed an LM2596 switching power supply. But the inductor values are discrete, in big steps, so I actually spent some 50 hours studying the theory and another 50 hours using a scope to measure the inductances./ to continue, … – tlfong01 31 mins ago
If you search the word inductance, in rpi.org.forum, you should find 20+ lont posts sharing my experiments in inductance. I forgot the frequency of my final version of DIY SMPS, but for sure less then 500kHz. So I am pretty sure that the OP’s a bit old UPS’s frequency should be less than 500kHz. So when I read the OP worrying 100MHz or 200Mhz, I did LOL. In short, I think my quick and dirty reply is very appropriate to the OP, considering my time constrain. In weekends, I often comment/chat with the OPs, sometimes 20 pages, lasting 2 weeks, before I propose my suggestions. / to continue, … – tlfong01 21 mins ago
One important reason I suggest the OP to try his replacement is that he has the BJT “lying around”, Actually I think it stupid to buy from eBay or AliExpress which might take 20 weeks. In about 10% of the answers I give, if I found the OP’s problem involving new devices which I have no experience or knowledge, I actually immediately placed order in factory flagship shop in ShenZhen, and get goods delivery within 24 hours. In case you are curious why I would spend my money to answer newbies questions, one reason is that I used to help under privileged learners / to continue, … – tlfong01 10 mins ago
who are living miserable lives (some of them are in jail but learn very hard under extremely harsh conditions. I was once a refugee and I understand living in poor conditions. Luckily I got chances in charities schools and work my way up. I am in debt to those once helping me, so it is my turn to pay back, By the way EE devices are dirt cheap from ShenZhen just over the border, I can usually get at 25% of price in Amazon. so it does not cost that much for me, Newbies and me learn together. … It is almost bed time, so I stopped here. I am just think aloud, my apologies for the typo errors. – tlfong01 just now Edit
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