Linear PSU Pass Transistor Question.

[maxbarjr]

I made a linear power supply for HAM Radio use. The transformer has a secondary voltage of 18-0-18 at 240VAC input. Transformer has no labels or markings except for the primary and secondary voltages. The secondary wire size is 3.5mm. The transformer core dimentions are 11mm(h)x 8mm(d)x13.5mm(w). So no idea how many amperes the transformer was. Anyway, I made a 13.8vdc psu base on an Astron RM-50M schematics. The only difference are the pass transistors as I can't source the 2n3771 so I used the 2N3055x6 with 0.1 5W emitter resistor. Power supply works ok except that the pass transistors gets pretty hot at 1min keydown at 100W. Heatsink with thermal paste applied. Heatsink thickness is 3mm with fins. Any advice, suggestions to improve is welcome.

Thank you in advance.

[eccerr0r]

Re: Linear PSU Pass Transistor Question.

Are you talking about a 100W (rated) transmitter or is your transmitter drawing 7.2 amperes at 13.8V? The former likely can draw 200W to transmit 100W radiated power.

A transformer supplying 18-0-18 likely uses a 2-diode full wave rectifier and will supply about 18V RMS DC (25.4V DC peak at no load). This needs to be dropped to 13.8V.

At 7.2A, the regulator would need to dissipate ~ 30W which would get warm, you probably can use a 40W incandescent light bulb as a reference to how hot they would get - though the 30W is spread across all the transistors and heat sink.

If your transceiver is trying to transmit 100W RF, then it will likely be drawing double the above at 14.4A and would heat the transistors at a full 60W, now a 60W light bulb indeed gets quite warm.

I would say this is "normal" if:
- your ham radio is working
- the output remains at 13.8V
- the transistors stay relatively cool when not transmitting (I'd suppose most transceivers are no more than 500mA-1A when receiving, which should cause minimal heat-up of the heatsink.)

100W is quite a large load, regardless if it's power draw or RF power radiated, and I would expect this heat-up.

[redwire]

Re: Linear PSU Pass Transistor Question.

It's hard to know the transformer's rating and what voltage it is making. The core seems very small for steel.

A chinese trick is to use too small transformer and full-wave bridge rectifier, so the DC voltage is very high at light loads, and then it sags under heavy load. The transformer's regulation is ~20%.

Consider adding a cooling fan for this much heat.

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

Are you talking about a 100W (rated) transmitter or is your transmitter drawing 7.2 amperes at 13.8V? The former likely can draw 200W to transmit 100W radiated power.

A transformer supplying 18-0-18 likely uses a 2-diode full wave rectifier and will supply about 18V RMS DC (25.4V DC peak at no load). This needs to be dropped to 13.8V.

At 7.2A, the regulator would need to dissipate ~ 30W which would get warm, you probably can use a 40W incandescent light bulb as a reference to how hot they would get - though the 30W is spread across all the transistors and heat sink.

If your transceiver is trying to transmit 100W RF, then it will likely be drawing double the above at 14.4A and would heat the transistors at a full 60W, now a 60W light bulb indeed gets quite warm.

I would say this is "normal" if:
- your ham radio is working
- the output remains at 13.8V
- the transistors stay relatively cool when not transmitting (I'd suppose most transceivers are no more than 500mA-1A when receiving, which should cause minimal heat-up of the heatsink.)

100W is quite a large load, regardless if it's power draw or RF power radiated, and I would expect this heat-up.

Thank you for your comments. Yes, it's a 100W RF transmitter. I should have added, intermittent transmit of 5sec on, 5sec off for 60secs to a 150W dummy load. Yes it goes to 2 diode for a fullwave rectification and filtered with a 64,000uF 40V capacitor. You are correct, on the peak voltage of 25.4V at no load. The regulator board is powered by a 1.5A separate transformer with 21-0-21 secondary to provide a 29V to the regulator circuit. Output so far is 13.85V without load, 13.80vdc with load (100W RF). Transistor get a bit warm when on receive. Idle current is about 1A. Does the 6 2N3055 pass transistors enought for this application?

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

It's hard to know the transformer's rating and what voltage it is making. The core seems very small for steel.

A chinese trick is to use too small transformer and full-wave bridge rectifier, so the DC voltage is very high at light loads, and then it sags under heavy load. The transformer's regulation is ~20%.

Consider adding a cooling fan for this much heat.

Not really very small transformer. It's very heavy. Maybe about 10kg. It's 4.5 inches tall x 5.5 inches wide x 3 inches thick.

[eccerr0r]

Re: Linear PSU Pass Transistor Question.

Well, that's one thing, transformer getting hot versus transistors getting hot. Is the transformer getting hot too?

But most likely this is normal for a linear dissipative PSU. How hot is "hot" and is the heatsink heating up (expected) vs just the transistors themselves (meaning your thermal interface needs work)? How good of a heatsink are you using - lots of surface area or just a flat piece of steel?

This being said, you may have other issues: though 2N3055 is a readily available, its safe operating area as a PSU transistor is smaller than of a 2N3771. At worst, this means the transistors will blow earlier. You're probably close to the limit of the 2N3055's with your current application I think, while using 2N3771s would give you considerably more headroom.

[budm]

Re: Linear PSU Pass Transistor Question.

So you are using 6 2N3055 in parallel with each transistor has it own 0.1 Ohm resistor, so how hot are the transistors? What is the ambient temperature?
Any pictures of the set up, especially the heatsink?
What are the Voltage readings when measure between E and C, and the Voltage drops on the 0.1 Ohm resistor?

[redwire]

Re: Linear PSU Pass Transistor Question.

... The transformer core dimentions are 11mm(h)x 8mm(d)x13.5mm(w)

OK cm not mm.


Astron RM-50A rated 37A uses eight 2N3771's. About 29V no load and 25V full load from the transformer. 2N3771 has 30% better heat transfer from the die to the case compared to 2N3055 for the 150W vs 115W rating.

2N3055 (assuming not fake) safe operating curve max. is about 10A at 12Vce but this is at 25C.

It depends on your heatsinking- but count on getting 1/2 of that, once you add in the thermal insulator and grease and heatsink temperature, the transistor die is getting pretty hot.

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

So you are using 6 2N3055 in parallel with each transistor has it own 0.1 Ohm resistor, so how hot are the transistors? What is the ambient temperature?
Any pictures of the set up, especially the heatsink?
What are the Voltage readings when measure between E and C, and the Voltage drops on the 0.1 Ohm resistor?

I used a Kenwood TM-281A at H power output of 65W into a dummy load. Here are some results of the measurements.

Between C and E: 10.2V with radio keyed, 12.5V unkeyed
Across the 0.1 Ohm resistor: 0.15 with radio keyed.
Temperature taken with a Fluke 287 meter and a Fluke 80BK-A probe: 140F 1min keyed.

Pictures of my homebrew power supply attached.

[eccerr0r]

Re: Linear PSU Pass Transistor Question.

Let me first say that your PSU looks nice. Much better than my homemade PSU (and my PSU isn't nearly as powerful either, only 2A!).

That poor transistor on the back is going to hurt, as that one is the worst cooled of them all! Is it just that one that's heating up badly? Is it just the steel part of the transistors heating up or is the heatsink also heating up with the transistors?

I also wonder if the diodes are hurting too, they would also be dissipating many watts too, but not as many as the transistors.

140°F is definitely in the concern zone but sort of expected with the current that's being passed. At 0.15V = I*0.1 I=1.5 amperes per transistor, dropping 10.2V =>15.3 watts (plus base current which adds another ~3%, so say 16W).

16W is a considerable amount of heat, and you have 6 of these transistors dumping 16W. Not sure if you've ever touched a CPU with the fan removed, they get quite warm too!

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

Let me first say that your PSU looks nice. Much better than my homemade PSU (and my PSU isn't nearly as powerful either, only 2A!).

That poor transistor on the back is going to hurt, as that one is the worst cooled of them all! Is it just that one that's heating up badly? Is it just the steel part of the transistors heating up or is the heatsink also heating up with the transistors?

I also wonder if the diodes are hurting too, they would also be dissipating many watts too, but not as many as the transistors.

140°F is definitely in the concern zone but sort of expected with the current that's being passed. At 0.15V = I*0.1 I=1.5 amperes per transistor, dropping 10.2V =>15.3 watts (plus base current which adds another ~3%, so say 16W).

16W is a considerable amount of heat, and you have 6 of these transistors dumping 16W. Not sure if you've ever touched a CPU with the fan removed, they get quite warm too!

You must be referring the the single transistor at the back just above the output terminals. It warms up a bit but not much. This transistor is the Q100 that drives the base of the pass transistors. It is not part of the 6 pass transistors. The heatsink and pass transistors has almost the same temps when measued. The rectifiers are just warm being fitted on those copper bus bars.

Oh yes, I have been repairing computers before and have seen a cpu really smoking without the heatsink and fan.

[stj]

Re: Linear PSU Pass Transistor Question.

so an interesting question - that probably needs an experiment.

can you regulate the output by messing with the input.where the current is lower?
maybe by phase clipping with a triac based dimmer.

it works for switching psu's - why not linear?

[budm]

Re: Linear PSU Pass Transistor Question.

140F (60c) at the case, may be OK, the safe junction spec is 150c is typical for 2N3055.

You can calculate the junction temperature if you know all other parameters.
You can study there here:
https://www.rohm.com/electronics-bas...or-temperature
https://cdn.badcaps-static.com/pdfs/...973d256525.pdf
https://cdn.badcaps-static.com/pdfs/...b33cb2f696.pdf
http://www.cypress.com/file/38656/download
https://toshiba.semicon-storage.com/...conductor.html

[kaboom]

Re: Linear PSU Pass Transistor Question.

the safe junction spec is 150c is typical for 2N3055.

As for most TO-3s and some of the "flatpacks," which are usually 125c.

But this is without dissipation!

At TjMax, devices are derated to zero watts on the collector.

[budm]

Re: Linear PSU Pass Transistor Question.

Heatsink.

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

Heatsink.

Thank you for sharing those documents. Very interesting. Thinking of adding 6 more 2N3055 on another heatsink mounted on the other side of the case. Will that reduce the heat on the heatsink and will it increase the power handing of the psu? Any improvement suggestions are welcome.

[eccerr0r]

Re: Linear PSU Pass Transistor Question.

The amount of heat is still the same, it's how much the heat is spread out between them.

Is your heatsink getting warm or are just the transistors?

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

The amount of heat is still the same, it's how much the heat is spread out between them.

Is your heatsink getting warm or are just the transistors?

Both the heatsink and the transistor. Measured the temps on them and almost the same.

[eccerr0r]

Re: Linear PSU Pass Transistor Question.

oh... if both are getting warm, then "working as intended" most likely.

Yeah it's probably best to spread them out more and at least add two more transistors, should help keep it within range; unless you want even more current (and your diodes/transformer can handle it) - then even more transistors would be great. Watch out for base drive though that 2N3055 for base drive should be sufficient for even more pass transistors.

[maxbarjr]

Re: Linear PSU Pass Transistor Question.

oh... if both are getting warm, then "working as intended" most likely.

Yeah it's probably best to spread them out more and at least add two more transistors, should help keep it within range; unless you want even more current (and your diodes/transformer can handle it) - then even more transistors would be great. Watch out for base drive though that 2N3055 for base drive should be sufficient for even more pass transistors.

Thank you.