Power transistors on ham radio PS

[AngraMelo]

Hey guys,
I got this project off the internet, it is from a power supply that is sold here in Brazil.
I have the 25amp model but it was not working, I got the schematic of the 20A because I could not find the schematics of the 25A model

I "fixed" it using the exact parts but now only the transistor which its emitter is connected to the base of the other 4 transistors seems to be heating up. I tried pulling 6a from it and this transistor gets very hot but the other 4 which are on the heatsink do not. Like room temperature cold.

Now, both the 25A and the 20A are sold with only the 4 transistors that are in parellel mounted on the heatsink, the pre driver(the mentioned transistor that is getting hot) is never mounted on a heatsink.

It´s very possible that I made I mistake but I just would like to hear opinions if this project is supposed to work like I described.

Thank you for your time!

[picburner]

The 4 transistors 2n3055 are brutally connected in parallel without balancing resistors on the emitters.
A such connection is unwise but if you really want to do that, you have to select the transistors so that they have the same gain.
Probably what you have replaced has a greater gain than the others and takes over all the current load.
That's why it warms up a lot.

[AngraMelo]

If you look at the last page of the schematic you will find that they did include resistors for the emitters, I think they are connected in a incorrect manner, right?

[picburner]

The resistors should be connected independently on each emitter (one resistor for emitter).
From your schematic it would seem that the emitters are all connected together and then go to the four resistors all connected in parallel.

[bd139]

Yes the original version is definitely wrong. One resistor per emitter.

Difficult to make out but I think the resistors there aren't being used for emitter degeneration but current limiting the LM723. Adds up to 7A. Hmm.

[AngraMelo]

You guys are right, I overlooked the resistor for each emitter.

Yes the original version is definitely wrong. One resistor per emitter.

Difficult to make out but I think the resistors there aren't being used for emitter degeneration but current limiting the LM723. Adds up to 7A. Hmm.

If the design is for 20A then there must be a mistake. Im thinking of just taking those resistor (0.33ohms) and individually set them for each 3055 emitter. Would that also bypass the current limiting properties?

[ArthurDent]

You have four 2N3055 on a heat sink being used as pass transistors and driven by one 2N3055 transistor that is not on a heat sink. if one or more of the 4 pass transistors has a problem, like a base to emitter short, those transistors are essentially not in the circuit and the one 2N3055 that isn't on the heat sink is now the pass transistor handling all the output current. That one 2N3055 will get very hot while the 4 pass transistors on the heat sink (that have no voltage drop across them) will remain cool.

Check that all of the 4 pass transistors are good, properly installed, and there are no shorts between pins or to the heat sink. 

[AngraMelo]

You have four 2N3055 on a heat sink being used as pass transistors and driven by one 2N3055 transistor that is not on a heat sink. if one or more of the 4 pass transistors has a problem, like a base to emitter short, those transistors are essentially not in the circuit and the one 2N3055 that isn't on the heat sink is now the pass transistor handling all the output current. That one 2N3055 will get very hot while the 4 pass transistors on the heat sink (that have no voltage drop across them) will remain cool.

Check that all of the 4 pass transistors are good, properly installed, and there are no shorts between pins or to the heat sink.

I did that and I found my mistake. I did not connect the collector of the 4 transistors properly. Now they all maintain a very similar temperature. Im thinking of removing the 4 low value resistors and connect them individually to each emitter of the 4 transistors.
My question is, how do I protect my PS from blowing up if the 3055 that is being used as a driver goes bust? or if one of the pass transistors goes bust?

[bd139]

You usually design it to fail. Add a "crowbar" to the output to protect the load (the valuable bit!)

I would read AoE chapter 9, conveniently the free sample chapter! https://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf

This covers LM723 etc and rationale behind this, crowbars, protection etc.

[AngraMelo]

You usually design it to fail. Add a "crowbar" to the output to protect the load (the valuable bit!)

I would read AoE chapter 9, conveniently the free sample chapter! https://artofelectronics.net/wp-content/uploads/2016/02/AoE3_chapter9.pdf

This covers LM723 etc and rationale behind this, crowbars, protection etc.

Thank you! I also have the book!

[coppercone2]

I think the authors were a bit fed up with bad power supplies since they decided that that chapter should be free lol

great book, if you know electronics somewhat or are rusty, it starts getting good at the end of the BJT and Mosfet sections and really interesting further on. Easy to read in 30 page segments or so. I really like all the explaination about hybrid amplifiers and precision circuits. the early pages with transistor approximations can be pretty boring but when it gets into the actual engineering like separating DC and signal gain to make some performance circuits and explaining this, is where it seems to differ from other books like Sedra smith etc. All those college books seem like a useful reference to the art of electronics, not the other way around...  I thought maybe you can offer one of those arcane tomes for a while to see the specifics on some particular topology, but just studying it? jesus h chirst, I am looking for anything else to pay attention to after like, 2 pages of that stuff.

[bd139]

Yeah it wouldn’t surprise me. There are so many absolutely shitty power supply designs out there it’s unreal. Honestly half the amateur radio sourced designs for anything are total steamers as well. I’ve made it a bit of a personal mission to smack them down a bit. A lot of ARRL texts are absolutely criminal!

I like Sedra / Smith. Needs copious red bull consumption though unlike AoE

[coppercone2]

the problem is that you start questioning if you have attention disorder with cedra smith.. but its just a hard read

[bd139]

Indeed. Hence the red bull 

[Wolfgang]

Sorry,

this is a bad power supply for many reasons:

- The cooler for the diodes is probably not large enough to cool the rectifier diodes properly. If you have 1V or forward voltage (only vor very large Schottky diodes)
  and 20A RMS, you have 40W on this small piece of metal - much too small.
- the arrangement of the 2N3055 pass transistors is thermally bad, because the space between the transistors is much too small.
  Even with a large cooler, this will create a hotspot in the middle.
- The driver transistor gets hot as well, but is not cooled by a heatsink
- Load balancing is missing. Every transistor needs an emitter resistor, roughly so that at the maximum current leads to a drop of about half of the BE voltage.
- Crowbar for output protection is missing

My suggestion: AoE chapter 9, or some reference designs from the ARRL for ham radio power supplies (I think there is a good one from Bill Sabin, IIRC).

[bd139]

Yep. Regardless of how shit the power supply is, crowbar is number one feature. All it takes is a shitty design to go C-E short on the pass transistors and your radio is going to let out the magic smoke double quick.

I've got one I've made that sits inline with powerpoles. Basically just an inline auto fuse, SCR and TL431 on a bit of FR4 shrink-wrapped. Design straight out of AoE.

[NiHaoMike]

Best design for amateur radio use would probably be a linear/switching hybrid. It would be linear in receive mode (low current) for minimal noise and switching in transmit mode (high current) for a more economical way to deliver lots of power. Could be done with a high current buck converter plus a current limited linear regulator in parallel set just enough to prevent the buck from operating below the current limit.

[Wolfgang]

There are some switching regulators (e.g., from LT) that have "discontinuous" mode for light loads.
For something like this, you dont need to know when the crossover is needed.

The major task for a switching PSU is RFI suppression and shielding. Homebrew a switching PSU is possible but several times more complicated than a linear one.

For my ICOM IC-7300 (100W) I use a JetFon PC30SWM switching PSU. Its not RFI free, but at least the garbage stays out of the amateur radio bands (I checked with
a spectrum analyzer).

For the cost of ca. 80€ you probably cannot homebrew a linear supply for the same power. If you want to learn from homebrew - different story.

[bd139]

You were lucky. I had the later digital MyDel version of that supply and it pisses all over 20m to the point you have to tune and use noise offset at the same time. Got rid of the damn thing and run off an SLA now topped up off a quality bench supply.

[Wolfgang]

Maybe, but I agreed for a right to return should the PSU be not quiet in the amatur bands.
The PSU I bought looks so very the same from many different vendors. Maybe they even cloned the clone ?
I bought mine from WiMo at the Ham Radio fair in Friedrichshafen. If they sell junk there, they always run the risk that the buyer goes to the next stand and makes measurements

[NiHaoMike]

It looks like what we really need is an open hardware power supply that's linear below 3A or so and seamlessly transitions to switching above that. Could probably base it around a buck converter with hysteresis control (no oscillator to generate noise when not switching) with a current limited linear regulator with the output set such that if it is able to hold up the supply rail alone, the buck converter stays off.

That idea could even be extended with a switcher that runs from mains, likely one with a secondary side controller driving the primary MOSFETs via a gate drive transformer. Then only a small mains transformer would be needed for the linear operation mode.

[bd139]

It’s called a sealed lead acid battery

[Bratster]

It’s called a sealed lead acid battery

That^

Then you also get the benefit of having backup power with zero interruption.
Can also toss a small solar charge controller on it and a tiny solar panel on your roof.

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