A Sudden Puff, and the Amplifier’s Final is Gone

We were eating dinner while the Softrock and amplifier were running WSPR at the far end of the living room. Suddenly we heard a loud bang from that area. I went over and smelled a funny odor. The amplifier blew its final MOSFET.

Initially I got a bit annoyed, because the amplifier is really useful (my antenna is low, so the extra power is helpful in making contacts). But then I decided that I will probably enjoy figuring out what went wrong, so I relaxed.

I can’t say I know what went wrong. I did a quick troubleshooting session and discovered the following facts.

  1. The amplifier produces no power.
  2. The final appears to be an open circuit (I didn’t do a lot of measurements, but it is not shorted).
  3. The rest of the amplifier seems fine; the power supply section is working, the bias supply is working, and the transmit/receive switching is working.

When the final blew up, the antenna seemed fine, and the Softrock and antenna kept working without a problem afterward. So I don’t think that the antenna suddenly presented a high SWR that caused the amplifier to fail. I’m not 100%, but I don’t see why that would happen after several hours of working fine.

So what happened to the transistor? I blew up transistors in two amplifiers I built before this one, but in all cases I was able to tell why they failed, whereas here I’m not sure.

Three things can destroy a power transistor: over heating, over current, or over voltage. The MOSFET was not destroyed by over heating. The fan was working fine, and the heat sink was barely warm just after the transistor failed. Earlier that day I tried to run the amplifier without the fan for one 2-minute WSPR transmit cycle, and the heat sink got quite hot. This tells me that the thermal conductivity between the transistor and the heat sink is good; it’s not that the transistor got very hot but the heat sink stayed cool. They were both cool.

This leaves us with over voltage, which can be caused by high SWR, or over current. The amplifier was powered by a 24V, 1.8A switching power supply. The 24V supply puts around 50V on the transistor’s drain with 1:1 SWR, so with high SWR, the drain voltage can probably exceed the 100V limit of the IRF510. As I wrote above, the antenna’s SWR did not seem to suddenly increase, so it’s a bit strange.

The only reason that I can think of for an SWR-related failure is if the output pole T/R relay got stuck on receive while the input pole switched. This would cause the amplifier to produce RF power without any load. In theory one of the components of the output filter might have failed, rather than the relay, but that seemsunlikely.

I didn’t hear of a similar relay failure, but I think it’s possible, especially since the relay is very old (I took it out from some circuit I built about 25 years ago).

Another possibility is that the amplifier went into some wild oscillations. I didn’t see any such things on the bench, but I only tested the amplifier with a 50 Ohm load. The antenna’s impedance should be close to 5o Ohm resistive, but it’s not exactly 50 Ohm, so perhaps the mismatch sent the amplifier into oscillations. But because worked fine for several hours prior to the failure, I doubt that this is what happened.

What do I do now? I think I’ll replace the relay, just in case. I’ll probably also add a 75V Zener diode across the MOSFET, hoping it will fail rather than the MOSFET if something similar happens again. I can also try to replace the IRF510 with a 200V device, like an IRF630. I don’t think that such a device will fail at any SWR with a 24V supply, but they have a much higher input capacitance, which makes them harder to drive. The 1W of the Softrock may not be enough. Another protective measure might be to run the amplifier on a 16V supply I have rather than 24V; the will make it less likely that the drain exceeds 100V.

There’s another little puzzle in this. The audible puff and the odor were probably caused by high current that vaporized the transistor. The high current, in turn, was probably caused by the transistor failing by getting shorted. The vaporization that followed turned the short into an open circuit. I could be wrong, but that’s what I think happened. The puzzle is where did the large current come from. The power supply is rated at 1.8A, and it’s still working fine. 1.8A is not enough to destroy an IRF510; it is rated at 5.6A. So the instantaneous current must have been much higher. So the burst of energy must have come from some capacitor, most likely the electrolytic in the amplifier (either 4700uF or 2200uF, I’m not sure). Does a large electrolytic capacitor have enough energy to vaporize a power MOSFET? I didn’t think so, but perhaps it can. Should I have used a smaller capacitor just so that it can’t destroy the transistor?

If readers with more experience than me have some ideas about what might have gone wrong and how to avoid similar mishaps, I’d love to hear from you.

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One Response to A Sudden Puff, and the Amplifier’s Final is Gone

  1. Pingback: Amplifier Update: Now Using an IRF630 « Eclectic Technical Experiences

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