Rapid heating of output stage transistors of amplifier

Thread Starter

RichW

Joined Nov 8, 2019
24
Hello,

I am hoping someone here may have some experience with RF amplifier operation.

I am using an amplifier operating at ~20-40 kHz (50 ohm), with ~10 ohm transducers. To match the impedance between the two I am using a step up transformer(SUT).

My tests were going well, I was able to run my transducer at ~20 kHz 100 W and monitor the temperature of the output stage transistors. The maximum temperature was ~44C.

However, something has now changed. When I try to repeat the experiment the temperature of my output stage transistors rapidly increases to beyond 60C, even at only 30 W power.

If I remove the step up transformer(SUT), and just have the amplifier running straight to the transducer, the matching is poor, but the increase in temperature does not happen. So the heating is not caused by mismatch of the amplifier and transducers.

We have looked at the SUT and it seems to be ok. We have looked at the amplifier and it seems to be ok. But when the amplifier and the SUT are used together there is a rapid increase in transistor temperature of the amplifier. So we can't use the amplifier for powers greater than 30 W because we don't want to overheat the unit.

We have no idea what is causing this problem. If anyone here has any suggestions it would really be appreciated.

Thank you for your time.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
Hi crutschow,

Apologies, what kind of schematic? I don't have any internal schematics because I'm not an electrical engineer. I can provide the specs of the amplifier though (please see below). I am having some issue between the SUT and the amplifier that wasn't seen previously that is causing the amplifier to over heat. I was just wondering if anyone had any similar problem / had any ideas of what it could be.

  • Manufacturer: T & C Power Conversion
  • Model #: AG 1024
  • Model: LF Generator & Amp
Description:
  • Model AG 1024 is a robust source of RF power for ultrasonic, laser modulation, RFI/EMI, plasma generation, laboratory and general industrial applications. Featuring state-of-the-art design of all amplifier stages and a built-in DDS signal source, it provides everything for a complete and reliable, controlled RF power delivery system. The manual & software are available on the manufacturer's website.
Specifications: (These specifications were taken from an online user manual and may not be accurate to this machine)
  • 20 kHz to 3 MHz
  • 2,000 Watt
  • Digital Meter, measures forward and reflected power.
  • Front Panel Control of Amplifier and Generator Functions.
  • Data acquisition: Status Monitoring & Power Measurement at Analog Port.
  • RS232 Communication: Full Control Of Amplifier & Generator Functions. AGC or Power Leveling: Gain Control to beter than 0.3dB
  • Controllable internal DDS signal source. Pulse and Sweep of RF internal singal generator
  • Specifications:
  • Class of Operation: Class B
  • Frequency: 20 kHz to 3 MHz RF
  • Power Output: 50 Ohm load: Up to 2,000W for 20 kHz to 3 MHz Pulse and low duty cycle
    Any load: Up to 1,800W, continuous operation.
  • Gain: 63 dB @ 2000W / 0.5 MHz ±1.5 dB 20 kHz to 1 MHz RF
  • Input Drive for AGC Recommended: 5 dBm to 0 dBm for ±0.5 dB gain flatness
    Input Drive Source Signal or function generator, analog computer input capable of up to 1 Vp-p @ 50 Ohm Input range: 30 to 0 dBm typical, +5 dBm maximum
  • Internal RF Source DDS oscillator: 20 kHz to 3 MHz, 1 kHz resolution
    Input and Output Impedance 50 Ohm 2:1 max INPUT VSWR 3:1 max OUTPUT VSWR
    Output VSWR Protection 400 W max reflected power limit for Load Impedance > 50 Ohm. Current level protection for Load Impedance < 50 Ohm.
  • Harmonic Level @ 1750W Better than 13 dBc for 3-d harmonic, any other better than –20 dBc
  • Spurious Output: 26 dBm equivalent noise level generated by internal circuits
  • RF Output Settings & Control: Front Panel EDITOR and function switches for manual control, RS232 port for GUI or other computer communication. Rear Panel SubD 25 Analog and Digital I/O . Port power scale 1V=250W. Rear Panel
  • BURST operation Pulse range: 1 to 500 usec Period: 1 to 50 milliseconds User settings via GUI and RS232
  • BURST external DC to > 200 kHz. User defined BURST scheme via SubD-25. See analog port description for more details.
  • SWEEP operation 0.02 to 3MHz. Min time 10 ms, max 10s. Settings and activation from GUI only.
    Output Blanking For pulsed applications, T&C amplifiers and generators offer blanking of the output signal for minimum noise RF spectrum
  • RF Connectors BNC Female: RF In N Female: RF Out
  • Cooling Forced air, temperature controlled, heatsink temperature monitored via RS232 GUI interface.
  • Acoustic level: 45dBa @ Max Fan Speed @ temp.
  • Case Designed to meet EMI and RFI shielding requirements steel chassis (frame), black conductive finish. Cover, Front and Back Panels AL with yellow conductive finish. Exterior: Front & Back Panel: T&C off-white. Cover: T&C black.
  • Environmental conditions Temp.: 10° to 40° C ambient Humidity: 80% Equipment intended for ISM applications in laboratory and light industrial environment.
  • AC Input Ratings: 2/N/PE ~ 200-240 V, 35 A, 50-60 Hz 2/N/PE ~ 200 V, 42 A, 50-60 Hz 2/N/PE ~ 346-415 V, 16 A, 50-60 Hz
 

AlbertHall

Joined Jun 4, 2014
11,594
I am using an amplifier operating at ~20-40 kHz (50 ohm), with ~10 ohm transducers. To match the impedance between the two I am using a step up transformer(SUT).
To get from 50Ω to 10Ω you would need a step down transformer (mis-tryping?)
Can you give us details/specification of the transformer?

Have you tried using the amplifier and transformer with no transducer?
If it still overheats then I would suspect the transformer.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
Have you tried using the amplifier and transformer with no transducer?
If it still overheats then I would suspect the transformer.
Obviously without a load the reflected power back to the amplifier is 100%. However, it did not cause the temperature of the amplifier to rise when there was just connection to the SUT and no load connected.

I have a feeling that the amplifier may have some internal problem (maybe at the output) which is causing it's impedance to change from 50 ohm. I guess if that was happening then it may not match with the input of the SUT. But I actually have no idea, since without the SUT, straight amp to transducer connection, the temperature does not rise. And, as just tested, just amp and SUT connected causes no temp rise.
 
Last edited:

Thread Starter

RichW

Joined Nov 8, 2019
24
Sorry, I missed this last time.

To get from 50Ω to 10Ω you would need a step down transformer (mis-tryping?)
Can you give us details/specification of the transformer?
The transducers impedance changes during operation, the step-up configuration works best for our process. The transformer is a coil with a number of turns, the switch of the transformer simply switches between the number of turns of the coil, there are no working parts.

I will also update, because I would like some other opinions on this. The amplifier can no longer give us the power we need. We believe that some of the MOSFET's in the amplifier may have originally failed and this caused the remaining MOSFET's in the amplifier to overheat - trying to provide the power we need. Now the rest of them are dying under the strain. Does this explanation make sense? Thanks again for all the ideas/feedback on this issue.
 

Papabravo

Joined Feb 24, 2006
17,242
Sorry, I missed this last time.



The transducers impedance changes during operation, the step-up configuration works best for our process. The transformer is a coil with a number of turns, the switch of the transformer simply switches between the number of turns of the coil, there are no working parts.

I will also update, because I would like some other opinions on this. The amplifier can no longer give us the power we need. We believe that some of the MOSFET's in the amplifier may have originally failed and this caused the remaining MOSFET's in the amplifier to overheat - trying to provide the power we need. Now the rest of them are dying under the strain. Does this explanation make sense? Thanks again for all the ideas/feedback on this issue.
Without a schematic diagram it is hard to know if your beliefs are accurate or just speculation. If you want to troubleshoot this device and have ANY chance of repairing it properly you must have a schematic and a proper Bill o Materials. One problem you will face is that many components that were previously available in abundant quantities are no longer being manufactured and the inventory on distributor shelves has been completely consumed. Acceptable, substitute parts may also in the same boat. Resurrecting this piece of equipment may end up costing you many times it's original or current value even if you time is worth nothing. This is because rare and hard to find parts can command an order(s) of magnitude over their original cost,

OTOH, with a schematic and a proper Bill of Materials a good technician and a purchasing agent can run down the cost and availability of acquiring replacement parts. Then you have a good estimate of the feasibility of even attempting to repair the unit. This does not appear to be a suitable DIY project considering the current impediments, and IMHO has a very low probability of success.

One more thing: I did read through the list of specifications most of which were very little help, but one thing stood out. It said the amplifier operates class B. That means the output stages are on all the time. Many RF amplifier operate Class C so the output devices operate for less than half a cycle reducing the stress and potentially extending their useful life.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
Without a schematic diagram it is hard to know if your beliefs are accurate or just speculation. If you want to troubleshoot this device and have ANY chance of repairing it properly you must have a schematic and a proper Bill o Materials. One problem you will face is that many components that were previously available in abundant quantities are no longer being manufactured and the inventory on distributor shelves has been completely consumed. Acceptable, substitute parts may also in the same boat. Resurrecting this piece of equipment may end up costing you many times it's original or current value even if you time is worth nothing. This is because rare and hard to find parts can command an order(s) of magnitude over their original cost,

OTOH, with a schematic and a proper Bill of Materials a good technician and a purchasing agent can run down the cost and availability of acquiring replacement parts. Then you have a good estimate of the feasibility of even attempting to repair the unit. This does not appear to be a suitable DIY project considering the current impediments, and IMHO has a very low probability of success.

One more thing: I did read through the list of specifications most of which were very little help, but one thing stood out. It said the amplifier operates class B. That means the output stages are on all the time. Many RF amplifier operate Class C so the output devices operate for less than half a cycle reducing the stress and potentially extending their useful life.

That's very helpful, thanks. I agree we need schematics and replacing parts may be expensive. The last time i had this sort of failure was when the 'push-pull' of the transistor did not occur and we got half a signal. We will try hook up the amplifier to a 50 ohm dummy load and check if any of the transistors are damaged.

I was really just trying to get an idea that, if, the transistors were damaged, would this put strain on the operational ones? And could this cause a rise in temperature of the output stage? I understand that it's a bit of a fuzzy question though.

Thanks for your time.
 

BobTPH

Joined Jun 5, 2013
4,032
Sorry, but if you are running a 50 Ohm output through s step up transformer to a 10 Ohm load, you are placing a load of less than 10 Ohms on the amo. You should not be surprised that it heats up.

Bob
 

Papabravo

Joined Feb 24, 2006
17,242
That's very helpful, thanks. I agree we need schematics and replacing parts may be expensive. The last time i had this sort of failure was when the 'push-pull' of the transistor did not occur and we got half a signal. We will try hook up the amplifier to a 50 ohm dummy load and check if any of the transistors are damaged.

I was really just trying to get an idea that, if, the transistors were damaged, would this put strain on the operational ones? And could this cause a rise in temperature of the output stage? I understand that it's a bit of a fuzzy question though.

Thanks for your time.
I don't know what the arrangement of transistors is in you output stages and I can't speculate about what happens in the condition of a single device failure. What you are describing does not sound like a rational design approach, that the rest of the parts should have to work harder in order to precipitate a faster failure of the remaining parts. An appropriate design would shut the device down to prevent further damage.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
Sorry, but if you are running a 50 Ohm output through s step up transformer to a 10 Ohm load, you are placing a load of less than 10 Ohms on the amo. You should not be surprised that it heats up.

Bob
We have seen some heating up to ~45C, which the amplifier can handle. This was when the mismatch between the amplifier and load cause reflected power (from load) to increase. But, as in the original OP, things changed where the output stage temp would increase beyond 60C. Then, following this, the amplifier now no longer gives the set power we need.

I don't know what the arrangement of transistors is in you output stages and I can't speculate about what happens in the condition of a single device failure. What you are describing does not sound like a rational design approach, that the rest of the parts should have to work harder in order to precipitate a faster failure of the remaining parts. An appropriate design would shut the device down to prevent further damage.
Again, I agree that our guess for failure may not be true. I will update when (or if) we figure out the problem. Thanks again.
 

Thread Starter

RichW

Joined Nov 8, 2019
24
My guess is that, when it was working before, the transformer was reversed (as it should be.)

Bob
Thanks, but we tried step and step down configurations, the internal temperature would increase for both. I don't believe this to be a matching issue since greater mismatch does not result in a greater increase in output stage temperature. The temperature now increases to 60+C regardless of the match between amp and load. BUT, previously when the system was working, then it was the mismatch that did cause the temperature increase (to ~40C).
 

BobTPH

Joined Jun 5, 2013
4,032
What is the construction of the transformer? (type of core, number of turns.) Does the AC impedance dominate the winding resistance, as it should? 40 Khz is a pretty low frequency if you are using a transformer designed for RF.

Bob
 

Ian0

Joined Aug 7, 2020
3,754
Seems like an strange choice of amplifier for the job - a decent MOSFET audio amplifier would manage 40kHz, happily drive a 10Ω load without the need for a transformer.
If I wanted to know why the present arrangement doesn't work well, I'd be having a serious look at the transformer like @BobTPH said. What frequency range was it designed for?
 
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