I've measured the heatsink temperature with a IR thermometer. It starts around 24 degrees and gets to around 40 degrees within 5 minutes. It seems to be around 42 at stead state.Some questions..
-Have you ever measured the case temperature at start-up and later on under fully loading condition ??
I can see what my power supply is putting out, and it looks like the average quiescent current drops from 160mA to 140mA over time.-In the case of 1, how about drain current ??? What is the deviation from cold state ??
I highly doubt it; it's nothing but copper coil with a matching network. Even if it did heat up its electrical properties shouldn't change significantly.-Does your load has temperature drifting ??
Each transistor is rated for 15W out, 55W dissipation. I'm not even close to either of those limits. And the current measurements I gave are for the entire amplifier (that is, both stages together). I've seen these devices pushed far harder than I am.I think 160mA Idq is too much for this transistor. Working in Class-A usually the maximum output power should be 25% from the maximum peak power, which for this transistor is 15W. So for Class-A the output power should be about 4W for a normal operation.
That might help temperature drift, but not enough I think. I'll still need something to correct for temperature.Sometimes reducing the Idq and placing the bias in Class-AB can meet almost the same linearity performance as Class-A.
I'm not seeing anything on this page besides the header. Is there some kind of flash plugin or applet I need to view whatever should be there?See the applet below to see how the thermal runway in a power MOSFET depends by some parameters.
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No, it's not too much. This exact device is used in tons of class A amplifiers which give 15W peak or more out (the MRF136 is specifically preferred for class A). The average power is much lower, as it is in my application (like maybe 2W average).55W is the maximum rating for the Total Dissipation Power.
Total dissipation power is not the same thing with maximum output power, which is 15W in this case.
You get now 8W output power, which is too much for a Class-A using this transistor.
So...? There's no reason it can't work at higher currents and power levels for short durations.If you check the application notes for this transistor you will see that the manufacturer recommends Idq between 25mA and 100mA.
Thought so. Can't see it on a workstation, I'll try it later at home.The applet link works fine..I think you need Java enabled.
How about contant current power source? Even using constant current source, it's also hard to keep power in 0.2dB tolerance.
Well, I was hoping for a closed loop method that doesn't require extensive measurement and calibration. Also I wonder how they even thermally couple the temp sensor to the transistor. Seems like a little mechanical variation would throw off the temperature reading and screw up the feedback. And I don't have the resources for another serial device (no more I/Os left on my MCU). To clarify, when I said temperature compensation, I wasn't really intent on a circuit that uses temperature as feedback, but still has performance that doesn't depend on temperature. Measuring drain current and using that as feedback seems easier from a design perspective.The schematic you posted is almost the same as the one from Intersil application note (page 4).
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They use a microcontroller with stored look-up table inside, which memorize different temperature and bias states.
Yes, this is what I was going to try (though I am going to put the sense resistor on the source, makes the error amp easier to design). But where does your error come from then? The current regulation should be very accurate... or is it that gain still changes with temperature, even if current is fixed?I used Idd sample with seriesing a vey small resisitor, say 0.1R, and use OpAmp to amplify this voltage difference on this resistor, then compare the voltage with a adjustable negative voltage to close loop, to control Vgs.
But this method have about 2dB error on the output power, that can't meet your requirement.
I should say that the current drops because I do have thermistor feedback in the bias network, and it's apparently too strong. If I used a current sensor, wouldn't it need to be an inverse PTAT (assuming my tempco is positive)?I repeat again here...
I ds drops down to 160mA to 140mA and this current must be compensated by adding Vgs voltage that will be proportional to temperature.The slope of theis voltage will be defined
by cut and try method because you'll never know this slope by model.
The simplest solution is to use PTAT voltage source for Vgs..
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