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OPA 548 goes into Thermal shutdown even on no load

Ambrose Rajendram
Prodigy 30 points
Other Parts Discussed in Thread: OPA547, OPA548

Hello,

I am using two OPA 547s to provide a boot strapped power supply. The supply voltages are +22V and -17.5V. The OpAmps are used to provide a bootstrapped power supply that are within 24V of each other(The negative will track the positive and stay 24V below it at all times, so when the positive is at +10V the negative will be at -14V, when the positive rises to +20V, the negative will rise to -4V).

The problem I have is that the Op Amps are running very hot - so hot that they go into thermal shutdown within a few seconds of powering on. There is no load connected at this time. The quiescent current is listed as 15mA max, so the power dissipated by the device should be 37.5*.015 = 0.59W. Each device has a 3W heatsink, but even without that, the TO220 package should be able to dissipate this.

I have checked the output of the Op Amp and there are no oscillations. 

Now here is the funny thing: If I put a 1Ohm resistor in series with my output, the heating stops and the devices run cool. I moved my feedback to after the 1Ohm resistor so that the output voltages are maintained. However, I just couldn't understand why this should work and am wondering if it will come back to bite me.

Any help will be appreciated.

- Ambrose

  • 0
    TI__Guru* 93105 points

    Hi Ambrose,

    If you can provide us a schematic of your OPA547 power supply circuit that would help us analyze the problem.

    Even though you indicate that the circuit isn't oscillating the behavior has the earmarks of an oscillation.  A high current can flow through the amplifier output stage transistors if the amplifier is oscillating at a high frequency. It would self heat and go into thermal shutdown. Since the OPA547 is going into thermal shutdown very quickly a temporary oscillation condition might be missed.

    Consider this: I would suspect that the OPA547 has a capacitance connected to the output pin, in your design. Without a resistive load and having just a capacitive load can reduce the phase margin and cause instability. Once a resistive load is added to the capacitor the load impedance is much different than a purely capacitive load condition. Stability would then be attained.

    Adding a resistance in series with an operational amplifiers output pin, after the feedback loop connection, is a common method of compensating an amplifier driving a capacitive load and preventing oscillation. Your test with the 1 Ohm resistor tends to indicate that the amplifier is oscillating without it.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 30 points

    Hi Thomas,

    Thank you for the quick response. You are right, we have a 10uF capacitor on the output with hardly any resistive load. I have attached the schematic for your reference. J42 and J43 connect the power supply to the rest of the circuit and were left open during testing.

    I will try out different resistive loads and also removing the capacitors and let you know what happens.

    Thanks again,
    Ambrose

     

  • 0 in reply to Ambrose Rajendram
    TI__Guru* 93105 points

    Hello Ambrose,

    A unity-gain (+1 V/V) buffer, driving a large capacitive load, can be challenging from a stability standpoint. They are found to oscillate fairly often. Nevertheless, it should be possible to compensate the stage such that it remains stable under a reasonable range of capacitive loads.

    The schematic you provided shows 10 uF and 0.1 uF capacitors connected from each OPA548 output to ground. Can you tell me the characteristics of each of these capacitors, i.e. type of capacitor, dielectric, manufacture part number, etc.? Also, what range of capacitances do you expect will be applied across the power supply outputs? This would be anything such as power supply decoupling capacitors on the load's PC board, or anything else that appears capacitive to the amplifiers driving the lines. The worst-case loading is a pure capacitance and any compensation scheme would focus on that condition.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 30 points

    Hi again Thomas,

    Sorry for the delay in my response. I got caught up in testing the rest of my board and couldn't get back to this.

    The 10uF cap is Murata MPN: GRM21BR71H105KA12L. the 0.1uF cap is Kemet Electronics Corporation MPN: C0603C104K5RAC. 

    These power supplies power 20 cross point switches type Analog Devices AD75019JP. Each of these has 0.1uF decap on both the positive and negative supply. These devices are used in banks of four (so only four will switch at one time). There is also another bulk 10uF cap on each supply at the far end. However, none of these were connected during my initial testing as my jumper was disconnected. 

    One additional observation is that the waveform looks perfect - there is no oscillation that I can see. I am using an Agilent MSO 9104A 1GHz Scope. I can post the waveform also if it will help.

    Another interesting observation is that we are using a similar circuit in eight other places on my board except that we have a DAC driving the OPA548. They work perfectly there with no warming up at all.

    Thanks,

    Ambrose