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OPA549: Non-linear working of OPA549

Patryk Kisiel
Prodigy 30 points
Part Number: OPA549

Hello,

I created a linear power supply based on OPA549 (scheme below) and it would work perfectly if there wasn't be one problem. When I supply an amplifier OPA549 with -5V and 15V everything is ok. I mean that output voltage is direct voltage. When I supply it with -5V and 30V linear output changes to switching mode witch I saw on an oscilloscope ( also below ). I can see that an OPA549 is turning off and off the output with high freg. I think it might be an thermal issue but I did apply heat sink and radiator wchich is cooling an amplifier, so it shuldn't be problem witch this. It isn't problem witch supplying becouse I tried to supply it in 2 diffrent ways and result was the same. Did anyone has the same problem?

Patryk

  • 0
    Guru 140200 points
    Hi Patryk,

    what capacitive load have you connected to the output of OPA549?

    The OPA549 is an OPAmp and OPAmps don't like big capacitive loads. A limited load capacitance can be handled by the help of Zobel network R6, C8. But bigger capacitive loads can result in instability.

    Do you see a change when modifying R6 and C8?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 30 points
    Hi Kai,

    I have a few high power resistors. During my tests I am using one, two or three connected in series 4,7kOhm resistors. I saw that when I connect as a load three resistors-14,1kOhm and the supply voltage is over 15V, problem doesn't exist, but when I connect two resistors- 9,6kOhm the switching occurs.


    Would it be a problem to change R6 and C8.


    Patryk
  • 0 in reply to Patryk Kisiel
    Guru 140200 points

    Hi Patryk,

    and absolutely no capacitive load at the output? Then remove the Zobel network for a test. For very fast-slewing input signals (during a fast turn-on, e.g.) the Zobel network can cause an additional load current spike!

    Are you sure that the OPA549 isn't heating up too much? What tells the ES pin?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 30 points
    Hi Kai

    Absolutely no capacitive load. Tomorrow I am gonna remove the Zobel network and focus on E/S pin. OPA549 is continuously cooled by small fan and it is connected to heat sink. It shouldn't be heating.

    Thanks Kai very much for your attention.
  • 0 in reply to kai klaas69
    TI__Guru* 93105 points
    Hi Patryk,

    When I closely examine the DSO output voltage and current images you provided it looks like the OPA549 is going into a repeating thermal shutdown/recovery pattern. The shutdown occurs when the output transistor junctions reach approximately 160 C, then cool to below 140 C while shutdown, and turn back on until the junctions heat again. Essentially, this is thermal on/off oscillation.

    The TO-220 package by itself can't dissipate but a few watts before thermal shutdown occurs. You do mention using a heat sink and a small fan, but that really doesn't tell us if it is sufficient for the output power conditions. The DSO waveform shows 2 Amperes output current and you indicate the oscillations occurs when the V+ supply is taken from +15 V up to +30 V. The OPA549 internal power dissipation is certainly higher with the increased supply voltage, increasing the heat being generated.

    If you are still having the issue, can you capture an image of the OPA549 mounted to the heat sink (and the fan position) so we can have a look? Also, if you have the heat sink specifications they would prove helpful.

    Regards, Thomas
    Precision Amplifiers Applications Engineering