Why does my power op amp IC (TI OPA549) thermally malfunction? IC works electrically perfectly, no oscillations or anything. By thermal malfunction I mean that Rj-c thermal impedance rises from default value (1.4°C/W) to about 6...7°C/W.
What kind of mechanism can cause such malfunction?
PCB has three identical channels (A, B, C) where two of them work flawlessly (B, C) but one (A) constantly malfunctions after short time of use (I have tried three chips).
Below you can see IC and its three important spots center (red), edge (green) and case (blue). With maximium stress ( ~ 40W), channels heat up as follows (measured with thermal camera):
70°C (red), 55°C (green), 40°C (blue)
+100°C* (red), 59°C (green), 40°C (blue)
*throttling, junction temperature reaches 150°C limit and stops.
Thing I have considered:
soldering: I have always screwed the case to the heatsink and after that carefully soldered the pins.
mechanical stress: Heatsink is heavy, but firmly attached to PCB.
case temperature is always same (40°C), so I can assume that the heatsink is working alright.
B and C channels are working flawlessly as I have designed.
Update: 29 June 2017
As Chupacabras brought up, I have noticed that problem has something to do with the Vcc line.
I previously had a common heatsink for all channels, but then I installed individual heatsinks for each chip. This had unexpected results:
- C channel works flawlessly
- B channel malfunctions as badly as before
- A channel is something in between.
I also figured out that when I was using linear lab supply, no malfunctions occurred. By default I have been using switching (65kHz) PSUs connected in series, where middle tap is ground.
However, I'm unable to measure any kind of differences in Vcc pins.