OPA197: OPA197IDBVT output giving -15V in field IC got damaged

Hi Kaushik

the 10n cap between pin3 and pin4 of OPA197 erodes the phase margin and can cause stability issues. I would remove it.

Kai

Hello Kaushik,

I did a stability analysis of your OPA197 circuit and it appears to have sufficient phase margin and it should be stable. You could check the output of the OPA197 on your working boards with a DSO and 10x probe, and make sure that there isn't any oscillation present. Even if the OPA197 were oscillating that wouldn't necessarily result in damage to the device, but it is better to avoid that situation.

There are high series resistances in series with both inputs so they should limit the input current to low levels should there be an unexpected input voltage event. That tends to point towards an electrical overstress (EOS) event at one of the OPA197 supply pins. EOS can damage an op amp, and if the output stage gets damaged the output can end up at one of the supply levels permanently.

One thing that we always recommend when the supply voltage characteristics haven't been fully characterized and may there is the possibility of an EOS event, is the inclusion of transient voltage suppressor diodes (TVS) from each supply pin to ground. You can see the way the diodes Zs1 and Zs2 are connected in the diagram provided below. The TVS diodes will turn on and clamp the supply voltage is a safe level should an EOS event occur. Since your application uses +/-15 V supplies the TVS diodes should have a reverse standoff voltage (VR) a little higher than the supply voltage.

An added benefit of the added TVS diode to each supply is there is always a current path through the op amp through one of the diodes should one supply come up later than the other. This avoids latching issues that can sometimes occur with some op amps when the supplies do not come up at the same time.

Regards, Thomas

Precision Amplifiers Applications Engineering

Thank you, sir, noted, will update and check it accordingly. 

Hello Kaushik,

You state:

Doubt: I have doubt on soldering pattern. Due to some reasons, we have soldered this ICs by manual solder instead of machine solder. Soldering temperature has been set around 320 ' C.

Is there any possibility of damaging of this ICs due to manual solder ??? The package is SOT-23-5pin.
Or any other reason?

So the OPA197IDBVT is being manually handled during the soldering operations - and it is the only IC being manually handled? If proper ESD handling procedures are not in place for manual handling, it is easy to damage an IC with electrostatic discharge (ESD). ESD can result in immediate failures, or latent failures. You need to check what ESD precautions are being used for all the handling operations associated with this manual soldering process.

Regards, Thomas

Precision Amplifiers Applications Engineering

Yes, sir, It is the only IC which was manually soldered. Other ICs were machine soldered.

I got the latent failure because PCBs had passed all factory tests. It was running fine till several days from installation. 

I will check out my soldering process. Will be back if having any doubts. Thanks for quick response. 

Hello Kai,

I will implement it and will send it to the field. 

Yes, sir, this is an outdoor application. All PCBs are installed at remote locations. Cable length is around 300mm but the sensor is fixed on a heatsink, very near to the IGBTs. IGBTs are switched at 1200V DC. A  case of IGBTs is 2.5KV isolated from the heatsink. We have old boards based on OP07-SOIC8. More than 1000 PCBs are running fine at the field. We got the problem with the new design. My problem is any change I will do will take 2 months to verify whether the problem is solved or not. Will go with TVS, will be back thank you.

Hi Kaushik,

aha! That's the killer! If there's enough stray capacitance between the IGBT and the heatsink and if the heatsink sits at a different potential compared to signal ground of OPA197 circuit, then dangerous equalizing currents can run into the OPA197 circuit and kill the OPA197!

Kai

Hi Kaushik,

the following two pictures shall show, how in your application dangerous currents can run into your OPA197 circuit:

In the picture above the central star ground point is chosen on the far right. So, if there exists a AC potential difference between the 2.5kV IGBT voltage and this signal ground an equalizing current can flow from the 2.5kV IGBT voltage via stray capacitance to the heatsink, from the heatsink via direct connection or stray capacitance to the PT100 sensor, via the PT100 cable to the input of OPA197 circuit and finally via signal ground copper traces of PCB back to the central star ground point on the far right. If the signal ground copper traces show some inductance and if the 2.5kV IGBT voltage contains spiky voltage pulses, then dangerous potential differencies within the signal ground routing of PCB can develop. These can damage the OPAmp at every terminal, not only at the input terminal. That's why TVS can be helpful, not only at the input but also at the supply voltage pins, as Thomas already recommended. But also via the output pin a dangerous overvoltage can enter the OPA197. Only a PCB with a solid ground plane can minimize the inductance of the signal ground routing and by this the dangerous ground bouncing effects.

But an even far better way to handle interference is to chose the central star ground point not on the far right, but at the place where the sensor signals enter (or leave!) the PCB:

Cover the PCB with a metal enclosure and connect the signal grounds and power supply grounds to the metal enclosure right at the point where the cables enter or leave the enclosure. If you use shielded cables connect the shields also to this point. By this method interference can no longer cause any dangerous currents to enter the OPAmp circuitry. Interference currents will only flow anymore on the solid metal enclosure or from shield to shield, but will no longer enter the metal enclosure or flow across the PCB. When doing it right, the metal enclosure forms a radio frequency plane and massively profits from the skin effect. This method is also the best cure against ESD...

Kai