OPA547: OPA547 can sometimes be damaged when drivening hydraulic valve

Hello Zongshan,

You stated "Driving proportional valve requires current of 0.3 mA, ambient temperature is about 65 degrees, no radiator." Is the OPA547 output current really 0.3 mA, or 0.3 Amps? If the current is 0.3 mA no external heat sink would be required. If the current is 0.3 Amps, the OPA547 junction temperature may be hitting its maximum rating.

The OPA547 circuit looks reasonable, but the proportional valve load on the Op amp is not known. There is a large 0.1 uF capacitor on the OPA547 and the proportional valve connects across it. I suspect the proportional value is an inductive load. Therefore, the actual load on the OPA547 would be a complex impedance. An inductive load when going from activated mode to inactive mode can produce a back EMF which not protected against can damage an Op amp's output. It would be helpful if you could connect a DSO to the OPA547 output and record the voltage at pin 6 when the valve is actuating on and off. It may 

D5 and D6 are included in designs to protect the Op amp output in such applications, but the 1N4148 is not the correct diode for the task. It is a low current, small-signal switching diode. D5 and D6 should be a very fast power rectifier diode. The OPA547 datasheet in Figure 11 shows the 11DQ06 Schottky rectifier diode and that should be a good choice. We now recommend using ultra fast silicon rectifier diodes for D5 and D6. Also, it is recommended that a transient voltage suppressor (TVS) diode be added from V+ to ground. Should D5 become forward biased due to a back EMF, the TVS diode will momentarily turn on an provide a path for the current to ground. 

If you can provide information about the proportional valve that might provide more insight into what kind of load it presents to the OPA547 output. Also, if you can provide information about the input control signal that would be helpful.

Regards, Thomas

Precision Amplifiers Applications Engineering