Electronics > Repair

Rigol DP832 power supply repair - U18 replacement

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giosif:
Hi,

I am trying to repair a DP832 power supply which has had its U18 IC burned to a crisp.
This is one of the 3 ICs on the top board which have had their marking rubbed off from factory (the one closest to the series pass transistor for channel 1).

Would anyone have any idea what make and model this IC might be (or some hints on how to identify it)?
Alternatively, would someone happen to have a spare such board they would be willing to extract the IC from? (cough... Dave? cough...)   ;D

Thank you!

Kleinstein:
My first guess for the parts doing the regulation would be an OP.  The resistors / caps and the power connection could give a hint. Chances are there are a few OPs with still some making intact, so one could identify the supply rails.

The other possible chips to expect are DACs and ADC - these should have a live to the digital part, usually not found with an OP. ADCs / DACs are usually less exposed to enough power to really burn the parts.
So chances are high to have an OP of some kind. I would also expect the power MOSFET to be blown as well and this way get higher voltage to the gate drive.

One may have to do some local reverse engineering to get an idea on which part would match. 

giosif:
Thank you for the hints!

I will investigate and try to determine at least the type of component (i.e. op-amp, DAC, other).
I will worry about finding a suitable replacement after that.

Interestingly enough, the power MOSFET seems to be ok, on a preliminary check.
I will check again, just to be sure.

m k:
Any step by steps somewhere?

After clearing the area check possible supply lines.
Then pins that are directly connected to the supply lines.
Then pins that have a single helper component in between.
Something can also be quite directly connected to the other chip or clear other circuit.

Other possible approach is to start listing parts it surely is not.

I was once digging up MCU types by comparing prints.
It was finally quite easy up to a point.
Help there was that I knew the maker and its/their habits.

What kind of a company Rigol is, own ways or by the example circuits?

TurboTom:
It's a single opamp in standard pinout, used as a current sense amplifier (its inputs are routed to the current shunt resistor, see Dave's teardown photo here:

https://www.flickr.com/photos/eevblog/9607771682/

Since the shunt is located in the positive output rail, and the OPAMP's input is divided only to 9/10ths, it would have to run on a fairly high voltage (which may explain its pyrotechnical demise) and needs to have a decently low offset voltage. I'ld say an OP27 or OP37, possibly of the better grouping (A) if available, should do the job. To be sure, I'ld measure the supply voltage with the damaged IC is removed. If it's more than 40V, rather go for something more exotic like the ADA4522-1. Also, check the resistors in its vicinity to be free of damage.

You will probably have to run a re-calibration of the affected channel after replacing the chip.

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