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DRV2700 HV piezo driver problem [SOLVED]
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Unixon:
I have a project with DRV2700 high voltage piezo driver in flyback configuration.
The problem is that the output voltage never exceeds about 65-78V when driven above that level (can reach 77-78V shortly but then slowly degrades to 65-66V).
Output voltage degradation depends on temperature, I could get 10-20V increase by freezing board around driver and transformer (but DRV2700 on that board eventually died).
High side Vpp after transformer before rectifier is about 110V, current consumption is 1.8W with no load.
Transformer is exactly the same as in demo board DRV2700EVM-HV500.
Feedback circuit works fine, voltage is well controlled in 0-65V range, H-Bridge works fine and consumes about 0.6-1mA in gate protection circuits.
Discharge circuit with Q2 is not involved in current consumption as Q2 is closed.

Related parts of schematics and board layout are in attachment.

Any ideas on what's wrong?
duak:
Does the DRV2700 get very warm?  Perhaps it is internally overheating and limiting current.

Does transformer T1 get very warm?  The resistance of the windings may increase enough to reduce the voltage at higher output currents.
hv222:
Is transformer soldered properly - pin 1 in right place? Can you share oscillograms of signal at SW pin?
Unixon:

--- Quote from: duak on August 17, 2019, 08:07:59 pm ---Does the DRV2700 get very warm?  Perhaps it is internally overheating and limiting current.

--- End quote ---
Not enough warm to fail finger test, but may be warmer than it should.
I need to get to my IR thermometer or a thermocouple to tell temperature precisely.


--- Quote from: duak on August 17, 2019, 08:07:59 pm ---Does transformer T1 get very warm?  The resistance of the windings may increase enough to reduce the voltage at higher output currents.

--- End quote ---
I think it does gets warm, because cooling this piece of board by touching with a fingertip or blowing air over causes up to 10V increase in output voltage.
Unixon:

--- Quote from: hv222 on August 17, 2019, 08:08:59 pm ---Is transformer soldered properly - pin 1 in right place?

--- End quote ---
Yes, I compared my assembled boards with TI's board and design and they do match transformer orientation.


--- Quote from: hv222 on August 17, 2019, 08:08:59 pm ---Can you share oscillograms of signal at SW pin?

--- End quote ---
Yes, sure, this will take a little time. One thing I can tell right away - they seem very suspicious - too close to meander.
I think I'm running into overcurrent/saturation here, but why TI's board doesn't with exactly same values in circuitry?
Unfortunately, I don't have the original demo board to measure reference voltages/oscillograms.
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