UtrasoundGuy
Bioengineer
- Nov 18, 2013
- 21
we have the following DC to DC converter.
It has been in use in our company for around 10 years without any major problems Im aware of.
The converter is capable of converting 15V DC to -200 to -400V DC (depending on trimpot turning)
Since Ive been working here for the last 2 years, we have been trying to get this device produced by 2 different vendors (one 2 different times) and the PCBs keep failing our incoming inspection.
The failure is that the output voltage is not stable at -220 and -400V which are test points in our test procedure.
An example of the output failure over the converter is shown below (AC coupling on oscilloscope is used to remove the -400V DC), the time is in seconds and volts are actually 10x what is shown and are 20-25V peak to peak fluctuation
After the tests are performed, and if everything checks out, the device then gets set to -300V where it gets used.
We are hesitant to use these PCBs if they are not passing inspection though,.
We are not sure why there are now difficulties with stability or what has changed.
Maybe the transformer is different although its speced the same but Im not sure how this would cause oscillation in the output waveform?
I believe part of the difficultly in debugging this is that it is a feedback system
So some symptom shows up throughout the board, its hard to track to just one part.
The current sense (pin 3) of the pwm controller appears as follows when it is stable
but when the waveform is oscillating, it appears as follows
you can see the blurriness in the waveform shows it is changing size and amplitude.
the pwm output from the pulse width modulation chip also changes as a result.
This is an in area where the voltage is changing rapidly and you can see the waveform of the pwm is changing on the output
The transformer end of the primary (attached to mosfet) appears as following the oscillation during the changing cycles
Any ideas on what to do or check or even where to start looking?
Can I start cutting traces and if so, what one is the best, all of the feedback?
any help is really appreciated
Jim
It has been in use in our company for around 10 years without any major problems Im aware of.
The converter is capable of converting 15V DC to -200 to -400V DC (depending on trimpot turning)
Since Ive been working here for the last 2 years, we have been trying to get this device produced by 2 different vendors (one 2 different times) and the PCBs keep failing our incoming inspection.
The failure is that the output voltage is not stable at -220 and -400V which are test points in our test procedure.
An example of the output failure over the converter is shown below (AC coupling on oscilloscope is used to remove the -400V DC), the time is in seconds and volts are actually 10x what is shown and are 20-25V peak to peak fluctuation
After the tests are performed, and if everything checks out, the device then gets set to -300V where it gets used.
We are hesitant to use these PCBs if they are not passing inspection though,.
We are not sure why there are now difficulties with stability or what has changed.
Maybe the transformer is different although its speced the same but Im not sure how this would cause oscillation in the output waveform?
I believe part of the difficultly in debugging this is that it is a feedback system
So some symptom shows up throughout the board, its hard to track to just one part.
The current sense (pin 3) of the pwm controller appears as follows when it is stable
but when the waveform is oscillating, it appears as follows
you can see the blurriness in the waveform shows it is changing size and amplitude.
the pwm output from the pulse width modulation chip also changes as a result.
This is an in area where the voltage is changing rapidly and you can see the waveform of the pwm is changing on the output
The transformer end of the primary (attached to mosfet) appears as following the oscillation during the changing cycles
Any ideas on what to do or check or even where to start looking?
Can I start cutting traces and if so, what one is the best, all of the feedback?
any help is really appreciated
Jim