Flyback SMPS on HT7A8C06 (holtek)

sijeophilip

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Hi,
We have designed a AC to DC SMPS with input supporting Universal Input (i.e. 90V to 285 V) and output of 24V/2A (Please find enclosed schematics for consideration) with external circuit for primary side OV and UV protection, Some of the boards are not working (i.e. the power supply is resetting again and again)and while we tried troubleshooting we probed the VCC (which you can see is from the bias winding of the transformer) of the IC w.r.t to primary ground using a Voltage probe of our oscilloscope.... But for some reason when we probe it either the Common Mode Choke(just to understand if the OV or UV is acting unexpectedly) or the Fuse on the primary side blows off... Can someone help us understand why the overcurrent scenario is encountered by connecting the probe of the oscillcope to the bias winding ...
Also would appreciate if anyone can advise how it should be done correctly.
 

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  • Power Supply Sch.pdf
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Solution
The oscilloscope is powered over the AC line
And this results in a galvanic connection between the oscilloscope probes and the DUT : "ground - Y_Cap - PE".
because the ground contact of the oscilloscope probe is connected to PE in the oscilloscope socket.

A necessary practice when working with AC/DC smps is to either have an oscilloscope with isolated probes
or use an isolation transformer.
Is your oscilloscope isolated from the 230V power supply?

[Moderator action: translated, please submit post in English]
 
Last edited by a moderator:

Is your oscilloscope isolated from the 230V power supply?

[Moderator action: translated, please submit post in English]
The oscilloscope is powered over the AC line i.e. benchtop tektronics DSO the probes are not isolated ... Please note the probes were connected to the 20V bias winding on the primary side when we observed the blowing of fuse of SMPS under test
 
Last edited:

The oscilloscope is powered over the AC line
And this results in a galvanic connection between the oscilloscope probes and the DUT : "ground - Y_Cap - PE".
because the ground contact of the oscilloscope probe is connected to PE in the oscilloscope socket.

A necessary practice when working with AC/DC smps is to either have an oscilloscope with isolated probes
or use an isolation transformer.
 
Solution
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