Top261 Question
My question is what happens when the bias supply is disconnected from a supply that is connected to a load?
I have a 60W design using a TOP261YN that UL was testing and when they removed the filter resistor in series with the bias supply the output current went to 11.8A. When I tested the supply in my lab. I noticed that with a resistive load across the output if the circuit is opened between the bias windings and the TOP switcher C pin the output is not what I expected to see. I expected that as the bias winding circuit is broken by removing series resistor that the output would shut off after a few ms. This is not what happens. Is there some light you could shine on this issue?
Comments
Here is the PCB layout (in pdf form) and the schematic (as previously provided).
Transformer spec. would not effect this operation as the construction is standard form for vertical 24VDC device.
Hi,
Looking at the schematic, it appears that resistor R9 is being opened as a part of the test. This will essentially float the "C" pin. This will lead to the part turning ON and OFF through the internal supply.
One work around for this situation is to eliminate R9 or have two resistors in parallel instead of one.
Regards
PI-Sarek
Okay that was the conclusion I came to as well. Thank you for the confirmation.
That sort of solves one problem however there is still and issue with if the D2 (bias winding rectifier diode) fails open then the circuit opens (also if tranformer was to fail or opto). When this happens I have found a catastrophic failure shortly occurs if the output is left open (no load). The parts literally explode :(
Here are some photos of the result of my lab testing. I assume there is something I am doing wrong as these parts and designs are showing UL compliance.
Hi,
In order to help us understand your problem, please provide us a copy of the circuit schematic and attach any additional information such as transformer design board layout etc.
Regards
PI-Sarek