Ozan,
Here you are applying 240V ripple at the voltage sense network. Q1050 connects/disconnects T1050 pin 2 and, another winding at pin 5 is connected through
CR1050 to your node "in". Although we don't have the parameters of the transformers T1050 and T1060, it is almost certain that when Q1050 turns off magnetic
energy in T1050 creates a current in CR1050 and current still continues to flow into your "in" node while Q1050 is off. There will be continuous current, with
some ripple, either from Q1050 or CR1050. The ripple voltage showing up at bottom of R1046 is probably much smaller.
Thanks for the explanation. I failed to consider the inductance in the transformer T1050. The manual did say that T1050 is part of the buck converter. I ran a simulation with the primary and secondary coils modeled as 10mH inductance, with mutual inductance coefficient 1 (perfect coupling). In this case, the ripple voltage between REF1 and REF2 (2467 diagram REF definition) will be about 20 volts. T1060 inductance also helps to reduce ripple. I will need to wait until I replace TL494 to do measurements again.
I haven't measured the ripple so I don't have a concrete number. It is also a difficult measurement to make safely, connecting scope probe ground to any of the
nodes inside the PSU needs isolating the PSU and even isolating the scope. I assume you are already aware of the internal connection between your scope
probe's signal ground and the earth terminal of the power plug. Without isolating the PSU you would be shorting any node scope ground touches to earth. There
are special probes with HV isolation for this purpose.
I learned this in a hard way. I am careful with the measurements. Still it is hard to do it right. Since opening the scope to try to fix it, I have damaged my low-power supply (accidentally passed the main voltage to its power output terminals), my Rigol probe (only the 10X setting has 300V p-p protection. I accidentally move the setting to 1X), and also the TL494 due to failure to connect the output to 2Ohm load. Another thing I realized is that the probe loop can pick up interference due to magnetic induction. So some of the noisy waveforms were factitious. Still I am having good fun, and learning a lot. Your comments certainly helped me a lot.
Luca
