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Hello! Yes, I think I found the problem. The current transformer in series with the main Tx was the reason for the main transformer saturation. I think that DC current information was lost due to current transformer (CT) placement. This small DC current probably moved the BH curve of the main...
I have actually built a simplified model of my converter. Model was simplified in a sense that I have not modeled the exact PWM IC behaviour and feedback circuitry. Also as my basic model don't include modeling of the saturation phenomenon of magnetics it has little use now. I think I nedd to...
TI recommends adding at least 1V to the ramp signal to prevent asymmetry in CD pulses. Application Report SLUA275 - September 2002. So I am using 3 resistors to add 2/5 of the current signal, 2/5 of the compensation ramp, and 1/5 of the ref voltage (1/5*5=1V).
CT core is B64290-L45-X38, T38...
Hello everyone!
Recently I was researching my PSFB converter and came across a phenomenon that was unclear to me.
Please see schematic, it is classical PCMC PSFB. The current sense transformer (CST1) placed in sires with main power transformer T103.
I configured error amplifier as a buffer...
Hi guys! I am glad to tell you that I've found the issue.
The main reason for such oscillatory behaviour was lack of slope compensation. The reason I didn't notice this from the beginning is that when the load current was low, the compensation ramp was sufficient, and when the load current was...
Thanks for the tips. I tried a unipolar transformer TCS2.1, 2.2, but the waveform from it is no good at all. For some reason the signal there has a reverse slope in every second cycle (QC, QB conducting). Please note the new schematic, with the colored numbers I have marked the measurement...
Unfortunately, my current probe only has a bandwidth of 100 kHz so I can't measure primary current directly. The most accurate measurements I have are those obtained with a current transformer.
Here is the voltage across R18 (current transformer load resistor) at the moment when the current...
Yes, I tried different CS resistor values of 20-70 ohms, but that "noisy" band is still pretty wide in all cases. How can this effect be reduced? Or is this a common problem with CMC, then more precise current limitation needs to be done with a separate circuit? What is the usual solution here?
Thanks for your sim, I will try to repeat this process in LTspice. Please note that the schematic is attached to my first post in the "Attachments" section.
Shorting the DC blocking capacitor makes no difference, I left it there just to be safe.
Yes, but in my setup the minimum load (6 ohm...
Hello,
I am designing a CMC PSFB converter based on UCC3895. I ran into a problem with cycle-by-cycle current limiting.
When the voltage at the CS pin reaches the 2V threshold, the duty cycle starts to change rapidly and asymmetrically. This causes audible noise and could even lead to primary...
It’s seems that 220nF at the totem poles gave a significant improvement. First of all, noisy sound is completely gone (or become almost indistinguishable) from 200 up to 700W range even with a 100nF C26 and converter regulating the output. But still at load lvl around 315W and below strange...
I am sorry for delayed answer. When I was messing around with feedback loop QC somehow popped. There was really unstable situation accompanied with loud crackle and then QC MOSFET just gone. I don’t really know how it could possibly happen since my CS pin is connected to the CST… Maybe Tx could...
So, the best I could find for now is TLP785 YE which CTR ranges from 50 up to 150%. I have also decreased R22 2.2k down to 1k to even more reduce gain (btw, is it a valid way...?). With C26 at 330 nF I could finally make it stabile under a certain (300-350W) load. But if I decrease load to...
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