I am copying a ti circuit using 28600 for a 400v-12v converter, the TI's example part number is PR798.5824.PR798B-2_SCH NC.pdf
but my board is on an off all the time, from the oscilliscope , the power supply pin of the chip fluctrates ,lower than shut down threshold; then it shut sdown for a while.later it turns on again.
however, as the chip relys on the bias winding of the transformer, if the circuit is not working , then the winding cannot provide the nesssery bias for itself.
so this is a bit hard to identify the problem.
any one has some clues/tips?
how the output capacitance of mosfet could affect the circuit? we only changed the mosfet of which output capacitance is 12pf compared with the original 50pf.
Is there an alternate source for the transformer used in the PMP5192 Rev. B design (AC input, 5 VDC output)? The transformer is a GCI part number: G094279LF. This device has a large lead time and I was wondering if there are alternate devices that were considered in a similar sized package.
Thanks - Allen
The transformer was a custom design for that application so the chances of finding an off-the-shelf version are pretty slim. We try to use standard core sizes, standard core materials, and standard bobbins so the lead time is minimized but not quite as short as standard catalog parts. Unfortunately, because these devices are custom, it's difficult to second source them as the actual design belongs to GCi.
Okay, thanks Lisa! :)
Just let you know, Ti has a good 12v-5v dc/dc part which is very small and meets many requiremnets in some embedded applications.
you can try to just add this one to get your 5v from 12v, you won't lose too much efficiency as the original 12v flyback needs minmum load current to run anyway.
For your information.
I'm curious as to why you think the UCC28600 needs a minimum load? The device is designed for minimum power consumption at no-load for green initiatives. The converter should regulate in a hysteretic burst mode for no-load and light load operation.
Your curiosity makes sense as I didn't describe it correctly.
In my application, UCC28600 circuit not only serves a 12v dc/dc but one auxillary winding of the transformer gives feed back on input voltage, say , 400v .
When UCC28600 works in green mode, the feedback input voltage is much lower since the chip works in intermittent mode as you know.
if the input voltage is not good, my downstream converter would not work.
Thus, I have to add a minimum load to force the chip to get out of the green mode in order to get continuous oscillation to make the filtered signals to the right level.
I hope this time I described it correctly.
I see, thanks for the clarification. Yes, the other slave outputs will lose regulation due to the extended time between burst cycles. Have you tried to minimize your "dummy" load by superimposing a small offset into CS so that the UCC28600 will transition out of burst at lighter load?
Hi Lisa, thanks for this smart idea.
this means creating a negative offset on CS , right?
it is not easy, do you have solutions?
Actually what you want to do is to add a positive offset to CS but only during burst mode, or lightload hysteretic operation. This would be the same procedure as reducing audible noise that is explained in the Design Considerations app note (SLUA399) section 4. The idea is that the STATUS signal is used to forward bias a diode that allows a small (100mV or so? I forget...) offset to be added to CS so that the converter thinks the current is high enough to kick it out of hysteretic mode and into FFM. You only want this offset superimposed on CS during burst mode because in all of the other modes there's an internal power limit offset already added to CS and if both offsets are there the controller will hit power limit. Luckily, STATUS is high during burst and low in the other modes...
Could you upload you updated shematic to guys, let us to share it ,thanks!
adding an offset to CS during burst mode is shown in Figure 5 and explained in section 4 of http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua399b&fileType=pdf
Where could I find the gerber files for this module discussed here?
I only find the schematic.
Here you are.
Thanks a lot.
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