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ucc28720 based power supply design

The following website: has a power supply design which I am considering using in one of my applications.

The top of the page says small form factor 10W offline flyback power supply reference design. Down the page on the left there is a spot

where you can download the schematic for this design. In the schematic there is a transformer that is labeled EPC17. Can anyone shed

any light on what this transformer really is. When I google EPC17 it says that it is a ferrite core and gives the dimensions of the ferrite core.

This transformer is a critical part of the design and without knowing exactly what it is the schematic is fairly useless.

Another thing I had a question about on this schematic is the inductor that is in parallel with a 6800 ohm resistor on the primary side. When I

googled the inductor I found that this inductor is a 1000uf cap and this makes the imoedence at 60 Hz equal to about .35 ohms. I don't understand

at this point why the 6800 ohms would be in there at all since it's effect will be very minimal when placed in parallel with a .35 ohm inductive impedence.

Thirdly, the two capacitors that are unlabeled in the secondary. Are they unlabeled because their sizes would depend on the current output

capability you desire?

If anyone can shed any light on any of the above it would be greatly appreciated.


  • Hi Jonathan,

    See attached transformer drawing.

    The inductor is in to filter switching ripple which can be expected to be in the 40kHz - 80kHz range and will have a much higher impedance at these frequencies.  The resistor is put in to damp out oscillations which occur around the self resonant frequency of the inductor when it's resistance peaks.

    If you download the BOM you can see the part number for the output caps. They are these parts

    EPC17 10W Flyback Transformer.pdf

  • Billy, Thanks for the quick response but I'm a little unclear on the transformer. There appears to be 10 pins on the transformer. Is there
    diagram somewhere that indicates which pins are for the primary, which for the secondary and which for the auxiliary coil. I believe this'
    transformer has three coils correct? Also what voltage can the primary input, 120VAC, 240VAC or both? And what is the windings ratio from the primary to the secondary and to the auxiliary coil. Additionally it is not clear to me from the data sheet you sent or from the BOM who makes this transformer.

    I would like to use this design to make a power supply that can input 120VAC to 240VAC and have as the output a 5V DC output capable of
    supplying 2A. Is this possible using this design topology.
  • Billy, I should have said in my previous response a minute ago, that I am not familiar with some of the terminology they have on the
    data sheet for the transformer that you sent. For instance what is the start and finish in the table and what is the FL acronym. Also
    they have number of turns listed on this table and the way that this parameter is entered into the table didn't make much sense to
    me either.

    Thanks again
  • Jonathan, the pin numbers for the winding are shown on the schematic and match the pin numbers on the transformer drawing, except that the secondary winding has pins have been replaced with flying leads (FL). ie. The terminations of this winding are not attached to any of the pins, they just go straight into the board.

    Note that the primary winding is split in two with pin 1 being used to connect the two halves. Start signifies which pin to connect the start of the winding to, and end signifies which pin to connect each end of the winding to. 

    The transformer has three windings, Pri (100T) Sec (6T) Auxiliart (21T)

    Per the reference design description on the webpage the design supports  85VAC-265VAC input range and 5V/10W outptut.

    These transformers were hand wound by the engineer. To get them wound by a magnetics vendor such as Wurth or TDK or GCI you would need to contact them with the drawing and ask them for samples.



  • Thanks for the response Billy. So this table the engineer has constructed is really a way of showing a potential transformer manufacturer how to actually construct the transformer, ie what pins to connect each end of each coil to is this correct?

    Also is it correct that that the primary coil has 100 turns, and the secondary has 6. This means that with a 135VAC input you have approx. 8.1VAC on the secondary. If you have 235 you'd have approx. 14.1 VAC on the secondary.

    The auxiliary coil which has 21 windings would then have approx. 28.35 VAC across it with 135VAC across the primary and approx 49.35 VAC across it with 235VAC on the primary. Are these statements correct? This then
    leads to another question: the UCC28720D must uses the R11 and R12 to determine the output voltage. So if
    the input is 235VAC on the primary then R11 and R12 would be one set of values if you want the output to be
    +5VDC. If 135VAC is across then R11 and R12 need to be a different set of values to get the +5VDC output.

    Also, the table says pin 5 is the finish and says that the wire is foil. What does this mean.

    Why does the table have the primary winding which starts at pin 7 unterminated. There is an entry in the
    table that says that it's finish is unconnected (NC). This does not agree with what the first entry in the table that
    says namely that the first half of the primary winding starts on pin 7 and ends on pin 1.

    Also, the schematic shows the secondary being connected to pins 2 and 3 of the transformer. Why does the table say that they are flying leads. This disagrees with the schematic.

    The EPC17 is a bobbin correct? Is this a standard bobbin can you get this from the different
    manufacturers that you mentioned in your above response?

    Can you tell me what the VA capability of this transformer is. I believe it has to be at least
    10VA (this is 5V x 2A). Is this correct? One reason that we are thinking we can use this design
    is because it is small. Is the engineer who designed this circuit pretty sure that the transformer can really fit in a package with the dimensions shown on the data sheet you sent (they were .728" x .748" x .492").

    Lastly, let me ask again so that I am completely clear since I may put considerable time and effort into getting a PCB laid out with this circuit and also in getting a transformer made. If I use this design exactly as indicated on the schematic it will give 5V DC output with a 2A current capability.
    The input could be anywhere from 85VAC to 265VAC and R11 and R12 would be two different sets
    of values depending on which primary input you need to hook up to?

    Thanks again for your help

  • Jonathan,

    The transformer drawing describes how to construct the transformer yes.

    As you can see from the schematic the input voltage is rectified to DC by D1, so the voltage applied to the transformer is not the AC line voltage. Also the output diode is reverse biased when the input voltage is applied across the transformer. The flyback transformer is really a coupled inductor. See the following links describing the operation of the flyback converter.

    The values of R11 and R12 do not need to be changed over the input voltage range, the duty cycle of the switch is varied to control Vout.

    There are three shield windings in the transformer to improve EMI. One of them is between pin 7 and NC, the others are foil windings connected to pin 5.

    I believe that pins 2 and 3 in the schematic should be flying leads.

    I'm not sure where to source the bobbin, possibly a distributor or the magnetic manufacturers as you suggest.

    As stated above, the flyback uses a couple inductor not a pure transformer. This design has been tested and the results are reviewable in the user guide / Test results file. As you can see from that the design is capable of supplying 5V/2A from 85Vac - 265Vac. R11 and R12 do not need to be changed.