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UCC28740 without optocoupler

Other Parts Discussed in Thread: UCC28740, UCC28710, TL431

Hello all,

We are looking to use the UCC28740 without an optocoupler for reliability reasons. We have considered UCC28710 but had too many problems with the sense pin that we decided to try the UCC28740 instead.

We are using the schematic from UCC28740EVM as a starting point, but with the following modifications:

1) since we are eliminating the opto, we are using the aux winding (BIAS) as main regulation source. We know regulation is not going to be good, but we use this to power relays, so we don't need accurate regulation. +/-10% is OK. Looking at the EVM sch we are connecting R21 and R15 to the VDD pin 1 of U1.

2) We are replacing the opto U2 with a PNP with E connected to opto emitter and C to opto collector. Base is driven cathode of TL431 (U3) through a base res. We are adding an emitter resistor to control gain.

3) We are planning on using the same compensation network to stabilize the feedback network.

We looked at the datasheet and the design guide is centered around opto usage. Does anyone have experience on this IC without an opto? Any feedback on this will be greatly appreciated!

  • Hi Mariano,

    I am surprised that you had problems with the UCC28710. The voltage on the sense pin is really determined by the signal out of the transformer and if this signal is not accurate then it becomes  difficult to control the output voltage.

    The UCC28740 uses an optocoupler feedback to control the output voltage and the sense pin to control the output current in constant current mode.

    If you replace the optocoupler with a transistor ( do you mean an NPN instead of a PNP?) then you will no longer have input/output isolation which is one of the main advantages of PSR type controllers.

    However the circuit you describe should work and the TL431 will set the output voltage provide you connect the two grounds together and use an npn transistor.

  • This is the circuit we are using. It is the same optocoupler version but with the addition of R125, Q6 and R126. It has to be a PNP in this case, please correct me if I am wrong.

    On the UCC28710 we could make it work but failed EMI immunity at 20V/m. We are having trouble keeping the sense signal clean, so we decided to try this IC to see if it is more immune to noise and transformer leakage.

  •  I have a clearer picture now of what you are doing now.

    The UCC28710  primary side controllers are difficult to use with multiple outputs. The problem is that the controller looks for the "knee" on the sense winding and this will depend on whichever of the outputs is least loaded. The end result is that it doesnt regulate very well. The UCC28740 is simpler in the sense that it is directly controlled by the current into the FB pin.

    So in that sense it will give more consistent voltage regulation.

    Im not sure whats going to happen in terms of EMI susceptibility. Board layout and packaging are going to be significant factors influencing the performance. Hope this helps.

  • John, thanks for your replies. We will give this a try and hope we do better with the UCC28740. We have tried this IC with an optocoupler at 20V/m (eval board) and worked with no problems. We hope it will do the same on our board.

    Thanks!