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I have designed the SMPS circuit with the help of flyback topology from Power Integrations.LNK3696P IC We get 2 outputs 12 V/400 mA and 5 V/500 mA [2 A at peak]. I am facing difficulties when I apply load at any output end of circuit. when I apply load at 5 V I get 15.6 V at 12 V output side. Please check the circuit. how can I minimize/eliminate the cross regulation.

Note: for Load I am using resistive load i.e. 10E/25W. is this because of wrong resistive load?

Thanks in advance.

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    \$\begingroup\$ You’ve hit upon a design feature of this configuration. Simple solution - add a post regulator. \$\endgroup\$
    – Kartman
    Aug 30, 2022 at 11:16
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    \$\begingroup\$ Have you tried simulating it with measured leakage and magnetizing inductance? How was the cross regulation? This is a common issue to solve in flyback converters and you tend to end up with a compromise or post-regulation as Andy suggests. \$\endgroup\$
    – winny
    Aug 30, 2022 at 11:46
  • \$\begingroup\$ @winny No I have not simulated it. I tried to do it on PI site but It is not showing PI expert with this particular IC. If I want to simulate this schematic then where can I simulate it ? \$\endgroup\$ Aug 30, 2022 at 12:16
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    \$\begingroup\$ LTspice or Falstad. If not with the real IC, something similar or if that's too show, fixed duty cycle. Won't give ideal results but you will see the effect. Do you have all the transformer parameters, especially leakage inductance. \$\endgroup\$
    – winny
    Aug 30, 2022 at 12:32
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    \$\begingroup\$ The true "regulation" is made on the 5V side. So to maintain a somewhat "regulated" voltage on the 12V side, try just a "bleeder" resistor (minimum current), but remember it is not a "stabilized" voltage. \$\endgroup\$
    – Antonio51
    Aug 30, 2022 at 12:42

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when I apply load at 5V I get 15.6V at 12V output side

Yes, this is an annoyance with simple fly-back converters that have dual (or more) outputs. Your regulation circuit is teed off from the 5 volt output hence, when that output gets loaded more, the opto and controller counteract the losses in the circuit to maintain the output at 5 volts.

But, they do this by activating the transformer primary for a longer time. That transfers more energy per switching cycle keeping the 5 volts happy but, the 12 volt output rises (as you have seen). You might find that the flyback transformer coupling factor is not as good as you would want it to be or, you might find that the DC resistances of primary and secondary are a little high. Not sure if you designed your own flyback transformer so I can't really comment more in that area.

So, with the basic circuit, all you can really do is tighten up on the transformer by making primary-secondary-secondary coupling improvements or by increasing winding cross sectional area to reduce resistive losses.

how can I minimize/eliminate the cross regulation

There are some compensation techniques but that involves another wound component on both outputs to reduce the 12 volt output when load current is taken on the 5 volt rail. I've seen it done so I know it works but, with that method the best improvement I've seen reduces the problem by about 50% or 60%.

Another thing to consider; make the 12 volt output more like 15 volts and use a buck regulator to bring it down to 12 volts DC.

Load I am using resistive load i.e. 10E/25W

I believe 10E means 10 Ω so yes, the load is correct for producing 0.5 amps on the 5 volt rail.

If you need anything clarifying then take the two minute tour and start the process of accepting the most appropriate answer to your earlier questions. You also have sufficient reputation to upvote good answers. That is the currency on this site for guaranteed future help on new questions.

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  • \$\begingroup\$ I really appreciate your answer. I am connecting Latching Relay of 12V at the 12V output. and on 5V I am supplying it to the controller. we designed it with the help of PI team. what are the compensation techniques? I want stable output at both ends. what is the reason for going 15V and not 12V. how can I make it to 15V output?. If I use Buck regulator then it will increase my BOM. Is my schematic correct? what are the solutions to make it cross regulation free. \$\endgroup\$ Aug 30, 2022 at 12:14
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    \$\begingroup\$ Please carefully read my last paragraph. \$\endgroup\$
    – Andy aka
    Aug 30, 2022 at 12:16
  • \$\begingroup\$ What are wound component ? \$\endgroup\$ Aug 30, 2022 at 12:17
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    \$\begingroup\$ Please carefully read my last paragraph. \$\endgroup\$
    – Andy aka
    Aug 30, 2022 at 13:32

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