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    Problem with LT8304 simple circuit flyback converter

    Hello all,
    Thanks for helping. I have been fighting with this circuit for a few days but I can't figure out what's wrong. It's a 12V flyback converter, I'm using LT8304 IC (doesn't require optocoupler or 3rd winding feedback) following the exact circuit and exact the same components from the datasheet (picture attached). It's a pretty simple circuit. I get the 12V at the output, It requires a minimum load to regulate (12mA, I have 1k resistor as load) but when I add more load(even another 1k resistor) the voltage drops and it's not delivering any current to the load. My input voltage is 30V.

    I checked and replaced the output diode, added more capacitance to the output but nothing works.

    Help will be much appreciated,

    Regards,

    Javier

    Click image for larger version. 

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    •   Alt15th February 2018, 01:00

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    Re: Problem with LT8304 simple circuit flyback converter

    have you got the right transformer and is the phasing correct? is your power source too limited? too much leakage on the Tx can interfere with operation...



    •   Alt15th February 2018, 01:46

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    Re: Problem with LT8304 simple circuit flyback converter

    Fortunately the LTspice simulator has this component in their library, and an example circuit is available on the manufacturer's website using this component:

    http://www.linear.com/solutions/7297

    Which, after few modifications to comply with the example of datasheet, it just confirms that at least in theory the values appear to be correct. Maybe the problem is with the assembly or the value of the components you are using could be different from what should be.

    Click image for larger version. 

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    Find attached the file used for above simulation, if you want to make experiments by yourself.
    --------------------------------------------------------------------------------------------------
    Part of the world that you live in, You are the part that you're giving ( Renaissance )



    •   Alt15th February 2018, 01:50

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    Re: Problem with LT8304 simple circuit flyback converter

    Hi Easy peasy and Andrew,

    Thanks so much for your replies, I gotthe transformer recommended in the application circuit WURTH 750315126, I checked my power supply directly with a load (can source up to 5 Amps without problems). Andrew, thanks so much for your simulation!! I changed the Rload to 12 ohms and shows that it should deliver 1A (attached).

    I checked again and again and I still can figure out why the circuit can't deliver current, I replaced the LT8304 but still the same problem, the circuit has only a few components so I don't know what to revise now - all the components are the ones recommended on the datasheet. Today I run a new test,I changed the transformer for a 24V ( WURTH 750315836), following the circuit on the datasheet( components don't change as the Vout depends on transformer ratio that changes to 1:1), I got the same problem, output voltage is ok but when I connect any load (2k resistor)the voltage goes down.

    Any ideas of what could be wrong? since the circuit is so simple i feel that I'm missing something silly...?



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    Re: Problem with LT8304 simple circuit flyback converter

    Did you purchase a genuine LT device? Most of the stuff on eBay, Alibaba and from other Chinese vendor sites are cloned rubbish. We once struggled with a customer design that could not make the performance, only to find some of the LT1028 opamps in the design was purchased through eBay. After we got some good parts, everything behaved as predicted.



    •   Alt20th February 2018, 02:33

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    Re: Problem with LT8304 simple circuit flyback converter

    It should work if you were to create your own pulse train which has variable frequency and duty cycle. This serves as a control circuit, biasing an N-device (in place of your IC). This N-device ought to be robust enough to endure voltages much higher than 30V. Even with your snubbing network there may be high voltage spikes coming from your transformer.

    To be safe attach a load before you power up. If load is absent then the output capacitor may charge up to destructive volt levels quickly after power-up.

    Experiment by changing frequency and duty cycle.
    The aim is to see if you can drive a light load to 12V.
    Attach heavier loads. Monitor components continually for rising temperatures.



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