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UC3842

Former Member

I like to know how can I make "hi cup" protection on SMPS flyback based on UC3842.

Secondary feedback with TL431 and FB pin is placed to ground.

The COMP pin is pull up with resistor from VREF.

In the COMP pin we connect the optocoupler.

We see from a lot of people place a series resistor from bias to vcc for power supply, is this the solution? make a UVLO?

 

Regards,

 

J.A.M.

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  • 0

    Hoi Jose,

     

    I don't know exactly what circuit you use but a nice way of making a hiccup mode is to pull the ''Comp'' pin to ground (Vcomp < 1V). You can achieve this by adding a discrete SCR (thyristor, 2 transistor circuit) and sensing on both the auxilairy supply voltage (Vout too high -> Vaux rises too high -> fire scr) and (Vout shorted -> Feedback too low -> fire SCR). You can add an RC-network with a considerable time constant (eg 22uF / 47kOhm) to keep the SCR activated for a long time when fired, resulting in a low hiccup frequency. If the above is to not clear enough i can upload a simplified diagram (if you want).

     

    Best regards,

     

    Enrico Migchels

    Power Conversion Design Engineer

    Heliox B.V.

    Best - The Netherlands

    www.heliox.nl

  • 0 in reply to enrico.migchels

    Hello Enrico, very nice response, is the same circuit I have, but with problems. I explain, I place two bipolar small signal transistors, a classic BC547 and BC557 with same letter for same gain configurated as SCR.

     

    I place a 62 k 1% resistor in combination of electrolic capacitor 10 uF for timming hiccup, but I have the problem that the power supply not operate well at full load. The specification of the smps is : 144 W 24 V on the output ( CCM ) and 230 Vac in the input. If you prefer I send to you a schematic for our power supply for better explain.

     

    I dont sure if I need make a ramp compesation for not get RHP and get unestable smps.

     

    I wait for your comments.

     

    Regards,

  • 0 in reply to Former Member
    Enrico your company have any relation with old philips or bobitrans in Spain?
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  • 0 in reply to Former Member

    Hi Jose,

     

    Understuud. It seems that your solution is pretty straight forward (except for the full load problems). You can upload the schematics, with some basic background information (any specific parameters of the PSU, nom/max/peak load and transformater values, Lp, turns-ratio). I have worked with the UC3842 controller in the past, but not recently as we now use the benefits of 'green' controllers which have a Vds valley switching and SOPS characteristics.

     

    Heliox B.V. does not have any legal connection to Bobitrans Spain and Philips but i worked at former Bobitrans Power Solutions and Philips Power Solutions, EPM,  Philips Power management. Bobitrans Power Solutions stopped their activities in the Netherlands a year ago. What is your relation to philips/Bobitrans?

     

    Best regards,

     

    Enrico Migchels

    Power Conversion Design Engineer

    Best, The Netherlands

    www.heliox.nl