UC3842

Question

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.

enrico.migchels · Accepted Answer

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