Hi, Please obselete TOPswitch...every co you go to thinks its the bees knees...but it has so many problems........please get rid of it so EE's like me dont have customers trying to force you to use it.......in this case for an FSGM0565 obselecence replacement.

When will TOPswitch go obselete?

So...

We are doing a 20W offline flyback. (FSGM0565 has gone obsolete)

We chose the thick-bodied  TOP256GN alternative because it is the package that our contractor thinks he can heatsink easiest to our setup. Power integrations don’t do ceramic TO220 like the FSGM0565.

Anyway, we now find that TOP256GN is not supported by their PI Expert or PI Designer.

We also realise that TOP256GN is a voltage mode controller. We are  fixed to a certain transformer, of which much stock has been bought.

At 90VAC in, the TOP256GN flyback will go into CCM……and it’s a voltage mode controller….the two together mean more  issues with feedback loop….because power integrations don’t seem to provide the modulator transfer function of TOP256, so how are we supposed to do the feedback loop calculation?

Not only that, but the TOP256GN needs us to supply the 2-8mA  of controller bias current via the feedback optocoupler (!)…..this itself creates issues with opto CTR when hot or aged.

You can add a NPN  in with the opto, Darlington style, to reduce the opto current...but you then have a wide toleranced NPN gain in your feedback loop...which is in voltage mode...

Also, the UVLO pin of the TOP256 is highly sensitive with a 4MEG resistor feeding into this sensitive UVLO pin…more issues that we don’t need.

Not to mention the fixed peak current limit which you cannot accurately vary.

Also, the TOP256GN has a big  load of hysteresis on the uVLO pin...so the part runs down to too low an input voltage.

Also, our primary peak current at max load , min vin, is just 0.82A...(to give 20W)...but the TOP256G forces us to have its 1.45A peak current........that corresponds to 43W of power output (for a 20W SMPS)...so our flyback is not even going to know its overloaded till its shipping >43W.......its going to well overheat.....and not go into overload protection.......who buys these TOPswitchs?

Does anyone know why power integrations TOPswitch chips are so popular?

I do like their Innoswitch, but would want it without the sync rect driver, and would just add the “hack” components so that you don’t have to have such a high NP/NS in certain cases. (to protect the sync rect sense pin). But innoswitch eSOP package is wrong for this job.

Can anyone give any reason why we shoudlnt just use a standard Flyback PWM chip with external ceramic TO220 FET?