INN3675C forward pin

For example, coudl the attached circuit be added to INN3675C, so that its FORWARD pin does not get overvoltaged when small np/ns values are used?

LTspice and PNG file

Hi, please find a better schem of the FORWARD pin clamp for innoswitch. Do you think this will be OK.

It appears that the innoswitch simply compares the voltage on the FORWARD pin, with the voltage on the VOUT pin, and when v(FORWARD) > v(VOUT), then the Innoswitch realises that the secondary has finished conducting, and so it therefore looks to turn on the FET at that time.......i am sure you would agree that the shown clamp wont interfere with this behaviour.

Hi, please find a better schem of the FORWARD pin clamp for innoswitch. Do you think this will be OK.?

It appears that the innoswitch simply compares the voltage on the FORWARD pin, with the voltage on the VOUT pin, and when v(FORWARD) > v(VOUT), then the Innoswitch realises that the secondary has finished conducting, and so it therefore looks to turn on the FET at that time.......i am sure you would agree that the shown clamp wont interfere with this behaviour.

Attached is a better way of getting around the forward pin limitation while still retaining quasi-resonant operation.

https://www.power.com/design-support/design-examples/der-903-72watt-charger-93pct-efficiency-using-innoswitch3-cp-powigan

Thanks, though that needs a split output winding...as such, it means more expense in the transformer. Please could you evaluate the circuit that i provide above at all?....which is for protecting the FWD pin from overvoltages.

Well, if you place a zener in series with the FWD pin, you will lose QR function at high line, and can switch at either a peak or a valley (or any place in between) on the discontinuous ring at high line, making thermal performance unpredictable. You can try this and see how well it works for you.

Thanks, so thats like any DCM flyback?

Well, it's like any DCM flyback that doesn't have the advantage of valley switching.

Thanks.

We need to do a 24vout flyback from 90-265vac. 20W. 

We are stuck with using the old NP/NS=84/19 (bias = 14 turns) transformer that we have thousands of, and was used for the previous one , whose (non PI) controller now obselete.

This txformer is not interleave wound.

So we think we can use Innoswitch if we raise it on a plinth so we can gap-pad it to chassis.

So , the referred voltage to sec is 373/ (84/19) = 84V...but as you know leakage ring can make this = 2x84v....=168v...and the innoswitch FB pin is only 150v rated.

So , may i ask can we use the above zener/BJT circuit to clamp the ring to the FB pin?

Its only 20W and we dont need Boundary conduction mode...just straight DCM at high mains, and CCM at lowest mains, is just fine.

We do like the output current clamp capability of innoswitch.

Please tell if this is OK?

We also see innoswitch has no external  feedback compensation components connected to it....so will it internally just work out the feedback control?

You will have to try it and see. This is a very non-standard use of the forward pin.

Thanks, surely its got to be possible ?

I mean we are only clamping off the leakage spike, not the actual reflected voltage.

I agree we wont have boundary switching any more but that doesnt bother us.

Neither PI Expert or PI XLS allows us to put in our 84/19 transformer though....(Lp=1.07mH)...Do you know why?

Please forward the details on the transformer - core, primary, secondary, and bias turns, and inductance, and I will check in PIXLS. I've been using PIXLS to design transformers for 10-15 years, and it hasn't steered me wrong.

Thanks, NP/NS/NAux = 84/19/14

Lp = 1.07mH

Llk measured as 70uH from pri with sec shorted.

Vin=90-265vac

Transformer is not interleave wound. (thousands of them already made, so cant change it)

Transformer is layered Primary, then Aux, then sec is the outer winding.

Vout = 24v

Power = 20W

(The FSGM0565 is currently doing this at 65khz, but its gone obselete)

Currently, Cin is 47uF 400v.

Cout is 1mF, 35v

Core type/size?

Thanks,  i have just been given a transformer with a datasheet which doesnt say the core details or core type etc........just i was told it has been in use with the FSGM0565 for 10 years and no probz.....but i think its an E25/13/7.....its potted in an plastic box which is 30mm x 30mm, so i cant even see the core......they wont let me disassemble one....they say its worked for 10 years so i shoudlnt need to know about it...

This is an obselecence replacement project for a 20w flyback which used to use the FSGM0565

In any case, to be honest, i know i can make it work with innoswitch, and dont need you to do a design......just i am asking can i clamp off the FORWARD pin in the manner i describe in the above schem?....ie, with the NPN and zener clamp.?

I dont need the boundary conduction mode, and dont need a sync rectifier fet.

(The txformer has done 40W 24v easily from 90-265vac input, and no ski sloping of primary current...in this app, we only need 20w.)

As I explained previously, this is sufficiently far afield from what we usually do that you will need to try it and see. 

Oh, and since you aren't using a synchronous rectifier, please remember to ground the sync rec drive pin. It looks for that condition at startup and allows use of a diode instead of a sync rectifier. If that pin is left open, the device may not start up properly.