Innoswitch3 offline flyback controller IC...heatsinking - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: December 02, 2017, 03:10:49 pm
content: Hello,Innoswitch3 offline flyback controller says it reduces space by sitting under the transformer on the PCB…(00:34 of the video)…….In this case, where then does the heatsink go?Also, “under the smps transformer” is usually out of bounds for a pwm controller IC, and this is because the switching node is inside the transformer, and you don’t want the pwm controller directly under it. (I appreciate the switching node also comes into the innoswitch pin, but it can be kept to the side of it, and not get near the internal sensitive circuitry.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 02, 2017, 04:36:05 pm
content: QuoteThe heatsink is the large source tab on the primary side and goes to the 0V copper on the primary side. As far as the primary side is concerned they are just like the LinkSwitch chips and they don't run very warm anyway, a 15W supply doesn't even need a heatsink. I stuck a finger onto the top of a linkswitch chip only last week just to see how warm it was running, to be honest I think the transformer and primary rectifiers were warmer, warm being around 50C. Before you going jabbing fingers at things on the primary side make sure you use an isolation transformer.QuoteThe switching node is not inside the transformer the switching node is the drain pin on the package. What exactly do you mean by "inside" the transformer, your choice of words could be better. If you're thinking of leakage flux from the transformer then Power Integrations have already got that covered. The reason you don't put components underneath the transformer is for safety i.e. creepage and clearance and I would hope that someone working with off line supplies would have learnt that by now, it's fundamental. BTW Power Integrations Innoswitch, very nice design, amazing what you can do with an asic and the really innovative bit is the magnetic link in the lead frame.

author: ocset
author_rank: Super Contributor
timestamp: December 03, 2017, 08:02:17 am
content: Thanks, in a flyback, the switching node does connect to the transformer, as you know.In our application, We need an actual heatsink, rather than the thin plated copper on the PCB.You are obviously correct when you say the isolation boundary must be respected, but as you know, the non isolated side of the innoswitch "is" under the transformer, so on the bottom side, things can go under the transformer, to a degree, and still allow the regulatory isolation barrier to be respected.Quote…the interesting point is looking at the gap on the body of the innoswitch, between the drain and source pins on the innoswitch…it is 3.35mm…which is what it needs to be for our lovely regulations.The interesting point is when we look at the gap (between drain and source upper legs) on the body of a TO220 FET, used in a vast number of offline power supplies,…this gap is 1.13mm…and a violation of our wonderful regulations. We wonder, sometimes, have these regulations been accosted by large corporations who are muddying and over-complicating our wonderful regulations for the purpose of keeping small startups out of the market?Also, you did bring up the subject of regulations, thankyou for that…here is a clip on these wonderful regulations……if you wish you only need watch from 3:43 to 4:43...please note the lobby groups at 4:25 of the above video...we wonder, just what is all that about?

author: ocset
author_rank: Super Contributor
timestamp: December 03, 2017, 08:35:40 am
content: QuoteThanks, i'd be most grateful if you could copy/paste where it says that in the regulations?Though i believe we can all see that in the case of the TO220 FET, the "exemption" is pretty non sensical... we both know that the clearance being on a fet body, and the clearance being the same on the pcb, is pretty much the same thing.There's a website which talks about this in more detail, one of the RF websites, but i cant find it now.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2017, 08:47:50 am
content: IEC 60950-1 contains rules for potted materials.I'd happily quote paragraph and section, but I'm not being paid that kind of money here...Tim

author: chris_leyson
author_rank: Super Contributor
timestamp: December 03, 2017, 10:08:22 am
content: If you want off line say 85V-265V at 15W to 25W or so you could try a LinkSwitch-HP the E and V packages have an exposed pad for heatsinking and primary side voltage sense gets rid of the optocoupler so saving some space. Pin spacing on the V package is just adequate for clearance and demo boards are available Farnell and Digikey. Wurth do lots of small off line flyback transformers if you don't want a custom wound part but I think only 15W for EE20 or similar core.

author: ocset
author_rank: Super Contributor
timestamp: December 03, 2017, 10:49:55 am
content: QuoteThankyou for telling of this.......and i am sure we agree what a nonsensical arrangement it all is....this website speaks of the TO220 conundrum....http://www.smps.us/pcbtracespacing.htmlhere is the quote...QuoteA TO220 fet is directly soldered to the PCB.....it should have the same rules, but doesnt....i mean, what if i make a little daughter PCB off the main PCB and violate regs on that daughter PCB...is that going to be ok?.....kind of like its "OK" on the body of the TO220.As you know, i am not disagreeing with the good writers here, this is the mess we all face.