How Can I Optimize the transformer for this SMPS - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 15, 2020, 11:56:19 am
content: Hi,I have designed a flyback converter based on LP3783B,the schematicsI have get the design spreadsheet from the manufacturer, and I put some numbers in it for my EFD15 core,everything is working, But If I get more than 500ma from output @ 240V AC, the controller would over heat and Over temperature would kick in about 10 minutes of for example 1A of output current,How can I Get something in the range of 7-9W out of this puppy with the EFD15 core,any Ideas?I have attached the design spread sheet for your reference.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 16, 2020, 09:54:57 am
content: Any Ideas? specially our Chinese members?

author: TimNJ
author_rank: Super Contributor
timestamp: September 17, 2020, 02:45:17 am
content: What's the efficiency at the problem load conditions? As blueskull alluded to, high leakage inductance will give you bad efficiency. Can you tell if your snubber is getting hot? (Don't touch it please.)Since the controller has an integrated BJT switch, and it's getting toasty at low current, it might be related to high switching loss, or an issue with the the BJT not getting sufficient drive current to turn on fully or quickly enough. Check if your Vcc is running low. I would imagine that if the Vcc is above the UVLO threshold it should be okay, but who knows.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 17, 2020, 06:24:42 am
content: QuoteThanks for the help, I have redesigned a new transformer, it's a bit better and can do about 650mA, the new spread sheet is attached,The measured primary inductance is around 630uH and the leakage is around 12.8uH, Both measured at 100Khz.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 17, 2020, 06:26:46 am
content: QuoteThe VCC is around 14-15V, How can I measure the snubber temp?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 17, 2020, 08:46:40 am
content: QuoteI have done it, it's about 12.66uHQuoteI have used this cap (0805 1kv 0805B102K102CT) for the snubber, It's small and High voltage! How can I measure it safely? should I use my scope?I have a 200W isolated transformer to power up the scope, but I have a 10x probe only.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 17, 2020, 10:20:02 am
content: Ok, I managed to measure the voltage on the 0805 part with a multi-meter,Also I have note a bug, the RS3MF was reverse mountedSO I corrected it too,Now it can deliver more to the output,The snubber cap voltage is 81V, But I noticed something,I can get around 6.5W form this with a drop in the output voltage, when the output is 4.5V, the output current is 1.5A, But I can get this 6.5W power at the input of around 180VAC, if I increase input voltage to 245VAC, it would overheat and switch of the output, after a few seconds it would turn back on again after cool-down.But if I lower the output load to around 4.5W, it would work in room temperature at 245VAC input,So Increasing the input voltage would make the dissipation higher and would cause the over temperature to kick in sooner,So how can I get more output? about 7-9W,Is it possible with EFD15?what about the snubber? is there a way to optimize it?I have measured the snubber voltage with 30Vac iput using my scope too, it was around 70V with multimeter and in scope it has peaks of around 110V.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2020, 11:36:09 am
content: If the relative moderate increase in voltage (180 to 240 V) causes so much trouble, this may be due to saturation. or getting too close to saturation. Even if not all the may in saturation the magnetic properties get nonlinear when at high magnetic field. One could check the secondary side of the transformer with the scope.