Switching transformer is hot - Page 1
Forum section: electronics / projects

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: April 03, 2021, 07:02:49 am
content: Hello everyone,I created switching transformer 39 turns primary , 16 secondary, effective are 211mm2. I calculate it for 360V to be at 250mT when having 50% duty cycle and 50khz frequency. Primary voltage is actually 300V so i should be at aprox 165mT.I used 0,5mm wire. One for primary, two for secondary. Peak power is 200W. Secondary voltage is 120V200W/300V ->0,67A200W/120V -> 1,67A0,5mm should handle 1,17A, If i take 6A for 1mm2If i remember correctly primary lenght was 1,5m and secondary 0,75m. Aprox...I already created thread at electrotechonline, but more people more know...Here is link to created threadhttps://www.electro-tech-online.com/threads/switching-transformer.161273/I think winding is getting hot. I took measurement with current sense transformer and values are aprox same as my calculations up.Core losses at 50Khz 165mT should be around 3W.I though selected wire will be ok.Can someone refute it for me?Also i using 2 switch forward topology.Thanks for helpCore detailsair gap 0,2mm / without (I have both cores, same result)https://semic.cz/!old/files/pdf_www/CF297.pdfhttps://semic.cz/!old/files/pdf_www/Lj_297ETD4917_CF.pdf

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: April 03, 2021, 07:50:30 am
content: tbf, too many "i think..", "should be..", "iirc..." for my taste. shoot your assumptionsfor example, to find out as to how much the windings contribute to the losses: calculate the equivalent DC current for each winding (include the skin effect correction) and test the heating of the windings with a DC source. did you calculate the temperature of the transformer?also, more cooks in the kitchen won't make for a better cake.

author: strawberry
author_rank: Super Contributor
timestamp: April 03, 2021, 08:12:26 am
content: two 0.5mm (diameter) copper wires in parallel and kapton or maylar tape will dosnubbers/interleaved windings could help ringingelectrical tape gets soft and deforms/penetrates/degrades even at ~50C

author: jonpaul
author_rank: Super Contributor
timestamp: April 03, 2021, 08:33:40 am
content: Specific info missing:What voltage and insulation grade?Spec of ambient temo min/max, eg 0-60C, -25 - 85C?Place thermocouples at hotspots, center leg of core, middle of windings and that actual temp curves.Measure power in/out in watts. Losses = Pin -Pout.Use FLIR or IR thermal meter to check surfact temp.Jon

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: April 03, 2021, 10:10:27 am
content: How can i calculate temperature of transformer, please?I did other calculations.I think 0,5mm wire is ok for 50khz, its +0,0014 ohms in primary winding (1,5m long)I ordered capton tape. I know electrical tape is not good for insulation due bad temperature resistivity.Measured temperature, it gets to 60C+. When it rise above 60C, shut it down. Its about minute to rise it.Core losses are aprox 3W.I tested 5A through secondary, it gets to 40C in several minutes and imedietely cool down when touch it with finger...Tested 2,5A through primary, it gets to 40C probably in 5 minutes.I have no idea why its so hot then, since Ip= 0,67A and Is = 1,67AWhat about D3. What if back current flow through D1Also what is normal working temperature?thanks*Checked diodes (MUR6060PT) - OK

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: April 03, 2021, 02:16:01 pm
content: have a look here.https://www.e-magnetica.pl/doku.php/thermal_resistance_of_ferrite_cores