SMPS thermal stability issue - Page 1
Forum section: electronics / projects

author: vitalis
author_rank: Newbie
timestamp: May 07, 2020, 04:26:11 am
content: Hello everyone! Recently I've decided to implement a power supply (60V, 600W) following to the schematics that is attached. PCB is hand made, at the beginning something has fail and ZD2 was burned out, as a result NCP1216 was also damaged, so I had to replace them both. Finally the PWM is working fine, at least it seems to be so, and I am able to adjust the output voltage (100k trimpot on H8: 1, 2 pins), but... R26 (2W) is overheating in a minute, so there is a risk to loose output caps, etc.I feel there is something wrong with the current sense or feedback cascade. I have no enough knowledge to understand what I did wrong.Any tips appreciated! thanks

author: Etesla
author_rank: Regular Contributor
timestamp: May 08, 2020, 04:02:22 pm
content: The purpose of R26 is to provide a minimum load to the supply, and to bleed off the output capacitors once the power supply is turned off. At a full output voltage of 60V, the power dissipated by R26 is V^2/R = 60*60/2000 = 1.8 watts, which is very very close to the 2A rating. A typical 2 watt resistor has a thermal resistance around 70K/W, giving you a temperature rise of about 70*1.8 = 135 degrees K, equal to 135 degrees C, which puts your resistor at a final temperature of 135+25 = 160 degrees C, or 320 degrees F, which is smoking hot. If you want this resistor to get less hot, use a higher power rated resistor that's still 2000 ohms, or increase your cooling efforts. I wouldn't mess with the resistors value as this power supply was probably designed with a minimum load requirement. As you said, it would also would be smart to get that resistor as far away from your output caps as possible to increase their lifetime.

author: vitalis
author_rank: Newbie
timestamp: May 08, 2020, 08:39:21 pm
content: @Etesla, thanks. "with a minimum load requirement" - good point. I had an idea to lower the stand-by voltage by changing R15-R16, and playing with the feedback. Also it is not clear how does FB works on the output side. I've tried to calculate the different cases on output between 40 and 60V, with an active load and without, but no luck so far.When the output voltage is 60V, IC2 cathode-anode voltage is 0.8V, and the feedback voltage is -0.3V.that looks quite wrong if I understand the NCP1216's datasheet correctly.Have no clue where to look, some knowledge is missing. Please point me on that if you have an idea

author: temperance
author_rank: Super Contributor
timestamp: May 09, 2020, 01:45:23 am
content: If this is your first attempt at designing a switching power supply? At least that's what it looks like. The gate drive is completely wrong. The mains input is wrong...The circuit itself is extremely easy to debug for someone with enough knowledge to design an off line power supply.Suggestion: first try something more basic before you kill yourself in an attempt to get something like this to work properly.

author: vitalis
author_rank: Newbie
timestamp: May 09, 2020, 02:10:51 am
content: correct. This type of power supplies at the first time. Thanks for the tips

author: temperance
author_rank: Super Contributor
timestamp: May 09, 2020, 02:45:46 pm
content: Here you'll find some hints on driving the MOSFET's and also on how to draw a readable schematic:https://www.st.com/resource/en/application_note/cd00043746-300w-secondary-controlled-two-switch-forward-converter-with-l5991a-stmicroelectronics.pdfhttps://www.onsemi.com/pub/Collateral/NCL30125-D.PDFAn app note on how to drive MOSFET's:http://www.ti.com/lit/ml/slua618a/slua618a.pdfFirst try to run experiments at much lower voltage. Try a 24VV/24V DC DC converter for example. Study the details of the waveforms and try to explain everything you see. Gate drive waveforms, drain-source voltage, current sense signal, diode recovery and how it impacts your design, study the power dissipation in each power component,...