TNY274PN explodes after removing one of noise suppression filter Capacitor(C2)
First of all I would like to thank you for your great products and kind related supports.
Now I have used PI expert Software for Designing Power Supply module in order to get 5 Volt DC with 250 mA output. It is has a wide range input(85-270 VAC) and in our country the nominal input voltage is 220 VAC.
Attached, you will find the related schematic image.
My question is, What is the exact functionality of element C2? I know that C2 with L1 are for suppressing differential noise.(If I am right)
But when I remove C2(in order to reduce circuit size) definitely, after plugging input to 220 AC, the TNY274 will be exploded, and seems that the Voltage(or current) increased too much that caused the TNY274 and other passive element like RF1 and L1 to be damaged!!! (TNY explodes and RF1 became open! In less than 1 second)
Thank you again for your kind attention.
Looking forward to hearing from you.
Comments
PI-Worf's post is excellent advice of what you should do, but here's why I think it exploded without C2:
With C2, the TNY274 and transformer see a (approximately) constant voltage input. As theTNY274 switches, the current drawn from C2 switches between 0 and the primary transformer current. L1 is feeding a constant current into C2 that is the average of the current drawn from C2, so C2 only sees the AC current.
When you remove C2, L1 is attempting to provide constant current to the transformer and TNY274. When the TNY274 tries to turn off, there is no where else for the current to flow through L1, so the current keeps flowing through the transistor even as the voltage rises. Since L1 is 1mH (a relatively large inductance), the current will not fall to 0V before the voltage rises over 700V and the TNY274 fails short-circuited.
Once the TNY274 fails short-circuit, you have line voltage with very little resistance to limit the current flow. RF1 should fail open-circuit fast enough to prevent a fire (although likely not fast enough to prevent more damage to the circuit).
If you remove C1 instead of C2, the TNY274 shouldn't explode like that, since it will still see an approximately constant voltage (instead of constant current from the inductor). However, as PI-Worf said, C1 and C2 also filter out the 120Hz ripple, so you may still need the full 4.4uF to have the circuit operate properly (as opposed to just not exploding).
I really appreciate your great analysis!!! Thanks a lot.
Somehow, I found that the voltage increases too much, but I could not analysis the whole story as you did so perfect.
Thanks for your support and also thanks for PI-Worf's post too that caused me to pass the problem.
The main function of C1 and C2 is to filter the 120 Hz ripple at the output of the full bridge rectifier. If you do not have them, then your circuit will crash as you already saw. There is a secondary function of the two input caps and the inductor L1. They form a PI filter to reduce conducted EMI.
You can use a single bulk capacitor but you need to remove L1 from the circuit and double the value of the single capacitor In other words. Instead of having 2 caps 2.2 uF, have only one but 4.4 uF. Of course this change will affect your conducted EMI.