Hello all,
we have used ucc28704 for our AC/DC 5V@2A adapter and we have "copied" the schematic of "UCC28704EVM-724 10-W USB Adapter", the same BOM and the same transformer (750315841 Wurth Elektronik). We have also followed the layout recommendation. Our issue is that the adapter doesn't work like the "UCC28704EVM-724 10-W USB Adapter User guide" says and after tunning some components values we are able to reach 5V@2A but after some seconds, output capacitors and transformer get hot (more than 65ºC) and Vout falls down.
The first time that I connect our adapter Vout is around 3V, I thought the issue was Vdd or Vs, after discarting Vdd I have to reduce Rs2(R12) from 36.5K to 19K in order to match Vout at 5.1V.
When I try to check the maximum current with an electronic load I realise that it protects itself at 0.9A, I appreciate that the switching frequency is 70-85KHz (measuring with an oscilloscope in the anode of the diode of the secundary side). So I think that it is in overcurrent protection.
The next step is to reduce Rcs(R15&R16) from 1.075 to 0.717 Ohms and the maxium current incrase up to 1.5A and 70-80Khz.
After reducing a little more Rcs up to 0.54 ohms, the maxium current incrase up to 2A but 45-50Khz but after some seconds the adapter protects itself.
I reduce RLC (R13) from 715 to 220 ohms like formulas says and I had to increase R10 from 20 to 50 ohms due to Vdd was bigger than 35V.
1. Is this tunning right? I would say that if Rcs is reduced more than 0.8 Ohm, Ipp is increased above the current satuuration of the transformer (800mA) and that is no good.
2. How could I fit a little more in order to reach 2A properly?
3. Is possible to fit the components values to have 2A with 70-85 Khz?
Thanks in advance!
P.D: I have checked everythig several times in order to avoid mistakes