problem with TOP222 Circuit
Posted by: homer5785
on
Hello! I refer to this Thread some weeks ago: http://www.powerint.com/en/forum/power-supply-design/replacement-top221
Now I have the SMD-prototype of this circuit but it didnt work. I need a Current of about 0,2A at 15V.
If I don't connect a load, I get a acceptable output voltage of 18V. I tried to connect a 1k Potentiometer but already at 1k the voltage broke down.
I use a Flyback Transformer with a primary inductance of 670µH and a winding ratio of 2,66.
What can be the cause for this problem?
Thanks in advance.
Andi
Comments
Sorry for the late answer.
1. the problem was the transformer, I connected the windings the wrong way. ;-) Now the application works.
2. The voltage of 18V is okay for my application. But you calculated the Resistor much higher than I had. Should I change this resistor? What is the right dimensioning? In the data sheet I found a supply current for the Control pin of about 1.2mA.
3. I paid attention to the design rules in the data sheet. Although I get a bad 100kHz ripple in my supply voltages. Maybe this is related to the second point?
Thanks again for your help
Andi
Nobody an idea?
Hmmm is this so difficult? Or why nobody answers?
Hi Andi,
First I would make sure that your transformer is properly wired as a flyback transformer, windings out of phase.
Secondly with a 12 V zener diode connected to the control pin your circuit will never work at 15 V. The reason is that the control pin voltage is about 6 V so with your 12 V zener your output voltage would have to be above 18 V to deliver any current into the C pin. If the output voltage drops below about 18 V at any time no current can be delivered to the C pin and you will lose regulation.
Assuming your transformer is correct, replace the 12 V zener with something more like 8 V and choose your resistor accordingly. Some like 15 V - 8 V - 6 V = 1 V at a maximum of 4 mA control pin current, R = 1 V / 4 mA = about 1 kOhm.
Hope this helps.