TOP243Y Low Temperarure Too Slow Rise Time
Dear PI Expert Support,
Our power supply, which is
Universal Input
TOP243Y
VIn 47uF
Feedback TL431
Output Voltage = 3.3 V, 10W
The power supply was designed using PI Expert Software.
works well at normal room temperature but at low temperatures (<0°), if it is powered after being left unpowered about an hour, it outputs 3.1 V just after the power-up and increases with an 0.01V/s(approximately) slope toward nominal output voltage 3.3V. After waiting a while and restart, the rise time becomes normal again. We think this is about self-heating effect. But the problem is about cold-start-up. It is really weird. It causes our system not to power-up normal because of abnormal voltage rise time.
Dou you have any suggestion for the problem?
Best Regards,
Comments
I further investigated the problem by measuring different points in the PS.
There are some strange oscillation at start-up at TL431 reference pin and over opto-coupler LED. It was like the current passed through opto-coupler is oscillating and not stable during 10 to 15 seconds after start-up and becomes stable after that time and so does the reference pin voltage but it is weird that the output voltage goes to 3.1 V with a sharp slope and rises slowly to final value 3.3V while the opto-coupler current is oscillating. As I wrote earlier, this occurs if only the PS is too cold (-15 C) at start-up.
The opto-coupler has an CRT of %100 at 1 mA IF. The bias resistor is 68R and has an 500R parallel resistor to opto-coupler LED. The regulator is TLV431A and the post filter is 3.3uH and 220uF electrolytic cap.
What may cause that oscillation at start-up when the PS is too cold?
Regards,
Koksal
Hi,
Please verify the rated operating temperature of all the parts used. If any of the parts are not rated for operation below 0 deg.C, they may cause unexpected operation until the parts warm up due to self heating.
One other possibility is that your power supply has low phase margin and operation at low temperature causes the optocoupler gain to rise which causes instability. Have you verified the gain and phase margin of your power supply? PIExpert assumes certain values of parasitic elements such as ESR of the output capacitor. In real life the values can be slightly different so it is important to measure phase margin.
Regards
PI-Sarek
This sounds more like a problem with your secondary feedback reference and error ampflifier circuit and not a cause of TOPSwitch