early feedback triggering
Hello,
I have been developing a 2.5W universal supply... It seems that during power up transient and under heavy loading the feed-back will trigger early at 13.5V to 14V instead of the 15.5V that the (LM431 & OPTO) is set to, the problem must be noise or ripple related?
The strange part is how the TNY picks up the "noise trend" and believes that the feedback is satisfied perpetuating this lower voltage... Problem corrects its self with changing in loading or does not happen at all with a slow ramp up of loading.
I can comprimise regulation accuraccy to eliminate this noise issue... should I go back to a simplistic Zener+opto config?
Matt Schuping
North Shore Safety
please contact:
mschuping@nssltd.com
440-205-9188
Comments
Hello Z.
I did lower the 10K further 3 20Ks... It seems to make the problem even worse?
Matt.
The LM431 can have upto 1 mA of bias current. 20k/3 * 1mA = 6.67 volts. This would bias the opto diode and cause the power supply not to regulate properly. You may have other issues, however the first thing you should do is reduce the 20k to 1k.
Z. Cochrane
The first thing I would try is to reduce the resistor value across the opto diode from 10k to 1k. This allows the LM431's bias current to flow through the resistor and not the opto's diode. This will help maintain regulation.
If you still have the problem, check the LM431 circuit. If the output voltage is too low, then the voltage at the LM431's reference pin should be less than 2.5 Volts. If this is the case, then the LM431's Cathode should not conduct current (other than the bias current 1 mA).
Z. Cochrane