Use of NTC thermistor and Bleeder resistance

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Hi, I am little weak in power supply design. But I am attempting to do as it seems all the things are made easy by PI. I am making 15VA power supply with 5V/2A 26V/0.2V output. But I could not see any information about bleeder resistance at output and NTC thermistor at input side in the design examples and the output of PI-expert-7. How can I get that? navin

Hi Navin,

an NTC in the input is generally not required for low power designs - the inrush current stays below the peak current rating of the rectifier diodes. As a general rule below 47 uF of input capacitance it's not required.

Output preload. For a single output design this is typically not required for PI designs - our Ecosmart modes eliminate the need for pre-load by reducing the power delivered to the output. For dual (or more) outputs a pre-load may be required on the output from which the feedback network is not connected - especially when one output is heavily loaded while the other is lightly loaded. This is due to the effect of leakage inductance peak charging the output that is unloaded. The best approach is to start with a resistor that gives a current that represents 1% of the full output current. Then adjust up or down until the voltage under worst case conditions is acceptable.