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I am trying to optimize a thermal detection circuit. But there are several problems.

The circuit for detecting 85 Celcius temperature and then disabled a PWM IC. When the FB pin is below 0.5V, IC is disabled and it waits to unplug the cable from the AC line. Due to there are some components that draw some current, it prevents IC to wake up. For example, if I changed R1-10k and R2-1k, it can not start. The IC has an HV start-up pin and it feeds the IC capacitor(C2) from the HV pin. When the IC wake-up, it supplies from aux-winding.

Another problem is related to the comparator start-up. How can I guarantee that during the first stage(IC is a start-up from an HV pin), the non-inverting input is higher than inverting input? Because if the comparator output is low instantly causes almost zero volts at FB pin. Therefore, the IC can never startup. I think, if I put a 1uF capacitor to parallel with R3, I can boost up the voltage on R4 in a short time. Thus, maybe I can not allow that situation. But in a very short time, the capacitor will draw a very high current and this can be a disaster for a start-up.

Also, when the temperature rise and NTC impedance drops, it will draw more current at that time. Even, if I unplug the cable from the line, the circuit will draw a high current as well because the NTC is not cool enough. And it is very difficult to obtain a precise temperature detection with an 11.5V supply because self-heating of NTC.

Can anybody help me with how can I optimize that circuit?

IC datasheet: enter link description here

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Neither the thermistor or the capacitor across R3 can draw a "high-current". The current is limited by R1 and R4 respectively to a few mA.

If self-heating of the thermistor is an issue reduce the current through it. How do you know that self-heating is an issue?

Also be careful about the limited common-mode range of U1 - a capacitor across R3 will cause the common-mode range to be exceeded. Much better to put the capacitor across R2. The common-mode range of U1 includes ground but not its positive supply rail.

U1 has an open-collector output and could connect directly to the PWM IC. T1 and T2 are not necessary.

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  • \$\begingroup\$ 1.Yes, but even a few mA currents can cause problems, the IC cannot start up. 2- When I compare with measured temperature and resistor value of NTC. 3- I could not understand that. The supply of LM293 11.5V-0V, the common voltage range of LM293 is (V–) (V+) – 1.5 from the datasheet. 4- Yes, I know but I improve the circuit with one more detection that is high voltage protection from the primary. \$\endgroup\$
    – analog_designer
    Commented Apr 3, 2022 at 18:58
  • \$\begingroup\$ @ivanmateo - What is the resistance of the thermistor at 85C? Increase the value of R3 and R4 to reduce the current at startup. What is "high-current". You didn't give a value. "High-current" could mean anything from milliamps to many Amps. You could still get rid of one of the transistor by reversing the phase of the comparator. \$\endgroup\$
    – Kevin White
    Commented Apr 3, 2022 at 20:08

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