UC1843: Cold start delay -40C

kasra Motesadi

Part Number: UC1843
Other Parts Discussed in Thread: UC1844,

I have a power supply with first stage UC1843 for boost and second stage UC1844 for Flyback and the output. problem is on cold temp test, power supply has a delay of several seconds to start and provide the needed voltages. measuring the pins, it turned out the COMP pins on both chips take time to come up to the level needed for chips to start making PWM. is there any inherent problem with cold weather on these chips? have you guys had any experience with delay in cold temperature?

over 1 year ago

0 Hong Huang over 1 year ago

TI__Guru 58035 points

Hi,

From our experience, these ICs work at cold similar to that at room temperature or hot. The delay is likely due to your external circuit particularly your feedback loop and your soft start circuit.

0 kasra Motesadi over 1 year ago in reply to Hong Huang

Prodigy 10 points

do you think tha feedback can prevent the comp to ramp up?

in-rush current protection circuit doesn't show any drop of voltage or any shutdown. not sure if it contribute to this problem!

0 Hong Huang over 1 year ago in reply to kasra Motesadi

TI__Guru 58035 points

Hi,

COMP is controlled by FB so yes and if you keep FB high, then COMP will stay low.

You need to trace FB and related circuit to find out.

0 kasra Motesadi over 1 year ago in reply to Hong Huang

Prodigy 10 points

feedback is already provided to the chip and exist. COMP go high (5V) and drop to zero quick, then it takes a few seconds to go back high for about 2.2v or so. when it is cold that ramp up from zero take several seconds. design uses the recommended design from TI design documents.

0 Hong Huang over 1 year ago in reply to kasra Motesadi

TI__Guru 58035 points

Hi,

There may be something wrong with your circuit. Please provide your schematics and waveforms of COMP, FB, OUTPUT (pulse), current sense.

0 Hong Huang over 1 year ago in reply to Hong Huang

TI__Guru 58035 points

Hi,

As long time no response, I will close this thread. If you need additional support, please re-open this thread or create a new thread.

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UC1843: Cold start delay -40C