UC3843: UC3843 flyback power supply PWM wave loss at high voltage input with light load

zoujiangyilang

Part Number: UC3843
Other Parts Discussed in Thread: UCC28C43

Hello

I am debugging a 5W flyback power supply based on UC3843
When the input voltage increases to 28V light load, the following waveform appears.
PWM loses the wave and the output ripple appears 3 or 5 times harmonic

What's going on here? Is the duty cycle of the PWM too small for high voltage light loads?

over 6 years ago

0 John Griffin1 over 6 years ago

TI__Mastermind 34465 points

Hello zoujiangyilang
At high line and light load the COMP voltage is very low. This cause a min pulse width on the gate drive.
In your application this min pulse width is about 400nS and your switching frequency is about 500kHz.
500kHz is the upper limit of the oscillator on the UC3843 and it appears that sometimes the gate drive is missing pulses.
The result is that you are getting subharmonics on the output ripple.
I think the UCC3843 is really struggling at this switching frequency and you should
(1) Use the UCC28C43 which has a 1MHz max frequency oscillator or
(2) Lower your operating frequency or
(3) Consider speeding up your gate drive circuit. This should allow you to obtain a pulse width less than 400nS

Regards
John

0 zoujiangyilang over 6 years ago in reply to John Griffin1

Genius 4560 points

Hello,John

I reduced the switching frequency to 300 kHz. The situation is better. The input voltage can be raised up but it is still not up to the required 50V.
I removed the 10 ohm resistor between the UC3843 output and the MOS transistor and the situation did not improve.
I would like to change the turns ratio of the transformer to increase the duty cycle at the high-end light load and it will maybe improve.

Regards
ZJYL

0 zoujiangyilang over 6 years ago in reply to zoujiangyilang

Genius 4560 points

Excuse me, what is the width of the smallest PWM that the UC3843 can issue? Is it 400ns?

0 John Griffin1 over 6 years ago in reply to zoujiangyilang

TI__Mastermind 34465 points

Hello ZJYL,
Sounds like you are on the right track.
Good Luck,
John

0 John Griffin1 over 6 years ago in reply to John Griffin1

TI__Mastermind 34465 points

Hello ZJYL,
You can measure the min gate drive directly on the output of the ic.
The signal you show is about 400ns min width but I don't know where you measure this.
Regards
John

0 zoujiangyilang over 6 years ago in reply to John Griffin1

Genius 4560 points

Thank you！

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UC3843: UC3843 flyback power supply PWM wave loss at high voltage input with light load