UCC25630-1EVM-291: How to set the start-up frequency

Satoshi

Guru 19645 points

Part Number: UCC25630-1EVM-291

Please let me know about how to set the start-up frequency for UCC25630-1EVM-291.

(My purpose is control the start up frequency and decrease resonant current)

Background is below;

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・Evaluating condition is based on default setting for UCC25630-1EVM-291.

・DC input: 390V and AC input: 100V

・Problem for start up, it was occurred large resonant current

　※Peak current for 120W output is 0.5A~0.6A, but startup is about 5A(Very large).

・I think that way to decrease resonant current is LL/SS pin: increase capacitor and adjust resistor(Burst mode setting).

　⇒These points are changed, but start-up resonant frequency was not changed(180kHz～190kHz) and current was not decrease.

・Waveform is attached below;

【Waveform for Start up】

Yellow： DCin:390V(100V/div) , Green： DCout:12V(2V/div), Purple： Resonant current(2A/div)

【Waveform for 120W output】

Yellow： DCin:390V(100V/div), Green： DCout:12V(2V/div), Purple： Resonant current(0.5A/div)

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Best regards,

Satoshi

over 7 years ago

0 Yalong Li over 7 years ago

TI__Prodigy 110 points

Hi Satoshi,

During startup, the switching pulses are determined by the LL/SS pin voltage and VCR pin voltage.
There are two ways to increase the switching frequency during startup, to reduce the inrush current:
1) Increase the LL/SS pin cap, and 2) reduce the capacitance of the capacitor divider on VCR pin (keep the ratio same) - so the internal 2mA current source will charge/discharge the VCR voltage faster.
In the EVM case, I think you can do #2 to achieve a higher switching frequency during startup.

Best regards,

Yalong

Power management

Power management forum

UCC25630-1EVM-291: How to set the start-up frequency