UCC28180: Oscillations under no load

Pravin Iyengar

Part Number: UCC28180

Hello All,

I'm setting up a PFC using the UCC28180 for universal input, 400Vout 3.5kW operation. But before doing so I wanted to test the controller at a lower voltage. To do so, I've modifie feedback elements and running at 30V DC input and 100V DC out. After steady state output voltage is achieved, the controlled applies minimum duty cycle. However, I see a large amount of oscillation across the Drain-source terminals of the FET. Further to this, I added a small resistor of 4k to the output. This reduced the oscillations as shown in Figure below. I am trying to understand the reason for these oscillations and how I can control them as I ramp up the voltage later.

CH1: Gate signal to PFC FET; CH2: FET Drain-Source voltage ; CH3: Output voltage

Any inputs appreciated.

Thanks

Regards,

Pravin

over 8 years ago

0 John Griffin1 over 8 years ago

TI__Mastermind 34465 points

Pravin,
The UCC28180 is a continuous conduction mode (CCM) pfc controller. You need to increase the load so that you are in the CCM condition.
In your test you will achieve this when the duty cycle is 0.7
In your waveforms the inductor current goes to zero and you are encountering ringing between the boost inductance and the capacitance on the drain source of the MOSFET.
Regards
John Griffin

0 Pravin Iyengar over 8 years ago in reply to John Griffin1

Prodigy 190 points

Hi John,
Yes, I increased the load and the oscillations disappeared. Does this mean a minimum load is always required during standby or is there an alternate solution?

Thanks
Pravin

0 John Griffin1 over 8 years ago in reply to Pravin Iyengar

TI__Mastermind 34465 points

Hi Pravin,

You do not need a minimum load. The output voltage should remain in specification while the oscillation occurs.

Regards

John

0 Pravin Iyengar over 8 years ago in reply to John Griffin1

Prodigy 190 points

Hi John,

Thanks. I was concerned if these oscillations would worsen if I remove the load and damage the silicon.

The output voltage looks to be well regulated.

Regards
Pravin

0 Bosheng Sun over 8 years ago in reply to Pravin Iyengar

TI__Expert 7310 points

Hi Pravin,

At light load, the PFC enters DCM mode, when inductor current reduces to zero, the inductor and switch node parasite capacitance start to oscillate, that's why you see the switch node voltage has oscillation. This happens in all the CCM PFC, has nothing to do with UCC28180. However, this should not be a concern as far as your THD still meet the spec, and your output voltage should still be very stable.

Regards,

Bosheng

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UCC28180: Oscillations under no load