UC2825: Minimum Duty Cycle before Pulse Skipping

Samuel Jaffe

Mastermind 25590 points

Part Number: UC2825A
Other Parts Discussed in Thread: UC2825

Hi E2E,

My customer is using the UC2825A in a buck configuration:

FETs switching at 150kHz
Output filter frequency at 300kHz
Duty cycle swing from 71% to 12% for wide input range

The issue:

At high input voltage (>100V), they immediately see a duty cycle of 6% which allows enough current to peak charge the output caps and hit current limit and force the converter into pulse skipping mode. We think if the converter ramped from 0-6% instead of jumping straight to 6%, we would not have this issue.

If they reduce the switching frequency to 75kHz, it works without this issue.

How can the customer avoid this issue and drive the FETs at 150kHz?

Thanks,

-Sam

over 8 years ago

0 Brian Johnson96 over 8 years ago

TI__Expert 3200 points

Hi Sam,

An AE will respond shortly.

0 Oliver Pan91 over 8 years ago

TI__Intellectual 2230 points

Have you try to increase the soft-start capacitor? At the beginning of start up, the SS capacitor starts to be charged and gradually increase allows the duty cycle to slowly increase. Thanks

0 Samuel Jaffe over 8 years ago in reply to Oliver Pan91

TI__Mastermind 25590 points

Oliver,

I have inquired about the soft start pin. I'll let you know what comes back.

In the meantime, is there a minimum duty cycle for this part? Any ideas as to why it would jump to 6%?

Thanks,
-Sam

0 Samuel Jaffe over 8 years ago in reply to Samuel Jaffe

TI__Mastermind 25590 points

Oliver,

Some feedback from the customer.

They are using the UC2825A, not the UC2825.
The soft start cap is 100nF

I'll recommend increasing this value and I'll get back to you.

Thanks,
-Sam

Power management

Power management forum

UC2825: Minimum Duty Cycle before Pulse Skipping