TPS2373-4EVM-758: Duty cycle cannot be adjusted

user5047327

Part Number: TPS2373-4EVM-758
Other Parts Discussed in Thread: PMP21318, TL431, UCC2897A

Hi ,

My design is exactly the same as 8228.PMP21318_RevA(001)_Sch.

On my circuit board, I found that the duty cycle is not adjustable, and the duty cycle is always the maximum output.Working normally at 12V 5A( 90% efficiency), and the ripple is normal 50mv.Among them, the circuit near TL431 has great interference, resulting in the lifting of load, TL431 K level is always 11v, almost unchanged, resulting in the optocoupler feedback voltage at the other end is always 3.3v (TP23), thus the duty ratio is the largest, always the same.What may be the cause of this problem?I am now testing the external power supply.

My schematic

原理图.pdf

Reference schematic

7411.8228.PMP21318_RevA(001)_Sch.PDF

The 12 v OUT ripple

TL431 K ripple

over 6 years ago

0 Darwin Fernandez over 6 years ago

TI__Mastermind 41770 points

Hi User,

Even with change in input input voltage range, the duty cycle is unchanged? What duty cycle is it operating at? Is it different from our test report (~60% measured on the primary FET)?

Thanks!

0 user5047327 over 6 years ago in reply to Darwin Fernandez

Prodigy 85 points

the duty cycle is unchanged with change in input input voltage range,but it's very small,the duty cycle is it operating at 51-53%.

0 Darwin Fernandez over 6 years ago in reply to user5047327

TI__Mastermind 41770 points

Hi User,

Today is a national holiday in the US. We will be able to have answer for you on this as early as Friday. Thanks!

0 user5047327 over 6 years ago in reply to Darwin Fernandez

Prodigy 85 points

Hi Darwin,

Thank you for your reply,just have fun.

0 Darwin Fernandez over 6 years ago in reply to user5047327

TI__Mastermind 41770 points

Hi User,

Looking at the schematic, I didn't see an issue. Also after reading your description above, the behavior seems normal. You are not in the max duty cycle range when FB is 3.3V. Please see below from the UCC2897A datasheet. If your output voltage is regulating with you input voltage and output current ranges, then your DCDC is not duty-cycle limited.

The control voltage is internally buffered and connected to the PWM comparator through a voltage divider to make it compatible to the signal level of the current-sense circuit. The useful voltage range of the FB pin is between approximately 2.5 V and 4.5 V. Control voltages below the 2.5-V threshold result in zero-duty cycle (pulse skipping) while voltages above 4.5 V result in full-duty-cycle (DMAX) operation.

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TPS2373-4EVM-758: Duty cycle cannot be adjusted