UCC28740: External circuit to enable/unable UCC28740

Noriyuki Takahashi

Genius 10950 points

Part Number: UCC28740
Other Parts Discussed in Thread: UCC28701

Hi team

Customer wants to enable/unable UCC28740. Do you recommend to use the circuit explained in below link?

Are there any good solutions else?

If you recommend this, please let me ask some questions about the circuit.

1. What is the resister value under Vdd? Is it 10kOhm? It seems to be 10N, but I am not sure what it is.

2. If VDD is less than VDD turnoff threshold, is it OK that UCC28740 is turned off and the Vout in the secondary side also goes to 0V?

Regards

Noriyuki Takahashi

over 4 years ago

0 Benjamin Lough over 4 years ago

TI__Mastermind 27095 points

Hi Takahashi-san,

I would suggest taking a look at this E2E post instead. There are some suggestions for how to implement enable/disable with UCC28740:https://e2e.ti.com/support/power-management/f/196/t/787246?UCC28740-How-to-Disable-Function-for-UCC28740-was-safety-Primary-Side-or-Second-Side

Best Regards,

Ben Lough

0 Noriyuki Takahashi over 4 years ago in reply to Benjamin Lough

TI__Genius 10950 points

Ben

Thanks!
Here is an additional question.

Customer thinks that below way that is explained in the link you told might be good for their system.

d) connect the cathode of the TL431 regulator to output GND

The output is really 0V? They have concern that the output will fluctuate at 0V, because the UCC28740 will enter a UVLO-shutdown-and-retry cycle.

If the way causes the fluctuation, are there any alternative ways to enable/unable UCC28740?

Regards,

Noriyuki Takahashi

0 Benjamin Lough over 4 years ago in reply to Noriyuki Takahashi

TI__Mastermind 27095 points

Hi Takahashi-san,

Another idea is to also put a gate driver IC that has an enable pin. When you disable the gate driver, you won't have the retry pulses anymore.

Best Regards,

Ben Lough

0 Noriyuki Takahashi over 4 years ago in reply to Benjamin Lough

TI__Genius 10950 points

Hi Ben

Do you mean that the output voltage can fluctuate at 0V even if the device is unabled by the below way?

d) connect the cathode of the TL431 regulator to output GND

Could you also recommend any devices that have a enable-pin instead of UCC28740?

Regards,

Noriyuki Takahashi

0 Benjamin Lough over 4 years ago in reply to Noriyuki Takahashi

TI__Mastermind 27095 points

Hi Takahashi-san,

I really don't think you will see much, if any, ripple in the output during the retry. The energy delivered during the retry should be extremely small. If your customer is really worried about this, then you could put a gate driver IC in front of the UCC28740 to stop the gate pulse from turning on the FET.

You could look at UCC28701 which has a NTC pin when if pulled below 0.95V, will force the controller to shut down.

Best Regards,

Ben Lough

Power management

Power management forum

UCC28740: External circuit to enable/unable UCC28740