UCC3895: About the built-in cycle-by-cycle current limiting function of the UCC3895 chip

7 EEE

Part Number: UCC3895
Other Parts Discussed in Thread: UCC2895, UCC256402, UCC28951, UCC28950

I use the 3895 chip as the main control chip of the LLC resonant converter, which adopts single voltage control. At the same time I wanted to use the CS pin for additional cycle-by-cycle current limiting in short circuit operation, but asymmetrical pulses appeared as shown. The output pulses of OUTC and OUTD are still complementary, but not 50%.

over 1 year ago

0 7 EEE over 1 year ago

Prodigy 10 points

According to the block diagram of UCC3895, it can be seen that OUTC and OUTD correspond to Q and Q. What I want to confirm is that when the cycle-by-cycle current limiting function of the CS pin is used in the voltage control mode, how does the output pulse change to achieve current limiting? Phase shift or reduce duty cycle of PWM wave? Symmetric or asymmetric?

0 Mike O' over 1 year ago in reply to 7 EEE

TI__Guru 58865 points

Hello,

I am reviewing your question and will get back to you shortly.

Regards,

0 Mike O' over 1 year ago in reply to Mike O'

TI__Guru 58865 points

Hello,

If you are designing an LLC resonant converter the UCC2895 is not the correct device. You could use a UCC256402 LLC controller to achieve this.

Out C and D are phase shifted to control the duty cycle. When the converter tries to demand 0% duty cycle the phase are shifted from 0 to 180 degrees. This might be what you are seeing. This operation is explained in the UCC2895 e2e thread link below.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/594581/ucc2895-asymmetric-duty-cycle-at-start-up

Regards,

0 7 EEE over 1 year ago in reply to Mike O'

Prodigy 10 points

Thank you for your reply. I use 28950 as the LLC control chip because my control method is PFM + phase shift control. At present, voltage regulation can be achieved under normal working conditions, but the above inconsistencies occur during the cycle-by-cycle current limiting stage. Symmetric situation.

0 Mike O' over 1 year ago in reply to 7 EEE

TI__Guru 58865 points

Hello,

I thought you were using the UCC2895. However, the asymmetrical duty cycle is still the same. The converter is trying to demand zero duty cycle and/or you are trying to demand over 90% duty cycle and are entering OCP protection. If you are using over 90% duty cycle I recommend you switched to the UCC28951.

The following application note chooses when you should select the UCC28950 over the UCC28951. https://www.ti.com/lit/pdf/slua853

Regards,

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UCC3895: About the built-in cycle-by-cycle current limiting function of the UCC3895 chip