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# TIDA-010080: Design help

Part Number: TIDA-010080

Hi Experts,

I am interested in using just the LLC stage and the output SR stage for a 600W DC-DC design. How would I calculate the input capacitance if I do not use a PFC stage?

Regards,

Gautham Venkat

• Hi Gautham,

You may use the below equation for calculating Cbulk(min). Please keep in mind that the original LLC design is optimized for a PFC input so if no changes are made to the LLC power stage, the LLC will operate much further below the resonant frequency and the rms currents in the primary and secondary will be considerably higher.

Best Regards,

Ben Lough

• What would Vbulk(min) be?

• Also what should fline be given that it is a DC input?

• Hi Gautham,

My previous answer assumed you were using rectified AC as the input to the LLC stage. If we are only looking at DC input, then the input capacitor selection would be based on the capacitor ripple current rating and if you need holdup time for the LLC to continue running for a few ms when input voltage is removed. The calculation for holdup time can be found in the design guide for TIDA-010080 (equation 10). The capacitor ripple current rating should be >= the value listed in equation 36.

https://www.ti.com/lit/ug/tiduet3/tiduet3.pdf?ts=1668450307229&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIDA-010080

Best Regards,

Ben Lough

• Hi Benjamin,

I had another question (not sure if this requires a separate thread?) but looking at the schematics, is this circuit needed for output overcurrent protection? Is it a necessity because the controller should be able to control current through ISENSE pin right?

• Hi Gautham,

The LLC controller has its own OCP protection built in via ISNS pin. I'm guessing in this reference design they wanted a very precise trip threshold for the OCP protection and perhaps that is why they wanted to sense the output current directly.

Best Regards,

Ben Lough

• Hi Ben,

Thank you for clarifying that. One more question, what is the purpose of D12 and C31  in the circuit below?

• Hi Gautham,

C31 is the resonant capacitor. D12 prevents the resonant capacitor voltage from becoming <0V.

Best Regards,

Ben Lough

• Hi Benjamin,

According to the BOM C31 is a thin film cap. The entire primary current flows through the cap, how could the TF cap handle the current approx 4.33A?

Regards,

Gautham Venkat

• Hi Gautham,

The entire resonant current flows through C31. Film caps are commonly used for resonant capacitors. Ultimately it depends on the current rating of the cap and the ESR of the cap you use. The resonant capacitor used is sufficient for the resonant current of this design.

Best Regards,

Ben Lough