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UCC256301: ucc25630

Zhengguo Zhou
Prodigy 30 points
Part Number: UCC256301

Hi,

we use ucc256301 to drive a LCC tank. several questions about C1 and C2 selection at VCR pin which are ......

1. the C1, C2 devider  samples from the resonant capacitor Cs. the across voltage of Cs include the DC block voltage (1/2 Vbus) and resonant voltage Vsrms. So the divided voltage of C1 and C2 for VCR pin should be counted the actual voltage of Vcs which are in the scale 0~6v?

2. How to take the ratio of the rasonant current to the ramp current in VCR divider C1 and C2? is any different to take the ratio in LLC tank and LCC tank ?

3. what effect when take a different diviied voltage of C1 and C2? opperating frequency, output power or others?

  • 0
    TI__Mastermind 26225 points
    Hi Zhengguo,

    Thanks for your interest in UCC256301.

    1. The Vcr capacitor divider sample the AC component of the resonant capacitor voltage. The AC component of the resonant capacitor voltage depends on the resonant current in the primary side power stage. The Vcr divider should be sized such that the Vcr pin itself is within 0-6V.

    2. Values between 0.1 and 0.6 are acceptable for the ratio between resonant current and ramp current. I do not expect a difference in selecting the value between LLC and LCC

    3. A larger divider ratio minimizes the contribution to the total Vcr pin waveform from the changing resonant capacitor voltage. In this case, the frequency compensation ramp will have a larger contribution. This will make the converter more closely mimic direct frequency control. A smaller divider ratio makes the resonant capacitor voltage have a larger contribution to the total Vcr waveform. This will make the converter more closely mimic charge control. This is will be some difference in your bandwidth depending on how you configure the divider ratio as a result.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 30 points

    Hi Ben,

    Thanks for your quick reply.

    For the question 1,  the divider do sample AC the voltage of the resonant capacitor Cs.  But the capacitor also have blocked DC voltage (1/2 DCbus)  and then  the DC blocked voltage will delivery to the dividing point of the divider of VCR pin.  Do we only calculate AC voltage and ignore DC voltage ?

    For the question 3,  you means The ratio of the C1/C2 divider only  contribute to the frequency control and charge control.and not affect switch frequency or others?

  • 0 in reply to Zhengguo Zhou
    TI__Mastermind 26225 points
    Hi Zhengguo,

    The Vcr capacitor divider only measures the AC component of the resonant capacitor voltage.

    The Vcr waveform will have a small effect on the burst mode threshold and its relation to the primary side current. The Vcr capacitance affects the balance between charge control and direct frequency control of the overall control scheme. There will be some difference in the relationship between primary side current and switching frequency depending on how you configure the Vcr divider. As such, there will be some difference in the switching frequency vs output power profile depending on your Vcr divider configuration.

    Best Regards,
    Ben Lough