UCC28056CDBVT datasheet(35/55 Pages) TI2

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ZC2

ZC1

ZC2

DSON

ZC1

ZC2

ZC1

ZC2

u ¨

CEquRMSHF

1.525

0.3 A

1.15 A

1.37 A

0.610

Out

68 F

136 F

HLF

100kHz _ ripple _ current _ rating

120Hz _ ripple _ current _ rating

CEquRMSHF

COutRMSLF

HLF

COutRMSHF

UCC28056

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SLUSD37E – OCTOBER 2017 – REVISED NOVEMBER 2019

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Electrolytic capacitors typically have a ripple current rating at twice Line frequency (120 Hz) and a different ripple

current rating at switching frequency (100 kHz). These ratings reflect the fact that the capacitor ESR is higher at

twice Line frequency and hence ripple current at this frequency leads to higher power loss than the same

amplitude of switching frequency ripple. Consider the equivalent high-frequency ripple current flowing in the

capacitor in order to select the correct capacitor.

(47)

The parameter KHLF is the ratio of high frequency to low frequency RMS ripple current rating for the particular

capacitor series to be used.

(48)

In this example design, for reasons of size and rating, two 68-µF, 450 V capacitors are selected from Rubycon

BXW series (450BXW68MEFC12.5X45), connected in parallel. In this way, both the capacitance value

requirement and ripple current rating are met with some additional margin.

(49)

(50)

9.2.2.3 ZCD/CS Pin

An external divider network attached to the ZCD/CS pin transfers both the attenuated Drain voltage waveform

(VDS) and the current sense signal (VCS) into the controller. This transfer is possible because the current sense

signal requires observation only when the switch is ON and the VDS signal is close to zero. While the Drain

voltage waveform requires sensing only when the switch is OFF and the current sense signal is close to zero.

(51)

Equation 52 describes the attenuated Drain voltage during the on-time period when the MOSFET is switched

ON.

(52)

The ON state resistance of the MOSFET (RDSON) typically has a similar value to the current sense resistor (RCS).

The attenuation of the divider (ZZC1, ZZC2) is 1/401 and hence the second term of Equation 52 may be neglected.

(53)

Hence the required current sense resistor value can be calculated from the maximum peak inductor current

obtained in section 9.2.2.2.1

Outside the TON period, when the MOSFET is switched OFF, the current flowing through the current sense

resistor is close to zero. In this case Equation 51 may be expressed as follows.

(54)

UCC28056 prevents the start of a new switching cycle until increasing negative slope is detected on the ZCD/CS

pin voltage waveform. The increasing negative slope indicates that the inductor current has fallen to zero so the

output diode is already OFF. Turn-ON switching loss is further reduced by synchronizing the start of each new

switching cycle with a minimum, or valley, on the Drain waveform.

UCC28056CDBVT Datasheet(PDF) 35 Page - Texas Instruments

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