UCC28C53_V01 datasheet(28/48 Pages) TI

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output ripple. The design of the transformer in this example sizes the inductance so the converter enters CCM

operation at approximately 10% load and minimum bulk voltage to minimize output ripple.

The inductor (LP) for a CCM flyback can be calculated using Equation 11.

P =

kVBULK :min;o2 × l

PS × VOUT

BULK

:min ; + NPS × VOUT p

0.1 × P

IN × fSW

(11)

where

• PIN is estimated by dividing the maximum output power (POUT) by the target efficiency (η)

• fSW is the switching frequency of the converter

For the UCC28C42 the switching frequency is equal to the oscillator frequency and is set to 110 kHz. Selecting

fSW to be 110 kHz provides a good compromise between size of magnetics, switching losses, and places

the first harmonic below the 150-kHz lower limit of EN55022. Therefore, the transformer inductance must be

approximately 1.8 mH. A 1.5 mH inductance is chosen as the magnetizing inductance, LP, value for this design.

Based on calculated inductor value and the switching frequency, the current stress of the MOSFET and output

diode can be calculated.

The peak current in the primary-side MOSFET of a CCM flyback can be calculated as shown in Equation 12.

IPK

MOSFET

VBULK :min ; ×

NPS × VOUT

VBULK :min ; + :NPS × VOUT ;

nVBULK(min)

2 × Lm

NPS × VOUT

VBULK :min ; + :NPS × VOUT ;

fSW

(12)

The MOSFET peak current is 1.36 A. The RMS current of the MOSFET is calculated to be 0.97 A as shown in

Equation 13. Therefore, IRFB9N65A is selected to be used as the primary-side switch.

IRM S

MOSFET

¨DMAX3

lVBULK(min)

LP × fSW

p2FFDMAX2×IPKMOSFET ×VBULK(min)

LP × fSW

G+

MAX × IPKMOSFET

(13)

The output diode peak current is equal to the MOSFET peak current reflected to the secondary side.

P K DIODE

= N

PS × IP K MOSFET

= 13.634 A

(14)

The diode average current is equal to the total output current (4 A) combined with a required 60-V rating and

13.6-A peak current requirement, a 48CTQ060-1 is selected for the output diode.

9.2.2.4 Output Capacitor

The total output capacitance is selected based upon the output voltage ripple requirement. In this design, 0.1%

voltage ripple is assumed. Based on the 0.1% ripple requirement, the capacitor value can be selected using

Equation 15.

OUT R

OUT ×

PS × VOUT

BULK

:min ; + NPS × VOUT

0.001 × V

OUT × fSW

= 1865 JF

(15)

To design for device tolerances, a 2200-µF capacitor was selected.

UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

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