UCC28C53_V01 datasheet(27/48 Pages) TI

27 / 48 page

The transformer primary to secondary turns ratio (NPS) can be selected based on the desired MOSFET voltage

rating and the secondary diode voltage rating. Because the maximum input voltage is 265 VRMS, the peak bulk

input voltage can be calculated as shown in Equation 4.

VBULK (max ) = ¾2 × VIN(max ) N 375 V

(4)

To minimize the cost of the system, a readily available 650-V MOSFET is selected. Derating the maximum

voltage stress on the drain to 80% of its rated value and allowing for a leakage inductance voltage spike of up

to 30% of the maximum bulk input voltage, the reflected output voltage must be less than 130 V as shown in

Equation 5.

REFLECTED

DS(rated)

BULK(max)

0.8

1.3 V

130.2 V

(5)

The maximum primary to secondary transformer turns ratio (NPS) for a 12 V output can be selected as

PS =

REFLECTED

OUT

= 10.85

(6)

A turns ratio of NPS = 10 is used in the design example.

The auxiliary winding is used to supply bias voltage to the controller. Maintaining the bias voltage above the VDD

minimum operating voltage after turnon is required for stable operation. The minimum VDD operating voltage for

the controller selected for this design is 10 V. The auxiliary winding is selected to support a 12 V bias voltage so

that it is above the minimum operating level but maintains a low level of losses in the IC. The primary to auxiliary

turns ratio (NPA) can be calculated from Equation 7:

PA = NPS ×

OUT

BIAS

= 10

(7)

The output diode experiences a voltage stress that is equal to the output voltage plus the reflected input voltage:

DIODE

BULK

:max ;

+ V

OUT = 49.5 V

(8)

TI recommends a Schottky diode with a rated blocking voltage greater than 60 V to allow for voltage spikes due

to ringing. The forward voltage drop (VF) of this diode is estimated to be equal to 0.6 V

To avoid high peak currents, the flyback converter in this design operates in continuous conduction mode. Once

NPS is determined, the maximum duty cycle (DMAX) can be calculated using the transfer function for a CCM

flyback converter:

OUT + VF

BULK

:min ;

PS p

MAX

F D

MAX p

(9)

OUT

MAX

BULK(min)

OUT

0.627

(10)

Because the maximum duty cycle exceeds 50%, and the design is an off-line (AC-input) application, the

UCC28C42 is best suited for this application.

9.2.2.3 Transformer Inductance and Peak Currents

For this design example, the transformer magnetizing inductance is selected based upon the CCM condition. An

inductance value that allows the converter to stay in CCM over a wider operating range before transitioning into

discontinuous current mode is used to minimize losses due to otherwise high currents and also to decrease the

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UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

SLUSER8 – JUNE 2022

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UCC28C53 Datasheet(PDF) 27 Page - Texas Instruments

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