Datasheet

UCC28600

SLUS646J –NOVEMBER 2005– REVISED JULY 2011

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Operating Mode Programming

Boundaries of the operating modes are programmed by the flyback transformer and the four components R

, R

OVP1

and R

OVP2

; shown in the Block Diagram/Application drawing.

The transformer characteristics that predominantly affect the modes are the magnetizing inductance of the

primary and the magnitude of the output voltage, reflected to the primary. To a lesser degree (yet significant), the

boundaries are affected by the MOSFET output capacitance and transformer leakage inductance. The design

procedure here is to select a magnetizing inductance and a reflected output voltage that operates at the

DCM/CCM boundary at maximum load and maximum line. The actual inductance should be noticeably smaller to

account for the ring between the magnetizing inductance and the total stray capacitance measured at the drain

of the power MOSFET. This programs the QR/DCM boundary of operation. All other mode boundaries are preset

with the thresholds in the oscillator and green-mode blocks.

The four components R

, R

OVP1

and R

OVP2

must be programmed as a set due to the interactions of the

functions. The use of the UCC28600 design calculator, TI Literature Number SLVC104, is highly recommended

in order to achieve the desired results with a careful balance between the transformer parameters and the

programming resistors.

Protection Features

The UCC28600 has many protection features that are found only on larger, full featured controllers. Refer to the

Block Diagram/Typical Application and Figures 1, 4, 5, 6 and 7 for detailed block descriptions that show how the

features are integrated into the normal control functions.

Overtemperature

Overtemperature lockout typically occurs when the substrate temperature reaches 140°C. Retry is allowed if the

substrate temperature reduces by the hysteresis value. Upon an overtemperature fault, C

on softstart is

discharged and STATUS is forced to a high impedance.