UCC2897 datasheet(21/32 Pages) TI

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UCC2897

SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004

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APPLICATION INFORMATION: SETUP GUIDE

In the UCC2897, controller slope compensation is implemented by sourcing a linearly increasing current at the

CS pin. When this current passes through the current sense filter resistor (RF), it is converted to a slope

compensation ramp which can be characterized by its dV

S dt . The

S dt

of the slope compensation

current is defined by RSLOPE according to:

SLOPE

where

D 2V is the peak−to−peak ramp amplitude of the internal oscillator waveform

D 5 is the multiplication factor of the internal current mirror

The voltage equivalent of the compensation ramp dV

S dt can be easily obtained by multiplying with RF. After

introducing the application specific m and dV

L dt values, the equation can be rearranged for RSLOPE:

SLOPE +

The UCC2897 controller is dedicated to control current mode active clamp flyback or forward converters in an

isolated power supply. The key advantage of the active clamp topologies is the zero voltage switching (ZVS)

of the primary side semiconductors. This operating mode reduces the switching losses of the converter, thus

facilitates higher switching frequencies or improves efficiency when operated at similar frequencies as its hard

switched counterparts. The simplified schematic below demonstrates the typical implementation of an active

clamp forward converter with high-side clamp utilizing a P-channel auxiliary switch.

Detailed analysis and design examples of active clamp converters are published in the references listed at the

end of this datasheet.

CBIAS

+VIN

−VIN

CCLAMP

QAUX

QMAIN

RCS

CIN

VIN

GND

OUT

VDD

AUX

Secondary−Side

Error Amplifier

and Isolation

Synchronous

Rectifier

Control

UCC2897

Bootstrap

Bias

Load

P−Channel

Gate Drive

UDG−03154

Figure 8. Active Clamp Forward Converter

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