THEORY OF OPERATION

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UCC28810

UCC28811

Transition Mode Operation

When the primary MOSFET switch is turned on, the drain current ramps from zero to a peak value determined by

the current reference generator output, which is the combination of the EAOUT pin voltage and the AC line

voltage waveform applied to the VINS pin. The EAOUT pin sets the power level to be transferred to the

secondary over the half sinewave cycle, and the current reference generator forces the peak switch current to

track the input line voltage to improve the power factor.

When the main switch is turned off, the peak current in the flyback inductor is transferred to the secondary side

and flows through the output diode to the output capacitors. This current drops to zero at the rate determined by

the output winding inductance and the output capacitor voltage. When the output current reaches zero, the diode

stops conducting, and all of the output windings and the drain of the MOSFET ring down towards ground. This

ringing is detected on the primary side by the TZE pin of the UCC28810 as it rings below approximately 1.4 V on

the bias winding. This triggers the next switch-on pulse to start very near the valley of the ringing waveform on

of the FET

Input Filter Damping Network

Offline flyback converters typically need common mode and differential mode input EMI filters to meet EMI

specifications. When a triac dimmer is used with a typical L-C EMI filter, the sharp turn on edge that is generated

by the triac phase control causes the LC filter to ring back and set up an oscillation between the triac and the L-C

filter. For this reason, the differential part of the filter is damped with a R-L network across the inductor. This can

also be accomplished with an R-C damping network across the capacitor. L3 and R5 are the damping

components in the schematic shown in Figure 17.

Figure 17. Input Filter Damping Network

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