UC3848L-S08-R datasheet(6/11 Pages) UTC

6 / 11 page

UC3848

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 11

www.unisonic.com.tw

QW-R103-033.F

FUNCTIONAL DESCRIPTION(Cont.)

(2) Switch Frequency Set

The maximum switch frequency is set to 68kHz. Switch frequency is modulated by output power POUT during IC

operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from

switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the subber circuit. The

magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower

switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light

load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between fSW

and POUT/POUT (MAX) as followed FIG.4.

FIG.4 The relation curve between fSW and relative output power POUT/ POUT (MAX)

(3) Internal Synchronized Slope Compensation

Built-in slope compensation circuit adds voltage ramp onto the current sense input voltage for PWM

generation, this greatly improves the close loop stability at CCM and prevents the sub-harmonic oscillation and thus

reduces the output ripple voltage.

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