AN4382 datasheet(16/70 Pages) STMICROELECTRONICS

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ZVS LC resonant stage description

AN4382

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DocID025437 Rev 1

ZVS LC resonant stage description

The resonant stage is an LC topology consisting of a half bridge operating in “Zero Voltage

Switching” condition supplying the resonant cell. The half bridge is connected to the PFC

output. The LC stage (Cres, Lres) is separated from the transformer. The transformer XF is

responsible for the energy transfer to the secondary side, where a rectifier circuit (Dout) and

a filtering capacitor transform the AC current into a ripple free DC current required by the

LEDs.

Figure 8. ZVS concept

The transformer XF is built in a way that maximize the winding coupling and operates

similarly to an ideal transformer where the secondary side current is proportional to the

primary side current and depends only on the transformer turn ratio (n = NPRI / NSEC). This

proportionality is the key to control the secondary side current from the primary side.

The selection of Lres and Cres is made in order to operate above resonance in any

condition therefore guaranteeing a soft switching behavior. The primary current has

a triangular shape so that it can be stated that the average value of the LED current is equal

to half of the peak of the resonant secondary current. Regulating the peak current during the

low side conduction period and operating a 50% duty cycle then the primary side control of

the output current is possible.

The transformer is coupled with an auxiliary winding which is used to measure at the

primary side the reflected voltage and detect abnormal conditions such as short-circuit or no

load.

AN4382 Datasheet(PDF) 16 Page - STMicroelectronics