AN-9738 datasheet(4/25 Pages) FAIRCHILD

Part #	AN-9738

Description	Design Guideline on 150W Power Supply for LED Street Lighting Design Using FL7930B and FAN7621S

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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AN-9738

APPLICATION NOTE

www.fairchildsemi.com

Rev. 1.0.0 • 4/20/11

4

This application note presents design considerations for an

LLC resonant half-bridge converter employing Fairchild’s

FAN7621S. It includes explanation of the LLC resonant

converter operation principles, designing the transformer

and resonant network, and selecting the components. The

step-by-step design procedure, explained with a design

example, helps design the LLC resonant converter. 0 shows

a simplified schematic of a half-bridge LLC resonant

converter, where Lm is the magnetizing inductance that acts

as a shunt inductor, Lr is the series resonant inductor, and Cr

is the resonant capacitor. Figure 8 illustrates the typical

waveforms of the LLC resonant converter. It is assumed that

the operation frequency is the same as the resonance

frequency, determined by the resonance between Lr and Cr.

Since the magnetizing inductor is relatively small, a

considerable amount of magnetizing current (Im) exists,

which freewheels in the primary side without being

involved in the power transfer. The primary-side current (Ip)

is the sum of the magnetizing current and the secondary-side

current referred to the primary.

In general, the LLC resonant topology consists of the three

stages shown in 0; square-wave generator, resonant

network, and rectifier network.

The square-wave generator produces a square-wave

voltage, Vd, by driving switches Q1 and Q2 alternately

with 50% duty cycle for each switch. A small dead time

is

usually

introduced

between

the

consecutive

transitions. The square-wave generator stage can be

built as a full-bridge or half-bridge type.

The resonant network consists of a capacitor, leakage

inductances, and the magnetizing inductance of the

transformer. The resonant network filters the higher

harmonic currents. Essentially, only sinusoidal current

is allowed to flow through the resonant network even

though a square-wave voltage is applied. The current

(Ip) lags the voltage applied to the resonant network

(that is, the fundamental component of the square-wave

voltage (Vd) applied to the half-bridge totem pole),

which allows the MOSFETs to be turned on with zero

voltage. As shown in Figure 8, the MOSFET turns on

while the voltage across the MOSFET is zero by

flowing current through the anti-parallel diode.

The

rectifier

network

produces

DC

voltage

by

rectifying the AC current with rectifier diodes and a

capacitor. The rectifier network can be implemented as

a full-wave bridge or a center-tapped configuration with

capacitive output filter.

Figure 7. Schematic of Half-Bridge LLC

Resonant Converter

Ip

IDS1

Vd

Im

VIN

ID

Vgs2

Vgs1

Figure 8. Typical Waveforms of Half-Bridge LLC

Resonant Converter

The filtering action of the resonant network allows the use

of the fundamental approximation to obtain the voltage gain

of the resonant converter, which assumes that only the

fundamental component of the square-wave voltage input to

the resonant network contributes to the power transfer to the

output. Because the rectifier circuit in the secondary side

acts as an impedance transformer, the equivalent load

resistance is different from actual load resistance. Figure 9

shows how this equivalent load resistance is derived. The

primary-side circuit is replaced by a sinusoidal current

source, Iac, and a square wave of voltage, VRI, appears at the

input to the rectifier. Since the average of |Iac| is the output

current, Io, Iac, is obtained as:

sin(

)

2

o

ac

I

t

π

ω

⋅

=

(3)

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