AN-9738 datasheet(3/25 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

3

IL

VGS

Average of input

current

fSW

t

Figure 4. Frequency Variation of BCM PFC

Since the design of the filter and inductor for a BCM PFC

converter with variable switching frequency should be at

minimum frequency condition, it is worthwhile to examine

how the minimum frequency of BCM PFC converter

changes with operating conditions.

2. Consideration of LLC Resonant

Converter

The attempt to obtain ever-increasing power density in

switched-mode power supplies has been limited by the size

of passive components. Operation at higher frequencies

considerably reduces the size of passive components, such

as transformers and filters; however, switching losses have

been an obstacle to high-frequency operation. To reduce

switching losses and allow high-frequency operation,

resonant switching techniques have been developed. These

techniques process power in a sinusoidal manner and the

switching devices are softly commutated. Therefore, the

switching losses and noise can be dramatically reduced.

Among various kinds of resonant converters, the simplest and

most popular is the LC series resonant converter, where the

rectifier-load network is placed in series with the LC resonant

network, as depicted in Figure 5. In this configuration, the

resonant network and the load act as a voltage divider. By

changing the frequency of driving voltage Vd, the impedance

of the resonant network changes. The input voltage is split

between this impedance and the reflected load. Since it is a

voltage divider, the DC gain of a LC series resonant converter

is always <1. At light-load condition, the impedance of the

load is large compared to the impedance of the resonant

network; all the input voltage is imposed on the load. This

makes it difficult to regulate the output at light load.

Theoretically, frequency should be infinite to regulate the

output at no load.

Figure 5. Half-Bridge, LC Series Resonant Converter

To overcome the limitation of series resonant converters, the

LLC resonant converter has been proposed. The LLC

resonant converter is a modified LC series resonant

converter implemented by placing a shunt inductor across

the transformer primary winding, as depicted in Figure 6.

When this topology was first presented, it did not receive

much attention due to the counterintuitive concept that

increasing the circulating current in the primary side with a

shunt inductor can be beneficial to circuit operation.

However, it can be very effective in improving efficiency

for high-input voltage applications where the switching loss

is more dominant than the conduction loss.

In most practical designs, this shunt inductor is realized

using the magnetizing inductance of the transformer. The

circuit diagram of LLC resonant converter looks much the

same as the LC series resonant converter: the only

difference is the value of the magnetizing inductor. While

the series resonant converter has a magnetizing inductance

larger than the LC series resonant inductor (Lr), the

magnetizing inductance in an LLC resonant converter is just

3~8 times Lr, which is usually implemented by introducing

an air gap in the transformer.

Figure 6. Half-Bridge LLC Resonant Converter

An LLC resonant converter has many advantages over a

series resonant converter. It can regulate the output over

wide line and load variations with a relatively small

variation of switching frequency. It can achieve zero voltage

switching (ZVS) over the entire operating range. All

essential

parasitic

elements;

including

the

junction

capacitances of all semiconductor devices, the leakage

inductance, and magnetizing inductance of the transformer;

are utilized to achieve soft switching.

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