AN-4151 Datasheet, PDF (1/17 Pages) Fairchild Semiconductor – Half-bridge LLC Resonant Converter

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Application Note AN-4151
Half-bridge LLC Resonant Converter Design Using
FSFR-series Fairchild Power Switch (FPS™)
Introduction
The effort to obtain ever-increasing power density of
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 [1-
7].
Among various kinds of resonant converters, the simplest and
most popular resonant converter is the LC series resonant
converter, where the rectifier-load network is placed in series
with the L-C resonant network, as depicted in Figure 1 [2-4].
In this configuration, the resonant network and the load act as
a voltage divider. By changing the frequency of driving
voltage V_d, 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 very 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.
Q₁
V_in
V_d
Q₂
L_r
n:1
C_r
R_o ⁺
V_O
-
Figure 1. Half-bridge, LC Series Resonant Converter
To overcome the limitation of series resonant converters,
LLC resonant converter has been proposed [8-12]. 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 2.
When this topology was first presented, it did not receive
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 10/9/07
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 application where the switching loss
is much more dominant than the conduction loss.
In most of the practical design, 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 much larger than the LC series resonant
inductor (L_r), the magnetizing inductance in LLC resonant
converter is just 3~8 times L_r, which is usually
implemented by introducing an air gap in the transformer.
Q₁
V_in
Q₂
L_r
n:1
L_shunt
( L_m )
C_r
R_o ⁺
V_O
-
Figure 2. 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 junction
capacitances of all semi-conductor devices and the leakage
inductance and magnetizing inductance of the transformer,
are utilized to achieve soft-switching.
This application note presents design considerations of an
LLC resonant half-bridge converter employing FSFR-series
FPS™. It includes explanation of LLC resonant converter
operation principle, 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.
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