Application Note AN 2012-09
V1.0 September 2012
Resonant LLC Converter: Operation and Design
250W 33Vin 400Vout Design Example
Sam Abdel-Rahman
Infineon Technologies North America (IFNA) Corp.
Resonant LLC Converter: Operation and Design
2
Application Note AN 2012-09
V1.0 September 2012
Resonant LLC Converter: Operation and Design
Sam Abdel-Rahman
Published by Infineon Technologies North America
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AN 2012-09
V1.0 September 2012
Author: Sam Abdel-Rahman
We Listen to Your Comments
Any information within this document that you feel is wrong, unclear or missing at all? Your feedback will
help us to continuously improve the quality of this document. Please send your proposal (including a
reference to this document) to: [Sam.Abdel-Rahman@infineon.com]
Resonant LLC Converter: Operation and Design
3
Application Note AN 2012-09
V1.0 September 2012
Table of contents
1 Introduction..................................................................................................................................................4
2 Overview of LLC Resonant Converter.......................................................................................................4
3 Design Steps................................................................................................................................................8
4 Bridge and Rectifier Selection .................................................................................................................11
5 Design Example.........................................................................................................................................13
6 Schematics and Bill of Material ...............................................................................................................17
7 References .................................................................................................................................................19
Resonant LLC Converter: Operation and Design
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Application Note AN 2012-09
V1.0 September 2012
1 Introduction
While a resonant LLC converter has several desired features such as high efficiency, low EMI and high
power density, the design of a resonant converter is an involved task, and requires more effort for
optimization compared to PWM converters. This document aims to simplify this task, and make it easier to
optimally design the resonant tank. This document provides an overview of LLC converter operation and
design guidelines. Finally, a comprehensive design example is given along with schematics, bill of materials,
experimental results and waveforms.
2 Overview of LLC Resonant Converter
This section offers an overview of the LLC converter operation and waveforms in the different modes. Figure
2.1 shows a Full-Bridge LLC converter with Full-Bridge rectifier. In a simplistic discussion, the switching
bridge generates a square waveform to excite the LLC resonant tank, which will output a resonant sinusoidal
current that gets scaled and rectified by the transformer and rectifier circuit, the output capacitor filters the
rectified ac current and outputs a DC voltage.
Figure 2.1 Full-Bridge LLC converter with Full-Bridge rectifier
2.1 Converter Voltage Gain
Converter gain= switching bridge gain * resonant tank gain * transformer turn ratio (Ns/Np)
Where the switching bridge gain is 1 for a Full-Bridge and 0.5 for a Half-Bridge.
The resonant tank gain can be derived by analyzing the equivalent resonant circuit shown in
Figure 2.2, the resonant tank gain is the magnitude of its transfer function as in Eq. 1.
Figure 2.2 Equivalent resonant circuit
2
2
2
2
2
2
2
2
_
_
111
1
)(
)(
),,(
QmFFxFm
mF
sV
sV
FmQK
xx
x
acin
aco
x
Eq. 1
Switching
bridge
LLC tank
Transformer
and Rectifier
Output
Capacitor
Cr
Lr
Lm
Np
S1
S2
S3
S4
Ns
D1
D2
D3
D4
+
Vsw
-
+
Vo
-
RoCo
Cr
Lr
Lm
Vo_ac
Rac
Vin_ac
Resonant LLC Converter: Operation and Design
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Application Note AN 2012-09
V1.0 September 2012
Where,
ac
rr
R
CL
Q
Quality factor
o
S
P
ac
R
N
N
R
2
2
2
8
Reflected load resistance
r
s
x
f
f
F
Normalized switching frequency
rr
r
CL
f
2
1
Resonant frequency
r
mr
L
LL
m
Ratio of total primary inductance to resonant inductance
One can plot the resonant tank gain K with the normalized switching frequency for different values of Quality
factor Q and any single value of m, as shown in Figure 2.3. The selection of the m value will be discussed in
a later section of this document, but m=6 was used as an example.
It can be seen in Figure 2.3 that low Q curves belong to lighter load operation while higher Q curves
represent heavier loads. It’s also seen that all Q curves (load conditions) cross at the resonant frequency
point (at Fx=1 or fs=fr) and have a unity gain.
Figure 2.3 shows that all gain curves has peaks which define the boundary between the inductive and
capacitive impedances of the resonant tank, hence we can define the inductive and capacitive operation
regions as shaded in the plot, the objective of defining both regions is because it is desired to maintain an
inductive operation across the entire input voltage and load current ranges, and never fall into the capacitive
region operation. Such requirement is due to that Zero Voltage Switching (ZVS) is only achieved in the
inductive region, in addition to that capacitive operation means that current leads the voltage, so the current
in the MOSFET will reverse direction before the MOSFET turns off, then after the MOSFET turns off the
reverse current will flow in the MOSFET’s body diode, which will cause a body diode hard commutation once
the other MOSFET in the bridge turns on, which in turn will cause reverse recovery losses and noise, and
might cause high current spikes and device failure. The capacitive operation can be prevented and will be
discussed in a later section of this document.
Figure 2.3
2.2 Modes of Operation
Since the LLC network gain is frequency modulated, the converter can operate in three modes depending on
input voltage and load current conditions, as listed below and shown in Figure 2.4:
0.1 1 10
0
1
2
3
K .2 m, Fx,
(
)
K .3 m, Fx,
(
)
K .5 m, Fx,
(
)
K .7 m, Fx,
(
)
K 1 m, Fx,
(
)
K 5 m, Fx,
(
)
Fx
Capacitive
region
ZCS
Inductive region
ZVS
m=6
Light load
Heavy load
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