2011 International Conference on Information and Electronics Engineering
IPCSIT vol.6 (2011) © (2011) IACSIT Press, Singapore
Design of a digitally-controlled LLC resonant converter
Jia-Wei huang1, Shun-Chung Wang 2 and Yi-Hua Liu1
1
2
National Taiwan University of Science and Technology, Taipei, Taiwan, R.O.C.
Lunghwa University of Science and Technology, Taoyuan County, Taiwan, R.O.C.
Abstract. Nowadays, liquid crystal display (LCD) panels are widely used in applications such as monitors,
notebooks and televisions. In large scale LCD TV (>30”), the power supply typically requires an output
power ranges from 200 W to 600 W. Generally, a half-bridge LLC resonant converter with zero voltage
switching (ZVS) capability is utilized as the DC/DC power stage. In this paper, the design of a digitallycontrolled LLC resonant converter is presented. The LLC resonant topology allows for ZVS of the main
switches, thereby dramatically lowering switching losses and boosting efficiency. A digital signal controller
(dsPIC30F2020) from Microchip corp. is utilized as the digital controller of the LLC resonant converter. In
order to design the compensation circuit correctly, the large-signal and small-signal model of the utilized
LLC converter is also derived in this paper using the phasor transformation method. Finally, experimental
results are presented to validate the correctness of the proposed system.
Keywords: LLC resonant converter, digital control, phasor transformation
1. Introduction
Nowadays, liquid crystal display (LCD) panels are widely used in applications such as monitors,
notebooks and televisions [1]-[2]. In large scale LCD TV (>30”), the power supply typically requires an
output power ranges from 200 W to 600 W. As the screen area increases, the power required for the 24-Vdc
rail continues to rise until it is no longer practical to implement the SMPS using a flyback topology. As a
result, a variety of higher-power topologies, including the half bridge LLC topology, has been considered to
achieve high efficiency in a compact space with low-EMI generation [3]-[4].
In this paper, the design of a digitally-controlled LLC resonant converter is presented. The LLC
resonant topology allows for ZVS of the main switches, thereby dramatically lowering switching losses and
boosting efficiency. A digital signal controller (dsPIC30F2020) from Microchip corp. is utilized as the
digital controller of the LLC resonant converter. Digital power also provides intelligent adaptability and
flexibility to satisfy any complex power requirement with the straightforward ability to monitor, process and
adapt to system conditions [5]. In order to design the compensation circuit correctly, the large-signal and
small-signal model of the utilized LLC converter is also derived in this paper using the phasor transformation
method [6]-[7]. Finally, experimental results are presented to validate the correctness of the proposed system.
The measured efficiency of the whole system achieved 87 %.
2. Hardware Description
Fig. 1 shows the circuit configuration of the proposed digitalized LLC resonant converter. From Fig. 1,
the whole hardware system can be divided into two major parts:  LLC resonant converter DC/DC power
stage and digital controller. Detailed descriptions about each part will be given in the following subsections.
2.1. LLC resonant converter
The half-bridge LLC resonant topology is utilized in this paper to convert the output voltage of power
factor corrector front stage (typically 400 V) into voltages needed to supply the functional blocks such as
backlighting, microprocessor and interfacing circuits, etc. LLC resonant converter is widely used in
consumer applications such as LCD TVs or plasma display panels with output power level ranges from 200
W up to 600 W. The advantages of LLC converter include:
• ZVS capability over the entire load range
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• Lower electromagnetic interference (EMI)
• Zero current turnoff of the secondary diodes
• No increased component count comparing to conventional half bridge topology.
From Fig. 1, the primary side of the LLC converter is a half-bridge configuration. The secondary side is
a center-tapped rectifier followed by a capacitive filter. Switches S1 and S2 are both driven by 50 % duty
cycle gating signals, with a small amount of dead time introduced between the consecutive transitions. The
circuit has three passive components, Lr, Cr and Lm, where Lm is the magnetizing inductance that acts as a
shunt inductor, Lr is the series resonant inductor, and Cr is the resonant capacitor.
Fig. 2 shows the typical waveforms of the presented LLC resonant converter. From Fig. 2, the operation
of half of switching cycle can be divided into four modes.
(1) Mode 1 (t0 < t < t1)：at t=t0, S1 turned on. During this mode, output rectifier diode D1 conduct. The
transformer voltage is clamped at Vo. Lm is linearly charged with output voltage, so it doesn’t participate
resonant during this period and V p = n ⋅ Vout . The current iLr and iLm increases. The energy flows through
the resonant tank and transformer and to the load. This mode ends when iLr current is the same as iLm
current. Output current reach zero.
(2) Mode 2 (t1 < t < t2)：at t=t1, , the two inductor current iLr and iLm are equal. Output current reaches
zero. Both output rectifier diodes D1 and D2 is reverse biased. Transformer secondary voltage is lower than
output voltage. Output is separated from transformer. During this period, since output is separated from
primary, Lm is freed to participate resonant. This mode ends when S1 is turned off.
(3) Mode 3 (t2 < t < t3)：at t=t2, S1 is turned off. During this mode, S1 and S2 are both off. The resonant
current iLr charges (discharges) the parasitic capacitance Coss1 ( Coss 2 ) of the power switches. When the
voltage across Coss1 equals Vin, the body diode of S2 is turned on.
(4) Mode 4 (t3 < t < t4)：The body diode of S2 is turned on in previous mode, which creates a ZVS
condition for S2. Gate signal of S2 should be applied during this mode. When S2 is turned on, iLr decreases
and this will force secondary diode D2 conduct and iout begin to increase. Also, from this moment,
transformer sees output voltage on the secondary side. Lm is clamped with constant voltage V p = −n ⋅ Vout , so
it doesn’t participate resonant during this period.
For next half cycle, the operation is same as analyzed above and is omitted here.
2.2. Digital controller
In this paper, the dsPIC30F2020 digital signal controller (DSC) from Microchip corp. is used to control
the presented LLC resonant converter. The proposed digital power supply offers a new freedom for users to
control the power output of the LCD power supply according to the power requirement, this leads to an
energy economy. When compared to an analog one, digital control boasts several advantages such as
• the possibility for implementing sophisticated control laws
• allows the designer to modify the control strategy without significant hardware modifications
• higher tolerance to signal noise and the complete absence of ageing effects or thermal drifts
• the possibility to provide man to machine interface (MMI)
The digital controller utilized in this paper is a conventional PID controller; the digital control algorithm
can be designed as:
u (n) = u (n − 1) + K 0 ⋅ e(n) + K1 ⋅ e(n − 1) + K 2 ⋅ e(n − 2)
(1)
where E(n) is the error signal and U(n) is the input signal.
It should be noted that the LLC converter works with variable frequency control. This implies that
power flow can be controlled by changing the operating frequency of the converter in such a way that a
reduced power demand from the load produces a frequency rise, while an increased power demand causes a
frequency reduction. Unlike the conventional PWM control strategies in which the duty cycle is the control
variable. For LLC converter, the control variable is the switching frequency. Therefore, for the presented
digital controller, the output of PID controller should be fed into the Period register of the utilized PWM
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module while the PDC (duty cycle) register should be set as half the value of that in Period register to obtain
50 % duty cycle.
Fig. 1 Circuit configuration of the digitalized LLC resonant converter
Fig. 2 Typical waveforms of LLC Resonant Converter
3. Small-signal Model of the LLC Resonant Converter
In order to design the parameters (Kp, Ki and Kd) of the compensation circuit, a small-signal model
should first be obtained. In this paper, the phasor transformation method as presented in [6] is utilized. In
[6], a modified phasor transform as shown in Eq. (2) can be used to apply to variable frequency operation.
j ωs ( t )dt
x(t ) = Re[ x (t )e ∫
]
(2)
where x (t ) is the time-varying phasor corresponding to x(t), which presents the amplitude of x(t), and
ωs (t ) is time-varying instantaneous frequency. Using Eq. (2), the phasor equations for resistor, inductor and
capacitor can be derived respectively. Using the same technique, the phasor presentation of a general
switching network and a rectifier with a transformer can also be obtained. Since the LLC resonant converter
only consists the above-mentioned parts, the large signal and small signal models of the LLC resonant
converter can be obtained and are presented in Fig. 3. Using this small-signal model, the transfer function
can be derived and the compensation circuit can be designed accordingly. Fig. 4 shows the open loop and
close loop bode plot of the presented system. From the close loop bode plot, the cut-off frequency is 20 kHz
and the phase-margin is 55.8 degree. The obtained compensation circuit parameters can be described as
(1 +
Gc ( s ) = Gc 0 ×
s
ωz
)(1 +
(1 +
s
ωp
ωz 2
s
)
≈ 5.34 +
)
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K
3.31 × 103
+ 11.89 × 10−4 s = K P + I + K D s
s
s
(3)
(a) Large signal model
(b) Small-signal model
Fig. 3 Large and small signal model of LLC Resonant Converter
(a) Open loop
(b) Close loop
Fig. 4 Open loop and close loop bode plot of the utilized LLC Resonant Converter
4. Experimental Results
In order to verify the correctness of the proposed system, some experiments are carried out. Due to
limited space, only selected waveforms are displayed in this section. The specification of the presented
prototype system is
1) Input voltage: 400 Vdc
2) Output voltage: 24 Vdc
3) Output current: 14.6 A (350 W)
Fig. 5 shows the measured key waveforms of the proposed LLC converter. From Fig. 15, the proposed
system operates in LLC resonant mode correctly. Fig. 6 shows the measured efficiency of the proposed LLC
converter. From Fig. 6, the efficiency of the proposed LLC is higher than 87 %. Fig. 7 shows the photo of
the proposed system.
(a) light load (3 A) (b) full load (14.6 A)
(VGS：10 V/div, VDS：500 V/div, Vp：500 V/div, ir：2 A/div, Time：10 μs/div)
Fig. 5 Measure key waveforms of the proposed LLC converter
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Fig. 6 Measured efficiency of the proposed LLC converter
Fig. 7 Photo of the proposed system
5. Conclusion
In this paper, the design of a digitalized LLC resonant converter is presented. Detailed description of the
LLC resonant converter stage and the digital controller is presented. Since the LLC converter works with
variable frequency control, the design consideration of implementing frequency control using commercially
available microcontroller is also presented. In order to design the compensation circuit correctly, the largesignal and small-signal model of the utilized LLC converter is also derived in this paper using the phasor
transformation method. According to the experimental results, the efficiency of the proposed LLC converter
is higher than 87 % for the whole load range.
6. References
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[3] F. Krismer and J. W. Kolar, “Accurate small-signal model for the digital control of an automotive bidirectional
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[5] Yan-Fei Liu; Meyer, E.; Xiaodong Liu, “Recent Developments in Digital Control Strategies for DC/DC Switching
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