Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 9, No. 3, 2024 205 Review Of Control Methods of Dual‐Output Half‐Bridge LLC Resonant Converter Ruibo Sun, Jiacheng Mai School of Electrical Engineering, Southwest Minzu University, Chengdu, 610041, China Abstract: Dual-output half-bridge LLC resonant converters are widely used in data centres, aerospace, portable electronic devices, new energy vehicles and other fields due to their superior performance of high frequency, high efficiency and high power density. This paper analyses the topology and operating principle of the dual output LLC resonant converter; introduces the traditional control methods and the new control methods in recent years. For each control method, its principle, characteristics, advantages and disadvantages are described. Finally, the future research direction and development trend of dual-output half- bridge LLC resonant converter are outlooked. Keywords: Dual-output half-bridge LLC resonant converter; Control strategy. 1. Introduction In many applications, outputs of different voltage levels need to be provided simultaneously, such as power adapters, server power supplies, etc. The dual-output half-bridge LLC resonant converter can meet this multi-output requirement, simplify the system structure, and reduce cost and volume[1]. The dual-output half-bridge LLC resonant converter can provide multiple output voltages in the same size, thereby increasing the power density of the system. At the same time, the use of resonant topology can improve conversion efficiency and reduce energy loss. The design of the dual-output half-bridge LLC resonant converter has strong flexibility and can adapt to a variety of application scenarios and voltage requirements. Through reasonable design, independent control and regulation of different output voltages can be achieved. By using the LLC resonant topology, the dual-output half-bridge LLC resonant converter can achieve better electromagnetic compatibility (EMC) and electromagnetic interference (EMI) suppression, improving the stability and reliability of the system[2]. As the functions of electronic devices continue to increase and diversify, the requirements for power systems are becoming higher and higher. As an efficient and flexible power conversion solution, the dual-output half-bridge LLC resonant converter has broad application prospects, especially in the fields of new energy, smart homes, electric vehicle chargers, and data centers. In summary, the dual-output half- bridge LLC resonant converter has important research significance and application value in meeting multiple output requirements, improving power density and efficiency, and improving system stability and reliability[3]. Determine the design and optimization methods of the dual-output half-bridge LLC resonant converter to meet multiple output requirements, including the selection of output voltage, the design of circuit topology, etc. By optimizing the design and control strategy, the power density and conversion efficiency of the dual-output half-bridge LLC resonant converter can be improved to meet the growing power density and energy efficiency requirements. Analyze the stability and reliability of the dual-output half-bridge LLC resonant converter under different operating conditions, and design effective control algorithms and protection mechanisms to ensure safe and reliable operation of the system. Study the needs of dual-output half-bridge LLC resonant converters in specific application fields (such as electric vehicle chargers, smart homes, data centers, etc.), and optimize the design to meet the special requirements in this field[4-8]. Study the circuit topology of dual-output half-bridge LLC resonant converter, including different design schemes such as parallel output and series output, and perform design optimization. Analyze the control strategy of dual-output half- bridge LLC resonant converter, including traditional control methods and advanced control methods, and conduct performance evaluation and comparison. Study the selection and performance characteristics of the power devices required for the dual-output half-bridge LLC resonant converter, including switching tubes, inductors, capacitors, etc., and analyze their impact on system performance. Study the selection and performance characteristics of the power devices required for the dual-output half-bridge LLC resonant converter, including switching tubes, inductors, capacitors, etc., and analyze their impact on system performance. Study the selection and performance characteristics of the power devices required for the dual-output half-bridge LLC resonant converter, including switching tubes, inductors, capacitors, etc., and analyze their impact on system performance. By clarifying the purpose and scope of the research, the development of research work can be effectively guided, ensuring that the research results can achieve the expected goals and have practical application and promotion value[9-11]. This article analyzes the topology and working principle of the dual-output half-bridge LLC resonant converter; introduces the traditional control method of the dual-output half-bridge LLC resonant converter and the new control methods in recent years; for each control method, its principles and characteristics are described and pros and cons. Finally, the future research direction and development trend of dual-output half-bridge LLC resonant converter are prospected. 2. Overview of Dual Output Half- Bridge LLC Resonant Converter 2.1. Basic principles and working principles Fig.1-1 is the schematic diagram of a dual-output half-bridge LLC resonant converter. The topology structure of the dual- output half-bridge LLC resonant converter mainly includes a DC voltage source, a half-bridge switching network, a resonant tank, 206 an ideal transformer, a rectifier circuit, a filter circuit and an output load; among them, Vin is the DC voltage source, Q1 and Q2 are Switching tube, Coss1 and Coss2 are switching tube parasitic capacitances, Lr is resonant inductance, Cr is resonant capacitance, Lm is excitation inductance, the transformation ratio of transformer T is N1:N2:N3, D1~D4 are the rectifier diodes of output branch 1 , D5~D8 are the rectifier diodes of output branch 2, C1 and C2 are the output filter capacitors of output branches 1 and 2 respectively, rc1 and rc2 are the equivalent resistances of the filter capacitors of the two output branches respectively, R1 and R2 are respectively are the load resistances of output branches 1 and 2; Vo1 and Vo2 are the output voltages of output branches 1 and 2 respectively, vab is the switching network output voltage, vCr is the resonant capacitor voltage, iLr is the resonant inductor current, and iLm is the excitation inductor current, ip is the primary current of the transformer. Vin Lm N1 N2 N3iLm Tip Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr vCr b a Figure 1-1. Dual output half-bridge LLC resonant converter schematic diagram Fig.1-2 shows the key waveform diagram of the dual-output half-bridge LLC resonant converter. To simplify the analysis, it is assumed that: 1) Except for the switching tubes Q1 and Q2 and the secondary-side output capacitors C1 and C2, the remaining components in the circuit are ideal components; 2) The frequency of the disturbance signal is much smaller than the switching frequency. As can be seen from Fig. 2, within a switching cycle (t0-t8), the dual-output half-bridge LLC resonant converter has eight operating modes. iLr iLm Q1 Q2Q1 Q2 iLr iD1 iD4 iD5 iD8 iD2 iD3 iD6 iD7 t0 t1 t2t3t4t5 t6 t7t8 t iLm Figure 1-2. Key waveform diagram of dual output half- bridge LLC resonant converter Mode 1 (t0—t1): At time t0, Q2 is turned off, and the circuit enters the mode 1 stage. The current flowing through the resonant inductor Lr is negative. At this time, the body diode of Q1 is turned on, creating a state for the ZVS turn-on of Q1. conditions. At this stage, energy is fed back to the input Vin. When the body diode of Q1 is turned on, iLr begins to gradually increase, the polarity of the primary winding of the transformer becomes up positive and down negative, and the polarity of the two secondary windings of the transformer is also up positive and down negative. The rectifier diodes D1, D4, D5 and D8 are turned on. The voltage on Lm is clamped by the output voltage. Therefore, only Lr and Cr participate in resonance, and Lm is charged at a constant voltage during this process. The equivalent circuit is shown in Fig.1-3. Mode 2 (t1—t2): When the current flowing through the resonant inductor iLr changes from negative to positive, it enters mode 2. Q1 has already input the gate drive signal in mode 1, so Q1 is forward-conducting at time t1. At this time, the rectifier diodes D1, D4, D5 and D8 are turned on, the voltage on the primary side of the transformer is clamped, and Lm is charged linearly. Does not participate in resonance. In this mode, the entire circuit is equivalent to the series resonant circuit energy of the resonant inductor Lr and the resonant capacitor Cr being transferred from the input end to the output end. At time t2, the current flowing through Lr is equal to the current flowing through Lm, and mode 2 ends. At this time, the current flowing through the rectifier diodes D1, D4, D5 and D8 becomes zero. The equivalent circuit is shown in Fig.1-4. Mode 3 (t2—t3): At time t2, the current iLr flowing through the resonant inductor Lr is equal to the current iLm flowing through the excitation inductor Lm. The rectifier diodes D1~D8 are reversely blocked, the output is isolated by the transformer, and the excitation inductor Lm begins to participate. Resonance, connected in series with Lr and Cr to form a series resonant circuit. Output capacitors C1 and C2 continue to power the output. Q1 turns off at time t3, and the mode 3 working state ends. The equivalent circuit is shown in Fig.1-5. Mode 4 (t3-t4): Starting from time t3, Q1 and Q2 are turned off and enter the dead time. The resonant current iLr charges the parasitic capacitance Coss1 of Q1 and discharges the parasitic capacitance Coss2 of Q2. At this time, iLm is greater than iLr. The difference between the two flows through the primary side of the transformer, and the rectifier diodes D2, D3, D6 and D7 begin to conduct. . At time t4, the discharge of Coss2 ends, the body diode of Q2 is turned on, and the mode 4 working state ends at this time. The equivalent circuit is shown in Fig.1-6. The next four working modes (t4-t8) are symmetrical to the above four working modes respectively, and will not be described again here. Vin Lm N1 N2 N3iLm T Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr b a Figure 1-3. Mode 1 (t0-t1) Vin Lm N1 N2 N3iLm T Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr b a Figure 1-4. Mode 2 (t1-t2) Vin Lm N1 N2 N3iLm T Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr b a Figure 1-5. Mode 3 (t2-t3) 207 Vin Lm N1 N2 N3iLm T Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D7 vab Coss1Q1 Lr iLr Cr b a D8 Figure 1-6. Mode 4 (t3-t4) 2.2. Circuit topology The circuit topology of dual-output half-bridge LLC resonant converter can be divided into dual-output half-bridge LLC resonant converter and dual-output full-bridge LLC resonant converter according to the primary side division of the transformer. The structure of the dual-output half-bridge LLC resonant converter includes two switching tubes and a resonant inductor, forming a half-bridge topology. It is relatively simple in structure, suitable for low and medium power applications, and has high power density and efficiency. The structure of the dual-output full-bridge LLC resonant converter includes four switching tubes and two resonant inductors, forming a full- bridge topology. It enables large power conversions and high efficiencies and is typically used in medium to high power applications. The circuit topology of the dual-output half-bridge LLC resonant converter can be divided into half-wave rectification, half-bridge rectification and full-wave rectification according to the secondary side of the transformer. Half-wave rectification usually consists of a rectifier tube; in half-wave rectification, only the positive half-cycle signal is retained, and the negative half-cycle signal is cut off. This rectification method is relatively simple, but the output DC voltage waveform will have large pulsations. In the field of power electronics, half-wave rectification is often used in some low-power and low-cost application scenarios. Half-bridge rectifier is a common rectifier circuit commonly used to convert alternating current to direct current. In half-bridge rectification, through the work of two rectifier tubes, only the signal of the positive half cycle is retained, while the signal of the negative half cycle is truncated. This rectification method is relatively simple and low-cost, and is suitable for some low-power application scenarios. In the field of power electronics, half-bridge rectifier is often used in some devices that require smaller power output, such as some small power adapters. Full-wave rectification is a rectifier circuit commonly used to convert alternating current to direct current. In full-wave rectification, through the work of four rectifier tubes, both positive and negative half-cycle signals are retained, thereby achieving complete rectification of the input AC signal. Compared with half-wave rectification, this rectification method has a more stable DC output and reduces the pulsation of the output waveform. Full-wave rectification is usually used in applications that require more stable DC output, such as power adapters, electronic equipment, etc. 3. Control Method Classification 3.1. Traditional control methods There are two main traditional control methods for dual-output half-bridge LLC resonant converters: PWM (Pulse Width Modulation) control and PFM (Pulse Frequency Modulation) control. a PWM control Pulse Width Modulation (PWM) control is a commonly used power electronic control method, often used to adjust the conduction time of switching devices (such as MOSFET, IGBT, etc.) in the circuit to adjust the output voltage or current [12]. By changing the duty cycle of the dual-output half-bridge LLC resonant converter, increasing or decreasing the voltage gain, and adjusting the output voltage, the switching frequency of the dual-output half-bridge LLC resonant converter remains unchanged under this control strategy. The gain range of duty cycle adjustment is small. When the duty cycle is small, the control signal of the dual-output half-bridge LLC resonant converter will be lost, which is not conducive to the conduction of the switch tube ZVS. Pulse width control is divided into symmetrical control and asymmetrical control. The symmetrical control switch tube drive signal is shown in Fig.2-1(a). The dual- output half-bridge LLC resonant converter switch tubes are complementary and conductive, and the duty cycle is the same. Symmetrical control When the switch tube dead time is not easy to determine, it is difficult to achieve ZVS conduction. The asymmetric control switching tube driving signal is shown in Fig. 2-1(b). The switching tubes of the dual-output half-bridge LLC resonant converter are complementary and conductive with different duty cycles, which can achieve ZVS conduction. During asymmetric control, the converter's The working mode is asymmetrical and the transformer utilization rate is low. Q1 Q2 Q1 Q2 D1 D2 t t t t (a) Symmetric modulation Q1 Q2 Q1 Q2 D1 D2 t t t t (b) Asymmetric modulation Figure 2-1. PWM control dual output half-bridge LLC resonant converter switch tube drive signal b PFM control Pulse Frequency Modulation (PFM), also known as voltage- type frequency conversion control, has a constant duty cycle of 50%. The disadvantage is that the voltage gain adjustable range is small and the frequency modulation range is narrow. When the system is in a low-gain state, the resonant frequency of the dual- output half-bridge LLC resonant converter is smaller than the switching frequency, and the diode cannot achieve ZCS turn-off [13]. Fig. 1-4 is the schematic diagram of the voltage-type variable frequency control dual-output half-bridge LLC resonant 208 converter. It can be seen from the Fig. that the output voltage vo1 of the sampling branch 1 is compared with the reference voltage vref, and the switching frequency is changed through the voltage- controlled oscillator Vco, and the driving circuit DC controls the on and off of switching tubes Q1 and Q2, thereby regulating the output voltage of the dual-output half-bridge LLC resonant converter. Its working principle is shown in Fig.2-2. Vin Lm N1 N2 N3iLm Tip Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr vCr b a vref AM vo1 Vco Vg1 Vg2 DC Figure 2-2. Schematic diagram of voltage-type frequency conversion controlled dual-output half-bridge LLC resonant converter The pulse-controlled dual-output half-bridge LLC resonant converter will adjust the operating frequency and thus the output voltage only when the output voltage changes. Therefore, its load transient response is slow and there is a high voltage overshoot during adjustment. 3.2. Other control methods a Average current type pulse frequency control The essence of average current type pulse frequency control [14] is also variable frequency control. In order to improve the load transient response speed of the dual-output half-bridge LLC resonant converter, a current loop is introduced to sample the inductance of the resonant cavity of the dual-output half-bridge LLC resonant converter. Current, because the inductor current phase leads the output voltage, when the inductor current changes, the converter adjusts the output voltage by changing the switching frequency. Therefore, compared to the voltage-type pulse frequency control, the average current-type pulse frequency control dual-output half-bridge LLC resonant converter has a faster load transient response than the voltage- type pulse frequency control dual-output half-bridge LLC resonant converter. . Its working principle is shown in Fig. 2-3. Vin Lm N1 N2 N3iLm Tip Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr vCr b a vref AM vo1 VCO Vg1 Vg2 DC iAV AM Figure 2-3. Average current mode controlled dual output half-bridge LLC resonant converter schematic diagram The average current type control dual output half-bridge LLC resonant converter control circuit consists of a current inner loop and a voltage outer loop. The current inner loop samples the resonant network inductor current through the average current sampling circuit to obtain iAV, and generates current compensation through the error amplifier AM. signal; similarly, the voltage outer loop samples the output voltage signal, and generates a voltage compensation signal through the error amplifier AM. The current compensation signal and voltage compensation signal are used as inputs of the voltage controlled oscillator VCO, and are output through the drive circuit DC to the dual output half-bridge LLC converter. Control the pulse signal and adjust the output voltage. b Dual frequency control Dual-frequency control [15] (Bi-Freqencuy Control, BFC) controls the dual-output half-bridge LLC resonant converter through two sets of pulse signals of different frequencies. Since the dual-frequency control strategy control loop does not require a compensation circuit, it has fast Faster response to changing load transients and lower voltage overshoot. The disadvantage is that under the dual-frequency control strategy, the output voltage of the dual-output half-bridge LLC resonant converter will produce low-frequency oscillation, which will increase the output voltage ripple. Its working principle is shown in Fig. 2-4. Vin Lm N1 N2 N3iLm Tip Q2 Coss2 D1 C1 R1 rc1 Vo1 + - D2 D4D3 D5 C2 R2 rc2 Vo2 + - D6 D8D7 vab Coss1Q1 Lr iLr Cr vCr b a PH Sample/Hold Circuit vo1 Logic Control Circuit PL vref CM PH PL Vg1 Vg2 DC 1 0 Figure 2-4. Dual-frequency controlled dual-output half- bridge LLC resonant converter The dual-frequency control dual-output half-bridge LLC resonant converter control circuit includes an output voltage sampling and holding circuit (Sample/Hold Circuit), a logic control circuit (Logic Control Circuit), a comparator (CM), and a drive circuit (DC). The dual-frequency controlled dual-output half-bridge LLC resonant converter achieves output voltage regulation through two sets of control pulses PH and PL with a duty cycle of 50% and frequencies of fH and fL. The output voltage sampling circuit samples the output voltage vo and compares it with the voltage reference value vref. When vo1>vref, the logic control circuit connects the control pulse signal PH with the frequency fH to the control circuit to reduce the output voltage. In the same way, when vo1