PFM-PWM Control Resonant DC/DC Converter Based on SG1525

1 Introduction
LLC resonant converter is widely used in the field of distributed power supply system because of its simple structure, wide input voltage range, high working efficiency, zero voltage conversion (ZVS) of primary switch tube and zero current conversion (ZCS) of secondary rectifier diode by relying on its own resonant characteristics without external buffer circuit, high integration of magnetic parts and low electromagnetic interference level. Military environment has strict requirements on the volume, weight and electromagnetic interference level of power converter, so LLC resonant converter should have good application prospects under military conditions. After analysis and research, SG1525 is used as cavity chip to realize frequency modulation control. SG1525 is a monolithic integrated PWM control chip with excellent performance, complete functions and strong versatility. It is widely used in switching power supply, and its working environment temperature is: -55～150℃. Through the reasonable design of its peripheral circuit, it realizes the function of PFM, and automatically enters the PWM working state under high voltage input and light load conditions, which improves the power supply performance under this working condition.

2 Main circuit topology and control principle
2.1 Main circuit structure
Due to the high output voltage, the main circuit adopts a symmetrical half-bridge LLC resonant converter with secondary voltage doubler rectification, as shown in Figure 1.

LLC谐振变换器采用固定占空比，通过改变开关频率大小来调节输出电压的变频控制模式。相比传统的PWM控制，变频控制由于保持占空比固定更适用于输入电压范围较宽的场合。而且当系统进入闭环时随着输入电压的加大，频率增大，回路电流减小，使得开关管的通态损耗减少。不同频率下变换器的工作性能各不相同。在LLC谐振变换器中，由于该谐振网络非单一的LC构成，固存在不同的特征谐振频率。当变压器向次级传递能量时，其初级电压被输出电压箝位，谐振槽路中只有Lr和Cr发生谐振，其谐振频率为串联谐振频率，其表达式为：

When the transformer does not transfer energy to the secondary, Lr, Cr and Lm resonate together, and the resonant frequency is: The series-parallel resonant frequency is:

According to fr and fm, the operating frequency fs range of the LLC resonant converter is divided into three regions: fs The circuit structure of the half-bridge LLC resonant converter is simplified and equivalently transformed, as shown in Figure 2.

In the figure, 1 and 2 are connected to the midpoint of the two switching tubes of the primary bridge arm and the midpoint of the bridge arm voltage divider capacitor in Figure 1. Since VQ1 and VQ2 are turned on alternately, the input of terminals 1 and 2 is a square wave with an amplitude of Ui/2 and a symmetrical positive and negative half-cycle. Re is the equivalent load converted from the secondary to the primary. The fundamental wave analysis method is used to obtain the input impedance of terminal 1.2:

Where: ωs is the switching angular frequency, ωs=2πfs.
When ωs<ωm, the impedance of the converter is capacitive; when ωs>ωm, the impedance of the converter is inductive. For LLC resonant converters, the switching frequency is usually set to work only within a range greater than fm. Because when the switching frequency is less than fm, the characteristic of the capacitive circuit is that the voltage lags behind the current, and the switch tube cannot achieve zero voltage turn-on at this time. [page]

From the above analysis, it can be seen that for LLC resonant converter, the switching frequency should be set to a range greater than fm, then the working frequency can be divided into two working frequency bands: fmfr. The converter can achieve zero voltage turn-on of the main switch tube under the two working frequency bands, which meets the requirements of soft switching technology. Figure 3 shows the voltage and current waveforms of the rectifier diode under the two frequency bands. Observing the switching state of the secondary rectifier tube, although the secondary rectifier diode can achieve ZCS under the two frequency bands, when fs>fr, VD2 is turned on immediately when VD1 is turned off. Even for fast recovery diodes, reverse recovery loss will be generated when the rectifier diode is commutated; for LLC converters when fmfr, the reverse voltage of the diode is always 2 times Uo after it is turned off. Compared with fs>fr, the loss of the diode will be relatively large. In addition, when fs>fr, the primary of the transformer always transfers energy to the secondary, and the magnetizing inductance does not participate in the resonance, so the operating frequency of the LLC resonant converter is usually designed within fm

2.2 Control circuit and working principle
2.2.1 Control principle of frequency modulation
by SG1525 The circuit structure of the conversion of PWM output mode to PFM output mode by SG1525 includes an internal PWM comparator, an oscillator, and an external variable frequency clock pulse generating circuit. The variable frequency clock pulse generating circuit can be composed of a comparator and some passive components and a logic gate circuit. The error voltage value ud obtained after the feedback voltage is compared with the reference voltage and PI adjustment is connected to the non-inverting input pin 2 of the internal operational amplifier of SG1525. The internal operational amplifier changes into a follower structure. Its output pin 9 signal is compared with the CT oscillation signal to obtain a duty cycle control signal with a frequency equal to half of the oscillation frequency. The larger the signal of pin 9, the greater the difference of 180° between the two outputs of pins 11 and 14. The larger the duty cycle of the phase signal, the greater the maximum duty cycle of each output is close to 0.5. The dead zone size can be adjusted by setting the resistance of pins 5 and 7 to control the maximum duty cycle. ud is sent to the variable frequency clock pulse generating circuit to adjust the oscillator frequency at the same time, as shown in Figure 4.

When the output voltage increases due to some reason (such as the load decreases or the input voltage increases), Ui decreases, and ud is input into the variable frequency clock pulse generating circuit. It is compared with the sawtooth wave generated by the oscillator to generate a narrow pulse T with a higher frequency. After T is shaped, a narrow pulse clock with a higher frequency is generated. Since this frequency is higher than the oscillation frequency originally set by SG1525, the narrow pulse is sent to the pin 3 of SG1525 to keep its internal oscillator working synchronously. When the narrow pulse arrives, the oscillator is immediately reset, that is, the sawtooth wave is pulled down in advance, and then enters the next oscillation cycle, which is equivalent to keeping the oscillation frequency synchronized with the pin 3 pulse, so the oscillator frequency increases. The peak value of the sawtooth wave is Ui. Since the voltage of the SG1525 pin 9 is also Ui, the control signal with the maximum duty cycle can always be obtained after the internal comparator of SG1525 is delivered, realizing PFM control. The PFM control process is shown in Figure 5. [page]

As shown in Figure 5, different Ui will generate clock pulses of different frequencies through the variable frequency clock pulse generation circuit. This clock pulse controls the reset of the oscillator, thereby generating a variable frequency output pulse with a duty cycle tending to 0.5, and different Ui corresponds to different frequency output clock pulses. Ui has a corresponding relationship with the frequency of the output pulse. By automatically adjusting Ui, variable frequency control can be achieved, and finally stable control of the output voltage can be achieved. In summary, SG1525 can realize PFM control mode.

2.2.2 Control principle of SG1525
In actual circuit operation, when the output is light-loaded or no-loaded, Uo is a stable output voltage, and the required switching frequency will be very large. On the one hand, it will increase the loss when light-loaded or no-loaded, and on the other hand, the maximum oscillation frequency that SG1525 can work is also limited (400 kHz). Therefore, it is necessary to solve the problem of stable control of Uo under no-load or light-load working conditions. The SG1525 used here adopts PWM control mode when light-loaded or no-loaded. The control circuit in Figure 4 is improved and designed, a circuit with a larger value is added, and a voltage threshold is set to realize the switching between the PFM mode and the PWM mode of the converter. The schematic diagram of the control strategy is shown in Figure 6. As the load decreases, Uo will increase, ud will decrease, and the switching frequency will increase to stabilize Uo. When Ui decreases further, Uref2 is the dividing point between the PFM output working mode and the PWM output working mode, which is also the critical point between the PFM and PWM control modes of the converter. When Ui continues to decrease and Ui

3 Experimental results
The main technical indicators of the DC/DC converter engineering prototype are: input DC voltage (270±30)V, output DC voltage 340V, rated power 340W, working efficiency greater than 90%. The specific parameters are as follows: fr=130kHz, parallel resonant frequency 70kHz, turns ratio n=1, Lm=197μH, Lr=55μH, Cr=33nF.

Figure 8a shows the voltage waveform of the switch tube and the resonant inductor current waveform when the output is full load 340 W, and Figure 8b shows the voltage waveform of the switch tube and the resonant current waveform when the output is 1.5 W (light load). The output voltage can be kept stable under no-load and light-load conditions.

4 Conclusion
A simple and feasible method of using SG1525 to achieve variable frequency control and PWM control under light load is proposed, and the correctness of the theoretical analysis is verified by experiments. The designed LLC resonant converter circuit has a simple structure and reliable operation, which can meet the requirements of wide temperature power supply.