Flyback converter based on synchronous rectification technology

With the continuous penetration of electronic technology and information technology in people's lives, the number of electronic products is increasing. Its energy consumption has greatly exceeded the energy used for lighting in people's lives. The National Energy Administration predicts that the national electricity demand in 2010 may reach about 4 trillion kWh, and the growth rate will exceed 8% or 9% in 2009. The national electricity demand is growing very fast, but the growth of power generation is limited. China is facing a serious power shortage problem. Saving energy can significantly reduce the required electricity, while reducing the number of power plants and reducing the pollution of waste gas, waste water and ash discharged by power plants to the environment. And power supply is an important link in saving energy.

Switching power supply is a kind of power supply that uses modern power electronics technology to maintain a stable output voltage by controlling the on-off time ratio of the switch. It is widely used in electronic devices such as computers, televisions, and cameras. The flyback converter has the advantages of simple circuit, input and output voltage isolation, low cost, and less space requirements. It has been widely used in low-power switching power supplies. However, when the output current is large and the output voltage is low, the traditional flyback converter has large conduction loss and reverse recovery loss of the secondary rectifier diode, and the efficiency is low. Synchronous rectification technology uses a dedicated power MOSFET with extremely low on-resistance to replace the rectifier diode. Applying synchronous rectification technology to the flyback converter can greatly improve the efficiency of the converter.

1 Principle of Synchronous Rectification Flyback Converter

The rectifier diode on the secondary side of the flyback converter is replaced by a synchronous rectifier SR to form a synchronous rectifier flyback converter. The basic topology is shown in Figure 1 (a). To achieve synchronous rectification of the flyback converter, the primary MOS tube Q and the secondary synchronous rectifier SR must work in sequence, that is, the conduction time of the two tubes cannot overlap. When the primary MOS tube Q is turned on, SR is turned off and the transformer stores energy; when the primary MOS tube Q is turned off, SR is turned on and the transformer transfers the stored energy to the load. The driving signal timing is shown in Figure 1 (b). In the actual circuit, in order to avoid the primary MOS tube Q and the secondary synchronous rectifier SR being turned on at the same time, there should be a delay between the turn-off time of Q and the turn-on time of SR; similarly, there should be a delay between the turn-on time of Q and the turn-off time of SR.

Figure 1 Synchronous Rectification Flyback Converter

2. Driving the Synchronous Rectifier

The drive of SR is an important issue in synchronous rectification circuits and needs to be reasonably selected. This paper uses discrete components to form a drive circuit. The drive circuit has a simple structure and low cost, and is suitable for converters with a wide input voltage range. The specific drive circuit is shown in Figure 2. The gate drive voltage of SR is taken from the converter output voltage, so the output voltage of the synchronous rectification converter using this drive circuit must meet the SR gate drive voltage requirements.

Figure 2 Driving circuit

The basic working principle of the drive circuit: The current transformer T2 is connected in series with the secondary synchronous rectifier SR in the same branch to detect the current of the SR. When current flows through the body diode of the SR, a current is induced on the secondary side of the current transformer, which is converted into a voltage through R1. When the voltage value reaches and exceeds the forward voltage of the emitter junction of the transistor Q1, Q1 is turned on. When the diode VD conduction voltage is reached, VD is turned on to clamp it. After the transistor Q1 is turned on, the output voltage drives the SR to turn on through the totem pole output circuit. When the sampling voltage of the current in the SR on the secondary side resistor R1 of the current transformer drops below the conduction threshold of Q1, Q1 is turned off and the SR is turned off.

The functions of the components of the synchronous rectifier drive circuit in the figure are described as follows:

SR is a synchronous rectifier, which is used to replace the rectifier diode;

T2 is a current transformer, which is used to detect the current passing through the SR. When current flows through the body diode of the SR, current is induced on the secondary side of the current transformer;

R1 is used to convert the current induced on the secondary side of the current transformer into voltage. At the same time, the value of R1 determines the magnitude of the current on the secondary side of the current transformer when the synchronous rectifier is turned on and off;

C1 and diode VD are used to filter and clamp the voltage on the secondary side of the transformer;

The bias resistor R2, the pull-down resistor R3 and the transistor Q1 form a switch circuit, and the saturation cutoff of Q1 is used to realize the conduction and shutdown of the synchronous rectifier SR;

Q2 and Q3 form a totem pole output circuit, which provides a large enough current to quickly increase the voltage between the SR gate and source to the required value, ensuring that the SR can be turned on quickly. At the same time, it provides a reverse current extraction loop when the SR is turned off, accelerating the SR turn-off.

3 Design of Synchronous Rectification Flyback Converter

3.6 Feedback Circuit Design

The feedback circuit uses TL431 with optocoupler PC817 as reference, isolation and sampling. In the circuit, the reverse input terminal 2 of the error amplifier inside UC3842 is directly grounded, and the transistor collector of PC817 is directly connected to the output terminal 1 of the error amplifier. The error amplifier inside the chip is skipped, and pin 1 is used directly for feedback. Then it is compared with the current detection input pin 3, and the PWM drive signal is output through the latch pulse width modulator. When the output voltage increases, the voltage input to the reference terminal of TL431 after the voltage division by resistors R5 and R6 also increases. At this time, the current flowing through the light-emitting diode in the optocoupler increases, the collector current of the PC817 transistor increases, the collector-emitter voltage of the transistor decreases, and the duty cycle of the output drive signal of pin 6 of UC3842 decreases, so the output voltage decreases, achieving the purpose of voltage regulation. Vice versa, the output is kept constant and is not affected by changes in input voltage or load.

The reference input voltage ref U of TL431 is 2.5V, and the current is 1.5μA. In order to prevent the current at this end from affecting the voltage divider ratio and avoiding the influence of noise, the current flowing through resistor R6 is usually taken to be more than 100 times the reference input current, so:

According to the characteristics of TL431, R5, R6, Uref and Uo have a fixed relationship:

The collector current Ic of the PC817 transistor is controlled by the forward current If of the light-emitting diode. According to the PC817 technical manual, when the forward current If of the diode changes around 5mA, Ic and If have a good linear relationship, and the collector-emitter current Ic of the transistor changes around 5mA. Therefore:

Where Uvref is the voltage of chip pin 8, 5V, and Ucomp is the voltage of chip pin 1. When calculating, take the maximum voltage of pin 1 when the system is stable.

The cathode-to-anode voltage Uka of TL431 needs to be greater than 2.5V for normal operation, and the forward voltage drop Uf of PC817 diode is 1.2V. So:

After calculation and simulation debugging, the resistance and capacitance parameters of the feedback circuit are obtained. Take R6 as 1KΩ, R5 as 3.8KΩ, R8 as 1KΩ, R9 as 120Ω, R7 as 150KΩ, and C4 as 1nF.

4 Simulation Analysis and Conclusion

The Saber simulation software is used to simulate the synchronous rectifier flyback converter designed in this paper. Figure 4 shows the primary MOS tube Q, the secondary synchronous rectifier tube SR drive signal and the secondary inductor current waveform when the input voltage is 200V and full load. It can be seen from the figure that after Q is turned off, SR is turned on after a very short delay, and when the secondary inductor current drops to near zero, SR is turned off. Figure 5 shows the distribution diagram of system efficiency when synchronous rectification and diode rectification are used respectively under full load conditions with input voltages of 100V, 200V, 250V, 300V and 375V.

The simulation results are consistent with the analysis of the working principle of the synchronous rectifier flyback converter and the synchronous rectifier tube drive circuit in this paper. At the same time, the simulation results prove that the drive circuit can well realize the synchronous rectification function, and the use of synchronous rectification technology can well improve the efficiency of the traditional flyback converter. When the input voltage is 100V and the load is full, the converter efficiency is as high as 87.7%.

Figure 4 Waveforms of Ugs (Q), Ugs (SR), is

Figure 5 Distribution of system efficiency