Design and performance test of a high-efficiency flyback switching power supply

Due to the problems of harmonic pollution to the power grid and low working efficiency caused by traditional switching power supplies, various switching power supply research institutions at home and abroad are currently striving to use various high-tech technologies to improve power supply performance [1]. Among them, the research and development approach of reducing electromagnetic pollution through power factor correction PFC (Power Factor Correction) technology and improving efficiency by using synchronous rectification technology in switching power supply design is particularly valued. References [2-3] specifically discussed active power factor correction (APFC) technology; Reference [4] reviewed the latest development of single-phase parallel technology; References [5-6] optimized the design of AC/DC converters and boost PFC converters with load current feedback and parallel PFC chips, but the designed power supply efficiency and power factor were below 85% and 90% respectively, and its performance needs to be further improved.

This paper designs and manufactures a high-efficiency and low-electromagnetic-pollution switching power supply prototype. Test results show that the power supply has excellent dynamic performance, high power factor and working efficiency, and is simple to control, so it has certain practical application value.

1 Switching power supply design

The structure of the switching power supply is shown in Figure 1. It mainly consists of three parts: 220V AC voltage rectification and filtering circuit, power factor correction circuit, and DC/DC converter.

The 220V AC is rectified and supplied to the power factor correction circuit. Boost type PFC is used to improve the input power factor of the power supply, while reducing the harmonic current, thereby reducing harmonic pollution. The output of PFC is a DC voltage UC, which can be converted into the required two output DC voltages Uo1 (12V) and Uo2 (24V) through DC/DC conversion.

As can be seen from the figure, the key to the design of this power supply system is to add a power factor correction circuit between the rectifier filter and the DC/DC converter, so that the input current is strictly controlled by the input voltage to achieve a higher power factor. At the same time, the design also uses synchronous rectification technology to reduce rectification losses and improve DC/DC conversion efficiency. The use of a flyback quasi-resonant DC/DC converter can not only enhance the adaptability to input voltage changes, but also reduce operating losses.

In order to ensure the performance of the switching power supply, some circuits are added during the actual production of the power supply: (1) Protection circuit. Prevent overvoltage, overcurrent or short circuit of the load itself; (2) Soft start control circuit. It can ensure that the power supply works stably, reliably and orderly, and prevent voltage and current overshoot during startup; (3) Surge absorption circuit. Prevent the output ripple peak-to-peak value from being too high and the generation of high-frequency radiation and high-order harmonics caused by surge voltage and current.

2. Selection of main components of switching power supply

2.1 APFC chip and control solution

The power factor correction circuit in the power supply is based on the TDA4863 chip produced by Infineon, and the circuit is shown in Figure 2. The switch tube VT1 uses an enhanced MOSFET. The specific control scheme is: feedback sampling from point A on the load side, introducing double closed-loop voltage series negative feedback to stabilize the input voltage of the DC/DC converter and the output voltage of the entire system.

2.2 Quasi-resonant DC/DC Converter

There are many types of DC/DC converters [7]. In order to ensure the safety of electricity use, this design scheme chooses an isolated type. Isolated DC/DC conversion forms can be further divided into forward, flyback, half-bridge, full-bridge and push-pull types. Among them, half-bridge, full-bridge and push-pull types are usually used in high-power output situations. Their excitation circuits are complex and difficult to implement; while forward and flyback circuits are simple and easy to implement. However, since the flyback type is more adaptable to input voltage changes than the forward type, and the PFC output voltage in this power supply system will change greatly, the UC/UO conversion in this design adopts the flyback method, which is conducive to ensuring that the output voltage is stable.

This design uses ONSMEI (ON Semiconductor) quasi-resonant PWM driver chip NCP1207, which always keeps the MOSFET turned on when the drain voltage is the lowest, improving the turn-on mode and reducing the turn-on loss.

Figure 3 is a DC/DC flyback converter circuit designed using the NCP1207 chip. Its working principle is as follows: PFC outputs a DC voltage UO, one path is directly connected to the transformer primary coil L1, and the other path is connected to the high-voltage terminal 8 pin of NCP1207 through a resistor R3, so that the circuit starts to oscillate and forms a soft start circuit; the 5-pin output drive pulse of NCP1207 turns on the switch tube VT, and L1 stores energy. When the drive is turned off, coils L2 and L3 release energy, and the secondary is rectified and filtered to supply power to the load. The auxiliary coil releases energy, and a part of it is rectified and filtered to supply power to VCC to form a bootstrap circuit, and the other part is sent to the 1-pin of NCP1207 after voltage division by resistors R1 and R2 to determine the soft opening moment of VT; the optocoupler P1 feeds back the signal from the output voltage, which is filtered by the integration circuit composed of resistor R7 and capacitor C2 and sent to the 2-pin of NCP1207 to adjust the stability of the output voltage. This is the voltage feedback link. Resistor R6 samples the main current signal, which is filtered by the integrating circuit composed of series resistor R5 and capacitor C4 and then sent to pin 3 of NCP1207. This is the current feedback link.

2.3 Synchronous Rectifier

The power supply system adopts current-driven synchronous rectification technology [8]. The basic idea is to use a low on-resistance MOSFET to replace the rectifier diode on the output side of the DC/DC converter to minimize the rectification loss. That is, the MOSFET driving signal is obtained by detecting the current flowing through itself. VT1 is turned on when the forward current flows through it, and is turned off when the current flowing through itself is zero, so that the reverse current cannot flow through VT1. Therefore, MOSFET can only conduct in one direction like the rectifier diode.

The main considerations for selecting synchronous rectifiers are large on-state current, small on-state resistance, large enough reverse withstand voltage (calculated according to the reverse voltage of the transformer secondary transformation at 24V), and short reverse recovery time of the parasitic diode. After analysis and calculation of the actual circuit, the MTY100N10E MOSFET produced by ONSEMI was selected, which has a withstand voltage of 100V, an on-state current of 100A, an on-state resistance of 11MΩ, a reverse recovery time of 145ns, a turn-on delay time of 48ns and a turn-off delay time of 186ns, which can meet the system working requirements.

3 Means of reducing energy consumption and electromagnetic pollution

3.1 Consumption reduction measures

(1) Utilize the superior performance of TDA4863 chip

The performance characteristics of TDA4863 are: when the input voltage is high, the APFC circuit on the chip draws more power from the power grid; conversely, when the input voltage is low, it absorbs less power, which suppresses the generation of harmonic currents and makes the power factor close to unity power factor; the chip also contains an active filter circuit that can filter out the harmonic currents generated by output voltage pulsation; the chip's micro-current operating conditions also reduce the loss of components.

(2) Voltage and current dual closed-loop feedback

Because the whole system forms a double closed-loop system, the DC/DC converter increases the input resistance and reduces the output resistance when outputting a stable voltage, thus achieving the purpose of closed-loop control. The converter presents a synchronous rectification mode at higher power, and the switch tube and the rectifier tube are both turned on at zero voltage at lower power. Synchronous rectification or zero voltage turn-on greatly reduces tube consumption.

3.2 Measures to reduce electromagnetic pollution

(1) Install an electromagnetic interference (EMI) filter on the AC side

The purpose of setting up EMI filters is to suppress high-frequency interference conducted on the power line and prevent harmonics generated by the power supply device from polluting the power grid.

(2) Install filter capacitor on the DC side

Four filter capacitors are connected in parallel at both ends of the rectifier bridge to weaken the impact of the rectifier part on the system operation.

(3) Optimize component layout and reduce wiring distance

In the primary rectification circuit, the diode is placed close to the transformer, while in the secondary rectification circuit, the diode is placed close to the transformer and the output capacitor.

(4) Reasonable grounding

On the one hand, in order to reduce the grounding impedance and eliminate the influence of distributed capacitance, the part that needs to be grounded should be connected to this end as close as possible during installation; on the other hand, the common ends of the low-frequency circuit, high-frequency circuit and power circuit should be connected separately and then connected to the reference ground end.

4 Analysis of prototype test results

4.1 Rectifier bridge and switch tube test waveform

The experimental circuit was tested using the Tektronix oscilloscope TDS5034B. Figure 4 shows the waveforms of the post-stage DC/DC converter when the load is 12V/1.53A and 24V/1.70A. Among them, udr and ud are the driving voltage of the switch tube VT1 and its drain voltage respectively, u5 is the voltage of the 5th pin of TDA4863, that is, the inductor zero current detection voltage, and ui is the rectifier bridge sinusoidal half-wave output voltage. As can be seen from the figure, the amplitude of ud is basically unchanged due to clamping, and it is a high-frequency rectangular wave; the envelope of u5 shows that the average current waveform of the inductor is close to a sine waveform. When ui is the valley point, the oscillation frequency f0 is significantly reduced, so the current reference signal is also at a low point, and when the output power is constant, the small peak current cannot increase u5; near the peak of ui, f0 is also low, because the current reference signal is also near the peak, the inductor current peak and output power are large, but because the output average power is constant, f0 is reduced.

4.2 Switching tube voltage waveform at different input AC voltages

Figure 5 shows the waveform of the drain voltage ud of the switch tube VT1 measured under different ui when the load is 12V/1.1A and 24V/3.2A. It can be seen from the figure that when ui is in the low voltage range of 90V~150V, ud is 252V and remains unchanged; when ui is in the high voltage range of 210V~260V, ud remains unchanged at 382V. This shows that the power supply system has achieved the goal of output voltage following the input AC voltage change.

4.3 Output ripple voltage waveform

Figure 6 shows the high-frequency and low-frequency ripple voltages of the APFC output. As can be seen from the figure, the high-frequency ripple voltage is about 3V, and when the low-frequency ripple frequency is 100Hz, the ripple voltage is about 10V. Since the subsequent stage is a flyback DC/DC converter, it has no effect on the output voltage.

4.4 Switching power supply main project test data

The experimental data tested under different loads and input AC voltages are shown in Table 1. In the table, Ui, Ii; UO, IO; Pi, PO represent the AC input voltage, input current; output voltage, output current; input power, output power of the entire power system respectively. The power factor cosΦ of the prototype is obtained by testing with a WT3000 high-precision power analyzer. The specific test situation is: when the power system is not started, cosΦ is only about 0.625, but when the system is working, cosΦ gradually increases and reaches above 0.952, and the peak point can reach 0.989. It can be seen that the power factor improvement of the power system is obvious.

Compared with ordinary switching power supplies, the flyback switching power supply designed in this paper has lower power consumption and electromagnetic pollution, and the power factor cosΦ measured on the prototype is higher than 0.95; when the output voltage is 12V and 24V respectively, the corresponding system output ripple voltage is measured to be about 104mV and 185mV; the THD value is as low as 3.75%, which meets the national EMI standard, and the efficiency range of the entire power supply system is 85.8%≤η≤87.9%. Therefore, the designed switching power supply has a high practical application value and can be applied to various small and medium power electronic devices.