Design and implementation of a practical high-performance switching power supply

With the rapid development of electronic information industry technology, switching power supply devices have been widely used. However, traditional switching power supplies also have problems such as pollution to the power grid and low working efficiency. Therefore, the use of new technologies to improve the performance of switching power supplies has become a research hotspot in the industry at home and abroad. In the design of switching power supplies, the research and development of reducing electromagnetic pollution through power factor correction (Power Factor Correction—PFC) technology and improving efficiency by using synchronous rectification technology are particularly valued. References [2] and [3] specifically discussed active power factor correction (APFC) technology; reference [4] reviewed the latest development of single-phase parallel technology; references [5] and [6] respectively optimized the design of AC/DC converters and boost PFC converters with load current feedback, single switch, parallel PFC chips. However, the efficiency of the power supply systems developed in the above references is only about 80%, and there are no reports on the experimental tests of the relevant power supply systems.

This paper takes reducing the power consumption and electromagnetic pollution of switching power supplies as the starting point, combines PFC technology, quasi-resonant DC/DC conversion and synchronous rectification technology, and designs and produces a high-efficiency, low-electromagnetic pollution "green" switching power supply device. It not only obtains a higher power factor and improves the impact on the power grid, but also significantly improves the working efficiency. It is also simple to control and has certain application value.

1 Overall design of switching power supply

The overall 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 power is rectified and supplied to the subsequent power factor corrector.

The Boost type power factor correction circuit is used to improve the input power factor of the power supply, while reducing the harmonic current and harmonic pollution. The output of the power factor correction PFC in Figure 1 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. The operation of APFC and 24V converter is controlled by sampling the output DC voltage Uo1 (12V).

In order to improve the performance of the switching power supply, some auxiliary circuits are added during the actual production of this power supply (not fully shown in Figure 1). The first is the protection circuit, which can prevent the load itself from overvoltage, overcurrent or short circuit; the second is the soft start control circuit, which can ensure that the power supply works stably, reliably and orderly, and prevent voltage and current overshoot during startup; the third is the surge absorption circuit, which can prevent the output ripple peak-to-peak value from being too high, high-frequency radiation and high-order harmonics caused by surge voltage and current.

2. Key technologies and core components selection

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; the synchronous rectification technology is used to reduce the rectification loss and improve the DC/DC conversion efficiency; the flyback quasi-resonant DC/DC converter is used to enhance the adaptability to input voltage changes and reduce working losses.

2.1APFC chip and control solution

The power system uses the Infineon APFC chip TDA4863 with excellent performance. The designed power factor correction main circuit and component parameters are 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. This design chooses an isolated type to ensure the safety of electricity use. The isolated DC/DC conversion form can be further subdivided into forward, flyback, half-bridge, full-bridge and push-pull types. Among them, half-bridge, full-bridge and push-pull are usually used in high-power output occasions. Their excitation circuits are complex and difficult to implement, while forward circuits and flyback circuits are simple and easy. However, since the flyback type is more adaptable to changes in input voltage 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 a flyback method, which is conducive to ensuring the stability of the output voltage.

The drain voltage of a common flyback converter is usually high when the MOSFET is turned on, which increases the turn-on loss of the MOSFET. This design uses ONSMEI's quasi-resonant PWM driver chip NCP1207, which always turns on when the MOSFET drain voltage is the lowest, improving the turn-on mode and reducing the turn-on loss.

Figure 3 is a flyback converter circuit designed using the NCP1207 chip. Its working principle is: one path of the PFC output DC voltage UO is directly connected to the transformer primary coil L1, and the other path is connected to the NCP1207 high-voltage terminal 8 pin through a resistor R3, so that the circuit starts to oscillate and form a soft start circuit; the 5-pin output drive pulse 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. Part of the energy released by the auxiliary coil L3 is rectified and filtered to supply power to VCC to form a bootstrap circuit, and the other part is divided by resistors R1 and R2 and sent to pin 1 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 pin 2 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 integration circuit composed of series resistor R5 and capacitor C4 and sent to pin 3. This is the current feedback link. Capacitor C6 plays two roles: one is to buffer the shutdown of the switch tube VT; the other is to form resonance with the primary coil to restore the magnetic core of the transformer.

2.3 Synchronous Rectification Technology

The power system uses 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, which can greatly reduce the rectification loss. That is, the MOSFET drive signal is obtained by detecting the current flowing through itself. VT is turned on when the forward current flows through it. When the current flowing through itself is zero, it is turned off, so that the reverse current cannot flow through VT. Therefore, MOSFET can only conduct in one direction like the rectifier diode. Compared with voltage-type synchronous rectification technology, current-driven synchronous rectification technology has good adaptability to different converter topologies.

The main considerations for selecting a synchronous rectifier are large on-state current, small on-state resistance, sufficient reverse withstand voltage (calculated based on the reverse voltage of the transformer secondary transformation at 24V), and short reverse recovery time of the parasitic diode. After analyzing and calculating 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 11m!, a reverse recovery time of 145ns, and a turn-on delay time of 48ns and a turn-off delay time of 186ns, respectively, which can meet the system working requirements.

3 Design points for reducing power consumption and electromagnetic pollution

3.1 Consumption reduction measures

(1) Take advantage of the superior performance of the TDA4863 chip.

The performance characteristics of TDA4863 are: when the input voltage is high, the APFC circuit on the chip absorbs more power from the power grid; conversely, when the input voltage is low, it absorbs less power, which suppresses the generated harmonic current and makes the power factor close to 1; the chip also contains an active filter circuit that can filter out the harmonic current generated by output voltage pulsation; the chip's micro-current working 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 power consumption.

(3) Line layout optimization.

The primary and secondary of all inductor coils in the system hardware circuit are wound with multiple strands of twisted wire, which reduces copper loss and effectively suppresses common-mode interference.

3.2 Measures to reduce electromagnetic pollution

(1) An electromagnetic interference (EMI) filter is installed 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 capacitors on the DC side.

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

(3) Set up a shielding layer.

Since the distributed capacitance effect is generated between the MOSFET tube and the rectifier device and the base plate and the heat sink through the insulating sheet, the electromagnetic interference is coupled to the AC input end to form a common-mode signal. Therefore, a shielding sheet is sandwiched between two layers of insulating sheets. That is, the shielding sheet is connected to the ground end to cut off the path for the common-mode interference to propagate to the power grid.

(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.

(5) 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 relatively close to the transformer and the output capacitor. In addition, the positive and load current conductors are laid in parallel to offset the external magnetic fields formed by each of them.

4 Prototype production and test results analysis

4.1 Prototype production

According to the circuit designed in this article, a physical product was produced. Protel was used to design and produce the experimental printed circuit board (PCB), and the components were installed and soldered. The front view of the PCB board is shown in Figure 4.

4.2 Rectifier bridge and switch tube test waveform

In the prototype experiment, the circuit was tested using the Tektronix oscilloscope TDS5034B. During the test, the load of the DC/DC converter at the rear stage was: 12V/1.53A; 24V/1.70A. Figure 5 is the test waveform, where udr and ud are the drive voltage of the switch tube VT1 and its drain voltage respectively, u5 is the voltage of the 5th pin of TDA4863, that is, the zero current detection voltage of the inductor, and ui is the rectifier bridge sinusoidal half-wave output voltage. It can be seen that 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 a 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, a very small peak current cannot increase u5; f0 is also low near the peak of ui, because the current reference signal is also near the peak, the inductor current peak and output power are both large, but because the output average power is constant, f0 is reduced.