A high power factor flyback AC/DC converter

Abstract: The reasons of low power factor in the electronic and power equipments are simply analyzed in this paper. A high frequency charge pump power-factor-correction(PFC) AC/DC converter is proposed. The principle of the circuit are discussed. The chip of TDA16846 was simply introduced. The simulation and experimental results show that unity power factor and low THD can be achieved, which proves that the performance of the converter is excellent.

Keyword：electronics Power;Power-Factor-Correction;charge pump;

I. Introduction

In the past two decades, power electronics technology has developed rapidly and has been widely used in the fields of electricity, metallurgy and chemical industry, coal, communications, and household appliances. However, with the increasing popularity of power electronic devices, power electronic devices inject a large amount of harmonics into the power grid, causing serious harmonic pollution to the power system and posing a serious threat to the safe operation of the power grid. This problem has attracted widespread attention from the society, and a series of harmonic limitation standards have been proposed, such as IEC 555-2, IEEE 519, IEC 1000-3-2, etc., to limit the current harmonics of power electronic and electrical equipment [1].

The main reason for the increase of harmonics and the decrease of power factor in power electronic devices is that the devices often use rectification and large capacitor filtering circuits. The rectifier diode can only be turned on when the power supply voltage is higher than the filter capacitor voltage. Therefore, the grid-side input current is a pulse wave with a high charging peak, as shown in Im in Figure 1. As a result, the grid-side input current has a large harmonic content and a low power factor. If a PFC circuit is added between the rectifier diode and the large filter capacitor, as shown in block diagram 2. When the input voltage passes through zero, the output of this circuit reaches the maximum value, and when the input voltage reaches the peak value, the output reaches the minimum value; and the current

It is a rectified sinusoidal waveform, and its output characteristics are shown in Figure 3 (

It is the sinusoidal double half-wave voltage obtained by the power grid through the bridge rectifier circuit.

is the output voltage of the PFC circuit). In this way, the grid-side input current waveform can be improved to make it as shown in the waveform of Imp in Figure 1, thereby achieving the purpose of improving the power factor.

Figure 1 Current waveform before and after PFC correction

Commonly used power factor correction adopts a two-stage solution. The first stage is the PFC stage, which usually uses a Boost converter. The PFC stage forces the line current to follow the line voltage to achieve a high power factor; the second stage is a power converter to adjust the output voltage. This solution can achieve high-performance power factor and fast output voltage regulation functions, and is relatively mature and suitable for various power applications. However, the disadvantages of this circuit are complex circuit structure, many components, high cost and low efficiency. In order to overcome these shortcomings, a single-stage power factor correction solution has been proposed in recent years. The PFC stage and the second stage share a switch tube, which can reduce the number of switch tubes and control circuits, reduce circuit costs, reduce weight and volume, and improve efficiency.

Figure 2 PFC converter block diagram

Figure 3 Characteristics of PFC circuit

This paper proposes a PFC circuit that combines a high-frequency charge pump circuit and a flyback converter. The circuit has a simple structure, low cost, easy control, and can effectively eliminate harmonics. It has broad application prospects in small and medium power electronic equipment.

2. The proposal of high-frequency charge pump circuit and its working principle

After theoretical investigation and experimental verification [2], the resonant circuit has an output characteristic similar to that in Figure 3. In this circuit design, the inductor

,capacitance

and diode

The charge pump circuit constructed realizes the PFC circuit function in Figure 2. The power converter in the latter stage adopts a flyback converter, and the two stages share a switch tube. The circuit is shown in Figure 4. Since the switching frequency is much higher than the input rectified voltage

and bus voltage

frequency, so in each switching cycle,

and

The value of can be regarded as a constant [3]. The circuit working voltage and current waveforms are shown in Figure 5. The working process of the circuit can be divided into 6 modes. Figure 6 is the equivalent circuit diagram of each mode. The specific analysis of the circuit is as follows:

Figure 4 Basic model of single-stage single switch

Figure 5 Voltage and current waveforms at various points in the circuit

Mode 1

exist

At this moment, switch S is turned on, and the DC bus voltage

Added to the primary side of the transformer, the primary inductor current

Linear increase. Since the voltage at point m

Less than

,diode

Cannot conduct.

,

The series resonance is formed to absorb energy from the power grid, and its equivalent circuit is shown in Figure 6(a).

Time, voltage

achieve

, this modal ends.

Modal 2

When the voltage

Rise to

,diode

Natural conduction, inductance

The stored energy is stored in a large capacitor

Since the rectified input voltage

Less than

, current

Gradually decreases. In this mode, due to

Clamped in

,capacitance

There is no energy change in the circuit. The equivalent circuit is shown in Figure 6(b).

Mode 3

exist

Moment, current

The difference between the input current and the capacitor

Discharge and reverse charge,

Voltage

Gradually decreases. When the transformer secondary rectifier diode

Conductivity, i.e.

This mode ends.

Mode 4

exist

time,

The equivalent circuit is shown in Figure 6(d). The transformer transfers energy to the load, and the current

At the same time, the inductance

Continue to the large capacitor

Transfer energy. When the current drops to zero, this mode ends.

Figure 6 Equivalent circuits of each mode

Figure 7 TDA16846 startup circuit

Mode 5

exist

time,

drops to zero,

Natural shutdown, the transformer primary winding voltage is zero, the capacitor

Voltage

Add to diode

The diode is turned off by the reverse voltage. The equivalent circuit is shown in Figure 6(e).

Through the transformer original variable inductance to the capacitor

Charging. This mode ends at

moment, the next mode starts.

3. Driving circuit

This paper uses the TDA16846 chip from Siemens to drive the switch tube. This chip combines the functions of PWM and PFC, supports charge pump circuits, and has the characteristics of high efficiency, simplicity, and reliability [4]. This chip has two modes: free oscillation and fixed frequency. This paper uses the fixed frequency mode to control the switch tube. This chip has the following characteristics:

1. No special startup circuit required

TDA16846 does not need a separate startup circuit, but is started by a diode connected to pin 2 inside the chip. Its internal partial structure is shown in Figure 7 (the circuit uses the transformer auxiliary winding to power TDA16846). The startup process of the chip is analyzed below. When the power is turned on, the auxiliary winding of the transformer cannot provide energy to the chip because the switch does not work, and the chip cannot work. The bus voltage

pass

, Chip 2 pin and its internal diode

To capacitor

When the capacitor

When the voltage on the chip reaches the chip startup voltage, the chip starts and the circuit begins to work.

From the above analysis, we can see that TDA16846 can be started without additional complex startup circuit, and its structure is simple.

2. Primary current simulation and current limiting function

TDA16846 can detect and limit the current of the switch tube through the resistor and capacitor connected to pin 2.

Give

Charging, the voltage at pin 2 of the chip when the switch is turned off

is limited to 1.5V, so

The charging time is basically the same as the switch on time, so the capacitor can be set

Charging time is

.capacitance

The charging current can be approximately expressed as:

(

Relative to

can be ignored). Then after charging

for:

(1)

inductance

It can also be considered that during the switch on period, the current value is:

(2)

From equation (1) and equation (2), we can get:

(3)

Formula (3) can calculate the maximum current that can flow through the switch tube. The maximum value of the control voltage 40 is the internal reference voltage of the chip 5V.

If it exceeds 5, the drive circuit is shut down and current limiting is performed.

4. Simulation and Experimental Results

In order to verify the feasibility of the above theory, a charge pump high power factor AC/DC converter was made according to the circuit in Figure 4 (due to the complexity of the circuit, some auxiliary circuits and control circuits are not drawn). Its main parameters are as follows:

Figure 8 shows the input current and voltage waveforms of the circuit simulated using Simetrix software. It can be seen that the input current waveform is no longer a spike pulse wave, but a good sine wave.

The power factor and harmonics of the circuit were measured using a virtual instrument test platform based on the HP I/O library. Figure 9 shows the waveforms of the input voltage and current, and Figure 10 shows the harmonic spectrum of the input current. It can be measured that the power factor of the circuit is 0.972, and the total harmonic distortion is 24.29%; the output voltage of the circuit is

, meeting the requirements of switching power supply.

Figure 8 Simulation waveforms of input voltage and current

Figure 9 Experimental waveforms of input voltage and current

Figure 10 Harmonic spectrum of input current

V. Conclusion

The above theoretical analysis and experimental research show that the AC/DC converter circuit combining a high-frequency charge pump and a flyback converter has a simple structure, excellent performance, low cost, and can achieve a power factor close to 1 and a harmonic content that meets the international standard IEC1000-3-2. It has broad application prospects in small and medium-power power electronic equipment.

References

[1] Yan Baiping, Liu Jian. Discontinuous conduction mode high power factor switching power supply[M]. Beijing: Science Press, 2000.

[2] Qian Jinrong and Lee Fred C. Charge pump power-factor-correction technologies[J]. IEEE Transactions on Power Electronics, 2000,15(1):121-128.

[3] Zhang Weiping et al. Green Power: Modern Power Conversion Technology and Applications[M]. Beijing: Science Press, 2001.

[4] Liu Shengli. Practical Technology of Modern High Frequency Switching Power Supply[M]. Beijing: Electronic Industry Press, 2001.9