Design of flyback switching power supply based on improved startup circuit

1 Introduction

Switching power supply With significant advantages such as high efficiency, low power consumption, small size and light weight, it has become the mainstream product of voltage-stabilized power supply. This paper designs a highly efficient single-ended flyback auxiliary power supply system with 4-way isolated output based on the current-type PWM control chip UC3844B, and designs a new control chip startup circuit to address the shortcomings of energy waste on the DC bus side in the traditional startup circuit. The experimental results show that the designed single-ended flyback switching power supply has good working performance, and the improved startup circuit can effectively shorten the startup time and improve the power efficiency.

2 UC3844B chip introduction

UC3844B is a high-performance fixed-frequency current-mode PWM control integrated circuit chip. The features of this integrated circuit are: an oscillator, a temperature-compensated reference, a high-gain error amplifier, a current sampling comparator, and a high-current totem pole output. It is an ideal device for driving power MOSFETs. Its internal structure and pin diagram are shown in Figure 1 [1].

Figure 1 UC3844B internal structure and pin diagram

The functions of each pin of the UC3844B chip with an 8-pin dual in-line package are as follows: Pin 1 (COMP) is the output of the error amplifier, which is used for loop compensation; Pin 2 (UFB) is the inverting input of the error amplifier, which is usually connected to the output of the switching power supply through a resistor divider; Pin 3 (ISEN) is the current sampling terminal, and a small resistor is usually connected in series with the source of the power switch tube as a sampling resistor. When the voltage on the sampling resistor exceeds a given value, the UC3488B turns off the output terminal; Pin 4 (RT/CT) is the oscillator, which is the common terminal of the external timing resistor RT and the timing capacitor CT. By connecting the resistor RT to the 8-pin Vref and the capacitor CT to the ground, the oscillator frequency and the maximum output duty cycle can be adjusted, and the operating frequency can reach 500kHz; Pin 5 (GND) is the common ground of the control circuit and the power supply; Pin 6 (OUT) is the output of the push-pull output amplifier, which can directly drive the gate of the power MOSFET, has a bidirectional drive capability of sourcing and sinking current, and a peak current of up to 1.0 A. Pin 7 (Vcc) is the power input terminal; Pin 8 (Vref) is the reference output pin, which provides charging current to capacitor CT through resistor RT [2]. UC3844B also includes overvoltage and undervoltage protection circuits. When the power supply voltage is lower than 10V, the chip stops working.

3 Switching Power Supply Principle and Design

3.1 Working Principle of Switching Power Supply

The working principle diagram of the switching power supply is shown in Figure 2 [3]. The +16V working voltage required by UC3844B at startup is provided by the R2 and C3 circuits. The 220V AC power is rectified by a bridge and filtered by a capacitor to obtain a +300V DC high voltage. After being stepped down by R2, it is connected to the 7th pin of U2. The charging process of C3 is used to gradually increase U2 to above +16V, thereby achieving startup. When the switching power supply enters normal operation, the high-frequency voltage on the auxiliary winding is rectified and filtered by VD2 and C4 to serve as the working voltage of the chip. UC3844B belongs to the current-controlled PWM. The voltage established by the current on the primary winding on the current detection resistor R10 is added to the non-inverting end of the current detection comparator and compared with the error voltage at the inverting end to control the duty cycle of the output pulse. Considering that the leakage inductance of the high-voltage transformer will generate a spike voltage when the switching power tube is turned off, a TVS, D5, R3, and C2 are used to form an absorption circuit to protect the switching power tube. The voltage feedback loop is mainly composed of the voltage regulator chip TL431 and the optocoupler PC817A. The feedback signal is obtained from the +5V output terminal. After passing through the voltage regulation of TL431 and the isolation of the optocoupler, it is sent to the voltage feedback pin of UC3844B to control the stability of the output voltage.

Figure 2 Schematic diagram of switching power supply

3.2 High-frequency transformer design

The technical parameters of the single-ended flyback transformer are as follows: operating frequency f=50kHz; the highest and lowest input voltages of the switching power supply transformer are Umax=375V; Umin=120V, the maximum operating duty cycle is Dmax=50%; the forward voltage drop of the rectifier diode is VFVD=0.6V; the output 4-way isolation voltages are: +5V/3A, ±5V/1A, 24V/0.5A.

(1) Calculate the primary peak current

Primary current peak Ipk:

(1)

Where: P0 is the transformer output power; η is the transformer efficiency, usually taken as 0.8.

(2) Calculate the primary inductance

Primary side inductor LP:

(2)

(3) Selecting the transformer core

The selection of magnetic core adopts AP method, which can be calculated as follows:

(3)

By calculating with the above formula and leaving enough power margin, we choose EI30 type transformer core.

(4) Calculate the number of turns of the primary and secondary windings

Primary Winding:

(4)

Where: Ae is the effective flux area; Aw is the maximum flux density.

Secondary Winding:

(5)

According to formula (5), the number of turns of the secondary winding for +5V output is 5 turns; the number of turns of the secondary winding for ±15V output is 14 turns; and the number of turns of the secondary winding for +24V output is 22 turns.

(5) Calculate the air gap length

The air gap length of the transformer is determined by the following formula:

(6)

3.3 Peak voltage absorption circuit design

When the power MOSFET is turned off, it will generate spike voltage and reflected voltage on the primary winding of the transformer. Combined with the high voltage on the DC side, it is easy to damage the power MOSFET, so a clamping circuit must be added to clamp it [4]. This design uses a transient voltage suppressor P6KE200 with a reverse breakdown voltage of 200V and an RS1M ultra-fast recovery diode with a reverse withstand voltage of 1 kV, and an RC resistor-capacitor absorption circuit to reduce the spike voltage.

3.4 Improved startup circuit design

As shown in FIG3(a), the conventional startup circuit uses a Zener diode DZ to limit the startup power supply of the control chip UC3844. When the control chip is in a stable working state, the current on the DC bus side still flows through the startup circuit, causing unnecessary energy loss.

Figure 3 Comparison between traditional startup circuit and improved startup circuit

Therefore, an improved startup circuit design is proposed, as shown in Figure 3(b). In the initial stage, transistor Q is turned on.

The DC bus voltage Vi charges the capacitor C4 through R16, and at the same time, the DC bus voltage Vi charges the capacitor C18 through the resistor R15. The voltage at Vb finally stabilizes at the following voltage:

Vb=12+Vi*R2/(R15+R2) (7)

Due to R15□R2, we can simply assume that Vb≈12V. Since the start-up and shutdown voltages of the control chip UC3844 are 16V and 10V, in order for Q to be shut down after the system works stably, the following conditions must be met:

(8)

After the transistor Q is turned off, the control system enters a stable working state. The chip UC3844 is powered by the feedback winding, and the DC bus current no longer flows through the startup circuit, greatly reducing the loss.

4 Experimental results and analysis

According to the above analysis, a multi-channel single-ended flyback switching power supply based on UC3844B is designed. The main parameters are as follows: switching frequency f=50kHz, DC input voltage fluctuation is 120V□375V, and DC multi-channel output voltage is +5V/3A, +15V/1A, +24V/0.5A. Figure 4 is a comparison of the startup voltage waveforms of the traditional startup circuit and the improved startup circuit.

As can be seen from the figure, when the starting voltage reaches 16V, UC3844B enters a stable working state and finally stabilizes at 12V. By comparison, it can be seen that the traditional control strategy requires 0.4s to reach its starting voltage of 16V, while the improved control strategy only requires 0.1s, which reduces the starting time and improves the control efficiency.

Figure 5 shows the peak current on the primary side and the driving voltage waveform of the MOSFET when the switching power supply is lightly loaded and heavily loaded. It can be seen from the figure that the adjustment period of the MOSFET is about 22μs, that is, the frequency is about 45kHz, and the duty cycle is about 40%, which meets the design requirements. By comparison, it can also be seen that when the switching power supply is lightly loaded, it works in discontinuous mode, and the primary current increases from zero; when the switching power supply is heavily loaded, it works in continuous mode, and the primary current does not pass through the zero point, but has a certain starting value.

Figure 4 Comparison of startup voltage waveforms of traditional and improved startup circuits: (a) Traditional startup circuit; (b) Improved startup circuit

Figure 5 Primary current and trigger pulse waveforms under different loads: (a) light load; (b) heavy load

5 Conclusion

This paper uses the current-mode pulse width modulation chip UC3844B to design a single-ended flyback multi-channel isolated output auxiliary power supply system and improves its startup circuit. The experimental results show that the startup time of the improved startup circuit is significantly shortened, which greatly reduces energy waste. At the same time, the switching power supply works stably, meets the design requirements, and has certain practicality.

References

[1] Ma Hongtao, Sha Zhanyou, Zhou Fenping. Switching power supply manufacturing and debugging [M]. Beijing: China Electric Power Press, 2010.

[2] Cheng Hailong, Li Yuren, Liang Bo. Design of power converter based on UC3842[J]. Power Technology, 2011, (35): 720-722.

[3] Xian Qingxin. Circuit Diagrams and Schematic Diagrams for Frequency Converters[M]. Beijing: Machinery Industry Press, 2009.

[4] Wu Guoping, Yang Rengang, Du Haijiang. Research on the design of a flyback switching power supply based on NCP1014[J]. Power Electronics Technology, 2010, (44): 78-50.

[5] Liang Cheng, Yunyue Ye, Zhou Zheng. Design of Improved Single Phase Flyback Switching

Power for PMSM Drive System[J]. IEEE Transaction on IE, 2011.

About the Author

Li Dapeng (1988-) Male, from Jining, Shandong, Master's degree candidate, research direction is power electronics and electrical transmission. ■