Design of power supply for LED display screen

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Design of power supply for LED display screen

Abstract: This paper introduces the design scheme of a special power supply for 5v/120aled display screen, briefly describes the application of lossless absorption technology and primary side current synthesis technology in the circuit, and gives the experimental results

key words: active power factor correction; Double positive cataclysm; Lossless absorption

1 introduction

led display screen is a new type of information display media which has developed rapidly. According to the market survey, the sales prices of high-efficiency electromechanical and ordinary electromechanical are 70 yuan/kw (9.0 yuan/kw) and about 40 yuan/kw respectively. With the continuous development of China's economy, it has been widely used in stations, hotels, banks, hospitals and other public places. The display power supply is an important part of the display, which is mainly used to provide the necessary working current for the display led to ensure the normal display of the screen. For simplicity, a power supply scheme with 3 to 4 display driver boards from a small power supply is usually adopted. Thus, a large display screen needs to be equipped with many power modules. For example, a 2m 1.5m screen body needs to provide 24 5v/20a module power supplies. The design has the following disadvantages

1) the wiring is complex. Each power supply shall be separately configured with AC input line and DC output line

2) poor redundancy of power supply in most cases, the contents displayed on the screen are text, animation and pictures, and the current consumed by each display driving board is different. Some power modules may be overloaded while others are unloaded. In addition, if a certain power supply fails, part of the screen will be black

3) the power supply has poor overload capacity and low utilization. The current consumed by the screen during operation varies with the content, color and brightness of the screen. The current is small most of the time. Although the large area and high brightness screen consumes a large amount of current, its duration is short. Considering that the LED is driven by constant current, the power supply voltage can be reduced as long as the drive board can work normally. It is better for the power supply to have the current limiting characteristic of drag down shape rather than the current limiting characteristic of steep shape, so as to ensure better overload capacity and higher utilization rate

considering the above points, a new power supply scheme is proposed as follows:

1) centralized power supply and N + 1 redundancy scheme

2) the power module is designed with appropriate output current, and the module can share the current. Ensure that the screen assembly process is easy to realize n + 1 redundancy

3) the power module has the current limiting characteristic of drag down shape to ensure better overload capacity and higher utilization rate

4) the power module has a flat shape and natural heat dissipation. It is easy to install on the screen and use the screen for heat dissipation

5) the power module is equipped with APFC to reduce the electrical interference and adapt to the electrical fluctuation

2 circuit design

the centralized power supply scheme can avoid the disadvantage of decentralized power supply, but it requires higher reliability of the power supply. Otherwise, once the power supply fails, the whole screen will be black, not part of it. The most active way to improve the reliability of the power supply is to improve the conversion efficiency, reduce the calorific value, and select circuits and devices with high reliability

2.1 ac/dc circuit design

the traditional ac/dc full wave rectifier circuit adopts rectifier + capacitor filter circuit. This circuit is a combination of nonlinear devices and energy storage elements. The waveform of the input AC voltage is sinusoidal, but the waveform of the input current is seriously distorted and appears as a pulse. The resulting harmonic current has a harmful effect on electricity, reducing the input power factor of the power supply. In this design, active power factor correction circuit (APFC) is used in the rectifier circuit to avoid the above shortcomings. Its circuit is shown in Figure 1

aluminum powder is widely used, in great demand and in many varieties Fig. 1 PFC lossless absorption main circuit

different from typical PFC main circuit, this circuit uses lossless absorption buffer network. The circuit reduces the switching loss of the switch tube, improves its stability and enhances its service life. It uses a group of passive components to make the switch realize zero current on and zero voltage off, which improves the working efficiency of the power supply and reduces the production cost compared with other resonant soft switching circuits

the working principle of this lossless absorption buffer network is explained by analyzing the working process of PFC main switch Q

1) when q is turned on, the current in Q increases from zero and rises slowly because the current in inductance L2 cannot change suddenly and the voltages of C2 and C1 cannot change suddenly. The current ID4 passing through D4 decreases gradually. Q realizes zero current switching on, and the conduction loss is small

2) when the current ID4 decreases to zero, D4 enters the reverse recovery state, and the current il2=il1 + ird4 passing through the inductor L2. The change rate of D4 reverse current ird4 is controlled by inductance L2, and the reverse recovery loss is reduced

3) the current in the main inductor L2 increases slowly, and the voltage UQ on Q decreases. Capacitor C2 discharges through D2, C1, L2 and Q, and voltage UC2 on C2 drops

4) when UC2 drops to zero, the energy in C2 completely turns to C1 and L2. The current saturation in L2 remains unchanged, UQ drops to zero, and Q completes the zero current switching on process

5) Q remains in the on state, which is the same as the switch state of ordinary PFC circuit

6) when q is switched off, the current IL2 in L2 flows to C2 through D1, C2 starts charging from zero, Q realizes zero voltage switching off, and the switching loss is small. Diodes D2 and D3 finally clamp UC2 at the output voltage VL

7) the energy stored in L2 during conduction flows to C1 through D1 and D2, and L2 gradually resets. When L2 is reset, the energy in C1 is output through D3

8) when the voltage at both ends of C1 becomes zero, D4 is conducting in the positive direction. Q complete the zero voltage shutdown process

9) Q remains off until a new switching cycle begins

The switching waveform of

q is shown in Figure 2; The measured on time and off time of Q are shown in Figure 3. (power load 22a)

Figure 2 Q switching waveform between D-S poles

Figure 3 Q on time and off time

from the above analysis, it can be seen that this lossless absorption network has the following characteristics

1) the maximum operating voltage of Q is equal to the output voltage VL

2) the withstand voltage of output diode D4 of PFC circuit is the sum of reverse voltage of VL and inductance L2

3) the current rise rate in Q, that is, the on loss of Q depends on the voltage at both ends of the inductor L2 and the inductance of L2

4) the voltage rise rate at both ends of Q, i.e. the turn-off loss of Q, depends on the current flowing through the capacitor C2 and the capacity of C2

5) the energy stored in L2 and C2 due to switch action is finally output to the load, ensuring the working efficiency of the converter

2.2 dc/dc main circuit design

dc/dc main circuit adopts single ended double forward circuit. Compared with other topological circuit structures, the single ended double forward circuit has a low voltage on the switch tube, so there is no need to worry about the common state conduction problem in the design of the control circuit, nor does it prevent the asymmetric one-way magnetic bias of the high-frequency transformer due to the fact that the circuit prevents personnel from entering the experimental site by mistake, that is, there is no transformer saturation problem, so it is a circuit with high reliability. Considering that the height of the whole machine does not exceed 60mm and the requirements of transformer process, installation and heat dissipation, the dc/dc converter adopts the structure of dual transformer and dual output inductance. The primary side of the transformer is connected in parallel, and the secondary side uses an output inductor respectively, as shown in Figure 4

Figure 4 dual forward lossless absorption main circuit

the lossless absorption network of this circuit is different from the lossless absorption network used in some ac/dc circuits. It only makes the switch complete the zero voltage switching off process. Take switch Q2 as an example (Q1 and Q2 have the same change state) to briefly describe the working principle of the network

1) during the conduction process

q1 and Q2 are turned on, except that one current passes through Q1, T1 secondary side and Q2, the other current flows through Q1, C5, L7, D10, C7 and Q2 to form an LC oscillation circuit, and C5 and C7 are charged. When the voltage UAB between points a and B is equal to the main circuit voltage VDC, the oscillation ends due to the unidirectional conductivity of d10. Inductance L7 limits the current change in C7 and C5. The current flowing through Q1 and Q2 is the sum of load current converted from secondary side to primary side and C5 and C7 charging current

2) during the shutdown process

q1 and Q2 are switched off, since the voltage to ground of point B is zero, C7 starts charging from zero, Q2 to ground voltage uq2 rises slowly, and Q2 is switched off at zero voltage. The voltage applied to Q2 is finally VDC due to the clamping action of diode D15. Therefore, the voltage at point B rises to VDC. Q2 realizes zero voltage shutdown process

the energy of the induced potential caused by the excitation inductance, leakage inductance and lead parasitic inductance of the transformer is returned to the power supply through C7 and D14, and the voltage on Q2 is maintained at VDC until the magnetic flux at the primary side of the transformer is reset. At this time, the voltage on Q1 and Q2 is vdc/2 respectively until the new working cycle

q2 has the same status as the common switch in the same period during opening and closing

Figure 5 shows the measured Q2 switching waveform. Figure 6 shows the measured Q2 zero voltage shutdown waveform

Fig. 5 D-S pole switching waveform of Q2

Fig. 6 turn off time of Q2

from the above analysis, the following characteristics can be summarized

1) the maximum working voltage of each switch in the circuit is equal to the power supply voltage

2) the voltage rise rate of Q1 and Q2 turn off depends on the capacity of capacitors C5 and C7 respectively

2.3 control circuit design

in order to ensure the safe and reliable operation of the power supply, TOP224Y is used to make a flyback switching power supply as the auxiliary source in the circuit design, as shown in Figure 7. Its two outputs supply power to the control circuit of ac/dc part and dc/dc part respectively

Figure 7 auxiliary power circuit

ac/dc control part uses PFC control chip uc3854b. UC3854 is closed in case of AC input overvoltage and undervoltage, PFC conversion DC voltage (400V) overvoltage and undervoltage, so that PFC part stops working. These fault signals are transmitted to the dc/dc control circuit through the isolation optocoupler, so as to protect the main switch tube in case of abnormal operation of the ac/dc part

dc/dc control part uses PWM control chip UC3846 and adopts peak current control mode. Compared with the voltage control mode, the peak current control mode has fast load response rate and pulse by pulse current limiting characteristics. It is easy to obtain the current limiting characteristics of drag down shape, which is very suitable for this application

In case of

n + 1 redundancy application, multiple modules must have current sharing function. The output current of the power supply is large, the power loss of the current signal taken directly from the DC output shunt is large, and the assembly process is complex. Therefore, this design adopts the method of primary side current synthesis

use a current sensor to take out the current signal at the primary side of the transformer when the switch tube is turned on. The signal includes the excitation current signal of the transformer and the current signal converted from the output inductance current to the primary side of the transformer. Since the current converted from the output inductor to the primary side is much greater than the excitation current of the transformer, it can be considered that what the current sensor takes out is the magnetizing current of the output inductor. This is the rising part of the output inductor current. As long as the falling part of the output inductor during freewheeling is simulated, the current signal of the output inductor can be obtained after synthesis, which is also the output current signal. The combined current signal can be used for current protection control and current sharing control

as shown in Figure 8, the current signal taken out by the current sensor is charged to a capacitor after high-speed one-way buffering. Turn off the constant current source when the switch is on, and turn on the constant current source when the switch is off to discharge the capacitor at constant current. After selecting appropriate circuit parameters, the voltage waveform on the capacitor is proportional to the current on the output inductor. After amplification, the output inductor current, that is, the output current, can be obtained

figure 8 output current synthesis circuit

3 experimental results

the test indicators of the prototype are as follows

input voltage range AC150 ~功能性消化不良吃什么药

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