This article focuses on the shortcomings of the existing LED drive circuit due to the existence of electrolytic capacitors to shorten its life, and proposes a design scheme for an LED drive circuit without electrolytic capacitors. This solution is based on the theme of prolonging the service life of the circuit, based on the combination of the switching power supply and the linear power supply, and taking advantage of the advantages of the shortcomings to take full advantage of their respective advantages. This design scheme makes full use of their respective advantages to replace electrolytic capacitor filtering, effectively solving the problem of short life of the existing LED driving circuit. This LED drive circuit does not have large-capacity electrolytic capacitors. Small capacitors can use long-life thin-film capacitors and other capacitive components, which make it have long life, high efficiency, low ripple current, and have high safety and stability. .
1 Introduction
LED (Light Emitting Diode) is a new generation of green lighting source, which has many advantages such as energy saving, environmental protection, high brightness, long life and so on. It is not only a new favorite of lighting sources, but also closely related to people's lives. Therefore, the development of a long-life driving power supply, the construction of high efficiency, low cost, high power factor is the key to the luminous quality and overall performance of LED lamps, and is also the need for the development of LED lighting technology. According to incomplete statistics, the life of existing incandescent bulbs is about 40 times shorter than LED lamps. Because the light emitting diode is not only a DC current driving device, but also a photoelectric converter, which has the function of converting photoelectricity. Its main function is to convert electrical energy into light energy by flowing current, so its advantage is higher energy saving efficiency and working life than general light sources. However, a large-capacity electrolytic capacitor is generally used in the rectifier circuit and filter circuit of the LED drive power supply. The service life of electrolytic capacitors is generally l05 ℃ / 2000h, which means that when the ambient temperature of the capacitor rises to 105 ℃, its service life is only 84 days. Even if it works in an environment with a temperature of 85 ℃, the service life is only 332 days, so electrolysis Capacitance is the main reason hindering the life of the LED drive circuit. In order to increase the life of the driving power supply, it is necessary to remove the electrolytic capacitor. For this purpose, a high-brightness LED driving power supply without electrolytic capacitor is proposed.
2. Working principle of LED drive circuit
The overall block diagram of this design scheme circuit is shown in Figure 1:
The circuit topology uses a flyback topology circuit and uses PWM to control the switching frequency so that it outputs a constant current and voltage to drive the LED lamp. Mainly includes: pre-stage protection circuit, EMI filter circuit, rectifier circuit, RCD clamp circuit, synchronous rectifier circuit, power conversion circuit, output filter circuit, feedback circuit, control circuit, etc.
In order to make the circuit less susceptible to electromagnetic interference, after connecting the EMI filter circuit to the pre-stage protection circuit, it will filter out the higher harmonics in the circuit and the surge in the circuit.
The input rectification part is composed of a bridge rectifier circuit and a π-type filter circuit. Because the diode has the characteristic of unidirectional conduction, the bridge rectifier circuit can convert AC power to unidirectional DC power. Under the action, output a stable DC voltage.
Then the control circuit adjusts and controls the output to reach the design value, and finally passes through the output filter circuit to reduce the output ripple to direct current, and finally outputs the direct current to the LED for use.
3. The specific design of the LED drive circuit
3.1 Design of input circuit
The indicators of this design circuit are: input AC voltage Vin: 90-264 VAC / 50-60Hz; output voltage Vo: 27VDC; output current Io: 0.68A.
As shown in FIG. 2, the input circuit includes a safety insurance device, an EMI noise filtering device, a bridge rectifier circuit, and a π-type filtering circuit.
As shown in Fig. 2, in order to reduce the electromagnetic interference in the frequency band of 1MHz, the EMI noise filter circuit is formed by the capacitors C1, C2 and the inductors L1, L2. The safety fuse is composed of a fuse and ZNR. When a peak current of a hazardous circuit is generated, the fuse will quickly cut off the circuit to protect the load; ZNR is a surge absorber. When static electricity and surge appear on the input end of the drive circuit, it will It becomes very high impedance, so it can protect the circuit behind. Bridge rectifier filter circuit, its role is to convert alternating current into direct current, and then the role of π-type filter to filter the ripple of voltage and current in the circuit.
3.1.1 Design of EMI filter
The EMI filter circuit diagram is shown in Figure 3. The EMI filter circuit consists of differential mode capacitors CX1 and CX2 in front of the rectifier bridge. It is mainly used to attenuate differential mode interference, and its value is generally large.
In order to reduce differential mode interference, a π-type differential mode filter composed of C1, C2, and L1 is added after the rectifier bridge.
The differential mode capacitor in the EMI filter circuit uses X safety capacitors, the safety level is X2, and its withstand voltage is 2500V, of which CX1 = 0.47uF, CX2 = 0.01uF. Common mode inductance LX1 is 7mH and LX2 is 1mH. The C1 and C2 filter capacitors of the π-type filter circuit use thin film capacitors withstand voltage of 450V, and the capacitance value is 0.22uF; the differential mode inductance L1 is 1mH.
It can be seen from Figure 4 that when the frequency is higher than 1KHZ, the noise signal will be significantly reduced. It can be seen that this circuit can effectively reduce high-frequency interference.
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