Aiming at the deficiency of limiting the service life of electrolytic capacitor in the existing LED driving circuit, a design method of LED driving circuit without electrolytic capacitor is proposed. The method adopts Panasonic mip553 chip with built-in PFC dimmable LED driving circuit, which is synthesized with external non isolated bottom chopper circuit as the basic circuit structure, and outputs stable current to meet the needs of LED operation. At the same time, the protection circuit is designed to protect the load. The experimental results show that the controller chip can work stably, and can achieve 27V constant voltage output and 350mA constant current output.
LED (light emitting diode) has become a new generation of green lighting source because of its energy saving, environmental protection, high brightness, long life and many other advantages. With the increasing maturity of LED lighting technology, it will eventually be used in all aspects of life and become a new favorite of lighting sources. However, the driving power supply with high efficiency, low cost, high power factor and long life is the key to the luminous quality and overall performance of LED lamps.
At present, the service life of LED lamps, which replace ordinary bulbs, is about 40 times that of incandescent bulbs, equivalent to 40000 hours. As LED is a DC current driven part, it directly converts electric energy into light energy through the current flowing, so it is also called photoelectric converter. Because there is no friction and mechanical loss, the efficiency of energy saving is higher than that of general light sources. However, when the AC power supply is turned on, the DC stabilized power supply of rectifier parts and smoothing circuit is generally used. The necessary electrolytic capacitor in the smoothing circuit will rise by 10 ℃ due to the surrounding temperature and its own heating, resulting in half the service life. Therefore, the electrolytic capacitor hinders the service life of LED lighting fixtures.
In order to improve the service life of the driving power supply, simplify the circuit, reduce the cost and improve the power density, it is necessary to remove the electrolytic capacitor. Therefore, a high brightness LED driving power supply without electrolytic capacitor is proposed in this paper.
1. Basic working principle of LED power supply
Buck converter and IPD control are used to realize switching power supply, output constant current and voltage and drive LED lights. The overall block diagram of the circuit is shown in Figure 1.
The main circuit is connected with a filter immediately after the mains power supply. Its function is to filter out the high-order harmonics and surges in the power supply, so that the control circuit is less disturbed by the power supply. The input rectifier part adopts an integrated rectifier bridge. Through the unidirectional conduction characteristics of the diode, the AC with the level floating above and below the zero point is converted into unidirectional pulsating DC, and then the DC voltage is output under the action of the filter capacitor and inductance. After the adjustment and control of mip553 and buck circuit, the voltage for LED is output.
2. Specific design of LED power supply
2.1 design of input circuit
In order to prolong the service life of LED driving power supply and match it with LED, the electrolytic capacitor in the circuit must be removed.
The design index of the circuit is: input AC voltage VM: 198-264vac / 50Hz; Output voltage Vo: 27VDC; Output Current IO: 0.35A.
The input circuit includes a noise filtering device, a safety device and an input rectifier, as shown in Fig. 2.
The noise filter device is mainly composed of capacitor C1 / C2 / C3 and inductance L1. Its function is to reduce electromagnetic interference (EMI) in the frequency band less than 1MHz. This device can also be linked after AC and before rectifier, and its filtering effect is the same. The safety device is composed of fuse and znr1. The fuse mainly prevents the circuit from being cut off quickly when the peak current endangering the circuit is generated to protect the load; Znr1 is a surge absorber, which absorbs static electricity and surge from the input to protect the subsequent circuit. The input rectifier device converts alternating current into direct current. The selection of input rectifier bridge: the voltage stress of rectifier bridge diode is:
Considering the margin, TSC gbl205 (VR = 600V, IFAN = 1a) is selected.
2.2 design of output circuit
The output circuit consists of a basic buck circuit and a zener diode dd1. As shown in Figure 3.
2.2.1 buck converter and its advantages
Buck converter, also known as step-down converter, series switching regulator and three terminal switching step-down regulator, is a single transistor non isolated DC / DC converter whose output voltage is equal to or less than the input voltage.
Input current is in operation, is > 0 when the switch is closed; When the switch is turned on, is = 0, so is is pulsating, but the output current IO is continuous and stable under the action of inductance, diode and capacitor. It is especially suitable for providing working current for LED.
Selection criteria of frd1: the rated current is greater than twice the output current, the rated voltage is greater than the input voltage, and the reverse recovery time should also be within 100ns. Considering the margin, the parameters of frd1 are 15a, 600V, TRR = 50ns. If T1 and cout are selected in a similar way, their parameters are t1:680 respectively μ H; Cout：1 μ F，50V。
2.2.2 zener diode dd1
In a certain range of low input voltage, if there is no such reverse device as dd1, there will be reverse current flowing through IPD at the moment of switch off, and IPD is not allowed to have this current, because this reverse current will lead to IPD damage.
Stress of dd1: IDD > 2 · IO = 2 × 0.35 = 0.7A, UDD > uo, reverse recovery time TRR < 100ns. Considering the margin, the selected parameters are 3A > 100ns. Considering the margin, the selected parameters are: 3A >
2.2.3 protection circuit
Mip553 has built-in protection circuits for overvoltage, overcurrent, overheating and led short circuit, but there is no design of protection circuit when LED is open circuit. The idea of the protection circuit when the LED is open circuit mainly includes zener diode protection, triode protection, bias coil protection, etc. Considering the cost and structure, the protection circuit with zener diode is selected in this paper. The circuit diagram is shown in Figure 3. When the LED is open circuit, the output voltage rises. If the output circuit has a protection circuit of a zener diode, the zener diode embeds the voltage of the LED under the voltage drop of the diode, so as to prevent the destruction of the output capacitor.
2.3 design of control circuit
The control circuit consists of mip553 and its peripheral circuits, as shown in Figure 4.
Mip553 chip realizes wide voltage 85 ~ 277v / AC input, built-in MOS, simple and stable structure, no electrolytic capacitor is required, supports isolation or non isolation scheme, single power output power of 6 ~ 30W, constant current output < > < >
The drain current of mip553 is controlled by pins Cl and ex, so the setting of resistors RCL and Rex connecting these two pins will directly affect the drain current. The maximum drain current can be determined by Rex. Considering that the maximum drain current flows through the LED, attention should be paid to setting the reference value.
Among them, it is assumed that the input voltage is 100V, the output voltage is 28V, the current is 400mA, and the maximum drain current is set to 1.0A.
The functions of CVDD, CEX and CCL are to stabilize the operation of mip553 and suppress external noise. Therefore, its value should be selected properly. CVDD can stabilize the voltage of Vdd and suppress the flicker of LED. Its characteristics are not affected by temperature and no additional noise is generated. The reference standard value is 1 ~ 10 μ Between F; CEX, suppress external noise from entering the ex pin, and its reference standard value is 470 ~ 1000pf; CCL, suppress external noise from entering CL pin. If its value is too large, PF value will be seriously affected, so its value should be less than 1000pf.
2.4 simulation results
The circuit is simulated by Multisim, and the results are shown in Figure 5.
As can be seen from Figure 5, the output voltage is stable at 27V and the current is stable at 0-35a, which meets the design requirements.
3 challenges of LED power supply
As a new type of electric light source, LED has obvious advantages in making large luminous three-dimensional characters and luminous signs. It has the advantages of low control voltage, low cost and high reliability. Although LED products have a growing development trend in domestic and foreign markets, LED lighting is an emerging industry after all. At present, it has not been widely popularized. Therefore, LED driving power supply inevitably has challenges in all aspects: first, because the forward voltage of LED will change with current and temperature, its “color dot” will also drift with current and temperature, In order to ensure the normal operation of LED, its driver is required to limit the current no matter how the input conditions and forward voltage change. Secondly, if LED dimming is required, pulse width modulation dimming technology is usually adopted, and the typical PWM frequency is 1 ~ 3kHz. Finally, the power processing capacity of the LED driving circuit must be sufficient, and the function must be strong. It can withstand a variety of fault conditions and is easy to realize.
LED is an energy-saving, efficient and environmental friendly green lighting. The research on its driving circuit is very important. In this paper, the LED driving power supply designed by mip553 is introduced, and the stability of its output current is proved by simulation, which has a good application prospect.