Innovative design of small Tips: LED drive circuit can be built without electrolytic capacitors

Abstract: In view of the shortcomings of the existing LED drive circuit with limited life of electrolytic capacitors, a design method of LED drive circuit without electroless capacitor is proposed. The method adopts a chip of Panasonic Panasonic MIP553 built-in PFC dimmable LED driving circuit, and is synthesized with an external non-isolated bottom edge chopper circuit as a basic circuit structure, and outputs a stable current to meet the needs of LED operation. At the same time, a protection circuit is designed to protect the load. The experimental results show that the controller chip can work stably and can achieve a constant voltage output of 27V and a constant current output of 350mA.

LED (Light Emitting Diode) has become a new generation of green lighting source with its advantages of energy saving, environmental protection, high brightness and long life. As LED lighting technology matures, it will eventually be used in all aspects of life and become the new darling of lighting sources. However, high efficiency, low cost, high power factor and long life drive power are the key to LED light quality and overall performance.

The LED lamp that replaces the general bulb on the market today is about 40 times longer than the life of an incandescent bulb, which is equivalent to 40,000 hours. Since the LED is a direct current driving part, the electric current is directly converted into light energy by the current flowing, and is therefore also called a photoelectric converter. Since there is no friction or mechanical loss, it is more efficient in energy saving than a general light source. However, when the AC power is turned on, it is generally a DC stabilized power supply using a rectifying part and a smoothing circuit. The electrolytic capacitor necessary in the smoothing circuit rises by 10 ° C due to the surrounding temperature and its own heat, resulting in a halving of the life. Therefore, electrolytic capacitors hinder the life of LED lighting fixtures.

In order to improve the life of the driving power supply, simplify the circuit, reduce the cost, and increase the power density, it is necessary to remove the electrolytic capacitor. For this reason, a high-brightness LED driving power source without an electrolytic capacitor is proposed.

1 The basic working principle of LED power supply

The BUCK converter and IPD control are used to realize the switching power supply, and output constant current and voltage to drive the LED lamp. The overall block diagram of the circuit is shown in Figure 1.

The main circuit part is followed by a filter after the mains supply. Its function is to filter out the higher harmonics in the power supply and the surge in the power supply, so that the control circuit is less interfered by the power supply. The input rectification part adopts an integrated rectifier bridge. The unidirectional conduction characteristic of the diode converts the alternating current whose level is floating up and down at zero to a unidirectional pulsating direct current, and then outputs a direct current voltage under the action of the filter capacitor and the inductor. After the adjustment and control of the MIP553 and BUCK circuits, the voltage for the LEDs is output.

2 LED power supply specific design

2.1 Input circuit design

In order to extend the life of the LED driver and match it to the LED, the electrolytic capacitor in the circuit must be removed.

The design specifications of the circuit are: 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 rectifying device, as shown in FIG.

The noise filtering device is mainly composed of a capacitor C1/C2/C3 and an inductor L1, and its function is to reduce electromagnetic interference (EMI) in a frequency band less than 1 MHz. This device can also be linked after AC AC, and the filtering effect is the same before the rectifying device. The safety device consists of a fuse and ZNR1. The fuse mainly prevents the circuit from being cut off to protect the load when the peak current of the circuit is damaged. The ZNR1 is a surge absorber that absorbs static electricity and surge from the input to protect it. The circuit behind. The input rectifier device converts the alternating current into direct current and the input rectifier bridge: the voltage stress of the rectifier bridge diode is:

Consider the margin, choose TSC GBL205 (VR=600V, IFAN=1A).

2.2 Output circuit design

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 buck converter, series switching regulator, three-terminal switching buck regulator, is a single-tube non-isolated DC/DC converter with output voltage equal to or less than the input voltage.

The input current is in operation, when the switch is closed, is “0”; when the switch is open, is=0, so is is pulsating, but the output current io is continuous and smooth under the action of inductor, diode and capacitor. of. Particularly suitable for providing operating current for LEDs.

FRD1 selection criteria: The rated current is greater than 2 times the output current, the rated voltage is greater than the input voltage, and the reverse recovery time is also within 100 ns. Consider the margin, the parameters of FRD1 are: 15A, 600V, trr=50ns. The method selects T1 and Cout, then the parameters are: T1: 680μH; Cout: 1μF, 50V.

2.2.2 Zener diode DD1

In the range of low input voltage, if there is no such reverse device like DD1, then there will be reverse current flowing through the IPD when the switch is turned off, and IPD is not allowed to have such current, because this A reverse current will cause damage to the IPD.

The stresses suffered by DD1: IDD"2·Io=2&TImes; 0.35=0.7A, UDD"Uo, reverse recovery time trr"100ns. Considering the margin, the selected parameters are: 3A/60V/75ns.

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