LED light sources have the advantages of energy saving, environmental protection, and various shapes, and are currently widely used. Then, what kind of Power Supply is used for the LED light source to ensure that it works safely and effectively, which is a very important issue. The most prominent advantage of the constant current power converter for LED lighting designed in this paper is that when one or several of the multiple LEDs in series are damaged (generally breakdown short circuit), the normal illumination of the remaining LEDs is not affected. If constant voltage control is used, when one or several of the multiple series LEDs are damaged (breakdown), the voltage across the remaining LEDs will rise and be damaged. Based on the considerations of LED protection and brightness adjustment, it is necessary to design a fully functional constant current power converter for LED lighting.
The circuit design of the constant current power converter for LED lighting designed in this paper is shown in Figure 1. It includes: rectification and filtering circuit, pulse width control (PWM) constant current circuit, overvoltage and constant voltage protection circuit and light control circuit. 220V AC is rectified and filtered to obtain about 300V DC voltage for the circuit. Since the rectification and filtering circuit is relatively simple, it will not be repeated here. The composition and working principle of the other circuits are described below.
The circuit adopts the low-cost, superior performance switching power supply control chip UC3842 as the core device. In order to improve the efficiency, the switch tube uses a high-power FET, and the load and the power supply are separated by a high-frequency isolation transformer T and a photocoupler. Good isolation performance. The circuit chip UC3842 adopts a fixed operating frequency pulse width controllable modulation method. There are 8 pins in total. The functions of each pin are as follows: (1) The pin is the output of the error amplifier, and the external RC component is used to improve the gain and frequency characteristics of the error amplifier. (2) The foot is the feedback voltage input terminal. The voltage of this pin is compared with the 2.5V reference voltage of the non-inverting terminal of the error amplifier to generate an error voltage to control the pulse width. (3) The pin is the current detection input terminal, when the detection voltage exceeds 1V. When the pulse width is reduced, the power supply is in the intermittent working state; (4) the foot is the timing terminal, and the operating frequency of the internal oscillator is determined by the external resistance time constant, f=1.8/(RTCT); (5) the foot is the common ground (6) The pin is the push-pull output terminal, the internal is the totem pole type, the rising and falling time is only 50 ns, the driving capability is ±1A; (7) the foot is the DC power supply terminal, with under- and over-voltage locking function, chip function The consumption is 15mW; the (8) pin is a 5V reference voltage output with a load capacity of 50mA.
PWM constant current circuit
The PWM constant current circuit is composed of a control chip UC3842, a high frequency isolation transformer T, sampling circuits R15, R16, R17 and the like. The overall circuit is shown in Figure 2. In order to make the UC3842 have stable output current control capability, the power supply adopts a separate regulated power supply mode. The DC input in the figure is regulated by a three-terminal regulator IC1 to provide a stable DC voltage of 18V. . The circuit uses a high-frequency isolation transformer as a transducer. It cooperates with a photocoupler (hereinafter referred to as optocoupler) to provide good isolation between the output of the circuit and the input of the AC. The control principle of the circuit is as follows:
When the load current (output current) increases, the voltage across the sampling circuit R17 rises, and the optocoupler IC3 connected in parallel increases the primary current, which reduces the on-resistance of the secondary 3 and 4 terminals, thus making the UC3842 2 The voltage of the foot rises, and the duty ratio of the PWM pulse outputted by the 6-pin is reduced by the internal circuit of the integrated circuit, and the conduction time of the control switch Q1 is shortened in one cycle, and the energy transmitted to the load through the switching transformer T is reduced. Therefore, the current output to the load is reduced; conversely, when the load current is decreased, the voltage across R17 is decreased, and the primary current of the optocoupler IC3 connected in parallel is decreased, so that the on-resistance of the secondary 3 and 4 terminals is increased, thereby The voltage of the 2 pin of the UC3842 is lowered, the duty ratio of the PWM pulse of the 6-pin output is increased, the conduction time of the switching transistor Q1 is lengthened in one cycle, and the current output to the load is increased, thereby achieving constant current output.
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