A brief analysis of the design of high-efficiency LED lighting

General LED lighting has a current limiting resistor in the driving circuit, and the power consumed by the resistor has nothing to do with the LED light emission. In order to improve efficiency, a switching power supply with constant current output should be used, and a power MOSFET should be used at the output stage.

　　Figure 1 is a block diagram of this drive circuit, which omits the triangular wave generator and error amplifier of the traditional circuit and uses a CMOS logic IC and a PWM modulator. When the clock CLK signal is low, the RS trigger is in a reset state and the output FET is turned off. When the CLK signal is high, the output FET is turned on. Its current is converted into a voltage by the resistor Rs. When this voltage is higher than the reference voltage Vref, the comparator output changes from high to low, the RS trigger is reset again, and the output FET is turned off again. As a result, even if the power supply voltage changes, the energy stored in the inductor is constant. Constant power output is achieved. Here, the voltage drop on Rs is used as a triangular wave voltage, and the PWM comparator directly compares this voltage with the reference voltage and acts as an error amplifier.

　　The actual circuit is shown in Figure 2, which is composed of discrete components, with the focus on the PWM comparator and voltage-doubler pulse output circuit.

　　The PWM comparator is composed of a NAND gate IC2a ​​and an analog switch IC3, which is the so-called chopper comparator. One input of the NAND gate is set to a high level to become an inverter. This inverter repeatedly works in the two states of "precharge" and "compare".

　　Pre-charge state

　　In the pre-charge state, capacitor C5 is charged by the reference voltage Vref provided by IC6 (LM385). At this time, the output is neither high level nor low level; it remains in the output state when the comparator is about to flip, and is combined with the ItS trigger to form a PWM circuit.

　　Comparison Status

　　The input voltage shifts the voltage level charged on C5 and inputs it to the inverter, where it is compared with the threshold voltage of the inverter and outputs a high or low level.

　　Voltage doubler pulse output circuit

　　As the name implies, this is to generate a pulse that is twice the power supply voltage. First, when the input is low level, capacitor C7 is charged and the output is 0V; when the input is high level, the voltage on voc and the capacitor is connected in series, and the voltage at the Y1X) terminal of inverter IC5 is 2Vcc; because the voc voltage is added to the Yss terminal, the output is 2Vcc.

　　In FIG. 2 , the 555 (IC1) time base circuit generates a clock pulse, and the charging voltage of C2 is changed through the brightness adjustment switch 51 to change the clock frequency by selecting two frequencies of 11.4kHz and 965Hz, so as to achieve the purpose of changing the brightness.

　　The output circuit and PWM comparator are directly connected to the power supply, but the power consumption is only 10uA when the light is not on.

　　The output is connected to 35 white light diodes, which are divided into five groups of series and parallel.