introduction Light-emitting diodes have been widely used since they were widely used as simple indicators until today. Market analysis believes that the development of LED is earlier, and will continue to mature, we will feel that because of the high-efficiency, low-cost LEDs available today, the industry has been greatly changed. The display industry has seen LEDs gradually penetrate into the backlight of small-sized displays. In the application, and further to the medium and large size LCD expansion. As LEDs are used in a variety of media, special attention needs to be paid to their electrical control methods, which helps maximize design efficiency. Despite significant advances in IC and LED driver design, there is still no one-size-fits-all LED driver. Today's smart designers and product integrators choose the most appropriate drive type for their application needs and features. Before choosing an LED driver for an OEM manufacturer, Endicott Research Group, for example, has prioritized the functional requirements that influence selection, including key dimensions, cost, power, power, noise, low light, and flexibility. Often, drive designers are more or less affected by the supply chain (such as the configuration of LED strings), which is unique not only in the display industry, but also in other solid-state light sources (SSL). This article highlights some of the standard structures used to drive OEM-LED backlights and discusses the thorny issues of conventional LED driver applications. 1. AC/DC LED driver for SSL source The International Electrotechnical Commission (IEC) and the American National Standards Institute (ANSI) have specified the power factor and input current harmonic components supplied to the AC input line. These regulations vary from application to application, but both exist for lighting equipment. influences. The power factor is defined as the ratio of the active power actually consumed by the appliance to the input apparent power, which emphasizes the phase relationship between the line voltage and the current. When the input current consumption of the electronic device is linear with the associated voltage (the load is purely resistive), the power factor is one unit (or 1), as shown in Figure 1(a). A power factor of 1 is ideal because the loss of use is reduced and the damage signal such as noise is very small. When the load has both nonlinear and reactive circuit components, the power factor is no longer one. If the load is linear, capacitive, and resistive, the input current phase leads the input voltage as shown in Figure 1 (b); if the load is linear, inductive, and resistive, the input current phase lags the input voltage. In other cases, the apparent power (the instantaneous voltage and current through the device) does not equal the actual consumed active power. This is where IEC and ANSI emphasize the difference between active power and apparent power, which is monitored by many power meters. 2, bridge rectifiers and filter capacitors in AC/DC converters Before considering the power factor and current harmonics, the single-phase voltage source consists of a bridge rectifier and a filter capacitor, which converts the AC input current into a somewhat fluctuating DC current, and the ripples pass through subsequent active or passive filters. Filtered to obtain a smooth DC current. This type of circuit is not expensive and simple to manufacture (currently widely used in low-power power supplies). When the sinusoidal input voltage reaches a peak value, the circuit composed of the bridge rectifier and the filter capacitor absorbs all input currents in a very short time. Highly deformed input current waveform (as shown in Figure 1(c)). The reason for this current pulse is that the bridge filter is only turned on when the instantaneous line voltage exceeds the voltage value on the capacitor (that is, the current is only turned on when the peak voltage is input). As a result of the power consumption being equal to the square of the input current multiplied by the line resistance, this pulsed input current reduces the performance of the power supply. When the circuit network uses this AC/DC conversion method, in addition to the reduced performance, the line voltage near the amplitude of the sine wave is deformed. Note that Figure 1 (c) and Figure 1 (d)), in order to eliminate the above problems, proposed an active power factor correction (PFC) circuit, Figure 1 (d) shows the typical active PFC circuit input voltage and current waveform . 3, power factor correction circuit In order to reduce the harmonic current generated by the AC/DC conversion method of the bridge rectifier and the filter capacitor, and to keep the current flowing when the input voltage has a phase difference, a circuit structure similar to that shown in FIG. 2 is used. This pulse width modulation (PWM) based circuit is very popular and well documented, but not the only PFC architecture. The main difference between the boost PFC and the boost converter is that the PFC circuit can modulate its input current consumption in order to generate an input current waveform for the resistive load. As shown in Figure 2, the input current waveform is sampled from the input voltage waveform (120Hz corrected input voltage in the US) and Q1 is modulated in time to produce a sinusoidal waveform that matches the input voltage in frequency and phase. The resistive network shown in Figure 2 is not always a scheme for modulating the input current, but it is a very popular one. The main reason is that the boost converter is the application basis of most active PFC circuits. This boosting action can apply all circuits above 0V up to the peak voltage, thereby limiting the distortion of the input current. Usually the PFC circuit requires a larger value filter capacitor (C1) to filter out the low frequency components when modulating the input current. These circuits also output voltages of 200 to 400V or higher (because the boosted input voltage is 110V or 220V), requiring a conversion stage between the output stage of the PFC and the load. We do not want the boost converter to have a high output voltage value, but this is inevitable in this circuit configuration. The switching converter includes an active PFC in addition to the current control circuit, and requires a complex filter at the input to reduce noise feedback and filter out noise. 