High brightness, low cost LED driver primary side regulation (two)

Basic concepts of primary end regulation

Fig. 2 Schematic diagram of the primary circuit of the flyback converter. In general, the discontinuous conduction mode (discontinuousconductionmode, DCM) output regulation performance is good, so it is the primary end of the first mode of operation. The key to the primary end regulation is how to obtain the output voltage and current information without direct detection. Once these values are obtained, the traditional PI control method can be easily controlled.

In the MOSFET turn-on time (TON), the primary end inductor (Lm) loads the input voltage (VIN). Thus, the MOSFET current (Ids) increases linearly from 0 to peak (Ipk). During this time, energy is transferred from the input to the inductor. When the MOSFET is turned off, the energy stored in the inductor causes the rectifier diode (D) to pass through. In the diode conduction time (TD), the output voltage (Vo) is loaded on the secondary terminal inductance (LmxNs2/Np2), the diode current (ID) from the peak (IpkxNp/Ns) decreased linearly to 0. At the end of the TD, all the energy stored in the inductor is released to the output. During this period, the sum of the output voltage and the diode forward voltage drop is reflected to the auxiliary winding end, which is expressed as (VoVF) xNa/Ns. As the forward voltage drop of the diode decreases with the decrease of the current, the diode current is reduced to 0 at the end of the diode turn-on time. Therefore, the voltage of the output voltage can be obtained by simply sampling the winding voltage at the end of the turn-on time of the diode.

Fig. 3 Schematic diagram of the internal module of the integrated power switch

At the same time, the estimation of output current needs some multiplication calculation. Assuming that the output current is equal to the average current of the diode, the output current can be estimated by the following formula: Io=Ipkx (Np/Ns) x (TD/2Ts). The output current estimator is used to obtain the peak value of the drain current through a peak detection circuit, and the output current is calculated by the diode conduction time (TD).

Integrated primary terminal controller

The primary end regulation PWM controller is a technology that is designed to deal with the primary side regulation off-line LED driver. This technology can significantly simplify the design requirements to meet the more stringent efficiency requirements, eliminating the need to increase the cost and reliability of external components, such as optical coupler and KA431. Figure 3 is the schematic diagram of the internal module of FAN102. The device has a tolerance for error amplifier for internal reference voltage of + 1%, according to the tolerance of external components will change the input current / voltage is reduced to a minimum, also with a temperature compensation circuit integrated external components change, regardless of the temperature changes can be obtained with high accuracy. The internal oscillator has a frequency hopping function to reduce the EMI, and can be used at the input end of the small line filter.