Primary-Side Regulation Technique for Low-Cost, High-Efficiency Off-Line LED Drivers

With the continuous innovation and rapid development of the lighting industry, coupled with the increasing importance of energy saving and environmental protection, high brightness (HB) LEDs have evolved from simple indicator lights to an important lighting source that surpasses traditional light sources. Compared with other lighting solutions, HB LEDs have several significant advantages, such as low energy consumption, long life and high lighting quality. However, if HB LEDs are to successfully replace ordinary incandescent lamps and enter the mass market, the cost of their driving electronics must be minimized.

Figure 1. Conventional secondary-side regulated offline LED driver

For the traditional lighting source, incandescent lamps, driving is a simple matter, just connect the bulb directly to the voltage source. Most energy is in the form of constant voltage, which makes the driving cost of incandescent lamps quite low. However, LEDs have a light output intensity that is proportional to its current and forward voltage drop, and varies with temperature. Therefore, LEDs need a constant current to drive and require additional circuitry . Traditionally, offline constant current drivers for LEDs are generally implemented using isolated flyback converters with output current regulation circuits (see Figure 1). The actual LED voltage is measured through a sense resistor and then compared with the reference voltage value to obtain an error voltage. This error voltage is transmitted to the primary side via an optocoupler and is used to control the duty cycle of the primary-side switching device. Although this solution can achieve excellent LED current regulation, the output regulation circuit requires an optocoupler, reference voltage, and sense resistor, which greatly increases the system cost and reduces the overall efficiency.

Primary side regulation (PSR) technology may be the best solution to minimize the cost of offline LED drivers. This technology can accurately control the LED current on the secondary side with only information from the primary side of the driver, eliminating the output current sensing loss and all secondary feedback circuits, thereby improving the efficiency of offline LED driver designs without incurring huge costs. In addition, this technology can regulate the LED driver output voltage without the need for secondary side feedback circuits, which is equivalent to providing a light-on overvoltage protection function, further ensuring the reliability of the driver. This article will discuss the basic working principles of primary side regulation technology and introduce a highly integrated primary side regulation PWM controller. Compared with traditional secondary side regulation methods, this controller has many significant advantages.

Figure 2 Primary-side regulated offline LED driver and its typical waveform

Basic Concepts of Primary-Side Regulation

Figure 2 shows the basic circuit diagram of a primary-side regulated flyback converter and its typical waveforms. Generally speaking, discontinuous conduction mode (DCM) has better output regulation performance and is therefore the preferred operating mode for primary-side regulation. The key to primary-side regulation is how to obtain information about the output voltage and current without direct detection. Once these values ​​are obtained, they can be easily controlled using traditional PI control methods.

During the MOSFET on-time (TON), the primary-side inductor (Lm) is loaded with the input voltage (VIN). As a result, the MOSFET current (Ids) increases linearly from 0 to the peak value (Ipk). During this period, energy is transferred from the input terminal and stored in the inductor. When the MOSFET is turned off, the energy stored in the inductor causes the rectifier diode (D) to conduct. During the diode on-time (TD), the output voltage (Vo) is loaded on the secondary-side inductor (Lm×Ns2/Np2), and the diode current (ID) decreases linearly from the peak value (Ipk×Np/Ns) to 0. At the end of TD, all the energy stored in the inductor is released to the output terminal. During this period, the sum of the output voltage and the diode forward voltage drop is reflected to the auxiliary winding terminal, expressed as (Vo+VF)×Na/Ns. Since the diode forward voltage drop decreases as the current decreases, at the end of the diode on-time, the diode current decreases to 0, so the auxiliary winding voltage at this time can best reflect the output voltage. Therefore, information about the output voltage can be obtained by simply sampling the winding voltage at the end of the diode conduction time, which can be obtained by monitoring the auxiliary winding voltage.

Figure 3 Schematic diagram of the internal module of the integrated power switch (FSCQ-series)

At the same time, the estimation of the output current requires some multiplication calculations. Assuming that the output current is equal to the average current of the diode in steady state, the output current can be estimated by the following formula: Io=Ipk×(Np/Ns)×(TD/2Ts). The output current estimator uses a peak detection circuit to obtain the drain current peak value and calculates the output current using the diode conduction time (TD).

Integrated primary-side regulation controller

Primary-side regulated PWM controllers, such as Fairchild Semiconductor 's FAN102, are a technology that specifically addresses the design of primary-side regulated offline LED drivers. This technology can significantly simplify the design challenges of meeting more stringent efficiency requirements and eliminate external components that increase cost and reliability issues, such as optocouplers and KA431. Figure 3 shows the internal block diagram of the FAN102. The device has an internal reference voltage with a tolerance of ±1% for the error amplifier, which can minimize input current/voltage changes based on the tolerance of external components, and an integrated external component temperature variation compensation circuit to achieve high accuracy regardless of temperature changes. Its internal oscillator has a frequency hopping function to reduce EMI, allowing the use of a small line filter at the input.

in conclusion

The proprietary technology that combines sampling and output estimation provides a precisely regulated, lower-cost implementation for a wide range of offline LED drivers—from streetlights to medical applications and even consumer electronics applications such as cabinet and table lamps. Now, these LED drivers can be smaller, lower cost and more efficient.