Author: Peter Hsieh / Leon Lee / Kevin Hsueh
In order to achieve high-reliability LEDs with high reliability, small size, high luminous efficiency, low power consumption and low cost, the industry has developed a PSR controller with a high-brightness LED driver solution. The PSR controller's outstanding constant current technology can accurately provide the appropriate voltage and current to improve the life of the LED. In addition, the PSR high integration solution can further reduce the size of the LED driver and PCB, thus starting in the industry. Lu Feng Mang.
The realization of low-power and high-efficiency technologies has attracted global attention. As lighting applications account for nearly 20% of the global power consumption, advances in lighting technology will also have a huge impact on energy conditions. Light-Emitting Diode (LED) Solid-State Lighting (SSL) is an environmentally friendly technology with excellent form factor, long life, high conversion efficiency, and power consumption 80% or 90% lower than conventional incandescent lamps.
In LED SSL, the LED driver plays an important role because it relies on it to provide the precise current needed to maintain a stable brightness. However, conventional LED driver solutions use a secondary feedback circuit to drive the voltage and current of the LED, but the secondary feedback circuit results in increased cost and size. This article describes a patented primary side regulation (PSR) technology that allows the PSR controller to accurately regulate the voltage and current of the LED driver in the primary side of the transformer without the need for a secondary-side feedback circuit. It also includes frequency jittering to reduce electromagnetic interference (EMI) and light-load standby mode to reduce standby loss. With this approach, PSR chargers can achieve smaller form factors, lower standby power, and higher efficiency than traditional designs such as ringing choke converter (RCC) and traditional pulse width modulation (PWM).
LED lighting has many advantages, but without proper voltage and precise current, the life of these components will not only be shortened, but also the power consumption and heat consumption will increase, which will eventually cause irreparable damage to the LED. Considering that the LED and the ordinary diode also have a large slope of the VI curve in terms of physical properties, the operating voltage of the LED is quite sensitive to the operating current. If the variation is large, the life of the HB LED unit is affected, so the current of the LED is illuminated. Very important. In this case, the PSR with excellent constant current technology is both important and beneficial for the life of the HB LED. LED drivers typically use a non-isolated buck converter or an isolated flyback converter.
In conventional LED control circuits, an off-line constant output current LED driver can be implemented using an isolated flyback converter in conjunction with a secondary side circuit to regulate the output current (Figure 1). Here, the LED current is measured by a Sense resistor Ro on the secondary side and provides the necessary feedback information by means of an optocoupler. The optocoupler forms an isolation between the primary side and the secondary side and couples the feedback signal to the PWM controller on the primary side. To achieve better output regulation, the PWM controller uses the optocoupler to receive the feedback signal on the secondary side. To determine the duty cycle of a metal oxide semiconductor field effect transistor (MOSFET). This solution provides accurate current control, but the disadvantage is that the larger number of components means greater board space, higher cost, and lower reliability. At the same time, the sense resistor Ro also increases power consumption and reduces constant current regulation power efficiency. Recently, the efficiency and energy saving requirements of LED drivers have become increasingly important, and LED applications have required smaller sizes, so conventional circuits no longer meet the relevant requirements. This article describes a primary side control method that reduces the number of components and increases efficiency.
Figure 1 LED driver with a conventional secondary-side regulated flyback converter
PSR technology is the best solution to minimize the cost of offline LED drivers, providing accurate current control without the need for optocouplers on the secondary side. The basic principle of PSR is to use an innovative method to replace the optocoupler on the secondary side with a reference coil to detect the output information. Figure 2 shows the basic circuit diagram of a flyback converter with a primary side controller and its main operating waveforms.
Figure 2 Basic circuit diagram and waveform of flyback converter with primary side controller
When the PSR controller turns on the MOSFET, the transformer current iP will increase linearly from zero to ipk, as in equation (1). During conduction, energy is stored in the transformer. When the MOSFET is turned off (toff), the energy stored in the transformer is transferred to the output of the power converter by the output rectifier, during which the output voltage VO and the forward voltage VF of the diode are reflected to the reference coil NAUX. The voltage on the reference coil NAUX can be expressed by the formula (2). At this time, a proprietary sampling technique can be used to sample the reflected voltage. Since the forward voltage of the output rectifier becomes constant, the relevant output voltage information can be obtained, and then the sampled voltage is compared with the accurate reference voltage. A voltage loop is formed to determine the on-time of the MOSFET and to accurately regulate the constant output voltage.
In the above formula, LP is the inductance of the primary coil of the transformer; VIN is the input voltage of the transformer; ton is the on-time of the MOSFET; NAUX/NS is the turns ratio of the reference coil and the secondary output coil; VO is the output voltage; VF is The forward voltage of the output rectifier. This sampling scheme also doubles the discharge time (tdis) of the transformer. As shown in Figure 2, the output current IO is related to the secondary side current of the transformer. IO can also be obtained by using ipk and tdis, as shown in equation (3). The PSR controller uses this result to determine the on-time of the MOSFET and to regulate the constant output current. The sense resistor RSENSE is used to adjust the value of the output current.
Here, tS is the switching period of the PSR controller; NP/NS is the turns ratio of the primary coil and the secondary output coil; RSENSE is the sensing resistor that converts the switching current of the transformer into the voltage VCS.
High-integration PSR controller solution for small-volume/low-cost HB LED system design
Here an HB LED driver is used to drive three series HB LEDs with an output specification of 12 volts/0.35 amps. Using the PSR controller FSEZ1016A, which integrates a PSR controller and a 600 volt / 1 amp MOSFET, will help reduce external component count, reduce signal noise on printed circuit board (PCB) MOSFET driver circuits, and reduce interference. The proprietary Green Mode feature provides non-conduction time modulation at light and no load conditions to linearly reduce the PWM frequency and minimize standby power consumption, making it easy to meet most green specifications. In addition, its built-in frequency-hopping function further enhances EMI performance.
Experiments show that with this method, constant current (CC) regulation accuracy can reach 1.8%, Fold-back voltage is 4 volts (Figure 3), for a large positive voltage (VDD) range, and CC capability and output voltage related. The efficiency is 77.66% at 115Vac input, 77.40% at 230Vac input, and 0.115 watt at no load. It can be seen that with the FSEZ1016A, a lighting solution with the lowest external components and the lowest cost can be obtained.
Figure 3 VI curve using the PSR controller
As the industry invests more in the development of energy-efficient electronic products, lighting applications require innovative technologies to replace traditional incandescent and halogen lamps. The advantages of HB LEDs are small size, high brightness, long life and environmental friendliness.
These advantages are all favorable factors that drive the product to gradually replace traditional lighting products. In order to improve the capability of the HB LED, the control circuit must use constant current to achieve LED driving. The PSR patented technology described in this article can accurately adjust the voltage and current of the LED driver in the primary side of the transformer by using the PSR controller, eliminating the need for a secondary side feedback circuit, resulting in a smaller size, longer life and more environmentally friendly products. . Experiments show that PSR can provide 1.8% constant current regulation accuracy. This PSR technology is the best solution for reducing the cost of offline LED drivers.
(The author of this article works for Fast Semiconductor)
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