These mistakes that are often made in the LED driver circuit must be avoided! Double-layer capacitor LED circuit analysis

For the novice, LED driver design is actually not an easy task. In this regard, Xiaobian specially summarizes some of the problems that design people need to pay attention to at work and personal design experience to share.

Since bipolar power devices are cheaper, generally about 2 cents, some designers use bipolar power devices to reduce the cost of LED driving, which will seriously affect the reliability of the circuit, because with the LED driver board With the increase of temperature, the effective working range of the bipolar device will shrink rapidly, which will cause the device to malfunction when the temperature rises, thus affecting the reliability of the LED lamp. The correct way is to use the MOSFET device, and the service life of the MOSFET device is far. Longer than bipolar devices.

Do you want to use electrolytic capacitors in the LED driver circuit? At present, there are supporters and opponents. Supporters believe that if the temperature of the board can be controlled well, the purpose of extending the life of the electrolytic capacitor can be achieved in turn. For example, a high-temperature electrolytic capacitor with a life of 105 degrees and a lifetime of 8000 hours is selected, according to the estimated life expectancy of the electrolytic capacitor. “Every time the temperature is lowered by 10 degrees, the life is doubled”, then it has a working life of 16,000 hours in a 95 degree environment and a working life of 32,000 hours in a 85 degree environment, in a 75 degree environment. The working life is 64,000 hours. If the actual working temperature is lower, the life will be longer! From this point of view, as long as the choice of high-quality electrolytic capacitors has no effect on the life of the drive power!

Other proponents believe that the low-frequency flicker caused by the high ripple current caused by electroless capacitors will cause physiological discomfort to some people's eyes. The large amplitude of low-frequency ripple will also cause some digital camera equipment to appear poor. A brightly lit grid of flickering lights. Therefore, high-quality light source lamps still need electrolytic capacitors. However, opponents believe that electrolytic capacitors will naturally age. In addition, the temperature of LED lamps is extremely difficult to control, so the life of electrolytic capacitors will inevitably decrease, thus affecting the life of LED lamps.

In this regard, senior engineers believe that the electrolytic input capacitor can be considered in the input part of the LED driver circuit. In fact, PI can be omitted by using LinkSwitch-PH of PI. PI's single-stage PFC/constant current design allows designers to save large capacity. Capacitor, in the output circuit, high-voltage ceramic capacitor can be used instead of electrolytic capacitor to improve reliability. When designing two-stage circuit, the output uses a 400V electrolytic capacitor, which will seriously affect the reliability of the circuit. It is ok to use a ceramic capacitor for a single-stage circuit. For industrial applications that do not pay much attention to dimming functions, high temperature environments, and high reliability, it is recommended not to use electrolytic capacitors for design.

MOSFETs with a withstand voltage of 600V are relatively cheap. Many people think that the input voltage of LED lamps is generally 220V, so the withstand voltage of 600V is enough, but in many cases the circuit voltage will reach 340V. In some cases, the 600V MOSFET is easily broken down. Influencing the life of LED lamps, in fact, the choice of 600V MOSFET may save some cost but the cost of the entire board, so do not choose 600V withstand voltage MOSFET, it is best to use MOSFET withstand voltage over 700V.

Some LED circuits use a two-stage architecture, PFC (Power Factor Correction) + isolated DC/DC converter architecture, which reduces the efficiency of the circuit. For example, if the efficiency of the PFC is 95% and the efficiency of the DC/DC section is 88%, the efficiency of the entire circuit will be reduced to 83.6%! "PI's LinkSwitch-PH device combines a PFC/CC controller, a 725V MOSFET, and a MOSFET driver into a single package, increasing the efficiency of the driver circuit to 87%. This device greatly simplifies board layout design and can save up to Go to the 25 components used in the traditional isolated flyback design! The omitted components include high-voltage large-capacity electrolytic capacitors and optocouplers. The LED two-stage architecture is suitable for the second constant-current drive circuit to be used to enable the PFC to drive the LED constant current. Older drives. These designs are outdated and no longer cost effective, so in most cases it is best to use a single stage design.

If the luminaire design is not very high, we recommend using an LED driver with integrated MOSFET, because the benefit of this is that the integrated MOSFET has less conduction and generates less heat than discrete. In addition, the integrated MOSFET is controlled. Together with the FET, it generally has a thermal shutdown function. When the MOSFET overheats, it will automatically shut down the circuit to protect the LED luminaire. This is very important for LED luminaires, because LED luminaires are generally small and difficult to carry out air.

In recent years, LEDs have been used for lighting, ultra-thin TV backlights, and the like, and are expected to be widely used as light sources for various devices. Among them, high-brightness LEDs can be used as a light source for camera photography. Today, high-brightness photographic light source LEDs are installed in portable devices such as smartphones, digital cameras, and digital video cameras, and are used as flare light for animation photography and flash for still photography. Due to the continuous improvement of LED performance in the future, it may be used more widely in shooting scenes.

LEDs have this feature, which adjusts its own brightness by controlling the amount of current and time it passes. Therefore, it is suitable for shooting scenes, making it possible to widely use flash for still photography and torch light for dynamic photography.

Figure 1 (left) shows the general LED flash circuit structure. The power of the LED is a battery, so the current flowing into the LED is limited by the performance of the battery. Therefore, brightness is also restricted. In order to cope with the improvement of LED performance and the situation of photography, brighter illumination is required, so a large current must flow to the LED. Here, FIG. 1 (right) is a circuit configuration in which an electric double layer capacitor (EDLC) is used as an auxiliary power source. In this circuit, the large current that the battery cannot provide can be supplied by the EDLC, so that the LED can emit brighter light regardless of the limitation of the battery.

Figure 1: Circuit structure of LED flash (left: no EDLC, right: EDLC used)

The EDLC in Figure 2 serves as an auxiliary power supply and shows the data when 8A of current flows into the LED. The result is more than 1,000 lux of brightness.

Figure 2: Luminescence characteristics when using EDLC to drive LEDs with high current (8A)

In order to supply a large current to the LED in a very short period of time, the characteristics of the charged particles used as the auxiliary power source require a large capacity and a low internal resistance. In addition, in order to provide a stable current in different environments, low internal resistance and stability in a large temperature range are also necessary.

The EDLC may realize a large capacity because the storage structure in which no chemical reaction is in principle can reduce the internal resistance as compared with the battery. The EDLC of Murata is the most suitable electrode material and structure. Although it is a small, ultra-thin light package, it achieves low resistance (tens of mΩ) and large-capacity capacitors in a large temperature range. Therefore, the large current A can be discharged with low loss in a large temperature range. For this reason, it is possible to provide the LED with a large current of more than 2 A which is difficult to provide for a battery of a general portable device between several tens of milliseconds and hundreds of milliseconds. (Refer to Figure 3)‖

Figure 3: Discharge characteristics (2.7V/700mF/30mΩ product)

Table 1 shows a list of EDLC products from Murata Manufacturing Co., Ltd., and Figure 4 shows an LEDflach demonstration board using our EDLC. This system can supply LEDs with a maximum current of 8A in 33 milliseconds. (can be controlled in 2A-8A/10-60 milliseconds)

Figure 4: LED Flash Demo Board with EDLC

LEDs used as photographic lamps in smartphones and digital video cameras are widely used. In these machines, the EDLC is used as an auxiliary power source for the LEDs for a brighter flash. On the other hand, the digital camera uses a xenon tube flash system, which has the characteristics of easy control, power saving and space saving, and is expected to replace LED. Up to now, circuit systems with high current and high-luminance LEDs require large-capacity and low-resistance EDLC.

The use of cameras equipped with cameras is expanding and is expected to occupy a more important position in our lives. Our company will continue to study the characteristics of the products, continue to propose small, thin and light-packaged LEDs with large capacity and low resistance, and continue to contribute to the convenience of equipped with camera equipment.