LED driver design experience

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

Do not use bipolar power devices

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.

Try not to use electrolysis

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.

The withstand voltage of the MOSFET should not be lower than 700V

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.

Try to use a single-stage architecture circuit

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.

Try to use MOSFET devices

If the LED luminaires are not designed to be very powerful, it is recommended to use LED driver products with integrated MOSFETs because the benefit of this is that the integrated MOSFETs have less conduction and generate less heat than discrete ones. In addition, the integrated MOSFETs are 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.