5 Keys to LED Drive Design

1. Do not use bipolar power devices

Doug Bailey pointed out that because bipolar power devices are cheaper than MOSFETs, typically around 2 cents or so, some designers use bipolar power devices in order to reduce LED drive costs, which can seriously affect the reliability of the circuit, because with the LED As the temperature of the driver circuit board increases, the effective operating range of the bipolar device will be rapidly reduced. This will cause the device to malfunction when the temperature rises, thus affecting the reliability of the LED lamp. The correct method is to select the MOSFET device and the service life of the MOSFET device. It is much longer than bipolar devices.

2. Try not to use electrolytic capacitors

Do you need to use electrolytic capacitors in the LED driver circuit? At present, there are supporters and opponents, and supporters believe that if you can control the temperature of the circuit board in order to achieve the purpose of extending the life of electrolytic capacitors, such as the use of high-temperature electrolytic capacitors with 105-degree life of 8000 hours, according to the prevailing electrolytic capacitor life estimation formula "For every 10 degrees of temperature decrease, life expectancy doubles," then it has a working life of 16,000 hours at 95 degrees, the working life of 32,000 hours at 85 degrees, and 64,000 hours working life at 75 degrees. The lower the operating temperature, the longer the life! From this point of view, as long as the use of high-quality electrolytic capacitors has no effect on the life of the drive power supply!

Other proponents believe that the low-frequency flicker caused by the high ripple current caused by the non-electrolytic capacitor may cause physiological discomfort to some human eyes, and the large-scale low-frequency ripple may also cause some digital camera equipment to appear poor. Flashing light and dark grid. Therefore, high-quality light fixtures still require 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, which will affect the life of LED lamps.

In this regard, Doug Bailey believes that in the input part of the LED driver circuit can be considered without electrolytic capacitors, in fact using PI's LinkSwitch-PH can save electrolytic capacitors, PI's single-stage PFC / constant current design allows designers to save large capacity Capacitance, in the output circuit, can use high-pressure ceramic capacitors instead of electrolytic capacitors to improve reliability, "Some people in the design of the two-stage circuit, the output uses a 400V electrolytic capacitor, which will seriously affect the circuit The reliability is recommended to use a single-stage circuit with a ceramic capacitor,” he stressed. "For industrial applications that do not care much about dimming capabilities, high-temperature environments, and require high reliability, I strongly recommend that no electrolytic capacitors be used for design."

3. The MOSFET voltage should not be less than 700V

Voltage-tolerant 600V MOSFET is cheaper, many believe that the LED lamp input voltage is generally 220V, so 600V voltage enough, but many times the circuit voltage will reach 340V, in the event of a surge, 600V MOSFET is easily penetrated, thus affecting the life of LED lamps, in fact, the use of 600V MOSFET may save some of the cost but to pay the price of the entire circuit board, so, "Do not use 600V voltage MOSFET, the best choice for the voltage more than 700V MOSFET. He stressed.

4. Try to use a single-level architecture circuit

Doug said that some LED circuits use a two-stage architecture, the "PFC (power factor correction) + isolated DC / DC converter" architecture, this design will reduce 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 overall circuit efficiency will be reduced to 83.6%! "PI's LinkSwitch-PH device integrates the PFC/CC controller, a 725V MOSFET, and MOSFET drivers into a single package, which increases the efficiency of the driver circuit to 87%!" Doug noted, "This device greatly simplifies board layout." The design can save up to 25 components used in the traditional isolated flyback design! The omitted components include high-voltage large-capacity electrolytic capacitors and optocouplers." Doug said that the LED two-stage architecture is suitable for the use of a second constant current driver The circuit can make the PFC drive the old LED driver constant current. These designs are outdated and no longer cost-effective, so single-stage designs are preferable in most cases.

5. Try to use MOSFET devices

If the designed LED lamp power is not very high, Doug recommends the use of integrated MOSFET LED driver products, because the advantage of this is that the integrated MOSFET has less on-resistance, generates less heat, and in addition, is an integrated MOSFET. It is the controller and FET together, there is generally a thermal shutdown function, when the MOSFET is overheated, it will automatically turn off the circuit to protect the purpose of LED lamps, which is very important for LED lamps, because LED lamps are generally small and difficult to air cooling. "Sometimes, it will happen that the LED burns because of overheating, but our plan will never be like this," he said.