Design strategy of LED driver without electrolytic capacitor based on VIPer17H

Many countries are actively developing LED led /' target='_blank'> lighting technology. To achieve a long life of high-brightness LEDs and minimize light decay, temperature control is the key, so compared with other light sources, LED lamps require a relatively large heat sink. In order to achieve ideal heat dissipation, direct contact between the LED and the heat sink can obtain the minimum thermal resistance. Exposing the heat sink to the air is a simple and effective design method, but it brings safety hazards. However, if an isolated LED driver solution is used, this problem will be solved. Under certain temperature conditions, the life of the LED can easily exceed 20,000 hours, so the life of the driver will become a key factor in the long life of the entire lamp.

As we all know, electrolytic capacitors are important components that affect the life of switching power supplies . Due to the structure of electrolytic capacitors , it is difficult to match the long life requirements of LEDs. Therefore, it is necessary to develop a driver without electrolytic capacitors. In addition, according to the Solid-State Lighting Fixture Program Standard Version 1.1 released by Energy Star in November 2008, the PF value of LED drivers is required to be greater than or equal to 0.7 for civil lamps and greater than or equal to 0.9 for commercial lamps.

This design uses ST's new VIPer series controller VIPer17H, and develops a 3.6W electrolytic capacitor-free LED driver after some modifications to the standard application circuit . The input is 100~240Vac/ 50~60Hz, the output is 10V/360mA (average value), and the circuit diagram is shown in Figure 1.

VIPer17 is a new generation of monolithic integrated controller from STMicroelectronics ( ST), which adopts BCD6 and SuperMESH process (Figure 2), has lower standby power consumption, and the switch part is a MOSFET with a withstand voltage of 800V. With complete protection functions, including OVP, OCP, OTP, SCP, Brownout, etc., the design can have more safety margin.

Among the 7 pins of this chip, the CONT pin is special and has two functions, as shown in Figure 3:

1. Set the maximum current value Idlim allowed to flow through the MOSFET;

2. Set the overvoltage protection point.

This 3.6W electrolytic capacitor-free design is achieved by changing the Idlim value using this pin. According to the working principle of the chip, when the CONT pin is connected to a resistor Rlim to GND, within a certain range, as the resistor value decreases, the current limit of the MOSFET will also decrease accordingly. The corresponding relationship is shown in Figure 4. In essence, as Rlim decreases, the current value flowing out of the CONT pin is changed, thereby changing the internal current limit. Therefore, if the CONT pin is connected to the output end of the input rectifier bridge through a resistor (such as R11 and R12 in Figure 1), the current flowing out of the CONT pin will change with the input voltage, thereby changing the current limit value. As the input increases, the current limit becomes larger, and vice versa. Through this step of change, we can see that the current value flowing through the switch tube changes with the input voltage to a certain extent, and the input characteristics are similar to the PFC circuit. Therefore, the PFC circuit structure can be used, with a small-capacitance input capacitor (usually a film capacitor). This design uses a 0.1mF film capacitor, and the control loop is similar to the PFC circuit setting, with a very low loop gain crossover frequency, so that a relatively high PF value and stability can be obtained. At this time, the electrolytic capacitor at the input end has been removed. Under the premise of meeting the EMC requirements, the value of this capacitor can be as small as possible, so that a higher PF value can be obtained.

With the continuous advancement of technology, ceramic capacitors are increasingly entering the application field of medium and low voltage electrolytic capacitors and tantalum capacitors. Ceramic capacitors have lower ESR and very long life, and these two advantages are very suitable for LED drivers. In this design, 5 10mF/16V ceramic capacitors are used as filter capacitors in the secondary. At the same time, ceramic capacitors are also used in the primary VCC power supply circuit. However, because the power supply current is very small, the capacitors themselves generate less heat, and the reduction in life is not as significant as the main circuit filter capacitors. Electrolytic capacitors can also be used to reduce costs.

At this point, the design of an LED driver without electrolytic capacitors is completed, and the test results are shown in Figure 5. In practice, due to the low capacity of the output capacitor and the control loop setting similar to PFC, the power frequency ripple of the output current is very large, even to zero, but this frequency is 100Hz, and the human eye cannot see the flicker; it should be pointed out that in order to maintain the constant average output current within a power frequency cycle, the peak current is 1.7 times the output average value, and the LED lamp is selected according to the average current when designing the lamp, so the peak current will cause an impact on the LED, but most LEDs can withstand such an impact as long as it is limited to a certain range. For example, SHARP's LED can withstand a current three times the rated value under this condition. In addition, another difficulty is to filter out the differential mode conduction noise. Due to the use of small-capacitance input filter capacitors and a working mode similar to PFC, the differential mode conduction noise is higher than that of ordinary flyback converters. Therefore, in Figure 1, in order to meet EMI requirements, X capacitors and differential mode inductors are added, which leads to an increase in volume and cost. However, in order to obtain a long life that matches LED lamps and meet relevant standards, this solution provides a new option.