Design no electrolytic capacitor LED driver strategy according to VIPER17H
Many countries are actively developing LED lighting technology. High-brightness LEDs have to reach long life and should be as small as possible, and the temperature control is the key, so the LED lamp needs a relatively large heat sink than other light sources. In order to achieve the ideal heat dissipation effect, the LED and the heat sink can be directly contacted, and the radiator is exposed to air is a simple and effective design method, but it has a safety hazard, but it is isolated. The LED driver solution will resolve this issue. Under certain temperature conditions, the LED life can be easily more than 20,000 hours, so the life of the drive will become a key factor in the long life of the entire lamp.
It is well known that the electrolytic capacitance is an important component that affects the power supply of the switch. It is limited to the structure of the electrolytic capacitance, which is difficult to match the long life requirements of the LED, so it is necessary to develop a driver of an electrolytic capacitor. In addition, according to the Solid Star Publishing Scheme 1.1 released in November 2008, the PF value of the LED driver is required, that is, the civil light is greater than or equal to 0.7, and the commercial use is greater than or equal to 0.9.
This design uses ST's new VIPER series controller ViPer17h, and has developed a 3.6W power-electrolytic capacitor LED driver after some modification of the standard application circuit. The input is 100 ~ 240Vac / 50 ~ 60 Hz, the output is 10V / 360mA (average), and the circuit diagram is shown in Figure 1.
Viper17 is a new generation of single-chip integrated controllers, which use BCD6 and SuperMesh process (Figure 2), with lower standby power consumption, the switch section is a MOSFET with a pressure of 800V, plus complete Protection features, including OVP, OCP, OTP, SCP, Brownout, etc., which allows you to have more security margins.
In the seven pins of this chip, the CONT pin is more special, with two functions, see Figure 3:
1. Set the maximum current value IDLIM that allows flow through MOSFET;
2. Set the overvoltage protection point.
This 3.6W powerless capacitor design is implemented using this pin to change the IDLIM value. According to the chip working principle, when the CONT pin is connected to the GND, in a certain interval, as the resistance value is small, MOSFET The limited flow point will also be smaller accordingly, and the correspondence is shown in Figure 4. From essentially, as RLIM is small, it is changed the current value of the CONT pin to change the interior limit point. Therefore, if the CONT pin is connected to the output of the CONT pin and the output of the input rectifier (R11, R12), the current flowing out of the CONT pin will follow the input voltage to change, and then change the limit stream value, input rising The limit stream is large, and vice versa. Through this change, we can see that the current value flowing through the switch followed the input voltage change to a certain extent, and the input characteristics are similar to the PFC circuit, so the circuit structure of the PFC can be used, and a small capacitance of the input capacitor ( Generally a film capacitor). This design uses a 0.1mF thin film capacitor, and the control circuit is similar to the setting of the PFC circuit, which is very low, which can achieve relatively high PF value and stability. At this point, the electrolytic capacitance of the input is already removed. Under the premise of meeting EMC, 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 medium and low-voltage electrolytic capacitors and tantalum capacitors, and ceramic capacitors have lower ESR and very long life. These two advantages are suitable for LED drivers. In the design, the secondary uses 5 10mF / 16V ceramic capacitors as a filter capacitor. At the same time, the primary VCC power supply circuit also uses ceramic capacitance, but since the supply current is small, the self-heating of the capacitor is less than the life of the main circuit filter capacitor, and the electrolytic capacitance can also be used in order to reduce costs.
At this point, an electrolytic capacitor's LED driver is designed, and the test results are shown in Figure 5. It is contemplated that due to the low capacity of the output, the power frequency ripple of the output current is very large, even zero, but this frequency is 100 Hz, and the human eye can not see flicker; It is pointed out to maintain the constant output of the average current in a power frequency cycle, and the peak of the current is 1.7 times the output average, and the lamp is designed to select the LED lamp according to the average current, so the peak current will impact LEDs. However, most LEDs can withstand such impact, as long as they are limited to a certain range, such as SHARP LEDs, to withstand the rated value of 3 times the current. In addition, another difficulty is to filter the differential mode conduction noise, due to the input filter capacitance of the small capacity, and the mode of operation of the PFC, the difference mode conductive noise is higher than the ordinary flying converter, so In Fig. 1, in order to satisfy the EMI requirements, X capacitors and differential mode inductors are increased, thereby bringing the increase in volume and the increase in cost. However, in order to obtain long life with the LED lamp, it meets the relevant standards, which provides a new option. Technology area
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