1. To choose a fast diode instead of an ultrafast diode, improve efficiency by restoring part of the leakage inductance energy.
2. Capacitor C3 is used to improve EMI performance.
3. Select resistor R10, which is used to supply 1 mA to U3 when the minimum output voltage is 6 V.
4. U1 selectable current limit point allows to optimize the current limit point and device size to suit the ambient temperature. For example, to reduce dissipation, the TNY280GN device can be used in the same design by changing C3 from 1 μF to 0.1 μF. Alternatively, in environments with high thermal performance, the TNY278GN device can be used by changing C3 from 1 μF to 10 μF.
5. The source can work correctly when the LED string voltage is between 6 V and 20 V. But because the output current is constant, the lower the string voltage, the lower the output power.
Design of LED Lighting Driver Based on Buck Converter
Although the output voltage may be higher or lower than the input voltage, the discontinuous buck-boost converter with peak current mode control is a good choice for LED drivers. However, when using such a buck-boost converter to design a driver, changes in the LED voltage will change the LED current, and an open LED will cause an excessively high voltage at the output, which will damage the converter. This article will discuss this converter design for LED in detail, and give a variety of methods to overcome its inherent disadvantages.
Light-emitting diodes (LEDs) have been used for many years, and with the latest technological advances, they are gradually becoming strong competitors in the lighting market. The new high-brightness LED has a long life (about 100,000 hours) and high efficiency (about 30 lumens / watt). Over the past three decades, the light output brightness of LEDs has doubled every 18 to 24 months, and this growth momentum will continue. This trend is called Haitz's law, which is equivalent to Moore's law of LEDs.
Electrically, LEDs are similar to diodes in that they are unidirectionally conductive (although their reverse blocking capability is not very good, high reverse voltages are easily damaged (LED) and have low dynamics similar to conventional diodes Impedance VI characteristics. In addition, LEDs generally have a rated current when they are safely turned on (the rated current of high-brightness LEDs is generally 350mA or 700mA). When the rated current is passed, the difference in LED forward voltage drop may be large, usually 350mA white The voltage drop of LED is between 3 ~ 4V.
Driving LEDs requires a controlled DC current. In order to make the LED have a longer service life, the ripple in the LED current must be very low, because a high ripple current will cause the LED to have a large resistive power consumption and reduce the LED service life. LED driving circuits need higher efficiency, because the overall efficiency depends not only on the LED itself, but also on the driving circuit. The switching converter working in the current control mode is an ideal driving scheme to meet the high power and high efficiency requirements of LED applications.
Driving multiple LEDs also requires careful consideration. Figure 1 is a series-parallel connection circuit for LEDs. Figure 1 (a) shows the parallel connection circuit of LEDs. Figure 1 (h) is a series connection circuit of LEDs. Because the dynamic impedance and forward voltage drop of each LED are different, if there is no external current sharing circuit (such as current mirror), it is impossible to ensure that the current flowing through the LED is the same; In addition, the failure of one LED will make the LED string Turning off causes all LED current to be distributed among the remaining LED strings, which will cause the current on the LED strings to increase, which may damage the LED. Therefore, for the above two reasons, it is generally not necessary to design a parallel LED circuit as shown in Figure 1 (a). Therefore, it is better to connect the LEDs in series. The disadvantage of this method is that if one LED fails, the entire LED string will stop working. A simple way to keep the remaining LED strings working is to connect a Zener diode (the rated voltage of which is greater than the highest voltage of the LED) in parallel with each (or each group) of LEDs, as shown in Figure 1 (b). In this way, after any LED fails, its current will flow to the corresponding Zener diode, and the rest of the LED string can still work normally.
Basic single-stage switching converters fall into three categories: buck converters, boost converters, and buck-boost converters. When the voltage of the LED string is lower than the input voltage, the buck converter Figure 2 (a) is the ideal choice; when the input voltage is always lower than the string output voltage, it is more appropriate to use a boost converter. Figure 2 (b) ; When the output voltage may be higher or lower than the input voltage (caused by output or input changes), it is more appropriate to use a buck-boost converter as shown in Figure 2 (c). The disadvantage of the boost converter is that any transient in the input voltage (which can increase the input voltage and exceed the output voltage) will cause a large current to flow in the LED (due to the low dynamic impedance of the load), thereby damaging the LED. Buck-boost converters can also replace boost converters because transients in the input voltage do not affect the LED current.