When the output voltage may be higher than the input voltage may be lower than the discontinuous buck-boost converter peak current mode control LED driver is a good choice. However, this buck-boost converter used to drive design, the LED voltage change will change the LED current to LED open circuit will produce high output voltage, thereby damaging the converter. This article will discuss in detail this applied to the LED lights converter design, and describe a variety of overcoming the inherent shortcomings of the method.
The light-emitting diode (LED street lights) applications have been for many years, with the latest technological advances, is becoming a strong competitor in the lighting market. The new high-brightness LED has a long life (100,000 hours) and high efficiency (about 30 lumens / watt). The past three decades, the LED light output brightness every 18 ~ 24 will double, and this growth momentum will continue, this trend is called Haitz law, which is equivalent LED Moore's Law.
Figure 1a: LED connected in parallel. Figure 1b: LED series connection.
Electrically similar to the LED and diode is a one-way conductive (although the reverse blocking capability is not very good, high reverse voltage is very easy to damage the LED), similar to the conventional diode low dynamic impedance VI characteristics. In addition, LEDs are generally safe conduction of the rated current (rated current of the high brightness LED typically is 350mA or 700mA). Rated current, LED forward voltage drop of the difference may be relatively large, typically 350mA white LED voltage drop between 3 to 4V.
The drive LED controlled DC current. In order to make the long life of the LED, the LED current ripple must be very low, and high ripple current causes the LED to produce large resistive power consumption, reduces the LED life. LED driver circuit is the need for higher efficiency, because the overall efficiency depends not only on the LED drive circuit. Work in the current control mode switching converter is ideal drive options to meet LED applications with high power and high efficiency requirements.
Drive multiple LEDs also need careful consideration. For the following two reasons, is not recommended as shown in Figure 1a as parallel LED strings: dynamic impedance and forward voltage drop of each LED is not the same, if there is no external current circuit (current mirror), impossible to guarantee that flow through the LED the same current; an LED failure will make the LED string is disconnected, resulting in LED current is distributed between the rest of the LED string, which will lead to increased current in the LED string, and may damage the LED.
Therefore, a better approach is the LED in series. The disadvantage is that if one LED fails, the entire LED string will stop working. An easy way to let the rest of the LED string to continue working is a Zener diode (rated voltage greater than the maximum LED voltage) shown in each (or group) LED in parallel, as shown in Figure 1b. In this way, any one LED fails, its current will flow to the rest of the Zener diode LED string can still work.
The basic single-stage switching converter can be divided into three categories: the buck converter, boost converter and buck-boost converter. When the LED string voltage is lower than the input voltage, the buck converter (Figure 2a) is the ideal choice; when the input voltage is always lower than the string output voltage, the boost converter is more appropriate (Figure 2b); when The output voltage may be higher than may be lower than the input voltage (caused by changes in output or input), the buck-boost converter (Figure 2c) is more appropriate. The disadvantage of the boost converter, input voltage transients (allows input voltage increases and exceeds the output voltage) will result in the LED upstream a large current (low dynamic impedance load), thereby damaging the LED. Buck-boost converter can replace the boost converter input voltage transient does not affect the LED current.
Buck-boost converter works
LED driver for low voltage applications, the buck-boost converter is a good choice. A variety of reasons, some of which are listed below: They can be used above and below the voltage of the input voltage to drive the LED grow lights strings (boost and buck); high efficiency (it is easy to reach more than 85%); discontinuous suppress the input voltage changes of work mode (to provide good line voltage regulation); peak current control mode allows the converter to regulate the LED current, without the need for complex compensation (simplified design); very easy to implement linear and PWM LED dimming; switch transistor failure will not damage the LED.
Figure 2a: buck converter. Figure 2b: boost converter. Figure 2c: buck-boost converter.
However, this approach there are still two shortcomings: the peak current controlled and discontinuous current mode buck-boost converter is a constant power converter, so any changes will cause the LED current in the LED string voltage corresponding change ; Another problem is that LED open circuit condition in the circuit to produce damage to the high voltage of the converter; In addition, additional circuitry is required to a constant power converter into a constant current converter, and protection in the case of no-load converter.
Figure 3 for the lifting pressure converter application circuit diagram, the controller has built-in used to set the switching frequency of the oscillator. The beginning of the switching cycle, Q1 turns on. Due to the input voltage VIN added to the inductor, the inductor current (iL (t)) from zero (initial steady state) began to rise.