High efficiency current drive for high brightness LED in lighting system

This paper discusses the simple circuit can drive current adjustable high brightness LED, the circuit uses non customized and highly integrated step-down switching regulator (MAX5035), can accurately control the current flowing through the LED. MAX5035 DC/DC converter in the 6.5V to 76V wide input voltage range to maintain 125kHz fixed operating frequency, is the ideal choice for automotive applications. Brightness control can be realized by analog (linear) or low frequency duty cycle (PWM).

High brightness LED development background

In recent years, high brightness LED (HB LED) in a variety of lighting system as the light source are becoming more and more popular, this is due to the high brightness LED has high reliability, service life can reach tens or even tens of thousands of hours, a few orders of magnitude higher than traditional incandescent lamps or halogen lamp lifetime. Based on this advantage, high brightness LED is widely used in automotive lighting, public signs and signal signs and architectural lighting.

High brightness LED is a special treatment of PN junction semiconductor devices, the forward bias can emit white light, red, green or blue (also may produce other color light). As the PN junction, they exhibit the V-I characteristics similar to those of the conventional diode, but have higher junction voltage drop. When the forward voltage reaches VF (2.5V from red LED to 4.5V of blue LED), the current flowing through the LED is very small; once the forward voltage reaches VF, the current will rise rapidly (the same as that of the conventional diode). Therefore, the current limiting measures must be adopted to prevent the LED from being damaged. At present, there are three kinds of basic current limiting methods, table 1 compares these three methods:

High brightness LED switching power supply

Figure 1 is based on fixed frequency, high integration PWM switching converter MAX5035 high brightness LED power supply schematic, the output current up to 1A. Another similar device MAX5033 output current can reach 500mA. The inductor based buck regulator is capable of accurately controlling the current flowing through the LED (or several series LED, with a total voltage of 12V). The switching frequency of the MAX5035 is 125kHz, the input voltage range is up to 76V (need to use higher rated voltage of the input capacitor and diode). This circuit can control and maintain a constant LED current over a wide input voltage range. Table 2 summarizes the design specifications of the circuit.

Figure 1 circuit in the control side of a voltage regulating LED current (Figure 2). Figure 3 shows the efficiency of this control architecture.

The control voltage and the voltage of the three shunt shunt resistors are common to the IC feedback (FB) pin. IC internal control loop so that the voltage on the FB pin remained at about 1.22V, therefore, the control voltage and current detection voltage must be maintained in 1.22V (by resistors R1 and R5 set), current control voltage higher will produce less.

In addition to this exception, the following equations can be used to design other output current and control voltage:

Where: VRFF = 1.22V, RSENSE is R2, R3 and R4 shunt resistance value (= 5).

In many cases, it is very convenient to use the low frequency (50Hz to 200Hz) PWM mode to adjust the LED current, and adjust the brightness by controlling the pulse width (Figure 4). Although LED keep the same brightness in each pulse period, the naked eye can detect changes in brightness, short but the advantages of this method is that the spectrum regulation remains unchanged, the amplitude adjustment time spectrum varies with the current flowing through the LED.

When using 100Hz PWM to control the waveform, the LED current pulse is shown in figure 4. Generally, the efficiency of the low frequency PWM dimming circuit is higher than that of the linear LED dimming circuit (Figure 2).

conclusion

Figure 1 shows IC (MAX5035, MAX5033) for constant current drive high brightness LED provides a cost-effective solution, the program has the following advantages:

* high switching frequency (125kHz) allows selection of small reactance devices (L1 and C2). * to achieve high conversion efficiency over a wide input voltage range. * output voltage up to 12V, can drive three series of high brightness green LED. * no mechanical radiator required. * voltage range can be extended to 76V, suitable for driving high brightness LED. * can be used for 24V signal lights and architectural lighting. * by changing the current sense resistor R2, R3 and R4, the output current can reach 1A. * built in switching power MOSFET, simplified design. * adjust the brightness by controlling the input and using the low frequency PWM signal.