Detailed explanation of the classification and characteristics of LED driver power supplies

Another approach is to combine a radial lead PTC thermistor of WHPTC and two fixed resistors to replace the above fixed resistors, as shown in the figure.

Since the majority of the LED current flows through the PTC thermistor itself, a larger radial leaded component needs to be selected. The PTC will heat up due to the current flowing through the resistor itself, and will therefore always reduce the current, regardless of the ambient temperature (see Figure 5). Connecting two or more chip PTC thermistors in parallel will split the current, but this solution still has limitations.

Figure 5 Temperature compensation drive circuit without IC

The current value is mainly set by the appropriate choice of two fixed resistors. These two resistors also play an important role in improving the circuit because they keep the tolerance of the generated LED forward current low. This is especially important in the normal operating temperature range, where the resistance tolerance of the PTC thermistor itself is still high. The second parallel fixed resistor also ensures that the PTC does not completely turn off the LED in extremely high temperature conditions, so the current does not drop below the value calculated by the following equation:

This performance is extremely important in applications such as automotive electronics, where safety requirements do not allow the lights to be completely switched off.

Background information: Temperature dependence of LEDs

Like all semiconductors, the maximum permissible junction temperature of LEDs must not be exceeded to avoid premature aging or complete failure. If the junction temperature is to be kept below a critical value, the maximum permissible forward current must decrease as the ambient temperature increases. However, if a heat sink is used, the forward current can be increased at a specific ambient temperature. The light output of LEDs decreases as the chip junction temperature increases. This is mainly the case for red and yellow LEDs, while white LEDs are less temperature-dependent. The luminous efficiency and forward current increase in tandem, but the high thermal resistivity of the LED mounted between the junction and the environment can reduce or even reverse this effect, because the emitted light decreases as the junction temperature increases.

In addition, when the junction temperature rises and the LED forward voltage keeps increasing synchronously with the temperature, the main wavelength of the emitted light will increase at a typical rate of +0.1 nm/K. Evaluation of the characteristics of various white light LED drive circuits In 1996, after Mr. Nakamura of Nichia Chemical discovered the blue light LED, the white light LED was regarded as the component with the most development potential for lighting sources. Therefore, the improvement of white light LED performance and commercial application immediately became the focus of research in various countries. At present, white light LEDs have been used in public places. Sidewalk lights, automotive lighting, traffic signs, portable electronic products, liquid crystal displays and other fields. Because white light LEDs also have the characteristics of rich three primary color temperatures and high luminous efficiency, they are generally considered to be very suitable for backlighting sources of liquid crystal displays. Therefore, various manufacturers have successively launched white light LED dedicated drive circuits and related components. In view of this, this article briefly explains the characteristics of LED dedicated drive circuits and future development trends. 1 Reasons for constant current drive

1.1 The luminosity of white LEDs is regulated by forward current

The forward voltage of white light LEDs is usually regulated to a minimum of 3.0V and a maximum of 4.0V at 20mA. That is, if a certain forward voltage is simply applied, the forward current will vary over a wide range.

Figure 1 shows the results of testing the forward voltage and forward current characteristics of three types of white light LED samples randomly selected from the products of LED companies A and B. According to the test results, if the six types of white light LEDs are driven by a 3.4V forward voltage, the forward current will vary greatly within the range of 10~44mA. Table 1 shows the electrical and optical characteristics of white light LEDs.

Since the luminosity and chromaticity of white light LEDs are measured using a constant current method, they are usually driven with a constant current to obtain the expected brightness and chromaticity.

Table 2 shows the ranks of optical coordinates (IF=25mA, Ta=250C).

1.2 Avoid forward current exceeding the allowable current value

To ensure the reliability of white light LEDs, it is basically necessary to try to prevent the forward current from exceeding the absolute maximum design value (rated value) of the white light LED.

In Figure 2, the maximum forward current of a white light LED is 30mA. As the ambient temperature rises, the allowable forward current continues to decrease. If the ambient temperature is 50°C, the forward current cannot exceed 20mA. In addition, it is difficult to control the current value flowing into the LED using a constant voltage drive method, so the reliability of the LED cannot be maintained.

2. Driving method of white light LED

FIG3 shows four commonly used power supply circuits for driving white light LEDs; FIG4 shows the Regulation accuracy characteristics of the above six randomly sampled white light LEDs after stabilization.

The test results in Figure 4 show that the load characteristics of the ReguLator appear at the VF corner of the white light LED, that is, the intersection point in the figure is the stable operating point of each white light LED.

2.1 How to drive the voltage regulator

The circuit in Figure 3(a) uses a voltage regulator and a BaLLast resistor to control the LED current. The advantage of this circuit is that there are many types of voltage regulators, the designer has more freedom to choose, and there is only one point of contact between the voltage regulator and the LED; the disadvantage is that the power loss caused by the BaLLast will lead to deterioration of efficiency. In addition, the forward current of the LED cannot be precisely controlled.

As can be seen in Figure 4(a), the forward current of six randomly sampled white LEDs has a wide distribution range from 14.2mA to 18.4mA, so the (average) forward current of the LED of manufacturer A is as high as 2.0mA. In contrast, although the ReguLator used in the circuit of Figure 4(b) has the advantages of small size and low cost, its disadvantage is that it may not meet the requirements of performance and reliability, which means that the practicality of this circuit is relatively weak.

2.2 Voltage Regulator drive method using constant current output

Although the circuit of FIG3( b ) can stabilize all currents flowing into the LEDs, a group of BaLLast resistors is specially provided in the circuit in order to match the electrical characteristics of each LED.

The MAX1910 in Figure 3(b) is a constant current output type voltage regulator. Although this circuit uses white LEDs from the same manufacturer and the same lot number (Lot), it has achieved excellent matching. However, when using LEDs from different manufacturers and lots, there will be a large difference in the distribution of characteristics. This current regulator uses a similar method to Figure 3(a) to control the drive current, but it can reduce the power consumption of the Ballast resistor by about half.

The test results in Figure 4(b) show that the current flowing into the six randomly sampled white light LEDs varies greatly from 15.4mA to 19.6mA. Therefore, the LEDs of both manufacturers A and B are driven with an average current of 17.5mA. The disadvantage of this circuit is that the power loss caused by the BaLLast resistor may remain, and the matching of the LED current cannot be achieved; however, overall, this circuit has both action characteristics and simplicity, so it has considerable use value.

2.3 Driving method using output type MuLti PuLL current regulator

The circuit of FIG3(c) can stabilize the current flowing into the LEDs, so there is no need to use a Ballast resistor, and the accuracy and matching of the current ReguLator are dominated by the respective current ReguLator.

The MAX1570 IC in Figure 3(c) can achieve the above current regulation to achieve the current accuracy of 2% standard and the current matching of 0.3% standard.

The current regulator composed of MAX1570 IC is a low drop-out type, so its operation efficiency is very high. The test results of Figure 4(c) show that when the driving circuit of Figure 3(c) is used, the stabilization current flowing into six randomly sampled white light LEDs is 17.5mA.

Although four connection terminals are required between the ReguLator and the LED, this circuit does not require a BaLLast resistor, so the packaging area can be effectively suppressed. This makes it very suitable for applications in areas such as small LCD panels where the packaging space is extremely narrow.

2.4 Using boost current regulator drive

The circuit in Figure 3(d) uses an inductor that can stabilize the current to form a so-called high-efficiency Step Up Converter. The biggest feature of this circuit is the Feed Back ThreshoLd voltage, which can reduce the power loss of the current detection resistor. In addition, the LEDs are connected in series, so the current flowing into the white light LED can be fully matched with the LED even under various requirements. The accuracy of the current basically depends on the Feed Back ThreshoLd accuracy of the Regu-Lator, so it will not be affected by the forward voltage of the LED.

The efficiency (PLED/PIN) of the current regulator composed of MAX1848 and MAX1561 ICs is: three LEDs + MAX1848, 87%; six LEDs + MAX-1561, 84%.