Problems and solutions of LED lighting

LED, known as the light of hope, has developed into the era of popular lighting since its invention in 1964. It has the advantages of energy saving, environmental protection, high brightness, low power consumption, long life, and impact resistance, and has a very broad prospect. In the initial stage of indoor lighting, it also has its inherent shortcomings, which are mainly manifested in the following aspects:

1. Problems caused by inconsistency:

In theory, all LEDs are diodes that can emit light, but in fact, the electrical properties of all LEDs are different. The production process of each manufacturer is inconsistent, and even very different. Even the process of the same manufacturer at different times is different; the purity of semiconductor raw materials used by different manufacturers is different, which makes the luminous intensity and driving current of LEDs not exactly the same, and the difference in overcurrent resistance and heat generation is naturally different; the difference in packaging process and packaging materials makes the overall heat dissipation capacity different, and there are also problems with thermal expansion and heat dissipation of combined materials. It is not difficult to see that there are still great differences between individual LEDs in the short term. If each lamp only uses one LED, it is easy to control. For example, the power indicator light on the TV and DVD; when we use LED to make lighting fixtures, we do not use a single LED, but use multiple, hundreds or thousands of LEDs in an array to connect to the circuit. Due to the difference in LEDs, there is always one that is damaged first. When several are broken (usually short-circuited), the current will increase and damage other LEDs. This is the result of the inconsistency band, and it is also one of the factors restricting its development.

2. Complex driving circuit issues:

a. In terms of voltage matching, LED is not like ordinary incandescent bulbs, which can be directly connected to 220V AC mains. LED is driven by a low voltage of 2-3 volts, so complex conversion circuits must be designed. Different LED lamps for different purposes must be equipped with different power supplies. b. In terms of current supply, the normal working current of LED is 18mA-20mA. When the supply current is less than 15mA, the luminous intensity of the LED is insufficient. When it is greater than 20mA, light decay occurs, causing LED aging, and heat generation, accelerated aging, and shortened life. When it exceeds 50mA, it will be damaged quickly. In order to extend the service life of LED lighting, integrated circuit power supplies, electronic transformers, and discrete component power supplies are often used. When driven by high current, high-power tubes or thyristor devices must be equipped, and protection circuits must be added. Constant current and constant voltage circuits must be used for power supply. In this way, the power supply circuit is complex, and the failure rate, component cost, production cost, and service cost will all increase. This greatly limits the market's competitiveness and purchasing groups.

2. Characteristics of LED lighting circuit:

A. Multiple LED lamps in series and parallel matrix structure; B. No need to control brightness; C. LED current changes within a certain range will not significantly affect illumination; D. The reason for LED damage is mostly excessive current, exceeding 50mA, and a small voltage change causes a large current change. Therefore, it is sufficient to control the overcurrent, which provides a basis for simplifying the circuit.

3. Solutions to the problem:

If the WHPTC overcurrent protector is used for protection, it can be known from the principle that when the current of the circuit exceeds the specified value, it will automatically protect quickly, and automatically reset after the fault is eliminated, without manual replacement. For LEDs, the change of voltage is not the direct cause of LED damage, but the increase of current is the real killer of LEDs. Using this feature, it has an absolute advantage in the circuit protection of LEDs, making simple power supply a reality. Practice has proved that it is effectively protected before the LED circuit fails. As shown in the figure below, with WHPTC, constant current and constant voltage circuits can be omitted. The device cost, production cost, and service cost are greatly reduced. It also greatly increases the market competitiveness of the product.

4. Model selection:

You can choose the required model from WH6-XXX to WH250-XXX with 20mA to 15A. Protect the LED without loss and extend its life. For example: WH250-020 is used in a 220V, 20mA LED circuit. When the current exceeds 40mA, WHPTC will protect for about 20 seconds.
Expert advice: When WHPTC is protecting, its follow-up current keeps the LED on, but the light is dim instead of extinguishing. This is an advantage that other protection circuits do not have. If the protection is caused by interference current or voltage, the LED will automatically return to normal lighting after the interference is over, without the need for manual replacement or maintenance.

5. WHPTC series products can be used for circuit protection of the following lighting, decoration, landscape and other lamps:

Various LED fluorescent lamps, bulb lamps, cup lamps, ceiling lamps, road and bridge lamps, traffic lights, tunnel lamps, guardrail lamps, aerial searchlights, laser lamps, floodlights, garden lamps, underground lamps, underwater lamps, corner lamps, foot lamps, curtain wall lamps, box lamps, disco lamps, bar lamps, floor tile lamps, square lamps, holiday lamps, craft lamps, line lamps, gas car lamps and other LED lamps.
The advancement of high-power LED technology has also made heat dissipation considerations in the design stage increasingly important. In order to avoid accelerated aging of LEDs, or in the worst case, complete scrapping, the LED itself cannot be overheated.

Once the heating efficiency of a high-power LED is higher than its luminous efficiency, the proportion of input power that generates heat instead of light is very high. Therefore, good heat dissipation must be considered during the design phase to ensure that the LED works reliably and allows it to work at higher ambient temperatures. When selecting an LED driver circuit, the heat dissipation of the device must be considered.

An important indicator to ensure that the LED chip does not overheat is the forward current. In actual use, the operating current is often set at a very low level to ensure that the LED does not overheat even at high ambient temperatures. However, if the operating current of the LED is not related to the temperature, it will bring a big problem: when the temperature is too high, the operating conditions of the LED exceed the requirements of its specifications. In addition, at very low temperatures, the current supplied to the LED will be much lower than the maximum allowable current.
Thermistor in LED drive circuit

Therefore, it is desirable to control the LED drive current and reduce the rated operating conditions of the LED. Some expensive LED driver ICs can achieve this function, which uses internal or external temperature sensors to sense the temperature and perform feedback control. We hope to provide a simple method by using PTC thermistors in LED driver circuits. It has the following advantages.
● At room temperature, the forward current is increased.
● Since the number of LEDs is reduced, the cost can be reduced. In addition, low-cost driver ICs can be used, or even driver circuits without integrated temperature management functions can be used.
● A circuit can be designed that does not require IC control but can still adjust the LED operating current according to the ambient temperature.
● Low-cost LEDs can also be used, but the rated operating conditions need to be reduced and a smaller safety margin needs to be provided.
● If an overheat protection function is added, the reliability of the LED will be improved.
● Heat sinks (sheets) and other methods can also be used for heat dissipation.

Comparison of topological structures before and after using WHPTC

A brief discussion on the aging of LED products When we use LEDs, we often encounter such problems. When the LED is welded on the product, it works normally at the beginning, but after lighting for a period of time, it will appear dim light, flicker, malfunction, intermittent lighting, etc., causing serious damage to the product. The reasons for this phenomenon are roughly:
1. When applying the product, there are problems with the welding process, such as the welding temperature is too high, the welding time is too long, and the anti-static work is not done well. More than 95% of these problems are caused by the packaging process.
2. It is caused by the quality of the LED itself or the production process. The prevention methods are:
1. Do a good job of controlling the welding process.
2. Perform aging tests on the product.

Aging is an important guarantee for the reliability of electronic products and is the last essential step in product production. LED products can improve their performance after aging, and help stabilize their performance in later use. LED aging test is a very important link in product quality control, but it is often overlooked and cannot be properly and effectively aged. LED aging test is a countermeasure taken based on the characteristics of the product's failure rate curve, namely the bathtub curve, to improve product reliability, but this method is not necessary. After all, aging test is at the expense of the life of a single LED product.

LED aging methods include constant current aging and constant voltage aging. A constant current source means that the current is constant at any time. If there is a frequency problem, it is not a constant current. That is an AC or pulsating current. An AC or pulsating current source can be designed to have a constant effective value, but this power source cannot be called a "constant current source". Constant current aging is the most consistent with the current working characteristics of LEDs and is the most scientific LED aging method; overcurrent impact aging is also a new aging method adopted by manufacturers. By using a constant current source with adjustable frequency and current for this type of aging, the quality and expected life of the LED can be judged in a short time, and many hidden dangers that cannot be picked out by conventional aging can be picked out. Effectively prevent high temperature failure-PTC thermistor used as LED current limiter In recent years, the development of light-emitting diodes (LEDs) has made great progress: it has developed from being used purely as an indicator to a high-power LED with a light output of more than 100 lumens. Soon, the cost of LED lighting will drop to a level similar to that of traditional cold cathode fluorescent lamps (CCFLs). This has led to a growing interest in LEDs for automotive lighting, LED light sources inside and outside buildings, and backlighting for laptop or TV LCD screens. The development of high-power LED technology has increased the requirements for heat dissipation during the design phase. Like all other semiconductors, LEDs must not overheat, lest their output decreases rapidly or, in the worst case, fail completely. Although high-power LEDs have higher efficiency than incandescent lamps, a significant portion of the input power still turns into heat rather than light. Reliable operation therefore requires good heat dissipation and requires that high temperatures be considered during the design phase. Temperature must also be taken into account when sizing the LED driver circuit: its forward current must be selected to ensure that the LED chip does not overheat even at the maximum ambient temperature. As the temperature rises, it is necessary to reduce the temperature by reducing the maximum allowable current, that is, reducing the rated value. LED manufacturers incorporate derating curves into their product specifications.

See Figure 1 for this type of curve.

Figure 1 LED frequency reduction curve

There is a drawback to operating LEDs with a power supply that is not temperature-dependent: in the high temperature region, the LEDs operate outside the specification range. In addition, when in the low temperature region, the lighting source is powered by a current significantly lower than the maximum allowable current (see the red curve in Figure 1). As shown in the green curve in Figure 1, controlling the LED current through a positive temperature coefficient thermistor (PTC thermistor) in the LED driver circuit is a major improvement. This can bring at least the following benefits:
* Increase the forward current at room temperature, thereby increasing the light output
* Because the number of LEDs can be reduced, a lower-priced driver integrated circuit (IC) or even a driver circuit without temperature management can be used to save costs
* A driver circuit design without IC control can be achieved, and this circuit can also change the LED current with temperature
* It is possible to use cheaper LEDs with higher derating values ​​and smaller safety margins
* Overheating protection function improves reliability
* Thermomechanical design with heat sink is simpler
Most LED driver circuit forms have one thing in common: the forward current flowing through the LED is set by a fixed resistor (see Figure

2). Generally speaking, the current flowing through the LED ILED depends on Rout, that is, ILED ~ 1/Rout. Since Rout does not change with temperature, the LED current is also not affected by temperature.
By replacing the fixed resistor with a circuit that changes with temperature, the temperature management of the LED current can be achieved. The following diagram illustrates how to use a PTC thermistor to improve the standard circuit.
Example 1: Constant current source with feedback loop
Circuit 1 in Figure 2 is a commonly used drive circuit. Its constant current source includes a feedback loop. When the feedback voltage across the adjustment resistor reaches VFB, which varies depending on the IC, the LED current does not change. The LED current is thus stabilized at ILED=VFB/Rout.

Figure 2 Traditional LED driving method

Figure 3 shows an improved version of the previous circuit: This circuit generates a temperature-dependent LED current by using a PTC thermistor. By properly selecting the PTC thermistor, Rseries, and Rparallel, this circuit is matched to the dedicated driver IC and LED combination. The LED current can be calculated by the following equation:
The circuit shown in Figure 3 illustrates the temperature dependence of the LED current (see Figure 3). Compared with a constant current source for a maximum operating temperature of 60 degrees, the LED current can be increased by up to 40% between 0 and 40 degrees after using a PTC thermistor, and the LED brightness can also be increased by the same percentage.

Figure 3 Temperature monitoring and current reduction using a PTC thermistor
Example 2: Constant current source without a regulating resistor in series with the LED
The circuit 2 shown in Figure 2 is another common constant current source circuit: the current is determined by the resistor connected to the driver IC. In this case, however, the regulating resistor is not connected in series with the LED. The ratio between Rset and ILED is determined by the IC specifications. Therefore, with a 20kW series resistor and a TLE4241G driver IC, the resulting LED current is 30mA. Figure 4 shows a modification of the standard circuit, which also includes a PTC thermistor, although a WHPTC thermistor is used here. At the sensed temperature, the component resistance can reach 4.7kW and the permissible error value is ±5°C (standard series) or ±3°C (precision series with permissible error value).
Figure 4 shows the LED current as a function of the ambient temperature. The fixed resistor Rseries has a small permissible error range and dominates the total resistance at low temperatures. Only at about 15 K below the sensed temperature of the PTC thermistor does the current start to decrease due to the increase in the resistance of the PTC thermistor. The current is about 23mA when sensing temperature (total resistance = Rseries + RPTC = 19.5kW + 4.7kW = 24.2kW). The PTC resistance rises sharply at higher temperatures, quickly causing a short circuit, thus avoiding failure due to overtemperature.

Figure 4 Temperature recording without shunt measurement
Example 3: Simple driver circuit without IC
As shown in Figure 2, Circuit 3, the LED can also be operated without a driver IC. The circuit shown is a single 200mA LED driven by a car battery. The regulator generates a stable supply voltage Vstab of 5 V to avoid supply voltage fluctuations. The LED operates at Vstab and the current is determined by the resistor element Rout connected in series with the LED. In this type of circuit, the temperature-independent forward current can be calculated by the following equation, where VDiode is the forward voltage of an LED:
Another approach is to replace the fixed resistor with a WHPTC radial lead PTC thermistor and two fixed resistors, as shown.
Since most of the LED current flows through the PTC thermistor itself, a larger radial lead component needs to be selected. The PTC will heat up due to the current flowing through the resistor itself, so it will always reduce the current regardless of the ambient temperature (as shown in Figure 5). Connecting two or more chip PTC thermistors in parallel will shunt 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 shut down the LED in extremely high temperature conditions, so the current does not drop below the value calculated by the following equation:

This property is extremely important in applications such as automotive electronics, as 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 with the temperature, the main wavelength of the emitted light will increase at a typical rate of +0.1 nm/K. Evaluation of various white light LED drive circuit characteristics After Mr. Nakamura of Nichia Chemical discovered blue light LED in 1996, white light LED was regarded as the component with the most development potential for lighting source. Therefore, the improvement of white light LED performance and commercial application immediately became the focus of research in various countries. At present, white light LED has been applied to public place walkway lights, automotive lighting, traffic signs, portable electronic products, liquid crystal displays and other fields. Since white light LED also has the characteristics of rich three primary color temperature and high luminous efficiency, it is generally considered to be very suitable for backlight illumination source of liquid crystal display. 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 brightness of white light LED is regulated by forward current
The forward voltage of white light LED is usually regulated to 3.0V at a minimum and 4.0V at a maximum when the forward voltage is 20mA. That is, if a certain forward voltage is simply applied, the forward current will vary widely.

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 optical coordinates rank (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 avoid the forward current exceeding the absolute maximum design value (rated value) of white light LEDs.

In Figure 2, the maximum forward current of a white 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 White LED Driving Method

Figure 3 shows four commonly used power supply circuits for driving white light LEDs; Figure 4 shows the ReguLation accuracy characteristics of the above six randomly sampled white light LEDs after stabilization.
The test results of Figure 4 show that the load characteristics of the ReguLator appear at the VF corner of the white light LED, that is, the intersection in the figure is the stable operating point of each white light LED.
2.1 Driving method using voltage ReguLator
The circuit of 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 greater freedom to choose, and there is only one point of contact with 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 light 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 in Figure 3 (b) can stabilize all currents flowing into the LED, a set of BaLLast resistors is specially set in the circuit 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 light LEDs of the same manufacturer and the same lot number (Lot) to achieve excellent matching, when using LEDs of 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 driving 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 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 disadvantages of this circuit are that there is a risk of residual power loss caused by the BaLLast resistor, and the LED current matching cannot be achieved; however, overall, this circuit has both operational characteristics and simplicity, so it has a considerable degree of use value.

2.3 Driving method using output type MuLti PuLL current Regu-Lator
The circuit of Figure 3 (c) can stabilize the current flowing into the LEDs individually, so there is no need to use BaLLast resistors, and the current accuracy and matching ReguLator are controlled by each current ReguLator.
The MAX1570 IC in Figure 3 (c) can make the above current ReguLation 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 stabilized 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 BaLLast resistors, so the packaging area can be effectively suppressed, making it very suitable for applications in fields such as small LCD panels where the packaging space is extremely narrow.

2.4 Drive method using boost current regulator
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 LED is 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 accuracy of the Feed Back ThreshoLd 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%. Another advantage of the Step Up Converter is that two connection terminals are required between the Regu-Lator and the LED, and the number of LEDs used will not be affected by the type of Step Up Converter, which means that designers will have more room for choice. Therefore, Step Up Converters are widely used in LCD panels of various sizes; the disadvantages of the circuit are the height of the inductor, the high cost of components, and EMI radiation interference.

3 Conclusion

The above introduces the commonly used driving circuits for white light LEDs, and explores the advantages, disadvantages and characteristics of each circuit in actual operation through experiments. Due to the limitations of LED structure, there are problems such as difficulty in controlling wavelength and driving current accuracy. With the increasing demand for white light LED backlight modules, how to improve the above wavelength and current accuracy problems and reduce the manufacturing cost of the driving circuit have become problems that must be overcome.