Constant current regulator for high brightness LED lighting solutions

In recent years, high-brightness LEDs (HB-LEDs) have made rapid progress, and these technological advances have been rapidly applied to end markets such as automobiles, buildings, and street lighting. Moreover, with the latest drive technology, high-performance LED lighting has become more reliable and efficient than traditional lighting forms, and is becoming more and more popular in the market. However, due to the lack of standardization, LED driving and control methods are various. The solutions used in many applications do not take into account the special needs of LEDs. Some methods, while able to cope with new certification needs, do not provide an excellent system solution. As a result, there are opportunities and needs for specific application solutions on the market.

First, the system program

The main components of a solid-state HB-LED lighting system can be simply divided into power conversion, control and drive, thermal management, optics, and of course the LED itself. Given that any of these components are not fully available and applied, a given HB-LED lighting system will not work efficiently. For example, without using a lens and a light guide to focus and process the light source, it does not meet the lighting specifications of the application. Similarly, if the thermal management problem is not carefully considered and addressed, the operating temperature of the system will be severely affected by the sudden rise of the LED junction temperature well above the maximum rated operating temperature of the component.

The voltage source in an HB-LED lighting system will vary with the type of application. For building and building applications, we can usually estimate the supply voltage to be AC. At the same time, outdoor lighting may be powered by unregulated power supplies such as AC power, 12V lead-acid batteries, or solar power. For automotive applications, the power supply is usually a 12V battery.

Although it is possible to use a voltage source to drive the LED without using some type of power conversion, this is not a good idea because normal voltage fluctuations can cause large changes in LED current. Taking into account the extremely steep voltage/current (V/I) curve and the large difference in the forward voltage (usually above 1V) of different batches of LEDs, it is necessary to use isolated or non-isolated power conversion sections.

Second, the LED steady flow

The main function of the led driver is to limit the current, regardless of the input conditions and how the forward voltage changes under various operating conditions. The drive itself and the overall system solution must meet the requirements for energy efficiency, current tolerance, form factor, size, cost, and safety. The chosen solution must also be easy to apply and strong enough to meet the extreme environmental conditions of a particular application.

Designers can choose from three different basic regulator topologies depending on the details of their application. They are:

1. Buck—Used when the minimum input voltage (Vin) is always greater than the maximum operating voltage of the LED string under all operating conditions.

Figure 1 Typical circuit diagram of the buck converter

2. Boost—Used when the maximum input voltage (Vin) is always less than the minimum operating voltage of the LED string under all operating conditions.

Figure 2 Typical circuit diagram of the boost converter
3. Buck-Boost or Single-Ended Primary Inductor Converter (SEP ic )—Used when there is overlap between input and output voltages. Advances in coupled inductors have made these solutions easier to apply in equally sized buck or boost topologies. Once mastered, the SEPIC topology offers more advantages than other common topologies, as well as higher energy efficiency, smaller form factor and lower cost.

Figure 3 Typical circuit diagram of a buck-boost converter

Third, the classification of LED steady flow scheme

1, the resistance

Resistance is the simplest and lowest cost steady current solution. In practice, they are not a practical solution because they rely on battery voltage, resulting in LED brightness variations, low energy efficiency and the necessary costly, and require time-consuming and labor-intensive LED coding.

2, linear regulator

The linear regulator is easy to design, provides effective current regulation and overcurrent protection, and provides an external current set point. It is a "medium" HB-LED system current regulation scheme. However, in today's energy-conscious era, for many devices, especially street lighting, buildings, and battery-powered applications, designers may think that they consume too much power and are unacceptably low-efficiency. Low-efficiency linear regulators almost always have thermal management problems, which often require some form of heat sink, but increase the overall design size and cost.

3, switching regulator

Switching regulators are the most expensive and technically complex LED current control solution. Unlike linear regulators and simple resistor-stabilization schemes, they are susceptible to electromagnetic interference (EMI), giving designers another challenge to overcome. However, switching regulators are highly energy efficient and provide brightness control for applications. For medium to high power solutions, or applications that need to handle a wide input voltage range, switching regulators are the only viable option.

4, constant current regulator

2-pin and 3-pin constant current regulators (such as those developed by ON Semiconductor) offer simpler and lower cost solutions than linear regulators and switching regulators, and offer a performance advantage over resistor solutions. The 2-pin component provides a fixed output, while the 3-pin version provides the ability to set the output with a simple external resistor. Its output current value is between 20 ~ 150mA, the maximum working voltage is 45V, to ensure that it can withstand the battery load dump voltage.

Figure 4 Typical circuit diagram of constant current regulator

Using a constant current regulator is like using linear and switching regulators, ensuring constant brightness over the wide range of LED voltages they support. They also protect the LED from overdrive at higher input voltages and significantly reduce or completely eliminate costly and problematic LED inventory coding issues. The wide input voltage range of up to 40V supports operation in a variety of applications and withstands associated supply voltage fluctuations. The constant current regulator can be configured as a buck, boost or SEPIC topology. If the current of the driven LED string is higher than the range that a single constant current regulator can support, these components can be connected in parallel to provide a solution.

The light emitted by the LED is proportional to its average output current. The constant current regulator also controls this current, providing additional light output regulation. Analog dimming or digital pulse modulation can be used to provide dimming. The analog dimming scheme combines the input PWM signal with the feedback voltage to reduce the average output current. The digital dimming scheme uses an input PWM signal to suppress the regulator's switching and reduce the average output current. The typical dimming frequency is 200 to 1000 Hz because the human eye cannot see subtle changes above the 200 Hz frequency, but can detect subtle changes below this frequency.
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