Analyze the technical challenges of LED lighting and solar charging

How to eliminate the LED flicker phenomenon when using PWM or analog dimming

Faced with the increasing penetration of high-power, high-brightness LEDs, electronic lighting designers must provide efficient, accurate and simple LED driver solutions. This task is made more difficult by the interchangeability of high-power lighting, such as automotive headlamps or large LCD display backlights, with commercial tandem LED arrays.

Traditionally, the use of accurate currents to drive high-power LED strings is incompatible with simplicity and high efficiency, often requiring some inefficient linear regulator scheme or more sophisticated multi-IC Switching regulator configuration. In addition, ensuring that each LED has uniform brightness without producing any flicker has also become a major design challenge.

There are two popular LED brightness control methods, analog dimming and PWM digital dimming. When analog dimming is used, the LED current can be adjusted from a certain maximum to about 10% of the maximum (10:1 dimming range). Since the chromatogram of an LED is related to current, this method is not suitable for some applications. However, PWM digital dimming switches between zero current and maximum LED current at a rate that is fast enough to mask visual flicker (typically above 100 kHz). This duty cycle changes the effective average current, resulting in a dimming range of up to 3000:1 (limited only to the minimum duty cycle). Since the LED current is either at its maximum or turned off, the method also has the advantage of avoiding LED color shift, which is common in analog dimming.

How to solve the heat dissipation problem of high-power LED lighting

The two most expensive and highest power LED lighting applications are the backlighting and headlights for large-screen LCD TV displays. Take a look at the standard LED car headlights used by Lexus, Audi, and even GM's Cadillac Escalade. The overall lighting structure of all these cars is very similar. Each automotive headlamp includes five LED-powered beams optimized for a variety of lighting requirements, including: low beam, high beam, turn assist lights, daytime running lights, and turn signal indicators.

Standard LED illumination beams typically will require 35W to 50W of power. This may not seem like a lot of power; however, LEDs provide 10 times the brightness of HID halogen lamps, so the LED's light output is equivalent to a 500W halogen lamp. The power required for a high beam is generally the same or slightly higher than a standard illumination beam, while the power required for a turn assist light, a daytime running light, and a turn signal indicator is lower. However, the overall automotive headlamps consume more than 200W of electrical energy, which may cause significant thermal power dissipation problems. This is really not a good thing, because as the operating temperature increases, the light output and working life of the LED will decrease rapidly.

There are many ways to deal with this heat dissipation problem. One is to add a large number of heat sinks to remove heat from the lights. However, this creates another set of problems, including an increase in cost and weight due to the use of heat dissipating materials. The most effective way to solve this problem is to use a very efficient driver (>93% efficiency) to minimize the heat dissipation of the LED driver circuit. This is not as difficult as it sounds, because a 50W high beam can usually consist of 14 1A LEDs in series. Since the forward voltage drop over the entire temperature range is approximately 4V per LED, the boost converter LED driver topology can boost the nominal battery voltage of 12V to just over 56V with 93% efficiency. Dissipating 3.5W of power, for this power dissipation value, it is easy to meet the requirements by laying a low-grade copper heat sink in the printed circuit board on which the LED headlights are mounted.

What are the key design challenges when charging batteries with electrical energy collected from solar panels?

As a power generation method that is practical in both commercial and residential environments, solar panels have been widely accepted. However, despite advances in technology, the cost of solar panels is still very expensive. Much of this high cost comes from the panel itself, where the size of the panel (and therefore its cost) will increase as the required output power increases. Therefore, in order to create the smallest and most cost-effective solution, it is important to maximize the performance of the panel.

In general, the energy harvested by the solar panel is used to charge the battery, which in turn will support the operation of the terminal application circuit in the absence of sunlight. To achieve the best design of a solar battery charger, it is necessary to understand the characteristics of the solar panel. First, due to the large bonding area, the solar panel will leak, and the battery will discharge through the panel in the dark. Moreover, each solar panel has a characteristic IV curve with a maximum power point, so energy extraction will be reduced when the load characteristics do not match the panel characteristics. Ideally, the panel will be continuously loaded at the maximum power point to take full advantage of the available solar energy and thereby minimize the cost of the panel.

In general, a Schottky diode in series with the panel can be used to solve the leakage problem of the panel. The reverse leakage is reduced to a very low value; however, the forward voltage drop of the Schottky diode (which consumes a lot of power at high currents) still causes energy loss. Therefore, an expensive heat sink and a fine layout are required to keep the Schottky diode at a low temperature. A more efficient way to solve this power dissipation problem is to replace the Schottky diode with an ideal MOSFET-based diode. This will reduce the forward voltage drop to as low as 20mV, significantly reducing power consumption while reducing the complexity, form factor and cost of the thermal layout. Fortunately, these goals are easily achieved because some IC suppliers have created ideal diodes with this specification (eg LTC4412 from Linear Technology).

However, two problems remain: “floating voltage control to fully charged batteries” and “loading the panel at the best power generation point.” These problems can often be achieved by using a switch mode charger and a high efficiency drop. The voltage regulator is used to solve it.

Linear Technology has developed a circuit consisting of the LTC1625 No RESNSE (no sense resistor) synchronous buck controller, the LTC1541 micropower operational amplifier, comparator and reference, and the LTC4412 ideal diode. The circuit is given below for reference:

The circuit of Figure 1 is placed between the solar panel and the battery to regulate the battery float voltage. An additional control loop based on the LTC1541 forces the charger to operate at the maximum panel power point. This increase in efficiency reduces the required panel size, thereby reducing the cost of the overall solution. The important advantages of this circuit are particularly prominent when there is a mismatch between the peak supply voltage of the panel and the battery voltage.

Figure 1: Peak Power Tracking Buck Charger Maximizes Efficiency

In order to meet the design needs of LED drivers and solar panel battery chargers, Linear Technology offers a wide range of products. The LT3595, LT3518 and LT3755 are some of these products.

An example of such a product and LED driver IC is Linear Technology's LT3595 buck mode LED driver, which has 16 separate channels, each capable of driving up to 10 50mA LEDs from up to 45V input. The composed LED string. Each channel can be used to drive 10 series LEDs to provide local dimming. Thus, each LT3595 can drive up to 160 50mA white LEDs. A 46-inch LCD TV will need to have about 10 LT3595s for each HDTV. Its 16 channels can be independently controlled and have one that can provide Individual PWM input up to 5000:1 PWM dimming ratio.