As a solution that is both environmentally friendly and energy-efficient, LED lighting has found its place in a wide range of applications such as cars, homes, office buildings, hotels, airports, and street lights. However, in addition to overcoming cost barriers, its large-scale commercial applications also need to solve technical problems such as dimming, flashing, heat dissipation, and color uniformity. In addition, the focus on clean energy and the decline in the cost of solar panels have also driven the current solar commercial boom in the industry. In order to help readers grasp this business opportunity faster and better, this publication specially invited Linear power supply expert Tony Armstrong to share his unique insights.
Q: When using PWM or analog dimming, how to eliminate the LED flicker?
A: Facing the increasing popularity of high-power, high-brightness LEDs, electronic lighting designers must provide efficient, accurate, and simple LED drive solutions. This task has become more difficult because high-power lighting (such as automotive headlights or large LCD display backlights) is interchangeable with commercial series LED arrays.
Traditionally, the use of accurate current to drive high-power LED strings contradicts the simplicity and high efficiency. It usually requires some inefficient linear regulator solution or a more elaborate and complex IC Switching regulator configuration. In addition, ensuring that each LED has a uniform brightness and does not generate any flicker has also become a major design obstacle.
There are two generally accepted methods of LED brightness control, namely analog dimming and PWM digital dimming. When analog dimming is used, the adjustment range of the LED current is between a certain maximum value and about 10% of the maximum value (10: 1 dimming range). Because the color spectrum of LEDs is related to current, this method is not suitable for certain applications. However, the PWM digital dimming method is to switch between zero current and maximum LED current at a rate fast enough to cover the visual flicker (usually higher than 100kHz). This duty cycle changes the effective average current, thereby achieving a dimming range of up to 3000: 1 (limited only by the minimum duty cycle). Since the LED current is either at a maximum or turned off, this method also has the advantage of avoiding LED color shift, which is very common when analog dimming is used.
Q: How to solve the heat dissipation problem of high-power LED lighting?
A: The two LED lighting applications with the highest usage and highest power are the back lighting of large-screen LCD TV displays and car headlights. You may wish to 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 car headlight includes 5 types of LED power beams optimized for various lighting requirements, including: low beam lights, high beam lights, turn assist lights, daytime running lights and turn signal indicators.
Standard LED lighting beams will usually require 35W to 50W of power supply. This may not seem like a lot of power; however, the brightness provided by LEDs is up to 10 times that of HID halogen lamps, so the light output of LEDs is equivalent to 500W halogen lamps. The power required for high beams is generally the same as or slightly higher than the standard lighting beam, while the power required for cornering lights, daytime running lights, and turn signal lights 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 rapidly decrease.
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 increased costs and weight due to the use of heat dissipating materials. The most effective way to solve this problem is to use a highly efficient driver (efficiency> 93%) to minimize the heat dissipation of the LED driver circuit. This is not as difficult as it sounds, because a 50W high beam usually consists of 14 1A LEDs in series. Since the forward voltage drop over the entire temperature range is about 4V per LED, the boost converter LED driver topology can increase the nominal battery voltage of 12V to just over 56V with 93% efficiency. This makes it only necessary to consume Dissipate 3.5W of power. For this power dissipation value, it is easy to meet the requirements by installing a low-grade copper heat sink in the printed circuit board where the LED car headlights are installed.
Q: What are the key design challenges when charging the battery with the electrical energy collected by the solar panel?
Answer: As a power generation method that is practical in both commercial and residential environments, solar panels have been widely accepted. However, despite the advances in technology, solar panels are still very expensive. A large part of this high cost comes from the panel itself. Here, the size of the panel (and therefore its cost) will increase as the required output power increases. Therefore, in order to create the smallest form factor and the most cost-effective solution, it is important to maximize the performance of the panel.
Generally speaking, the energy obtained by the solar panel is used to charge the battery, and the energy storage of the battery will in turn provide support for the operation of the end application circuit in the absence of sunlight. If you want to achieve the best design of a solar battery charger, you must 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 dark conditions. Moreover, each solar panel has a characteristic IV curve with the maximum power point, so when the load characteristics do not match the characteristics of the panel, energy extraction will be reduced. The ideal situation is that the panel will be continuously loaded at the maximum power point to make full use of the available solar energy and thereby minimize the cost of the panel.
In general, a Schottky diode connected in series with the battery board can be used to solve the leakage problem of the battery board. Reverse leakage is reduced to a very low value; however, the forward voltage drop of the Schottky diode (which consumes a lot of power under high current conditions) can still cause energy loss. Therefore, it is necessary to use an expensive heat sink and a fine layout to keep the Schottky diode at a low temperature. A more effective 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, thereby significantly reducing power consumption, while reducing the complexity, size and cost of the heat dissipation layout. Fortunately, since some IC suppliers have already produced ideal diodes with this specification (for example: LTC4412 provided by Linear Technology), the above goals can be easily achieved.
However, two problems still exist, namely: "Floating voltage control of fully charged batteries" and "Loading the panel at the optimal power generation point". These problems can often be solved by using a switch-mode charger and a high-efficiency buck regulator.
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