All LEDs are driven electrically before they can emit quality light. This is one of their basic characteristics which differentiate them from other kinds of light sources. In this write-up, we’re going to be focusing on the DC drive circuitry for LEDs.
In the first place, the battery is a typical DC source. It’s usually used for flashlights. It can as well serve as the output of a converter. Actually, DC is quite different from AC. The difference is that DC has much lower voltage than AC. Hence, the regulations governing the use of DC are much easier to follow. You don’t need to worry about EMI since the voltages are quite safe. The input voltage of a typical DC drive circuitry is usually steady. However, the downside is that the current in the DC drive can be very high. Hence, the source impedance is a very vital factor in its design.
Meanwhile, the fundamental determinant factor on what type of converter to use for a DC drive is usually set by the relative values of the LED voltage as well as the supply voltage. Three kinds of cases are involved here.
• First, the supply voltage is usually higher than the LED voltage.
• Secondly, the supply voltage is simply lower than the LED voltage
• Thirdly, the supply voltage can sometimes be higher than the LED voltage while the reverse can also be the case in some occasions.
In most cases, the supply voltage may be higher than the LED voltage where there’s never a time in operation when it’s lower.
A typical example of when the supply voltage is always higher than the LED voltage is seen in a flashlight that uses 4 D cells and also runs a single 3 W white LED. The D cells are usually 1.5 V each when they are still new. However, they tend to lower in voltage when they get older. They can produce a total battery voltage of 3.6 - 6.0. The LED’s forward voltage will then be 3.2 V at 350 mA. The rate will also begin to decrease with the reality of the increasing temperature on ground. In such cases, a buck converter will then be used.
A typical example of the second case when the supply voltage is always lower than the LED voltage can be seen in a flashlight that uses 2 C cells and also drives a set of 75 mm white LEDs. The C cells may have the same voltage range as in the case of D cells. Hence, the battery voltage range will be at 1.8 – 3.0 V. The LEDs here will have a voltage of 3.2 V for each of them. This will amount to a total string voltage of 22.4 V. In such cases, a boost converter would be used.
On the third case where the supply voltage is sometimes higher and sometimes lower than the LED voltage, we can also take an example of a flashlight with 3 D cells to check it out. The total battery voltage here will be 2.7 – 4.5 V. The LED voltage would be in the middle of this range. In such a case like this, a buck-boost converter would be used.
A look at Battery Basics
The three examples discussed above are based on batteries. Hence, there’s a need to grasp some basic pieces of information about battery make-ups. Basically, the word “battery” refers to a sealed device that powers other devices and gadgets. For instance, we have laptop batteries, flashlight batters, clock batteries and so on. You can easily purchase them in supermarkets around your vicinity. Some of them are big enough while others are small.
In technical terms, the word “cells” is used in denoting batteries. A battery is simply the end device that powers another device or unit. It can be made up of 2 or more cells. It can showcase in series of cells depending on the make. Some batteries may also contain single cells especially the types found in toys. Having said this, it’s clear that a laptop doesn’t use batteries per se. it rather uses three or four 4.3 V lithium cells in technical terms. On the other hand, a simple flashlight with 2 D cells uses a battery to run.
From this explanation, the word “battery” and “cell” can be confusing. People tend to use them interchangeably but there’s a difference found between the two. Actually, the simplest electrical information about a battery or a cell is that it serves as a voltage source. A cell can change its output voltage depending on the current load it carries. The state of its charge, age and temperature also affects its output voltage. There are also different types of cells with different features and make-ups. All of these facts come into play when designing a DC drive for LEDs.
Meanwhile batteries can be affected by the ambient temperature. When the cells keep getting colder, their voltage will begin to drop while their impedance continues to rise.
There’s also aging effect on most rechargeable cells. As you keep charging and discharging them from time to time, they will begin to lose their charging ability. After hundreds of charging cycles, they are likely to lose much capacity and finally become dead. Some nickel/metal hydride batteries do experience a memory effect. If you charge them partially before they are fully re-charged, they usually lose some of their storage capacity. This can continue to weaken them until they are completely dead.
Single Series Strings for LEDs
Majority of LED drive circuits usually make use of single series string of LEDs. This usually sets the output voltage of the circuit since all LED forward voltages are also in series. Their input voltage also determines the topology. In any case, there’s always a downside to having all the LEDs in series. If any of them actually fails to open, the entire string will be affected. In most cases, the entire string will also be open and no light will be generated.
From the above discourse, you must have gathered some information on practical DC Drive Circuitry for LEDs. You still need to keep researching on the topic in order to know more.