LCD display backlight, general lighting, street lamps and other uses need to drive multiple high-brightness LED at the same time. These LEDs need to be connected in series and parallel, and need to be driven.
For example, when driving 28 LEDs, there are six possible connection methods. One is the first series of 14 LED (LED string) and then in parallel with the two series of LED string (14 series × 2 parallel). In addition, there are 7 series × 4 parallel, 4 series × 7 parallel, 2 series × 14 parallel, 28 series × 1 parallel, 1 series × 28 parallel connection.
What kind of connection method is best? To answer this question, you need to understand the advantages and disadvantages of serial and parallel.
The advantage of series is that the brightness is uniform
The advantage of series is the same string of LED brightness uniformity. Because the current supplied to all LEDs is the same. Moreover, the driver IC only need to output a current, you can light all the LED in series, the circuit structure is simple. In addition, even if an LED short circuit fault occurs, other LED can continue to light is also its advantages.
The disadvantage is that the drive voltage is high. 14 LED in series, assuming a LED forward voltage drop (VF) is 4V, the drive voltage needs 56V. If 28 in series, up to 112V. In addition to part of the semiconductor manufacturers of products, high drive voltage LED driver IC optional range is very narrow.
In addition, there is a LED failure after the break, the LED can not light all the shortcomings.
The advantages of parallel have two. One is able to drive multiple LEDs at low voltage at the same time. If it is 2 series × 14 parallel structure, then, 8V drive voltage can drive 28 LED. Another advantage is that when a series LED fails for some reason, other series LEDs can continue to light up. That is, it is possible to make a circuit that is less susceptible to failure.
There are two shortcomings. One is parallel to the multiple LED string supply current is difficult to control to the same value, so the brightness of each string of LED is likely to produce differences. The other is the drive circuit is more complex. If the number of parallel is 14, you need 14 LED string wiring. Moreover, in accordance with the circuit structure, all LED string must be a separate voltage control and current control, resulting in increased costs.