Comparison of the advantages and disadvantages of LED series and parallel

For example, when driving 28 LEDs, there are six possible connection methods. One is to first connect 14 LEDs in series (LED string) and then connect two of these series-connected LED strings in parallel (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, and other connection methods.

Which connection method is the best? To answer this question, you need to understand the advantages and disadvantages of series and parallel connection.

The advantage of series connection is uniform brightness

The advantage of series connection is that the brightness of LEDs in the same string is uniform. This is because the same current is supplied to all LEDs. In addition, the driver IC only needs to output one current to light up all LEDs in the series, making the circuit structure simple. In addition, even if one LED shorts out, the other LEDs can continue to light up, which is also an advantage.

The disadvantage is that the driving voltage is high. When 14 LEDs are connected in series, assuming that the forward voltage drop (VF) of one LED is 4V, the driving voltage needs to be 56V. If 28 are connected in series, the voltage is as high as 112V. Except for the products of some semiconductor manufacturers, the selection range of LED driver ICs with high driving voltage is extremely narrow.

In addition, there is a disadvantage that if one LED fails to open, all the LEDs connected in series cannot be lit.

There are two advantages to parallel connection. One is that multiple LEDs can be driven at low voltage at the same time. If it is a 2-series × 14-parallel structure, then 8V drive voltage can drive 28 LEDs. Another advantage is that when one series LED fails for some reason, the other series LEDs can continue to light up. In other words, a circuit that is not easily affected by failure can be formed.

There are two disadvantages. One is that it is difficult to control the power supply current of multiple LED strings connected in parallel to the same value, so the brightness of each LED string is likely to be different. The other is that the driving circuit is more complicated. If the number of parallel connections is 14, it is necessary to wire 14 LED strings. Moreover, according to the circuit structure, all LED strings must be individually voltage-controlled and current-controlled, which increases costs.