Discussion on the advantages and disadvantages of high voltage LED

Recently, many websites have been reporting on a "high-voltage LED", believing it to be a new type of LED with many advantages. Some even believe that it will make "today's low-voltage LEDs fade out of the future LED general lighting market" and high-voltage LEDs "dominate the future LED general lighting". Is this really the case?

We know that LED in Chinese means "light-emitting diode", which is fundamentally just a "diode". And it is a diode that works in the forward direction. In the past, there were only high reverse voltage diodes, which means high reverse breakdown voltage. If it works in the forward direction, then the higher the voltage, the greater the current. However, the biggest feature of this high-voltage LED is high voltage and low current. So what's going on?

1. What is High Voltage LED

After a closer look, it turns out that it is just a lot of 20mA low-power LEDs connected in series, which become the so-called high-voltage LED. Connecting many low-power LEDs in series is nothing new, in fact, it has been used in many lamps for a long time. The only difference is that in the past, lamp manufacturers connected the already packaged low-power LEDs in series. For example, Figure 1 shows that Shanghai Longxing Company connected 80 surface-mounted 0.1W low-power LEDs in series to obtain the characteristics of high voltage and low current, which are used in LED bulbs.

Figure 1. 80 0.1W low-power LEDs connected in series

Now, LED manufacturers provide a series connection of low-power LEDs and call it "high-voltage LED". It is just one type of integrated LED. In fact, there have been various integrated LEDs in the past. Different numbers of LEDs are connected in series and parallel to obtain LEDs of various powers and voltages. It can be said that the earliest company Bridgelux in the United States has launched this kind of integrated LED. That is, many low-power LEDs are connected in series and parallel on the substrate to obtain a high-power LED. They call it an LED array. For example, the 30W LED array BXRA-C2000 they launched in 2009 is actually 25 1W LEDs connected in 5 series on the chip. Its size is 25.3x22.3mm, the diameter of the light-emitting surface is 17.5mm (Figure 2), the forward voltage is 16.6V, the forward current is 1.75A, and the thermal resistance is 0.5°C/W.

Figure 2. Integrated LED from Bridgelux, USA

The main difference between high-voltage LEDs and this type of integrated LED is that high-voltage LEDs are all connected in series, while integrated LEDs are connected in series and parallel. The characteristic of integrated LEDs is that a large chip is cut into many small LEDs by a groove method. The depth of the groove is about 4-8μm, and the groove cannot be too wide to avoid reducing the light-emitting area. After the grooves are opened, in order to lay the wires connecting each LED, these grooves must be filled with an insulating layer, and then the corresponding aluminum wires are laid according to the requirements of series or parallel connection.

2. Performance indicators of high voltage LED

At present, we only know that Taiwan's Epistar is producing this kind of high-voltage LED. Its indicators are shown in the following table:

Epistar HVLED Index 3. Advantages and Disadvantages of High Voltage LED

So let's take a look at the advantages and disadvantages of this high-voltage LED.

1. Power dissipation and heat sink size: Some reports claim that the voltage of a 1W high-voltage LED is 50V and the current is 20mA; while the voltage of an ordinary low-voltage 1W LED is 3V and the current is 350mA, so "the power dissipated by a high-voltage LED with the same output power during operation is much lower than that of a low-voltage LED, which means that the cost of the heat dissipation aluminum housing can be greatly reduced." This statement is obviously not true. The size of the dissipated power is mainly determined by the luminous efficiency of the LED, not by its nominal power. The nominal power is not equal to the input power. If you want to determine the size of the heat sink, you should calculate it at the same luminous efficiency. It is generally believed that for the current luminous efficiency of 100lm/W, its true electro-optical efficiency (that is, the efficiency of converting electrical energy into light energy) is only about 30%, that is, only 30% of the electrical energy is converted into light energy, and the remaining 70% of the electrical energy is converted into heat energy and needs to be dissipated through the heat sink. Therefore, for a 1W high-voltage LED and an ordinary low-voltage LED with the same luminous efficiency, the part that becomes heat energy is 0.7W, which needs to be dissipated through the heat sink. Therefore, the size of the heat sink required for this high-voltage LED will not be different under the same input power and the same driving power supply efficiency.

2. Efficiency of AC/DC converter: The same article states that "the lower the input and output voltage difference, the higher the conversion efficiency from AC to DC". The article also states that when the input is 220V, the high-voltage LEDs only need 4 in series to be 200V, which is only 20V different from 220V. However, when using low-voltage LEDs, even if 10 are in series, the forward voltage drop is only 30V, which is very different from 220V. Therefore, "if high-voltage LEDs are used, the efficiency of the transformer can be greatly improved, thereby greatly reducing the power loss during AC-DC conversion. This reduction in heat loss can further reduce the cost of the heat dissipation housing." In fact, anyone who has done AC/DC constant current drive knows that the efficiency of the AC/DC converter has almost nothing to do with the final output voltage. It is possible that the current on the secondary side of the transformer will increase some copper loss, but this is very small and will not affect the design of the heat sink. There are many factors that really affect the efficiency of AC/DC converters. For example, the efficiency of non-isolated converters is much higher than that of isolated converters (because non-isolated converters usually do not use transformers at all. As long as transformers are used, no matter how much the ratio is, the efficiency will be greatly reduced); in addition, the loss of the output rectifier diode of the isolated flyback type will also affect the efficiency. In order to improve the efficiency of the rectifier diode, it is best to use Schottky diodes. If the output is 200V high voltage, it is difficult to buy such high-voltage Schottky diodes. Even if they are bought, the price is very high. If the same inefficient ordinary rectifier diodes are used, the efficiency cannot be improved. . Therefore, it is not accurate to think that high output voltage can greatly reduce the power loss of the converter and even reduce the cost of the heat sink.

3. "High-voltage LEDs reduce the area of ​​LEDs" is true. For example, for a 36W LED, if 36 1W LEDs are used, it will occupy a large area, but if an integrated 36W high-voltage LED is made, it will only take up a small area (Figure 3). This seems to be an advantage, but it may not be an advantage from the perspective of heat dissipation. Because if the luminous efficiency of the two is the same, then the heat they will dissipate is the same. And dissipating a lot of heat in a small area will be a big problem in the design of the heat sink.

Figure 3. Difference in area between integrated LED and ordinary LED

In fact, manufacturers of integrated LEDs have noticed this problem and have made many improvements. The first is to reduce its thermal resistance. The thermal resistance of a general 1W LED is about 6-9°C/W, while the thermal resistance of an integrated LED can be reduced to 2°C/W or even less. Secondly, its backplane is made of copper to improve its thermal conductivity. However, these measures cannot fundamentally improve the basic situation of high heat in a small area. In order to quickly export heat, the only way is to use a heat pipe. This will increase the cost of the radiator. Similarly, since the base plate of the high-voltage LED is made of copper, the part of the radiator that is in direct contact with it must also be made of copper instead of aluminum, because the expansion coefficients of the two are different, and direct contact will produce gaps and affect thermal conductivity. If a copper radiator is used, the cost will increase. Therefore, this high-voltage LED will not only not reduce the cost of the radiator, but will increase the cost of the radiator.

4. “High-voltage LEDs do not need a transformer at all.” Many reports compare high-voltage LEDs with AC-LEDs. Therefore, it is believed that high-voltage LEDs do not need a constant current power supply at all (Figure 4).

Figure 4. High-voltage LEDs and AC-LEDs are often mentioned together.