High-power LED beads

1. Power dissipation and radiator size: some reports claim that the voltage of 1W high-voltage LED is 50V and the current is 20mA; However, ordinary low-voltage 1W LED has a voltage of 3V and a current of 350mA, so “the power dissipated by high-voltage LED with the same output power is much lower than that of low-voltage LED, which means that the cost of cooling aluminum shell can be greatly reduced. Obviously, this is not true. The amount of dissipated power is mainly determined by LED luminous efficiency rather than its nominal power. Nominal power is not equal to the input power. 

If you want to determine the size of the radiator, it should be calculated 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 changing electric energy into light energy) is only about 30%, that is, only 30% of electric energy is converted into light energy, and the remaining 70% of electric energy is converted into heat energy and needs to be dispersed through the radiator.

Therefore, for 1W high-voltage LED and common low-voltage LED with the same luminous efficiency, their heat energy is 0.7w, which needs to be dispersed through the radiator. Therefore, the size of the radiator required by the high-voltage LED will not make any difference with the same input power and the same drive power efficiency.

2. Efficiency of AC/DC converter: in the same paper, it is believed that “the lower the input and output pressure difference, the higher the conversion efficiency of AC to DC”. This paper also thinks that, because only four high-voltage leds in series are 200V when 220V is input, the difference between 220V and 220V is only 20V. With low-voltage LED, even 10 series positive voltage drops are only 30V, which is quite different from 220V.

Therefore, “if high-voltage LED is adopted, the efficiency of the transformer can be greatly improved, which can greatly reduce the power loss during ac-dc conversion, and this heat loss can further reduce the cost of cooling housing.”

As anyone who has ever done AC/DC constant current drive knows, the efficiency of AC/DC converter has almost nothing to do with the final output voltage. There may be some copper loss due to the high current in the secondary of the transformer, but it is too small to affect the design of the radiator. 

There are many factors that really affect the efficiency of AC/DC converter. For example, the efficiency of the non-isolated converter is much higher than that of the isolated converter (because usually the non-isolated converter does not use transformer at all, as long as the transformer is used, no matter what the ratio is, the efficiency will be greatly reduced);

In addition, the isolation of the flyback type of output rectifier diode loss may also affect efficiency, in order to improve the efficiency of the rectifier diodes, had better use schottky diodes, and if the output is 200 v high voltage, so it is very difficult to buy this kind of high-pressure schottky diode, even buy the price is very high, if the same inefficient ordinary rectifier diode, so its efficiency is not likely to improve. Therefore, it is not accurate to say that high output voltage can greatly reduce the power loss of the converter and even reduce the cost of the radiator.

3. “high voltage LED reduces LED area“, indeed. For example, for a 36W LED, if 36 1W leds are used, it will occupy a large area, while for an integrated 36W high-voltage LED, only a small area is needed (figure 3). This may seem like an advantage, but it’s not necessarily an advantage from a cooling point of view. Because if the luminous efficiency of both is same, so the heat that they will send out is same. And want to send out a lot of quantity of heat in a very small area, this will be a big problem in radiator design.

In fact, integrated LED manufacturers have noticed this problem and have made many improvements. The first is to reduce the thermal resistance. General 1 wled thermal resistance in about 6 to 9 ° C/W, and integrated LED thermal resistance can be reduced to 2 ° C/W or less. Secondly, its back plate is converted to red copper to improve its heat conduction. However, these measures cannot fundamentally improve the basic situation of small area and high calorie. The only way to get the heat out quickly is to use a heat pipe. 

And this can increase the cost of radiator instead. Similarly, since the base plate of high-voltage LED adopts red copper, the radiator part in direct contact with it must also adopt red copper instead of aluminum, because the expansion coefficient of the two is different, the direct contact will produce gaps and affect the heat conduction. If the use of copper radiator will increase the cost.

Therefore, this kind of high-voltage LED will not only reduce the cost of the radiator, but also increase the cost of the radiator.

4. “high-voltage LED may not need transformer at all“. In many reports, high voltage LED and AC-LED in the same. Therefore, it is believed that high-voltage LED does not need constant current power supply at all (figure 4).

We know that first of all, not only high-voltage LED and ac-led can not use constant current source, and all LED can not use constant current source, but the use of constant current source will greatly reduce the service life of LED. This is because the negative temperature coefficient of LED volt-ampere characteristic makes the positive current of LED increase with the temperature rise of LED, which makes the junction temperature rise of LED higher and reduces the service life of LED. 

High-voltage LED is no exception. So this is not true. Sacrificing life to reduce costs doesn’t seem like a good way to do it. Because let’s say you could have 50,000 hours of work, and now you have 25,000 hours of work, that’s a tripling of the cost for the user. Much more expensive than saving a constant current source.

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