Digital camera flashes tend to be LED-based. Where are the key components?

The application fields of LED (light emitting diode) are steadily expanding , such as signal lights, electronic signs, display backlights, lighting products, etc. It is estimated that in the near future, it can also be used as a flash light for digital cameras .

Until now, digital cameras have long used xenon lamps as flashlights because they can emit strong light in an instant and are inexpensive. However, due to the principle of xenon discharge, the interval and length of light emission cannot be easily and flexibly adjusted.

LEDs can easily achieve a variety of flashes, such as micro-lighting suitable for close-up shots, multiple flashes that can be easily optimized for indoor and backlit situations, and torch-like continuous flashes that are very useful when shooting videos. The only drawback is that it currently cannot meet the light intensity standards required by digital camera manufacturers. (In fact, smartphones with digital camera functions have begun to be equipped with LED flashes , because smartphones do not require as much light as digital cameras.)

In this context, Rohm found a way to make digital camera flash LED, because it finally developed a key component. So, what exactly is this key component?

The answer is a thin (multilayer) electric double layer capacitor (EDLC). The electric double layer capacitor (EDLC) developed by Rohm can instantly supply the large current required by the flash of digital cameras. In terms of the flash guide number (GN), which indicates the amount of flash light, the LED flash module composed of this EDLC and LED has a GN value of 4 or more. The GN of the flash for compact digital cameras using xenon lamps is usually 4 to 6, and "digital camera manufacturers require the same GN value" (Rohm). The company is considering starting sample shipments in the next few months.

This time, Rohm adopted a multilayer EDLC structure (Figure 1) to achieve a GN value of more than 4. Since the distance from the collector tip to the electrode is very short, the resistance can be reduced and the power density can be easily increased. According to the company, the multilayer product developed this time can increase the power density to 40,000 W/kg, which is about 60 times that of the cylindrical product.

Figure 1 shows how the power density is improved by adopting a laminated structure. The original cylindrical structure has a long distance from the collector tip to the electrode, resulting in high resistance and making it difficult to increase the power density (a). However, the laminated structure shortens this distance, resulting in low resistance and improved power density (b).

Regarding multilayer EDLCs, in addition to Rohm, three other companies are currently developing such products, and some manufacturers have already started mass production (the multilayer EDLCs of other companies seem to have achieved the same power density as the products developed by Rohm this time).

However, Rohm said that other companies' products "mostly use highly toxic substances in the electrolyte to reduce resistance." When overvoltage is applied to EDLCs that use highly toxic substances, toxic gas will be generated. The use of highly toxic substances should be avoided as much as possible.

Therefore, in order to achieve higher power density without using highly toxic substances, ROHM reduced the three types of internal resistance (Figure 2).

The first is to improve the conductivity between active materials and between active materials and collectors, thereby reducing body resistance.

The second is to improve the surface treatment process of the collector, inhibit the formation of oxide film, and reduce the interface resistance to basically zero.

The third is to increase the speed of ion adsorption and detachment from active materials during charge and discharge, thereby reducing ion diffusion resistance. Through these improvements, the idea of ​​equipping digital cameras with EDLC has become a reality.