**4. Electronic converters used to supply high brightness LEDs**

The application of electronic converters switched to supply high brightness LEDs is critical because these converters have higher efficiency than linear converters. Thus, there are several possibilities of implementing converters DC/DC insulated and not insulated. The converters that are chosen must attend all the control and the power delivery to the high brightness LEDs [8]. Resonant converters help to reduce the peak power; they have low switching losses and low electromagnetic interference. Therefore, these topologies are useful for LED applications.

Depending on the characteristics of the electronic converters applied to LEDs more favorable characteristics should be observed to its applicability. Several converters can be used to supply the LEDs, preferably those that have a natural control over the output current.

DC/DC converter for high brightness LEDs should have a current source characteristic at its output, which already limits the inrush current, acting as an inherent protection circuit. To minimize the variations current it is preferable that its output current is not pulsed, which also minimizes the filter output current and allows the use of capacitors with longer life.

#### **4.1. Converters not isolated commonly used for supplying high brightness LEDs**

Buck converter, shown in Figure 8, is widely used in power high brightness LEDs. The current source attribute in the output makes this very interesting electronic converter, essentially because its output current can be continuous. Thus, the output capacitor C may have a small value and is unnecessary to use an electrolytic capacitor, which has the characteristic of a considerably short lifetime.

The output inductance L can be designed to provide a small ripple current, maintaining stable optical characteristics and the temperature of the LED junction.

**Figure 8.** Buck converter.

the spectrum than the red), chromophores (hemoglobin) has bands of high absorption and the

The water begins to absorb wavelengths longer than 1150nm and acts tissue at wavelengths in the range of red and NIR, wherein the tissue penetration becomes maximized (Figure 7). Furthermore, blue, green and yellow light may have significant effects on the cells because its

**Figure 7.** Optical window in tissue due to reduced absorption of red and NIR wavelengths (600-1200 nm) by tissue

The application of electronic converters switched to supply high brightness LEDs is critical because these converters have higher efficiency than linear converters. Thus, there are several possibilities of implementing converters DC/DC insulated and not insulated. The converters that are chosen must attend all the control and the power delivery to the high brightness LEDs [8]. Resonant converters help to reduce the peak power; they have low switching losses and low electromagnetic interference. Therefore, these topologies are useful for LED applications.

Depending on the characteristics of the electronic converters applied to LEDs more favorable characteristics should be observed to its applicability. Several converters can be used to supply

DC/DC converter for high brightness LEDs should have a current source characteristic at its output, which already limits the inrush current, acting as an inherent protection circuit. To

**4. Electronic converters used to supply high brightness LEDs**

the LEDs, preferably those that have a natural control over the output current.

wavelength less than 600nm [7]. For these reasons they are called "optical window".

therapeutic interaction is proven every day.

238 Advances in Bioengineering

chromophores.

If the output capacitor is removed from this type of DC/DC converter, the current on the LEDs is no longer purely DC because it contains a pulsating component. If the converter is a Boost or Buck-Boost converters, LED load is powered by an almost square with a sufficiently high reactance wave. The electronic converter CUK, in its basic structure consists of a fusion of two converters (Boost and Buck) in series using only a controlled switch. The union in series of these two converters permits the entry and exit can operate in continuous conduction mode and the static gain of the converter is the same Buck-Boost converter. Buck converter output enables obtaining a low current ripple in the LED, even for a small amount of C. ZETA converter comprises a Buck-Boost converter in series with an input Buck converter output. Similarly the CUK converter, the Buck converter on the output allows to obtain a low ripple current on the LED. SEPIC converter is composed of a Boost converter at the input, in series with a Buck-Boost converter output. All of these topologies may be used to supply the high brightness LEDs, some have more positive characteristics for the LEDs, such as the control of the output current.

#### **4.2. Converters isolated commonly used for supply high brightness LEDs**

Currently there is a considerable range of converters that can be used to supply LEDs, such as that with galvanic isolation. This sort of application employs the Flyback, Push Pull, Forward and Resonant converters [9]. The Figure 9 shows a system that supplies power to the LEDs using galvanic isolation.

**Figure 9.** Representation of a LEDs supply the galvanic isolation system.

#### **4.3. Topologies used in research**

After reviewing several possible topologies for using as a power source for high-brightness LEDs two converters were chosen, one with galvanic isolation, the Flyback, and one without galvanic isolation, the Buck.

Flyback converter is more robust and has the advantage of being isolated, but it may show some noise in the output. Buck converter controls the output current better and offers a good response, but it has the disadvantage of not being isolated.

In summary, these two converters have been selected because their characteristics are more suited to supply the arrangements proposed by LEDs. The expected results should be quite satisfactory [9, 20].

#### *4.3.1. Flyback converter*

The Flyback converters of levels below 100W of power are widely used for the several applications and also for lighting with LED, normally, operating in discontinuous mode. This mode of operation is appropriate to control the current. The proposed topology is observed on Figure 10 and it was developed to supply the array of LEDs, which produce red light [9].

The red color has a greater wavelength (in the range of 647 to 780nm) and penetrates more deeply into the tissue. Thus, it is indicated for healing and recovering deep tissues [9].

The Flyback converter employed in the experiments owns a universal voltage input and its maximum output voltage is 5V.

The maximum output current is 2A. His frequency of switching is 100kHz.

The proposed arrangement of red LED contains 90 high-intensity LEDs of 5mm, with wave‐ length in the range of 400 to 730nm. The current in each LED is around 20mA.

High Brightness LEDs Supplied by Electronics Converters Used in Tissue Healing and Cell Rejuvenation http://dx.doi.org/10.5772/59817 241

**Figure 10.** LEDs powered by a Flyback converter.

The source was designed to support up to 100 LEDs.

These tests are being conducted in patients with proper authorization and with the participa‐ tion of five doctors, two surgeons, and three dermatologists, at Hospital Regional do Oeste in Chapecó, SC - Brazil.

The voltage produced on the LED was 4.1V and current of the LEDs around 570mA. The values obtained were close to the simulation and design [12, 13].

#### *4.3.2. Buck converter*

**Figure 9.** Representation of a LEDs supply the galvanic isolation system.

response, but it has the disadvantage of not being isolated.

After reviewing several possible topologies for using as a power source for high-brightness LEDs two converters were chosen, one with galvanic isolation, the Flyback, and one without

Flyback converter is more robust and has the advantage of being isolated, but it may show some noise in the output. Buck converter controls the output current better and offers a good

In summary, these two converters have been selected because their characteristics are more suited to supply the arrangements proposed by LEDs. The expected results should be quite

The Flyback converters of levels below 100W of power are widely used for the several applications and also for lighting with LED, normally, operating in discontinuous mode. This mode of operation is appropriate to control the current. The proposed topology is observed on Figure 10 and it was developed to supply the array of LEDs, which produce red light [9]. The red color has a greater wavelength (in the range of 647 to 780nm) and penetrates more deeply into the tissue. Thus, it is indicated for healing and recovering deep tissues [9].

The Flyback converter employed in the experiments owns a universal voltage input and its

The proposed arrangement of red LED contains 90 high-intensity LEDs of 5mm, with wave‐

The maximum output current is 2A. His frequency of switching is 100kHz.

length in the range of 400 to 730nm. The current in each LED is around 20mA.

**4.3. Topologies used in research**

galvanic isolation, the Buck.

satisfactory [9, 20].

240 Advances in Bioengineering

*4.3.1. Flyback converter*

maximum output voltage is 5V.

The second converter developed has the Buck configuration as shown in Figure 11, with the following characteristics: Input Voltage DC-13V (after one stage rectified by with a Flyback converter) and the output voltage reaches 6V and maximum output current reaches 1A [20]. The frequency of switching is 52kHz. The source has total isolation, even in short-circuit conditions in its terminals [10].

**Figure 11.** Buck converter.

This topology has the same versatility of a Flyback converter and supplies the array of LEDs.

The voltage produced on the LED was 3.8V and current of the LEDs around 580mA. The values obtained were close to the simulation and design [11].
