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DC/DC Converter for commercial refrigeration LED lighting.

1 INTRODUCTION

Commercial refrigerators and freezers that are used in grocery stores around the world to keep frozen foods, dairy products, meats, and many other foods fresh require lighting fixtures inside the cases in order to illuminate the products inside. These lighting fixtures are often located between the refrigerator doors across from the food. This area inside the case is called a mullion. The mullions can be seen below in Fig. 1. There are mullions located between doors and there are mullions located at each end of the case. In a three door case, as seen in Fig. 2, there would be a total of four lighting fixtures, two at each end and two in the center.

In the recent past, finding an efficient lighting fixture with a long life to install in commercial refrigeration systems was difficult. Before light emitting diode systems became available for refrigeration lighting use, fluorescent systems were the common choice of lighting for the cases. Fluorescent lamps are commonly known for their long life and efficiency, but what is not well-known is that these lamps do not perform well in cold environments, like in a commercial refrigerator. Commercial refrigerators typically run in the temperature range of -25C to 5C. These temperatures are not ideal for fluorescent lamps. Because of this, light emitting diode systems are becoming extremely popular for these refrigerators and freezers. The efficiency of LEDs is not affected by low temperatures like fluorescent systems are. This makes it ideal for these refrigerators and freezers. In a commercial refrigerator or freezer environment, a fluorescent lamps light output can drop by 25 percent of the full output. This makes the system less efficient than fluorescent lighting systems that are installed in room temperature environments. This is not ideal for stores that have large quantities of refrigerators. Systems that include LEDs can often be more than 50 percent energy efficient than fluorescent systems. One example of the possible savings can be seen in Fig. 3. This data was taken from General Electric (GE) Lighting's RV30 Refrigeration Display Lighting product. An image of the RV30 product can be seen in Fig. 4. It can be seen that the energy savings from just one 5-door case can have a large impact on energy savings over a period of time.

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Another problem with fluorescent fixtures in refrigerator and freezer cases is that the lamps do not scatter the light evenly over the products. The design of a fluorescent tube is not optimized for the short distance that the light has to travel to illuminate the food in these cases. Cases that use fluorescent systems often have bright spots in front of the lamp and are dim between each mullion (General Electric 2008). LED systems can be made with optic lenses that will scatter the light in the proper directions. Because of this, all the food in the cases is uniformly illuminated. Fig. 5, seen below, shows the difference between a fluorescent lamp system, shown on the left, and an LED system (a GE's Gelcore system), shown in the right. Looking at the fluorescent system, we can see that the light is not as uniform in the case compared to the LED system.

LEDs often have longer lives compared to other lighting technologies. One of the common failure modes for LEDs is due to high thermal junction temperatures. Luckily, this is not much of an issue inside of a refrigerator or freezer environment. LEDs are not negatively affected by low temperatures, and actually perform better and live longer in these environments. As shown in Fig. 6, LED systems often have a rated life of 50,000 hours in this type of environment. Fluorescent lamps often will not last half as long as these LED systems. A GE T8 Cold Temperature fluorescent lamp has a rated life of 20,000 hours. In large stores that have many refrigeration cases, the extended life of the LED systems is very important because there are reduced maintenance costs compared to the fluorescent systems.

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It is clear that LED systems are better suited for refrigeration and freezer cases compared to other lighting systems. Fluorescent lamps are not as efficient, require more maintenance, and do not disperse light as uniformly as LED systems. Incandescent systems would be far less efficient than even a fluorescent system and have extremely short lives. Incandescent systems, as fluorescent systems, are not appropriate for these cases.

2 PROPOSED REFRIGERATION DISPLAY LIGHTING

With LED technology advancing at the fast rate that it is, the refrigeration LED systems have the opportunity to become more efficient than the preceding LED systems. Quickly, over the years, LEDs have been able to produce more and more output light for every watt that it consumes, or lumens per watt. Because of this, the proposed design can produce the same amount of light with fewer LEDs and less energy consumption. In addition, the proposed design will have dimming capabilities. This will allow occupancy sensors to control when the lights will be full on or when they will be dimmed to a level of 20 percent. When there are no consumers around the refrigeration cases, the lighting will be dimmed in order to become even more energy efficient. This will allow the stores to save even more money on lighting costs than if the systems were on all the time. Using pulse width modulation (PWM) creates the dimming signal for the system. The PWM dimming is controlled using the AC to DC power supply at the front end of the system. The power supply controls the dimming of the system. It has two wires that are attached to the occupancy sensor for the system. When the occupancy sensor is activated, the power supply outputs a 12Vdc constant voltage. When the sensor is not active, the power supply will output a 12Vdc, 20 percent duty cycle PWM signal. This 20 percent duty cycle allows the system to be in dim mode. The AC to DC power supply is often located outside of the mullion, where the light bars are installed. There is often an average of 5 ft of wire between the AC/DC power supply and the light bar assembly. The power supply can be used to run a load up to 60W, so there can be more than one light bar connected up to each power supply. Each light bar system consists of the LEDs and a DC to DC converter that is required to run the light bar. There are two different light bar configurations: one for the center mullions and another for the end mullions. These light bars both have the same LEDs and electronic DC/DC driver in them. There are a few differences between the two light bars. The optical system that directs the light in the case most efficiently is different for each system. The center bars require the light to be directed over a larger area, or angle, than the end light bars. The drive current that the LEDs are driven at is also different for the two bars. The LEDs are driven at a lower drive current on the end bars, because the light has to be directed onto an area of about half the area of the center bars. Finally, the mechanical shapes of the two systems are very different. The center mullions and the end mullions are shaped differently. The center mullions are flat and the end mullions have a "L-shaped" curve to them. Because of the difference in shapes, the two light bars must be shaped differently, while they contain identical components. A block diagram of the proposed system is shown in Fig. 7.

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The proposed DC/DC converter is located at the end of the light bar before the first LED. As seen in Fig. 8, the electronics are located on the first module of the system. The first module of the system includes the DC/DC converter circuit followed by the first LED of the system. Each converter module is over-molded with thermally conductive plastic material as shown in Fig. 9. This allows the system to have optimal thermal performance. Following the converter module are the LED modules. The system includes four to nine LEDs in each light bar. The number of LEDs in each bar is dependent on the length that the light bar will be. Each LED is strategically spaced between one another to optimize the optical performance of the system.

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All the LEDs in each light bar are connected in a series configuration, no matter what the length of the bar is. This makes sure that each LED is running at the same current and each is outputting similar light levels. This is very important to ensure that the optical system performs efficiently. In a center bar, each LED is located in the center of a FR4 material printed circuit board assembly. The size and dimensions of the board can be seen below in Fig. 8. Each power converter board (PCB) has a connector on the bottom layer that connects each board to one another using 20 gauge wires. This allows the production line to use the same PCB LED board assembly for any length light bar simply by changing the length of the wire between each board. There is an over-molded module spacer placed between each LED over-molded module. This covers up the wires between each module and adds support for the system.

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When manufactured, each light bar must go through a particular process. Each light bar has a particular length and LED current that it is specified to be. The appropriate DC/DC converter is chosen in order to output the correct current. The appropriate wire length is chosen to make sure that the light bar will be the correct length. The casing for the modules is then cut to the correct length. This length can be 30", 48", 60", or 70". The PCB assembly modules are electrically connected to one another. They are then sent to be over-molded. Once each module is over-molded, the assembly is inserted into the casing. Throughout the assembly process, the light bars must go through a series of tests in order to insure that each component and the final assembly of the light bar function correctly. A representation of the assembly process can be seen below in Fig. 10. This particular light bar is an end assembly.

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3 PROPOSED DC/DC CONVERTER

The DC/DC converter in this system is designed in a buck-boost configuration (Mohan 2003). Block diagram of the proposed configuration is shown in Fig. 11 (Wu 1998). Center mullion system and end mullion system in the commercial refrigerator are powered differently. The center mullion system is higher powered than the end mullion system in order to extend light over a larger area than on the ends. In different configurations, there can be anywhere from three to nine LEDs in series. When there are three LEDs in series the buck-boost converter shown in Fig. 12 will be functioning in a buck mode and when there are nine output LEDs in series the converter will be in a boost mode. The control of the DC/DC converter was developed around the functionality of an IC chip, MPQ2483 (Systems 2009).

The MPQ2483 is a current mode regulator, so the driver outputs a constant current to the LED load. A functional block diagram of the chip can be seen in Fig. 13. The MPQ2483 has a 0.28 ohm internal power MOSFET switch. The drain of the switch is connected to the SW output pin. The chip applies an internal voltage to the gate of the switch to drive the MOSFET and at a defined switching frequency. The chip switches at 1.35MHz in this particular circuit. This is the maximum switching frequency of the chip. This value can be lowered if necessary by adding resistance from pin 5, RSET, to VSS. A 1nF capacitor is necessary in order to bypass this pin to VSS. Leaving this pin open will allow the internal MOSFET to switch at 1.35MHz. This higher frequency will allow for less output ripple in the DC current. Lowering the frequency will make the chip more efficient and create less noise, or EMI, in the circuit.

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The MPS chip regulates the voltage across current sensing resistors between the FB pin and the VSS pin in order to control the output current. Regulating a voltage of 0.198 from FB to VSS sets the output current. These pins can be referenced in Fig. 14. By simply changing the values of the RSET resistors located between the FB and VSS pins, the output current can be varied. This is important, because for each center and end light bar there will be various output currents. This is done in order to make the optical performance equal at each light bar length.

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4 INPUT CIRCUIT--POLARITY CORRECTION

In order to make the product easy to install, both input wires to the dc/dc converter needed to be able to be connected to either the positive wire or the negative wire of the 12Vdc signal (Fig. 11). The input of the circuit consists of a full bridge rectifier consisting of two diodes and two MOSFET switches shown in Fig. 15. This is a very important aspect to the design. This lighting system is often installed in refrigerators that already exist in the field. With these systems, there is wiring that was previously used for a fluorescent system. The fluorescent wires that come out of the mullion to connect to the lighting system are often the same color. These wires run through the mullion and into the power supply. In order to make sure that the installer does not make a mistake when wiring the system, the input to the DC/DC converter was designed not to be polarity sensitive. The full bridge rectifier allows the 12Vdc to be applied in either direction.

5 SYSTEM DIMMING

Many consumers for the product would like to be able to dim the refrigerator cases while shoppers are not near the cases, and fully light the cases when shoppers are around. This is done using an occupancy sensor on the refrigeration cases. In order to dim the LEDs, a DC or PWM signal must be present on pin 8 of the chip, the EN/DIM pin. In this design, the occupancy sensor is connected to the external AC to DC power supply. The AC to DC power supply controls the dimming of the system based on the occupancy sensor feedback. When the power supply detects that there is movement by the refrigerator cases, the power supply outputs a constant 12Vdc signal to the LED light bars. When the sensor does not detect that there is anyone by the refrigerator cases, the power supply outputs a 300 Hz PWM signal with a duty cycle of 20 percent. This causes the MPQ2483 chip to output 20 percent of the set current. Since the main input voltage is a 300 Hz PWM signal with a 20 percent duty cycle during the dimming period, the circuit must be manipulated in such a way to activate the EN/DIM pin and also keep the MPQ2483 powered up during the PWM cycles. The circuit was designed in a way that allows the input PWM voltage to go through a voltage divider circuit using resistors R1 and R2, as seen below in Fig. 16. The MPS chip's EN/DIM pin is activated using a PWM signal between 100 Hz and 2 kHz. The amplitude of this square wave must be greater than 1.4V. The voltage divider provides a 3V square wave signal on the EN/DIM pin. When this dimming is activated, the output of the chip turns the LEDs on and off with the 300 Hz dimming PWM signal. In order to avoid large current spikes on the leading edge of the dimming signal, a large capacitance was added to the input of the MPS chip. Without a large amount of capacitance at this pin, a spike of 10A or greater was seen on the leading edge of the dimming signal. This 220uF capacitor, C9, cleans up the noise at the input pin and gets rid of this large current spike. This large capacitor is also helpful in order to keep the chip turned on during the off time of each cycle of the PWM signal. The 220uF capacitor insures that [V.sub.DD] is kept above the 4.5Vdc that it takes to keep the chip on. The capacitor charges up during the 20 percent on time of the PWM signal and discharges during the 80 percent off time of the PWM signal, allowing the chip to continuously stay on during PWM dimming.

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The schematic with input polarity correction circuit, dimming capabilities and current-controlled DC/DC buck-boost converter is shown in Fig. 16.

6 EXPERIMENTAL RESULTS

Efficiency and dimming functionality of the circuit were checked to ensure that the design would meet the strict energy regulations of the product. It was found that the efficiency of the system was close to 90 percent when used for 48", 60", and 70" light bars. It was determined that these numbers were satisfactory in order to ensure overall system efficiency specifications and system lumen output requirements. The efficiency measurements for this system can be seen in TABLE I

It was important to make sure that when a 20 percent duty cycle was applied to the MPQ2483, that the output current would be 20 percent of the maximum current (output current during 100 percent operation). From Figure 17 it is apparent that the measured dimmed current is very close to the expected value (duty cycle of the PWM signal). The MPS chip switching frequency was verified along with the output ripple current.

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Inductor current and a voltage across the diode, D1 are shown in Fig 18 and Fig. 19 respectively.

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It can be seen that there is a lot of noise being generated by the diode during the 1.35MHz switching cycles. In order to fix this, a 220pF capacitor and 15-ohm resistor snubber was added across the diode.

When this snubber across the diode was added, the overshoot and undershoot of the voltage was almost eliminated. Also, the oscillations on the rising and falling edge's of the voltage signal were eliminated. This was very important, because this will allow the diode to operate within its rated values and have a life that will meet the product's life specifications.

Because of the type of commercial application that the product will be used for, stringent EMI standards must be met. The product must comply with FCC EMI regulations for the United States and the CISPR European standards for conducted and radiated EMI. Test results are being conducted and any necessary design changes are being made to ensure that these important standards are met. Proposed additions to the circuit to help EMI can be seen below in Fig. 20.

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7 CONCLUSION

In this paper a DC/DC buck-boost converter operating in the continuous conduction mode for commercial refrigeration LED lighting with dimming capabilities is proposed. The current control of the DC/DC converter was developed around the functionality of an IC chip, MPQ2483. In addition the input circuit with polarity correction was developed to ease the installation process. The experimental results show that the efficiency of the proposed converter is 88 percent. This configuration also allows consumers to easily retrofit the product in existing fluorescent lighting-based freezer and refrigerator cases and it is easily adapted to many new freezer case styles.

doi: 10.1582/LEUKOS.2010.07.02004

REFERENCES

[GE] General Electric Co. 2008. Immersion LED Display Lighting. Retrieved from GE Lumination: http://www.lumination.com/literature/Immersion_RV30_Brochure_ WEB_100609.pdf

Mohan UR. 2003. Power Electronics. New York: J Wiley. 802 p.

M-P Systems. 2009. MPQ2483 - Frequency white LED driver data sheet.

National Semiconductor. Switching Regulators. Retrieved from National Semiconductor: www.national.com

Wu T-F, Chen Y-K. 1998. Modeling PWM DC/DC converters out of basic converter units. IEEE Trans Power Elec. 16(5):870-881.

Nina R. Scheidegger (1) and Ana V. Stankovic (2)

(1) General Electric-Lighting Cleveland State University, (2) Cleveland, Ohio Cleveland, Ohio
TABLE 1.

Vin       En        Pin      Vout      Iout      Pout      Ploss

12.0     1.209    14.508     23.28     0.550    12.804     1.704
12.0     1.557    18.653     29.80     0.545    16.241     2.412
12.0     1.031    12.372     19.92     0.551    10.976     1.396

Vin     Efficiency     Configuration

12.0      88.3%        Boost using 7 Nichia LEDs
12.0      87.1%        Boost using 9 Nichia LEDs
12.0      88.7%        Boost using 6 Nichia LEDs
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Author:Scheidegger, Nina R.; Stankovic, Ana V.
Publication:Leukos
Article Type:Report
Geographic Code:1USA
Date:Oct 1, 2010
Words:3515
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