edge.rit.eduedge.rit.edu/edge/P08456/public/Electrical Design... · Web viewBenoit Hennekinne: Circuit Design Table of Contents Background on High Brightness LED3 LED Bulb System3

P08456: Lighting System for an Underwater ROVElectronic Detailed Design Review

Team Members:

Jeremy Schiele: Project Manager

Jonathan Lent: Housing Design

Justin VanSlyke: Mounting System

Ryan Seeber: Control System

Benoit Hennekinne: Circuit Design

P08456: Lighting system for an Underwater ROVElectronic Detailed Design Review

Table of Contents

Background on High Brightness LED........................................................................................3LED Bulb System.......................................................................................................................3LED Driver Design.....................................................................................................................5Power Supply Design..................................................................................................................7Temperature Sensor Design........................................................................................................9Connections between boards.....................................................................................................11Fuse consideration....................................................................................................................12Power dissipation......................................................................................................................12

Update Name Autor Rev. DateCreation Benoit Hennekinne A 10/10/2007Revised Benoit Hennekinne B 10/ 31/2007

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Background on High Brightness LED

In recent years, high-brightness (HB) LEDs have gained prominence as the lighting source for a variety of applications. HB LEDs are rugged and reliable semiconductor devices capable of several tens of thousands of cycles—up to 100,000 hours of operation. That performance represents an operating life that is orders of magnitude longer than conventional incandescent and halogen lamps. Thus, HB-LED applications can be found in automotive lighting, public and commercial signage, and architectural lighting.

HB LEDs are PN-junction devices especially processed to produce white, red, green, and blue light when forward biased (amber and a few other colors are possible as well). As PN-junction devices, LEDs exhibit characteristics similar to those of conventional diodes, but with higher voltage drops across their junctions. Little current passes through an LED until the forward voltage reaches a required VF, which varies from 2.5V for red LEDs to about 4.5V for blue LEDs; when the VF is reached, the current increases very rapidly (as in conventional diodes).

Consequently, the designer must employ current limiting to prevent possible damage. Current limiting can be implemented with three basic methods, each with advantages and disadvantages.

The first method is to use a resistor. This method is not expensive and use only one large component but it can not control current accurately and provides high power dissipation in the resistor.

The second method is to use an active linear control, but this solution is more expensive than the simple resistive current limiter and dissipates about the same power as a resistor limiter for the same supply voltage. Moreover it may require mechanical heat sinking of the active pass device.

The third method is to use a switching regulator control with a control loop which regulates LED current precisely. This solution allows dimming by amplitude control or low-frequency PWM. Moreover it does not usually have a mechanical heat-sink, which saves cost and complexity.

LED Bulb System

The bulb concept uses 6 individual LED bulbs, of which a set 3 can be controlled at a given time by a single LED driver. This 6 x radial pattern provides even lighting with up to 2 colors of LEDs (white + one other). The choice of the LEDs was made with respect to these considerations: brightness, low power, luminous efficiency (calculated in lumen per watt), size and angle.

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Brightness

Low Power

Luminous EfficiencySize

Angle

0

5

10

LaminaLUXEON REBELEdison Opto EPBW-4E00

Figure 1: Comparative diagram of the three best HB LED

The smaller LEDs are more efficient, providing more luminous flux per power consumed (lm/watt). The overall bulb will be able to consume up to 10.2 watts, with each individual bulb using up to 1 Ampere and 3.6 Volts. The intensity produced will be close to 600 lumens, but this intensity will cause premature failure of the LED, at a sustained current of 700 mA each bulb would give off 100 lumens, so the light unit producing 300 lm. The multiple LED design also allows for the secondary color in each module to be selected independently of the others, so many color options are possible for alternate uses on the Robotic Platform, etc.

However a HB LED uses only 15-20% of the power for the light and the remainder of the power is dissipated in heat. That is why this concept requires the study of the heat dissipation on the led board in order to find solutions for a well dissipation of heat.

Figure 2: This small HB-LED has been chosen: Luxeon Rebel from Lumiled by Philips

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LED Driver Design

In order to control the brightness of a light, LED current must be adjustable and controllable. In many cases it is advantageous to dim an LED by pulsing its current at a low frequency (50Hz to 200Hz) and controlling the width of the pulses. Though the LED illuminates with the same brightness during each pulse, the eye perceives a dimming as the pulse narrows. The light spectrum, moreover, remains constant, unlike the case of dimming by amplitude modulation in which the light spectrum shifts as the LED current varies.

Figure 3: Examples of PWM Dimming from the Datasheet of the MAX16820

The LED driver used will be able to accept the full 24 V and regulate the power consumed by the LED bank. Its dimming capability will be controlled using a PWM signal from the Atmel AVR microcontroller contained within the housing. The driver must be a low cost continuous buck driver with a wide input voltage range, enabling it to be used on platforms that use larger or smaller voltage sources (batteries).

The choice of the led driver has been made with the following specifications: - 1A output at least,- VOUT ≥ 12V,- VIN = 24V,- Allow PWM Dimming (dedicated input),- Step-down (buck) regulator with efficiency as high as possible,- Switching regulator control,- Bulk as tiny as possible.

According to these specifications the MAX16820 from MAXIM has been chosen since it is the most efficient product for this application. The MAX16820 operates from a 4.5V to 28V input voltage range. A high-side current-sense resistor adjusts the output current and a dedicated PWM input (DIM) enables a wide range of pulsed dimming. The high-side current-sensing and an integrated current-setting circuitry minimize the number of external components while delivering an LED current with ±5% accuracy. This device operates up to 2MHz switching frequency, thus allowing for small component size. The MAX16820 operates over the -40°C to +125°C and is available in 3mm x 3mm x 0.8mm, 6-pin TDFN packages.

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Figure 4: PWM Dimming and ILED variation in the MAX16820

From the datasheet of the MAX16820, the schematic was designed. The calculation of the necessary electrical components has been made according to the datasheet.

- RSENSE is a 1% power resistor.

RSENSE=

12∗V SNSHI+V SNSLO

I LEDThe maximum allowed current is 1A. So I LED= 1A. V SNSHI = 210 mV (typ) and V SNSLO = 190 mV (typ)

Finally, RSENSE = 0.2 Ω, 1%, 1W.

- The inductance L is calculated from this equation:

f SW=(V ¿−n∗V LED )∗n∗V LED∗RSENSE

V ¿∗∆V∗L

with n = 3; VLED = 3.6V RSENSE = 0.2 Ω; VIN = 24V; ∆V = V SNSHI-V SNSLO= 20 mV; fSW = 0.2 MHz

Finally, L = 47.25 µH.

From these calculations, the following table was built.

Input Definition Parameter Input UnitInput DC Vin 24 V

Forward Voltage Freewheel Diode Vdiode 0,5 VLED forward Voltage VLED 3,5 V

No. of LED n 3 IntegerOutput Current Io 1 A

Switching Frequency fs 0,2 MHz

Ouput Definition Parameter Output UnitAdjustment Work Condition Yes/No Can design

Ripple Current ΔI 0,16 ASense Resistor Rs 0,20 Ω

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Designed Minimum Inductor L 47,25 µH

From the previous calculations, the electrical schematic was designed.

Figure 5: Schematic of implementation of LEDs & Driver

The selection of the line of LEDs is done through a select input from the microcontroller which drives a power transistor.

The difference between VIN and VOUT is important because larger differences result in an efficiency decrease. That’s why the use of 3 LEDs in series is interesting because each LED needs a VF of 3.6V; 10.8V in all. So this application is more efficient with 3 LEDs in series than with several lines of one LED each.

Power Supply Design

All the digital components require a 5V input. The 24V power supply must be decrease until 5V. The gap is important. Using a step-down (buck) regulator will reduce the heat dissipation and the efficiency will be better. The LM2674M-5.0 from National

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Semiconductor is adapted to make this power conversion. The electrical components are chosen according to the specifications of the datasheet.

Figure 6: Power Supply Schematic

The efficiency from a 24V input is up than 88% (90% according to the datasheet). The wanted current for digital components is not really important compared to the current used for LEDs. Consequently, the loss is insignificant.

Figure 7: Efficiency of the LM2674M

The Supply Power board should be the same for the lighting system and the thruster. That is why a dedicated output +24Vout have been created. The 24V input signal is filtered and separated from the other signals to prevent peaks of current and noise from the polarization of the motor.

Figure 8: +24V filtered

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In order to decrease the noise in signals, analog and digital signals have been separated as shown in the figure below:

Figure 9: Separation of analog and digital signals

Temperature Sensor Design

To control the temperature on the LED board, a temperature sensor is placed in the middle of the board. To have a good processing of the temperature information and a fast response, the surface mounted chip MAX6626 is used. The MAX6626 is a 12 bit temperature-to-digital converter. The MAX6626 combine a temperature sensor, a programmable overtemperature alarm, and an I²C-compatible serial interface into single compact packages. It converts its die temperatures into digital values using internal analog-to-digital converters (ADCs). The result of the conversion is held in a temperature register, readable at any time through the serial interface. A dedicated alarm output, OT, activates if the conversion result exceeds the value programmed in the high-temperature register. A programmable fault queue sets the number of faults that must occur before the alarm activates, preventing spurious alarms in noisy environments. OT has programmable output polarity and operating modes. The MAX6626P OT outputs are open drain, and the MAX6626R OT outputs include internal pull-up resistors.

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Figure 10: OT Alarm Output and Reset Diagram (from MAX6626 datasheet)

The MAX6626 has a 12-bit internal ADC. In the worth case, the accuracy of the temperature value is ±4°C in a -55°C to +125°C temperature range. The conversion time is 133ms. Five registers are included: a pointer register, a configuration register, a temperature register and 2 registers to memorize the value of the high temperature and the low temperature.

Figure 11: MAX6626 Programmers Model (from MAX6626 datasheet)

From the datasheet of the MAX6626, the electrical schematic was designed.

Figure 12: Temperature Sensor Schematic

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Connections between boards

Here is a block diagram which show connections and wires between the different electrical boards.

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LED Board

LED LED1 LED2 SDA SCL OT +5VD DGND

LED LED1 LED2 SDA SCL OT +5VD DGND

LED Driver Board

+24V_filtered AGND PWM SELECT SDA SCL OT +5VD DGND

+24V_filtered AGND PWM SELECT SDA SCL OT +5VD DGND

µC Board

+24V_filtered AGND +24V_out GND_24V DATA+ DATA- +5VD DGND

+24V_filtered AGND +24V_out GND_24V DATA+ DATA- +5VD DGND

Power Supply Board

VE_IN (24V) GND DATA+ DATA-


Fuse consideration

It should be useful to add a fuse to protect the battery against short circuit if the light housing is broken. A fuse is a small safety device in an electrical circuit which causes it to stop working if the electric current becomes too high, thus preventing fire or other dangers. The inline fuse is placed between the control box and positive terminal on the +24V cable which is connected to the light housing. If a power surge or short circuit occurs, the thin metal filament in the fuse will burn out, stopping all electrical current. This will prevent the wires and electronic systems on the ROV from overheating and damaging themselves. The fuse can carry a maximum of 10 A prior to breaking the circuit.

Power dissipation

LEDs and active components used generate heat. A HB LED uses only 15-20% of the power for the light and the remainder of the power is dissipated in heat. That is why this concept requires the study of the heat dissipation on the led board in order to find solutions for a well dissipation of heat.

Continuous power dissipation of active components is shown in the table below:

Component Continuous Power dissipation

MAX6626 727 mW (+9.1mW/°C above +70°C)MAX16820 1454mW (+18.17mW/°C above +70°C)LM2674M 280mW

Table 2: Power dissipation of active components

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edge.rit.eduedge.rit.edu/edge/P08456/public/Electrical Design... · Web viewBenoit Hennekinne: Circuit Design Table of Contents Background on High Brightness LED3 LED Bulb System3