P08456: Lighting system for an Underwater ROV
Electronic Detailed Design Review
P08456: Lighting System for an Underwater ROV
Electronic Detailed Design Review Packet 2
Team Members:
Jeremy Schiele: Project Manager
Jonathan Lent: Housing Design
Justin VanSlyke: Mounting System
Ryan Seeber: Control System
Benoit Hennekinne: Circuit Design
Table of Contents
Background on High Brightness LED
LED Bulb System
LED Driver Design
Power Supply Design
Temperature Sensor Design
Connections between boards
Fuse consideration
Power dissipation
Update Name / Autor / Rev. / DateCreation / Benoit Hennekinne / A / 02/17/2008
Revised / Benoit Hennekinne / B / 02/21/2008
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 poweras a resistor limiter for the same supply voltage. Moreover it may require mechanicalheat 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.This method will be used in the electrical design.
LED Bulb System
The bulb concept uses 6 individual surface-mounted 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.
Figure 1: Comparative diagram of three powerful surface-mounted 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 over to 600 lumens, but this intensity will cause premature failure of the LED, at a sustained current of 350 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.
In a first time, Luxeon Rebel LEDs from Philips were chosen according to the study of HB LEDs on the market by using a scored matrix (see Figure 1). After , these bright LEDs were no longer available at the brightness required because of manufacturing issues. Therefore, white CREE XLamp XR-E LEDs, which are similar characteristics, were adopted instead of white Luxeon Rebel LEDs. For the green ones, Luxeon Rebel LEDs were kept in the design.
Figure 2: Luxeon Rebel from Lumiled by Philips /
Figure3: XLamp XR-E HB LED from CREE
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 4: 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 LT3474 from Linear Technology has been chosen since it is the most efficient product for this application.The LT3474 operates from a 4.5V to 36Vinput voltage range. The LT3474 is a fixed frequency step-down DC/DC converterdesigned to operate as a constant-current source.An internal sense resistor monitors the output currentallowing accurate current regulation, ideal for driving highcurrent LEDs. High output current accuracy is maintainedover a wide current range, from 35mA to 1A, allowing awide dimming range.Unique PWM circuitry allows a dimming range of 400:1,avoiding the color shift normally associated with LEDcurrent dimming. Short circuit and thermal shutdown provide additional protection.The LT3474 operates over the -40°C to+85°C.
Figure 5: ILED variation in function of the temperature in the LT3474
The schematic was designed from the datasheet of the LT3474. The calculation of the necessary electrical components has been made according to the datasheet.
Figure 6: 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.
Thedifference between VIN and VOUTis 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 VFof 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 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 at 88% (90% according to the datasheet). The required current for the 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
In order to decrease the noise in the signal, the 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 over-temperature alarm, and an I²C-compatible serial interface into single compact packages.It converts its die temperatures into digital valuesusing internal analog-to-digital converters (ADCs). Theresult of the conversion is held in a temperature register,readable at any time through the serial interface. A dedicatedalarm output, OT, activates if the conversion resultexceeds the value programmed in the high-temperatureregister. A programmable fault queue sets the number offaults that must occur before the alarm activates, preventingspurious alarms in noisy environments. OT has programmableoutput polarity and operating modes. The MAX6626P OT outputs areopen drain, and the MAX6626R OT outputsinclude internal pull-up resistors.
Figure 10: OT Alarm Output and Reset Diagram (from MAX6626 datasheet)
TheMAX6626 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
Connections between boards
Here is a block diagram to show the connections and wires between the different electrical boards.
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 asmall safety device in an electrical circuit which causes it to stop working if the electric currentbecomes too high, thus preventing fire or other dangers. The inline fuse is placed between the controlbox and positive terminal on the +24V cable which is connected to the light housing. If a power surge or short circuit occurs, the thin metalfilament in the fuse will burn out, stopping all electrical current. This will prevent the wires andelectronic systems on the ROV from overheating and damaging themselves. The fuse can carrya maximum of 10 A prior to breaking the circuit.
Power dissipation
The 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 (see mechanical design and drawings).
Continuous power dissipation of active components is shown in the table below:
Component / Continuous Power dissipationMAX6626 / 727 mW (+9.1mW/°C above +70°C)
LT3474 / 1454mW (+18.17mW/°C above +70°C)
LM2674M / 280mW
Table 2: Power dissipation of active components
1