Department of Science and Technology Institutionen för teknik och naturvetenskap
Linköping University Linköpings universitet
g n i p ö k r r o N 4 7 1 0 6 n e d e w S , g n i p ö k r r o N 4 7 1 0 6 -E S
LiU-ITN-TEK-G-14/081--SE
Over voltage protection device
for ROV
David Kantzon
Sebastian Lahti
LiU-ITN-TEK-G-14/081--SE
Over voltage protection device
for ROV
Examensarbete utfört i Elektroteknik
vid Tekniska högskolan vid
Linköpings universitet
David Kantzon
Sebastian Lahti
Handledare Kjell Karlsson
Examinator Lars Backström
Upphovsrätt
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LINK ¨OPING UNIVERSITY
Abstract
Department of Science and Technology
Bachelor of Science
by David Kantzon Sebastian Lahti
Supplying power to equipment always imposes a risk of damaging it. This risk is appar-ent in every application whether it be an industrial or a home appliance. One of these harmful occurrences is something like lightning which introduces a potentially harm-ful voltage in the system. To reduce the risk of damage significantly an over-voltage protection device is needed. Devices which deal with this problem are common in any electrical equipment and this report deals with the construction and evaluation of one such unit.
The device or equipment needing protection in this case is an industrial underwater robot built by Ocean Robotics. In order for this robot to operate safely it needs several protective measures where an over-voltage protection system is one of them. This system provides protection against over-voltages appearing on the main cable supplying the robot, where the input voltage ranges from 200 to 600 volts. As the desired voltage output range from the over-voltage system is 250 to 350 volts it must be able to handle significant power in some cases.
Due to the restrictions on functionality given by the contractor a novel way of achieving the goals was tested. The completed prototype can only dissipate 70 watts of power which is far from the required 1500 watts which was specified in the sheet of requirements. However, this system could be used to protect equipment with lower requirements for power handling capability and with added time and effort it could possibly meet the requirements for this project as well.
Acknowledgements
Ocean Robotics for providing this opportunity as well as guidance during our work
Lars Backstr¨om for being lenient and understanding
Pinkesh Sachdev at Linear Technologies for answering our questions
Contents
Abstract i Acknowledgements ii List of Figures v List of Tables vi Abbreviations vii 1 Introduction 1 1.1 Purpose . . . 1 1.2 Aim . . . 2 1.3 Methodology . . . 21.4 Scope and limitations . . . 2
2 Background Theory 4 2.1 Effects of excessive voltage. . . 5
2.2 Protective measures . . . 5
2.2.1 What needs to be protected? . . . 6
3 Design Overview 7 3.1 Hardware Design and Choices . . . 7
3.1.1 MOSFETs . . . 9
3.1.2 Digital pot . . . 10
3.1.3 Voltage regulators . . . 10
3.1.4 STM32 . . . 11
3.1.5 Transceiver Isolation barrier. . . 11
3.1.6 Measure pin. . . 12 3.1.7 24 Volt input . . . 12 3.1.8 PCB Design. . . 13 3.1.9 Component selection . . . 13 4 Evaluation 14 4.1 Simulations . . . 14 4.1.1 Transient simulation . . . 14
4.1.2 Observing output voltage during a spike . . . 17 iii
Contents iv
4.2 Physical testing . . . 18
4.2.1 Operational test . . . 19
4.2.2 Test with load - One MOSFET . . . 20
4.2.3 Test with load - Two IGBTs in parallel . . . 21
4.2.4 Test with load - Second test with two IGBTs in parallel . . . 21
4.2.5 Test with load - Alternative connection . . . 22
5 Result and discussion 23
6 Improvements 25
7 Conclusion 27
A Specification of requirements 28
B Circuit diagrams for high speed and regulator components 30
List of Figures
3.1 Circuit diagram . . . 8
3.2 PCB design . . . 13
4.1 Simulated circuit . . . 15
4.2 Simulated results with 250V clamp . . . 16
4.3 Simulated results with variable clamping . . . 17
4.4 Test configuration . . . 18
B.1 Microcontroller and peripheral components . . . 31
B.2 Communication interface. . . 32
B.3 Regulator circuits for microcontroller and communication interface . . . . 32
List of Tables
4.1 Table of parameters . . . 20
4.2 Test with single MOSFET . . . 20
4.3 Test with two IGBTs. . . 21
4.4 Test with two IGBTs. . . 21
4.5 Two controllers with one IGBT each . . . 22
Abbreviations
LAH List Abbreviations Here
PCB Printed Circuit Board
IGBT InsulatedGate Bipolare Transistor
MOSFET Metal Oxide Semiconductor Field Effect Transistor
ADC Analog to Digital Converter
PSU Power Supply Unit
ROV Remotely Operated Vehicle
SOA Safe Operating Area
MATLAB Matrix Laboratory
Chapter 1
Introduction
In modern marine industry, robots have taken over many of the roles traditionally held by people. Tasks in deep waters such as exploration and maintenance are better suited for machines where the risk for loss of lives is eliminated. These underwater vehicles must function without fault due to the dangers involved in their work e.g. if the equipment being serviced is an oil well. Malfunction at great depths may also make salvaging the vehicle difficult or in some cases impossible.
Since the main method of supplying power to machines is by cable, it is also a link in the chain where errors might occur. The power supply may malfunction but general operation might also incur undesirable conditions such as switching noise or load dumps when the machine requests less power than is supplied and so on. Many of these condi-tions lead to an excessive voltage being supplied to the equipment. These voltages may come in the form of short spikes or be present during a longer interval of time. If this is the case then the equipment must be protected so it does not break down due to the excess energy.
The components diverting this energy, often in the form of heat, must be carefully selected to provide adequate protection under all conditions.
1.1
Purpose
The purpose of this thesis work is to construct an over-voltage protection device which is able to handle a host of different harmful conditions. These conditions range from short
Introduction 2 spikes to sustained periods of excessive voltage. Some form of measurement capability and connectivity with the operating system is also desirable. Furthermore the device must be robust since it will be in use without recurring maintenance.
Design and construction must take available space, separation of high and low power components as well as heat into consideration.
1.2
Aim
The aim of this project is to design an over-voltage protection device which can dissipate large amounts of power when the supplied voltage exceeds the desired one by 50V. It must be able to dissipate 1500W but 3000W or more is desired. Expected voltage spikes in the system have a dV /dt of 300V /1ms which the device must handle as well as sustained over-voltage conditions in the range of 5-10 minutes, preferably longer. The device should have an adjustable clamp voltage ranging from 250 to 350 V. More-over, the device must be able to operate for at least 4400 hours, preferably 10000 hours, without malfunction.
These values were obtained from a specification sheet supplied by Ocean Robotics.
1.3
Methodology
Theoretical study of over-voltage protection systems is done to understand conventional methods and their limitations, AltiumDesigner will be used for designing a schematic and PCB. The PCB will be ordered from a manufacturer chosen by Ocean Robotics. Moreover, software such as MATLAB and LTSpice IV are used to design and verify functionality.
1.4
Scope and limitations
Most of the time spent working on the project was dedicated to the parts required for functional over-voltage protection. It was decided that any software needed for commu-nication and monitoring would be developed if the specifications for power dissipation
Introduction 3 were met. In the event that the power requirements were not met any communica-tion and monitoring would be useless. Many components required for communicacommunica-tion were predetermined by Oceans Robotics since they had previous experience with their functionality.
Chapter 2
Background Theory
Electronic equipment is dependent on power to operate. While voltage and current brings life to a circuit it can also cause harm. One of the most common sources of failure is exceeding the equipments rated working voltage. The most common example of an over-voltage is lightning. Lightning strikes and other similar types of transient over-voltages are often referred to as surges. Transient over-voltages have a short
dura-tion of a few microseconds after which they disappear [1].
Surges appear when large amounts of energy are released in a system. This may be due to the switching of capacitive and inductive elements, connection or disconnection of power-lines and general switching in and out of other circuit elements. Furthermore, poorly designed supply- and distribution-grids can increase the susceptibility to distur-bances.
Transient over-voltages can enter the equipment from the outside through power lines and other mediums. However, they may also arise from inside. One such example is the operation of electric motors as they are inductive elements. Moreover, a sudden decrease in load conditions where more energy is supplied than is needed could cause
an over-voltage[1, p. 2]. Large enough transients may damage or immediately destroy
equipment. Weaker transient over-voltages applied repeatedly over time will also shorten the lifetime of the equipment. Apart from surges over-voltages can also appear contin-uously over time .
Background Theory 5
2.1
Effects of excessive voltage
When an over-voltage occurs the most common cause of damage is that the power dissi-pated in a component goes beyond some maximum value. An excessive voltage can also break down insulators or make a brief connection between conductors separated by air
if the electrical field is sufficiently large [2]. During conduction, large currents may flow
and harm or short circuit devices.
To protect devices from exceeding their rated values components such as a variety of diodes, varistors, gas discharge tubes and so on are used depending on the applica-tion. What they all have in common is that until the applied voltage reaches a certain level they conduct very little current. Once this voltage called the clamping voltage is reached they rapidly start to conduct larger currents to protect equipment further down the chain. Another form of protection is called a crowbar device. These devices lower
the voltage to a certain level after the trigger-voltage has been achieved [2].
2.2
Protective measures
Devices such as varistors come with a certain clamping voltage from the manufacturer. To achieve a higher overall clamping voltage, one can either get a bigger device or con-nect several devices in series. Furthermore, current-handling capability can be increased by placing several devices in parallel. The problem with parallelling is that there is some variance in the clamping voltage between devices. This may lead to clamping at the wrong voltage and in turn not have all the devices clamping at the same time. The remedy here is to buy matched sets to ensure that all device parameters are tightly
Background Theory 6 Thermal fuses are often used to prevent protective devices from conducting over sus-tained periods of time. If too much current flows through the fuse, it breaks and no power is supplied to the load after failure.
When the load drops rapidly the device might not be able to lower the input volt-age fast enough which could lead to too high surges throughout the circuit. When these surges arise they will destroy the circuit.
Lastly, most conventional methods of protection are not made to withstand continu-ous periods of over-voltage. When the protective device conducts, heat is generated which might cause the device to deteriorate and possibly lead to a catastrophic failure. After seeing a number of excessive voltages the clamping voltage of a device may also start to change. Changes like these may lead to some devices conducting when they should not, eventually leading to failure, thus leaving equipment without protection.
2.2.1 What needs to be protected?
The ROV is powered through a so called tether-cable. The cable supplies power from a six pulse rectifier with an additional twelve pulse rectifier acting as a booster. The booster circuit adds voltage when the six pulse rectifier is not supplying enough power by increasing the voltage supplied from the source. Since the rectifier circuit is on a long cable, the changes that happens in the source will not happen instantaneously. The rectifier circuit does not have a control loop for continuously supervising the applied voltage. Because of this the possibility of extended periods with excessive voltage is increased. This increases the risks of over-voltage in the ROV which is why it needs a protective circuit.
Chapter 3
Design Overview
This chapter details the choice of components as well as some rudimentary theory needed to understand these choices.
3.1
Hardware Design and Choices
The Linear Tech LTC4366 is a surge stopper that protects loads from surge transients and continuous over-voltage. This is done by controlling an external MOSFET effec-tively using it as a variable resistor. In the case of an over-voltage any excess voltage is
dropped over the MOSFET. Figure 3.1 shows the circuit diagram of the configuration
used in this application. For circuit diagrams of the microcontroller and communications parts please refer to Appendix B.
The two resistors Rin and Rss allow the circuit to float up with the supply voltage
enabling the device to operate at higher voltages. RF B1 and RF B2 form a resistive
di-vider connected between the OUT and FB pins on the LTC4366. The values on these two resistors decide at which voltage the device should start clamping.
The reason for choosing the LTC4366 instead of conventional components such as varis-tors or similar components was because of easier scalability and control over the clamping voltage. The LTC4366 can operate at any voltage as long as the external components are sized properly. Furthermore, the clamping voltage can be set with more precision
Design Overview 8 compared to the predefined values of discrete components.
During start-up the LTC4366 operates from the voltage applied at the input. When the voltage is sufficiently high it switches to operating from the output voltage. In this mode of operation the control voltage to the MOSFETs will need to exceed the output voltage for it to conduct. This is achieved by an internal charge pump charging the
capacitor Cg to the desired value.
Figure 3.1: Circuit diagram
The difference between the LTC4366-1 and 2 is that the LTC4366-2 has an auto-retry function after a shutdown due to a fault. On the T imer pin there is an external ca-pacitor. The size of the capacitor decides the allowed duration of the over-voltage fault before shutdown. In this case the external capacitor was not used and as such both versions of the 4366 could be used. The reason for neglecting the timer capacitor was
Design Overview 9 that no shutdown was to be allowed.
All calculations done in order to obtain the values seen in Figure 3.1 were done
ac-cording to the design example in [4, p. 16]
3.1.1 MOSFETs
Choosing the right MOSFET for the application is very important as the transistors must handle considerable amounts of power. Among the parameters to consider are
drain-source breakdown voltage (BVDSS), drain-source on state resistance (RDSon), threshold
voltage (Vth) and safe operating area. Apart from the aforementioned parameters power
handling capability is also important.
The drain-source breakdown voltage BVDSS denotes the maximum voltage that the
MOSFET can block between its drain and source terminals while in the off state. Op-eration at voltages above this is not guaranteed and may result in avalanche breakdown
causing large currents to flow through the device [5, p. 12]. BVDSS is important in this
particular application since the full supply voltage will appear over the MOSFET in the case of an output short or over-voltage condition. Because of this a transistor must be
chosen with a BVDSS rating higher than the maximum supply voltage.
Drain-source on state resistance dictates the resistance that the device presents to the circuit when in the fully on state. A low RDSon is preferred since this will limit the voltage drop over the device when conducting large currents and consequently lower the
amount of power dissipated [5, p. 12].
The gate-source voltage at which the MOSFET starts to conduct, Vth, is called the
threshold voltage [5, p. 12]. If the voltage applied is less the transistor will conduct very
little to no current. Vthis also used in calculating the values for some other components
used with the LTC4366. Since there are variations in this voltage between components of the same type, parallelling them to achieve higher power handling can cause problems. This difference can cause the load over each MOSFET to differ making one component dissipate more heat than the others.
Design Overview 10 Safe operating area (SOA) describes under which conditions the device may operate without breaking down. This includes the drain-source voltage and current as well as
time of operation [5, p. 10]. These values were ignored in this particular application
since continuous power dissipation results in going out of these bounds. What kind of power which can be dissipated continuously is dependant on cooling and other factors which must be determined through testing.
More on MOSFET parameters can be found in [5].
In this project IXFN44N100Q3 MOSFETs were chosen because they met the required voltage and current handling capability. They were also picked in large part due to a ”bigger is better” mentality as this would result in a larger thermal mass and as such increased cooling. Another reason for choosing this transistor is because better transistors were not available at the time. Waiting for these to become available would have made testing impossible in the time frame of the project.
3.1.2 Digital pot
The clamping voltage is decided by the resistive divider. Increasing RF B1 will lower
the clamping voltage and vice versa. To control the output voltage one could wire
a potentiometer as a variable resistor in series with RF B1. The value of the fixed
resistor then sets the upper limit of the output voltage. Any resistance added by the potentiometer will lower this limit.
An analog potentiometer works in a testing environment but something that could be controlled remotely is desired.
3.1.3 Voltage regulators
All voltage regulators used in the project were made by TRACOPOWER. This is in large part due to the fact that they require no peripheral components to function although decoupling capacitors were used in some areas of the board. TRACOPOWER devices were also recommended by Ocean Robotics because of their familiarity with the products along with the aforementioned lack of requirements for peripheral components.
Design Overview 11 Apart from a 3.3V supply for logic components the transceiver connection needs an isolated 5V source. For this application a 5V regulator from TRACOPOWER was
chosen. The regulator has a required input voltage of 9-34V.[6]
3.1.4 STM32
The STM32 is a microcontroller whose main function is to handle the communication between the user and the protection device. It supervises the output voltage and controls the clamping voltage.
The STM32 is chosen for its interchangeable capabilities, this means that it is possible to use any STM32 with different properties and still have the same pin-out. It is also in the specifications received by Ocean Robotics. For this particular project an STM32F303C was used.
3.1.5 Transceiver Isolation barrier
Communication interfaces such as RS-232 or CAN are susceptible to noise and other disturbances. While the logic operates at 3.3V there is no telling what voltages may appear in a cable connecting devices together in a harsh environment. Differences in potential lead to currents flowing which if sufficiently large could damage or destroy equipment. Because of these potential differences the data interface needs to be galvani-cally isolated. Data must be able to cross the isolation barrier while preventing the flow of current.
A common solution is the use of optocouplers. An optocoupler consists of a light-emitting source and a photosensitive device which will turn on or off when subjected to light. Each side has its own power supply that are isolated from each other as well. Because there is no physical connection between the two sides, no current may flow between them. Any over-voltage appearing on the transmission line is also blocked. For the communication interface in this particular application a MAX3313 RS-232 transceiver from Maxim Integrated was used. In order to create an isolated interface an
Design Overview 12 ADUM1201 from Analog Devices was also added. These two devices were chosen be-cause they had been used by Ocean Robotics in other projects and as such were familiar with them.
3.1.6 Measure pin
It is desirable to measure the output voltage for logging purposes as well as warning the user in case of an over-voltage. The measure pin (PB2 pin 20) serves as an input to an ADC. This input is fed by a voltage divider which provides 0-3.3V depending on the output voltage of the circuit. The acquired data should then be sent over either RS-232 or CAN.
The two resistors used in the divider were chosen in such a way that they could handle the high voltages while still providing an acceptable signal for the ADC.
3.1.7 24 Volt input
To be able to use the STM32 without connecting the 300V power supply a 24 Volt power input was added as a safety and testing measure. This input could also be used in the event that the resistive divider scheme did not work.
Design Overview 13
3.1.8 PCB Design
Figure 3.2: PCB design
The circuit board was designed by trying to keep the low power components on one half and the high power component on the other half. Keeping low and high power components on separate parts of the board also separates high and low speed components at the same time. This is a common practice when designing circuit boards. The low power components were also connected to a ground-plane for easier routing.
3.1.9 Component selection
Some devices such as the microcontroller and communication interfaces were chosen because Ocean Robotics had previous experience with them in other projects. This simplified the design process as no thought had to be put into what kind of voltages they had to handle and so on.
Any components in the high voltage part were chosen with respect to power and voltage handling capability. This in turn imposed some requirements on the physical size.
Chapter 4
Evaluation
The circuit was evaluated via simulation as well as physical testing. This section high-lights the differences between the two as well as the problems encountered during testing.
4.1
Simulations
As stated in the methodology, LTSpice IV was used to do all the simulations. LTSpice IV contains models of every device produced by Linear Technologies as well as all common parts such as resistors, capacitors and so on. This enables the user to focus on actual development without the need to find all the appropriate models.
4.1.1 Transient simulation
In order to verify that the calculated values of various components were correct they were simulated. The values of all the components were obtained using formulas from
the LTC4366 datasheet in MATLAB[4].
Evaluation 15
Figure 4.1: Simulated circuit
The simulated circuit can be seen in Figure 4.1. Because the MOSFETs used on the
physical board were not available as models, devices with similar characteristics were used in simulation. Moreover, the simulated circuit uses four devices in parallel whereas the physical circuit only uses one or two in the various tests. In addition, the various devices will not present different resistances or have differing threshold voltages because of ideal models. As such, no information on whether these common occurrences will cause problems can be found.
In this simulation the input voltage was set to start at 300V. After a period of time a spike topping out at 600V with a dV /dT of 300V /1ms appears, after which the voltage settles at 400V. The maximum voltage and dV /dT was obtained from the maximum expected values in the specification sheet. Furthermore, the output voltage was set to
clamp at 250V using R7 and R1. R7 is equivalent to RFB1 in Figure 3.1 and R1 acts
as the potentiometer. Moreover, with R1 at or close to zero the clamping voltage is set to 350V with a subsequent lowering of the output voltage as R1 is increased. R1 is
Evaluation 16
Figure 4.2: Simulated results with 250V clamp
Figure 4.4 shows the curves for the input and output voltages. In the blue curve the
input voltage with the added spike going up to 600V can be seen. The output voltage is seen in the green curve. The initial part is where the circuit is in its startup phase after which the voltage stays at the specified clamp voltage.
No information on power dissipation was gathered from this simulation since such pa-rameters would have to be obtained through physical tests.
Evaluation 17
4.1.2 Observing output voltage during a spike
After verifying that the circuit worked, at least in a virtual environment, a test was done to see how fast the output voltage increases when a spike is applied. The same circuit as in the previous test was used but the clamp voltage was set at 350 V by lowering R1. Input voltage was set to the same waveform as in the previous test.
Figure 4.3: Simulated results with variable clamping
In Figure4.3 the waveforms of the input and output voltages can be seen. The output
voltage follows the input very closely in the beginning with a small difference possibly due to the internal resistance of the MOSFETs. As the spike appears the output voltage ramps up to 350V and stays there even though the input has settled at 400V.
Evaluation 18
4.2
Physical testing
The tests focused on sustained periods of over-voltage. To generate controlled transients of sufficient magnitude special equipment is needed which was not available. One can assume that if the device can withstand long periods of over-voltage it should also be able to withstand shorter spikes.
During testing the two MOSFETs were mounted to a fan-cooled heat sink. Both resistors responsible for dropping the voltage down to a suitable level for other electronics were also mounted on a smaller fan-cooled heat sink. To measure the temperature of the various components a laser measurement tool was positioned approximately 30 cm away.
Evaluation 19
4.2.1 Operational test
A first test to determine whether the circuit was at all operational when mounted on the board was done with a large power resistor as a load. The potentiometer controlling the clamping voltage was set to allow the maximum clamping voltage of 350V.
Voltage applied to the circuit was slowly ramped up but no significant voltage was detected at the output. When measuring the voltage over the LTC4366 it appeared that
the supplied voltage was too low. Since the values on Rin and Rss were chosen close to
the maximum allowed values according to calculations found in the data sheet [4, p. 16]
it was decided that these should be lowered. New values were taken from an example on page 19 in the datasheet which had similar constraints on input and output voltage.
After replacing Rin and Rss the circuit started operating correctly and the voltage
measured across the load resistor followed the input voltage up to the limit set by the resistor divider. Changing the value of the potentiometer also resulted in changes to the output voltage.
During this test the two resistors responsible for dividing the output down to levels suit-able for low power electronics became overheated and subsequently destroyed. Because of this it was decided that any low power electronics in the circuit should have their own 24V feed line.
Evaluation 20
4.2.2 Test with load - One MOSFET
In this test only one MOSFET was connected to determine what kind of power it could sink before breaking. The output voltage was set to a certain value while the input was ramped up in steps to slowly increase the power dissipated by the MOSFET. A 500W halogen lamp served as a load in this experiment.
To determine the power dissipated P = U · I was utilized where U is the voltage over
the MOSFET (Vin-Vout) and I is the current drawn by the load. Table4.1describes the
various parameters.
Vin (V) Input voltage
Vout (V) Output voltage
I (A) Load current
T (C) MOSFET case temperature
Pdis (W) Dissipated power
Table 4.1: Table of parameters
Vin (V) Vout (V) I (A) T (C) Pdis (W)
191.1 189.7 1.604 26.7 2.2456 193.2 189.7 1.599 29.5 5.5965 195.3 189.7 1.599 33 8.9544 197.4 189.8 1.598 35.6 12.1448 201.5 189.8 1.598 40 18.6966 205.7 189.8 1.597 49 25.3923 207.7 189.8 1.597 52 28.5863 209.8 189.9 1.597 56 31.7803 214 190 1.598 62 38.352 218.1 190.1 1.599 64 44.772 224.3 190.2 1.599 82 54.5259 228.5 190.3 1.599 89 61.0818 232.7 190.4 1.6 100 67.68
Table 4.2: Test with single MOSFET
The MOSFET could withstand a load of about 70W before overheating as seen in
Table 4.2. Even though the case temperature measured did not exceed the maximum
Evaluation 21
4.2.3 Test with load - Two IGBTs in parallel
Since one of the two acquired MOSFETs had failed there was no way to test two of them in parallel. Two IGBTs with similar characteristics were soldered to the board and tested instead. IGBTs were chosen due to the fact that they are voltage controlled devices. Akin to the MOSFET they have their characteristic parameters such as threshold voltage but also share similarities with bipolar transistors. All measurements were conducted in the same manner as earlier tests.
Vin (V) Vout (V) I (A) T1 (C) T2 (C) Pdis (W)
200 190.4 1.604 31 33.5 15.3984
210.4 190.4 1.604 40.5 40 32.08
200 190.4 1.603 33 34.6 15.3888
218.7 190.4 1.6 67 32 45.28
Table 4.3: Test with two IGBTs
The temperatures shown in Table4.3indicate that the load is not shared symmetrically
by the two devices.
4.2.4 Test with load - Second test with two IGBTs in parallel
This test was done in the same manner as the other test conducted with the IGBTs. The difference here is that the current flowing through each transistor was also measured.
Vin (V) Vout (V) I1 (A) I2 (A) Itot (A) T1 (C) T2 (C) Pdis (W)
191.5 190.4 0.44 1.161 1.601 32 32 1.7611
201.8 190.4 0.433 1.167 1.6 35 36 18.24
210.2 190.4 0.36 1.24 1.6 36.2 52 31.68
218.5 190.4 0.26 1.3404 1.6004 31 70 44.97124
Table 4.4: Test with two IGBTs
Looking at Table 4.4 it is clear that the load is once again not shared symmetrically
Evaluation 22
4.2.5 Test with load - Alternative connection
This test was done with two PCBs, where each board had its own controller and IGBT. The two boards were connected to each other through the FB pins on the controller circuit. This approach did not solve any issues. The current in the table shows the total current drawn by the circuit.
Vin (V) Vout (V) I (A) TP CB1 TP CB2 Pdis (W)
201.6 188.8 1.593 26 27 20.3904
209.8 188.3 1.593 27.7 29.8 34.2495
218.1 188.5 1.593 29.6 30.9 47.1528
230.6 200 1.67 60 30 51.102
Chapter 5
Result and discussion
This chapter describes some of the problems encountered during the project.
Although the circuit was functional and capable of some continuous power dissipation it did not meet the specifications. A peak power dissipation of around 70W was achieved before device failure occurred. This is far from the desired power dissipation of at least 1500W. To achieve this level of protection more transistors need to be added if the cur-rent design is to be used. Furthermore, whatever the protection scheme used, the device needs to be bigger.
From the specifications sheet a dimensional limit of around 10x5x4 cm was obtained. Designing the device with roughly this size in mind was done to avoid any complications later. These complications might arise if the device did not fit the box in which it was to share space with other electronics.
The box itself was to be made out of aluminium and fit in the ROV, thus submerged in water during operation. However, with this in mind, the allotted dimensions for the protective device are still small considering the amount of power that might have to be dissipated for extended periods of time. Furthermore, no actual testing with the box submerged in water could be done since final dimensions depended on other electronics being added. The improved cooling of a submerged box might have increased the power handling capability.
Results and Discussion 24 The main reason why the circuit does not work as intended is because the actual specifi-cations of the MOSFETs do not apply when using the transistor in this manner. Using the MOSFETs in parallel was a bit harder than expected. The reason for this is because the on-resistance or threshold voltage of each MOSFET might differ. Even though this difference might be small it could start a snowball effect which eventually leads to one or more MOSFETs failing leaving devices down the line without protection.
Chapter 6
Improvements
This section explores different improvements or changes that could be made in order to meet the requirements. Some overlap with the previous chapter do occur but with more focus on the actual solution rather than the problem.
An idea that was explored in this project was to connect more transistors in parallel to increase power handling. However, this is complicated by the fact that there exists variations between devices from the same series. Because of these variations e.g. in the threshold voltage, the transistors do not share the load symmetrically.
By increasing the number of devices in parallel the effect of the variations in param-eters should decrease. Even though the load may not be shared symmetrically across all transistors the difference should still be smaller than before. In addition, a device with tight parameter distribution could be used to further insure a symmetrical sharing of the load between devices. However, since the power handling does not scale linearly with the amount of devices used the exact number needed would have to be determined experimentally.
Another idea is to use a number of circuits in series. Each circuit would contain the necessary components such as controller, MOSFET and so on. These would then be configured so that each step in the circuit clamps at a certain voltage akin to a resistive ladder. Connecting circuits in this way should remove the issue of different transistors having differing threshold voltages since each transistor now has its own control circuit.
Improvements 26 If the clamping voltage for each circuit increases stepwise the load should also be shared by all the devices.
Most other components need not be changed from circuit to circuit except the ones that control the actual clamping voltage. This is where problems arise since controlling the output voltage from the entire circuit would require controlling a large number of sub-circuits.
To actually control the clamping voltage a digital potentiometer would be ideal since it could be set by the micro-controller housed on the same PCB. However, most digital potentiometers only allow for supply voltages up to 60V. Since the voltage over the circuit relative ground would be over 200V this would not work. One solution would be to let the potentiometer along with its regulator float with the supply voltage. This still leaves the problem of interfacing logic signals with a device driven by 3.3 V. No matter the configuration, be it series, parallel or a combination of the two, some sort of automatic control should be added to the power supply. By sensing the current drawn from the PSU and knowing what kind of voltages appear during these load conditions, the PSU could increase or decrease the supplied voltage to the ROV. In conjunction with this a circuit such as the one designed in this project could be used to handle any over-voltages that might occur from the control system failing or being too slow.
Chapter 7
Conclusion
There are a number of ways to protect circuitry from excessive voltages. All of these come with their various pros and cons. This work details the design and evaluation of a protective device using somewhat unconventional technology because of the restrictions imposed in the sheet of specifications. It might not be the most efficient solution but it provides the functionality required although at a much too low power handling capability. All in all, this device could be used to protect circuits from high voltage spikes and extended over-voltages as long as cooling needs are met. In its current form it is not suitable for handling power above 70W but with increased available space and cooling this could very well be achieved.
Appendix A
Specification of requirements
Appendix A 29
Ocean Robotics International
Address: Teknikringen 7 Tel: +46 (0)761 86 86 30 www.ocean-robotics.com S-583 30 Linköping Fax: +46 (0)13 15 20 66
SWEDEN
Teknisk spec överspänningsskydd
Ett överspänningsskydd som ska kunna hantera olika utspänningar genom att vara ställbart och kunna klippa överspänningsnivåer, även kontinuerliga sådana inom visa gränser.
Inspänningsnivåer
200VDC – 600VDC
Utspänningsnivåer
250VDC – 350VDC, ställbart genom t.ex. mjukvara, spänningsdelare, något annat…
Spänningsspikar
De största spikar som förväntas i systemet har en dV/dt på 300V/1ms. Spikar måste klippas innan de hunnit bli för stora men ska inte kunna hanteras kontinuerligt.
Effekthantering
Då överspänningen är högst 50VDC över inställd utspänning ska skyddet klara att kontinuerligt sänka en effekt på:
Krav: 1500W Önskemål: 3000W
Mätning
Spänning ska kontinuerligt mätas, både för skyddet(?) samt för loggningsändamål.
Hårdvara för mjukvara
Ev. mikroprocessorer i systemet ska helst vara av modell STM32F303. Kommunikation ska ske över isolerade gränssnitt (CAN, RS-232) då kommunikation med omvärlden behövs. Lågspännings-elektronik bör drivas av den övervakade spänningen.
Kylning
Skyddet ska monteras i en aluminiumlåda, delar som behöver kylning kan skruvas direkt i väggen. Alltsammans kommer sedan att vara i vatten under drift, så bra kylning finns.
Storlek
Då alltsammans ska monteras på en relativt liten undervattensfarkost krävs att hänsyn tas för att välja komponenter som är så små som möjligt och förpackningen kan bli så liten som möjligt. För laborationskort krävs inte liten storlek men det måste beaktas så inga överraskningar kommer då slutlig produkt ska utvecklas.
Appendix B
Circuit diagrams for high speed
and regulator components
Appendix B 31
Appendix B 32
Figure B.2: Communication interface
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