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High Current Pulse Generator Team DEC13-06 Design Document
High Current Pulse Generator
Team DEC13-06 Page 1
Team Composition
Members:
Wen Ya Ting(Audrey) Ho Hsu (Lily) Li-yeh Yang (Leo) Shih-yao Yen (Lawrence)
Tatung University Tatung University Tatung University Tatung University
Stephen Chiev Greg Bulleit Matt Stegemann
Iowa State University Iowa State University Iowa State University
Advisors:
Robert Bouda
Mani Mina
John Pritchard
Client:
High-Speed Systems
Engineering Laboratory
High Current Pulse Generator
Team DEC13-06 Page 2
Table of Contents
Executive Summary ................................................................................. 3
Requirements ............................................................................................ 3
Functional Decomposition ..................................................................... 3
Detailed Design ......................................................................................... 4
Simulation and Testing ........................................................................... 7
High Current Pulse Generator
Team DEC13-06 Page 3
Executive Summary
Our goal of this project is to further research into high current pulse generators that
could be used as for Transcranial magnetic stimulation (TMS). The magnetic fields
used in TMS applications are pulsed at very short time intervals. A high current
pulse is sent through an electromagnetic coil to create these fields. The goal of this
Senior Design team is to create a device that can deliver such a pulse. This device will
have controllable parameters (such as pulse width and amplitude) and will be able to
manage inductive loads.
This document will cover the overall design of our circuit. We have broken down the
circuit into its basic parts; the power supply, power storage, switching device, and
switching device control. For each of these parts we have broken down our design
choices and considerations.
Requirements
Functional
1. Control of pulse width and amplitude
2. Initial device capable of 25A monophasic
3. 400 s for maximum pulse width
4. Biphasic implementation
5. Higher current design or device
Non-Functional
1. Single device capable of monophasic and biphasic
2. The size and the weight of our machine would not be too big and heavy. It would
be easy to move and carry.
Functional Decomposition
The basis of our design comes from the idea of storing a large amount of energy in a
capacitor and being able to control its discharge through a load. Our device can be
broken down into four different parts; power conversion and storage, switching
device, load, and switching control. The first part power storage and conversion
covers both of our power supplies and our capacitors used to store the energy for the
High Current Pulse Generator
Team DEC13-06 Page 4
pulse. The second part our switching device covers the large device we chose to
handle the circuits current and its protection diode. The third part load covers the
different loads we have used for the device. The last part switching control
encompasses the microcontroller and the different circuitry we are using to create
the switching devices gate voltage and pulse.
Figure 1: System level diagram.
Detailed Design
Input/Output
The device has four inputs and three outputs. The inputs are the wall outlet for
power, parameter control, a switch to select mono or bi-phasic operation, and the
button to initiate a pulse. The outputs are connections for the load device, a serial
High Current Pulse Generator
Team DEC13-06 Page 5
connection over USB to read the circuits device parameters, and last a possible LCD
to display the current device parameters.
Inputs
120Vrms AC wall outlet
Button
Mono/Bi-phasic switch
Pulse length control
Pulse height control
Outputs
Load
Serial (USB)
Possible LCD
Power Conversion and Storage
Our device is power from a normal 120Vrma wall outlet. To easily store power we
need to convert the AC input voltage to a DC voltage. In our device we have used a
transformer to drop the AC voltage from 120Vrms to 40Vrms. We drop the voltage
because the capacitors become prohibitively expensive at high voltages. Once the AC
voltage is reduced we use a full wave rectifier and our capacitors to create a DC
voltage of 56V. Our secondary power supply is smaller and is used to power our
switching control. This design uses a transformer to create a 15Vrms signal that is
rectified to become a 21V DC voltage.
Switching device
In our design our switching device must be able to handle the full current flowing
through the load. The device we chose is an insulated-gate bipolar transistor (IGBT)
for its high speed and high voltage control. The device can handle up to 20V on its
gate and 600V from collector to emitter. The device can also handle 200A for short
pulses. In parallel with our switching device is a protection diode. This diode will
break down with voltages high than 70V. These voltages can occur when an inductive
load is used and its path to ground is suddenly cut off.
Switching control (Arduino)
High Current Pulse Generator
Team DEC13-06 Page 6
An Arduino board was chosen to control the parameters of the pulse and to trigger
the pulse itself. This board is easy to use and allows for plenty of flexibility in the
design. The main purposes it will serve are:
Monophasic or Biphasic select
Control pulse amplitude
Control pulse width
Output pulse trigger
Serial display
Arduino Inputs
Two analog inputs will be used to control the width and amplitude parameters. The
voltage detected at each of the inputs will correspond to specific pulse and amplitude
ranges. A digital third input will be used to select either monophasic or biphasic. The
last digital input will be used to detect when to send to the pulse.
Arduino Outputs
A digital output will be used to send the initial pulse. This pulse will switch off a BJT
that will allow current to flow to the IGBT switching circuit.
Figure 2: A system level diagram of the Arduino inputs and outputs.
Arduino
D1
D2
A1
A2
PWM
D3
TX
Power Amp
Filter/Amp
Inputs Outputs
High Current Pulse Generator
Team DEC13-06 Page 7
A pulse-width-modulation (PWM) output will control the current flow to the gate of
the IGBT during the pulse, thus controlling the amplitude. The duty cycle of the
PWM will be controlled by one of the analog inputs. This PWM wave will then be
filtered and amplified to act as an analog voltage, depending on the duty cycle. This
voltage will be used as VDD in the circuit of Figure 2.
The last output will be a serial output to an LCD display. This will display
information such as the mode select (monophasic or biphasic) and the pulse width.
Simulation and Testing
Software: National Instruments (NI) Multisim
To simulate and test our design, we use National Instruments (NI) Multisim
software. Multisim is a very capable and widely used SPICE program. We have
access to Multisim on the campus lab computers and also through remote desktop.
The wide range of component selection and intuitive interface make it a very useful
tool in our design.
Pulse Circuit and Simulation Results
VDD
R
BJT
VA
VB
high
low
HIGH
LOW
VA
VB
Figure 3: Function of the digital output and the power amp circuit. VA
is connected to D3 from Figure 2 and VB goes to the IGBT.
High Current Pulse Generator
Team DEC13-06 Page 8
Figure 4: Multisim schematic of a complete pulse circuit.
Figure 4 shows a complete pulse circuit. The following table lists the relevant
components:
Component Purpose V3 Simulates voltage from wall outlet T1 Transformer to step down the wall voltage D1 D4 Full wave rectifier for conversion of AC to DC C1 and R2 Capacitor for energy storage with simulation of
built in series resistance L2 Inductive load R1 Parallel load resistor to minimize voltage
ringing Q1 IGBT for switching purposes V1 Substitute for Arduino power amplifier output D5 Protection diode
Table 1: A list of the components and their purposes for the circuit in Figure 4.
Simulating this circuit allows us to send a current pulse through the load. The
voltage source V1 controls what the output current waveform will look like. In
Multisim, we are able to adjust the sources pulse width and amplitude to produce
different current outputs.
High Current Pulse Generator
Team DEC13-06 Page 9
Figure 5: Load current for a pulsed amplitude of 9 volts and width of 375 microseconds. Total pulse time is close to 400 microseconds.
Figure 5 shows a sample output of the current through the load. The waveform
shows us the total pulse width will not be the same as the pulse width from the
source V1. There is additional time added due to the discharge of the inductor.
Figure 6: Output current pulse, with inclusion of the voltage spike at the end of the pulse.
High Current Pulse Generator
Team DEC13-06 Page 10
Figure 6 shows the current output again (in red) and also the resulting voltage spike
(in green) from the discharging inductor. This voltage spike occurs at the collector of
the IGBT. Having the protection diode D5 greatly reduces the voltage spike and thus
prevents the IGBT from being damaged.
Arduino Power Amplifier and Filter Circuit
Figure 7: Multisim schematic for the power amplifier and filter circuits associated with the Arduino outputs.
The voltage source V1 in Figure 4 is a substitute for the above circuit in Figure 7. This
circuit lets us simulate the PWM output and the digital output from the Arduino.
High Current Pulse Generator
Team DEC13-06 Page 11
Figure 8: Output from the filter circuit of Figure 7. This waveform shows that it takes about 100 milliseconds for our output to settle to the desired voltage.
Figure 8 is the output of the filter section from Figure 7. The waveform shows us
about how much time is needed until the output settles to the desired voltage.
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