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High Current Pulse Generator
Team DEC13-06
Design Document
High Current Pulse Generator
Team Composition
Members:
Greg Bulleit
Stephen Chiev
Ho Hsu
Matt Stegemann
Yating Wen
Li-Yeh Yang
Shih-Yao Yen
Advisors:
Robert Bouda
Mani Mina
John Pritchard
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High Current Pulse Generator
Table of Contents
Executive Summary ................................................................................. 3
Requirements ............................................................................................ 3
Non-Functional Requirements ............................................................. 3
Overall Design ........................................................................................... 4
Individual Parts ........................................................................................ 5
Input/Output ............................................................................................11
Resources ................................................................................................. 12
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High Current Pulse Generator
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 down 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
Our final goal is to create a Mono/Bi phasic Pulse Generator. The purpose of this Pulse
Generator is to be of use for TMS products in the future. The Pulse Generator will be have a
main power source from the wall outlet, which will power the Arduino, charge the Capacitor,
and power the IGBT.
Functional
1.
2.
3.
4.
Control of pulse width and amplitude
Initial device capable of 25A mono-phasic by end of spring semester
Biphasic implementation
Higher current design or device
Non-Functional
1. Single device capable of mono/bi phasic pulse generation
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High Current Pulse Generator
Overall Design
Our current design is based on the original design we were given as a starting point for
our project. The device works on the basis of storing a large charge and then being able to
control the discharge through a load. The first part of the device is the power delivery and power
storage. Our supply of power is a normal 120V AC wall outlet. To supply our circuit with power
we need a DC voltage and out design use transformers to lower the voltages and rectifiers to
convert the AC voltage to DC. To store the power and be able to give high current pulses we
chose to use large capacitors. The size of the capacitors will change our maximum pulse length
and maximum current deliverable to the load.
The second part of the circuit is the switching device and switching control. For the
switching control we want to be able to control pulse length and amplitude. The device we chase
to control the pulses is an arduino micro controller. The arduino allowed us to easily control
pulse width and amplitude along with the ability to show us what the parameters are. The
switching device we chose is an IGBT. It can handle large currents and can switch quickly
enough for our pulse length.
Transformer/Rectifier
We are going to power the pulse generator through the wall outlet, but the wall outlet has an
output of 120vrms. This vrms is too high for our pulse generator to function, so that is why we
have decided to use a step down transformer to decrease the voltage to about 40 vrms. The
rectifier is used to change the power from AC to DC source. The main purposes of the
transformer and rectifier are:
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


Convert power from AC to DC
Step down the voltage to a usable level
Power all parts of the pulse generator
Arduino/Control/Trigger
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
Filter/Amp
Send Pulse
Mono/Bi Select
Pulse
Width
Pulse Amplitude
PW
M
D1
D2
A1
A2
Inputs
Arduino
D3
TX
Power Amp
To IGBT
Serial Display
Outputs
Capacitor
The basic charging circuit has two parts: capacitor and resistance. In order to know the
characteristic of charging circuit, we offer a DC source to do the charging. In picture one, DC
source charge the capacitor. After the capacitor is fully charged, S1 off and S2 on, the capacitor is
discharging through the resistance. We can see the result as below:
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High Current Pulse Generator
We can obviously see that after one time constant, the capacitor can charge to almost 63
percent of total charge, vice versa.
Time constant is an important factor of charging capacitor, we know that time constant
equal to resistance times capacitor (t=R*C), in order to have much more obvious charging and
discharging procedure, we have to keep time constant from being too big. In our project, we are
dealing with large current so that we can induce stronger magnetic field when current flow
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High Current Pulse Generator
through the load. Therefore, we choose a super capacitor so that it can storage enough voltage
and discharge it in a short time.
In original charging circuit, there will still some voltage left in capacitor in case the
capacitor is not fully discharge. Due to the small internal resistance of super capacitor, the time
to discharge through the internal resistance will be too long, therefore, we design a circuit to let
the capacitor fully discharged after using the machine.
Charging Capacitor Circuit Task List
1. Automatically discharge after using so that we don’t have to force the capacitor to the
ground to reduce the danger.
2. Ability to charge and discharge in a short time.
3. Eliminate time delay of RC circuit.
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High Current Pulse Generator
This circuit diagram is our design of charging capacitor part:
1. At first, we need to charge the capacitor. So J1(IGBT) off, J2 turn to the power side and
J3 off. In this circuit, the capacitor is charging by the power (37v)
2. When we received the signal, the capacitor should discharge its chargeto the load. So
J1(IGBT) on, J2 turn to the load R1 side and J3 off. In this circuit, the capacitor is
discharging the charge to the load and we got the current.
3. After the signal disappear, the power would be off. Then we need to discharge the charge
which is left in the capacitor. And J1(IGBT) would be off, J2 has no different in both side
and J3 on. In this circuit, the capacitor would be discharge its charge to the register R2.
It made to prevent from discharging the capacitor by ourselves. That would be less
dangerous to discharge by this circuit.
IGBT
We use the AC voltage supplier to trigger BJT and let the 25A flow through the circuit,
and we choose point 3 as output So clearly , we can see the variation of voltage versus time in
diagram2 when the circuit conducts , the voltage of the point 3 rise up quickly however , when
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the circuit is cut off the voltage falls down to 25k volt which is calculated from timing the
current and the inner resistance of IGBT
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Input/Output
Input
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.
Output
VDD
high
VA
low
R
VB
HIGH
VA
BJT
VB
LOW
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.
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.
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Resources
http://datasheet.seekic.com/pdfimage/2N6/2N6975INTERSILIntersilCorporationpdf1.jpg (Capacitor
DataSheet)
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