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Transcript
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 Team DEC13-06 Page 1 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 Team DEC13-06 Page 2 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 Team DEC13-06 Page 3 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: Team DEC13-06 Page 4 High Current Pulse Generator 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: Team DEC13-06 Page 5 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 Team DEC13-06 Page 6 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. Team DEC13-06 Page 7 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 Team DEC13-06 Page 8 High Current Pulse Generator 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 Team DEC13-06 Page 9 High Current Pulse Generator Team DEC13-06 Page 10 High Current Pulse Generator 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. Team DEC13-06 Page 11 High Current Pulse Generator Resources http://datasheet.seekic.com/pdfimage/2N6/2N6975INTERSILIntersilCorporationpdf1.jpg (Capacitor DataSheet) Team DEC13-06 Page 12