Varistor Characterization using Non-Linear Components

Transcription

1 Varistor Characterization using In this lab, a non-linear resistor will be used to model a Varistor. An alternate method will also be evaluated using a non-linear voltage dependent current source. The Varistor will then be used in an application circuit. Open a new Simplorer project, rename the Project to be Non_Linear_Components, name the default design to be Varistor then File -> Save As to desired location Add the following components Voltage Source (one) Resistor (one) Voltmeter (one) Voltage Dependent Current Source (one) Basic Elements/Circuit/Sources/IC: Controlled Current Source NOTE flip this Vertically Arrange them as shown below Double click on the Voltage Meter, select the Output/Display tab and select to Show Pin for the Voltage that is being measured V Double click on Controlled Current Source, select NonLinear Voltage Control and Select Use Pin for the Control Component -1

2 Note this sets up the current source to be controlled by an external voltage source with a non-linear behavior Flip the Controlled Current Source Horizontally and connect the circuit as shown below Note that a Varistor is basically a voltage dependent current, so the Controlled Current Source can be used which monitors the voltage across itself, and defines a current based on that voltage. Note the plot below shows the non-linear IV characteristic of a Varistor, the device passes very little current until a large voltage is reached across its terminals, then a large current is created. Note the Resistor component can also be made to be non-linear where the current is a function of the voltage across it (i = f(v)) -2

3 Double click on the Resistor, name it Varistor, select the Nonlinear function, then select the Characteristic button to define the non-linear relationship between the current and voltage Select Dataset then the Datasets Button to add data Select Add Select import Dataset then select *.txt file in the pull down menu, go to Where the VI_char.txt file exist, and Open it. -3

4 Select Space as the Separator And begin import at line 2 so the Title is not imported, select OK The data will now be imported and Shown, name the dataset to be $VI_char Select OK, done, OK, OK to get back to the schematic Note the symbol on the Resistor will now change to show it is now a non-linear resistance where i = f(v) -4

5 Double click on the Controlled Current Source, select the Characteristic button to define the non-linear relationship between the current and the voltage that controls it. Select Dataset and the Datasets button to define the non-linear relationship Select the previous define non-linear relationship $VI_char Select Done, OK, OK, to get back to the schematic Double click on the Voltage Source and select Time Controlled Sine, Spice Compatible, Amplitude = 240V, Frequency = 60Hz, select Ok Leave TR analysis settings at the defaults (Tend = 40m, Hmin=10u, Hmax=1m) -5

6 Zoom out on the schematic to make room for plots Add one Rectangular plot Draw -> Report -> rectangular plot do not add a signal yet, simply close, then re-size it as shown Select the plot on the schematic and copy it (Ctrl + C) then paste it two more times (Ctrl + V) as shown below Double click on the top plot, and define the signals to be for the voltage E1.V and the varistor current Varistor.I -6

7 Double click on the lower left plot, and define it so to display the Voltage-Current relationship for the Controlled Current Source Un-select the X axis default button, and select to use the voltage across the Controlled Current Source IC1.V Select the Y axis to be the current in the Controlled Current Source IC1.I, Add Trace, Close Select the same plot (lower left), and choose the visibility for the Header only RMB -> View -> Visibility, select the Legends tab, select only the Header box Rename this plot in the Project Manager window under the Results section to be Varistor - CCS for Controlled Current Source. -7

8 Double click on the lower right plot, and define it so to display the Voltage- Current relationship for the non-linear resistor Un-select the X axis default button, and select to use the voltage across the non-linear resistor Varistor.V Select the Y axis to be the current in the non-linear resistor Varistor.I, Add Trace, Close Select the same plot (lower right), and choose the visibility for the Header only RMB -> View -> Visibility, select the Legends tab, select only the Header box Rename this plot in the Project Manager window under the Results section to be Varistor - NLR for non-linear resistor. -8

9 Run the TR simulation, the results should appear as shown below File -> Save Note in the top plot, the current thru the non-linear resistor (Varistor) is very small until the voltage across it reaches the breakdown region (for both positive and negative voltage), then current starts to flow thru the device. Note in both lower plots, the I-V relationship is that of a Varistor for both the Controlled Current Source and the non-linear resistor -9

10 Varistor Application Circuit In this section, the Varistor will be used in a switched inductive application to show its usefulness as transient protection for the switch. When the current thru an inductor is interrupted quickly via a switch, there will be a very large induced voltage due to the relationship of VL = Ldi/dt (where di change in current (large change) as a function of time dt (very small change) yields the large voltage spike. Create a new Design in the existing Simplorer Project Non_Linear_Components using Project -> Insert Simplorer design, rename the new design to be Switched_Inductor The following Circuit will now be created -10

11 Add the following components to the Switched Inductor design and place them based on the previous schematic or as shown below Two Voltage Sources One Voltage meter and One current meter Basic Elements/Measurement/Electrical. Note rotate the current meter One resistor One inductor One BJT NPN transistor Basic Elements/Circuit/Semiconductors Device Level/BJT/NPN6: One PWM time function Basic Elements/Tools/Time Functions/PWM: Pulse-Width Mod.. Add the Varistor model that was created in the previous section by opening the previous design Varistor, select the Varistor (created from the non-linear Resistor), copy it (Ctrl + c), then re-open the new design Switched_Inductor and paste it (Ctrl + v) The placements of the components should appear as shown below -11

12 Note the voltage source used to drive the gate of the BJT will be a PWM signal derived from the PWM Time Function. One option would be to select the EMF Value of the Voltage source to be a pin, then connect the output pin of the PWM block to this EMF input pin for the Voltage source. The output of the PWM time function by default is between 0 and 1, therefore may not be enough to turn on the gate of the BJT. The output of the PWM block will be scaled to 0 to 5V in the Voltage source. Double click on the PWM time function, define the period to be 10mS and leave the duty cycle to be the default of 0.5 Select the Output/Display tab for the PWM function, and un-select the box Show Pin for the Output VAL (this will remove the output pin from the symbol), select OK Double click on the Votage source used to drive the gate of the BJT, and name it Vgate, add the EMF Value to be PWM1.VAL*5 (this will take the output of the PWM block and multiply it by 5 to yield the 0 to 5V gate drive. Select Spice Compatible, Select OK Double click on the inductor and name it L_load and give it a value of 2mH Double click on the resistor and name it Rload and give it a value of 1 Ohms Double click on the other voltage source and give it a value of 24V, set Spice Compatible Chose to display the values of the Inductor, resistor and 24V source -12

13 Connect the components and add ground so the circuit appears as shown below File -> Save Zoom out on the schematic to make room for adding plots Add a stacked plot via Draw -> Report -> Rectangular Stacked Plot Select to add 3 signals: Vgate.V, AM1.I, and L_load.I Add another Rectangular Plot Draw -> Report -> Rectangular Plot Select to add VM1.V Set up the TR analysis for Tend=10mS, Hmin=1uS, Hmax=10uS Run the TR analysis, the results should appear as shown below -13

14 Note from the results, when the switch is turned off, the Inductor current will decrease rapidly yielding the voltage spike across the switch. Note the Varistor in this case will provide a path for the inductor current during turn off to limit the voltage spike magnitude to under 250V. Select the Varistor, and disable it using RMB -> Deactivate Open, this will take the Varistor out of the circuit Re-run the TR simulation and note the results below Note the voltage spike across the switch is now close to 40kV without the Varistor. Note also the inductor current starts to ramp up again even thought the switch is being turned off (Vgate = 0), this is a result of the large voltage spike across the BJT causing it to break down and turn on again Double click the BJT, change the Breakthrough Collector-Emitter Voltage to be V Re-run the TR analysis and note the inductor current will not ramp back up now Re-activate the Varistor in the circuit, and re-run the TR analyses, File -> Save -14