### Transient Analysis

```PSPICE Lecture – Transient Analysis
PSPICE – Transient Analysis
Topics to be presented:
• Transient Analysis
• Analysis of 1st-order circuits
• Analysis of 2nd-order circuits
• Transient and Parametric Analysis
1
PSPICE Lecture – Transient Analysis
Transient Analysis in PSPICE
Recall that there are 4 types of analysis in PSPICE.
• Bias Point (DC analysis
where you place voltages,
currents, and power on the
schematic)
• DC Sweep (vary a source or
component)
• AC Sweep (vary frequency)
• Transient (vary time)
2
Simulation Settings
window shows 4
analysis types
3
PSPICE Lecture - Transient Analysis
Transient Analysis – A transient analysis is used to graph various quantities
versus time. Recall that whatever is varied in PSPICE will be placed on the xaxis when graphs are created. So graphs created using a transient analysis will
always have time, t, on the x-axis.
Example: Use a transient analysis to graph the source voltage, resistor
voltage, and capacitor voltage in the circuit below until they reach steady-state.
t=0
+
VR
-
1 k
100 V
+
_
1 F
+
VC
_
PSPICE Lecture - Transient Analysis
1) Create the Project and draw the circuit: Create a project and draw the
circuit shown below. Refer to earlier PSPICE lectures if you need help
creating a project.
Notes:
• Switch: Use part Sw_tClose from EVAL library for the switch
• Nodes: Label the nodes so that they can be referred to when graphing.
• Ground: Recall that all analog circuits require the 0 ground.
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PSPICE Lecture - Transient Analysis
2) Initial Capacitor Voltage: You must add the initial capacitor voltage, even if it
is 0V. To do this:
• Double-click on the capacitor to open the Property Editor (shown on the
left below)
• Select the part (column) named IC and select the Display tab
• Change the Display Format to Name and Value and select OK
• IC = should now appear on the schematic. Double-click on it and enter
the value (20V in this example)
5
PSPICE Lecture - Transient Analysis
3) Capacitor polarity: Capacitors has fixed + and - terminals in PSPICE. This is
important when initial conditions are added. The 20V initial condition just added
might act like -20V if the capacitor is upside down. To check the polarity:
• Select PSPICE – Create Netlist
• Select PSPICE – View Netlist
• The Netlist shown below indicates that capacitor C1 is connected from node 0
(+) to node C (-), so it is upside down! (The positive node is listed first.)
•
Right-click on the capacitor and select Mirror Vertically
•
Check the Netlist to see that the capacitor now has the correct polarity.
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PSPICE Lecture - Transient Analysis
4) Create the Simulation Profile: Recall that exponential functions take 5Tau to
decay, so we often want to perform a transient analysis for 5Tau. For this example:
Length of transient analysis = 5Tau = 5RC = 5(1k)(1uF) = 5ms
•
•
•
•
•
Select PSPICE – New Simulation Profile
Give the Simulation Profile a Name (any name is OK but using the schematic
name is a good idea)
Under Analysis Type select Time Domain (Transient)
Under Run To Time: Enter 5ms (no spaces!)
Select OK.
Fine point: By default approximately 100
points will be used to create each graph, so
Maximum step size = (Run to time)/100.
In this example the blank box indicates that
the Maximum step size is 5ms/100 = 50us.
If you wished to use twice as many points
you could enter 25us into the Maximum
step size box.
8
PSPICE Lecture - Transient Analysis
5) Analyze the circuit and graph the results
• Select PSPICE – Run to analyze the circuit. The graphing window should
appear. Since we did a transient analysis from 0 to 5ms, time should vary from
0 to 5ms on the x-axis.
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PSPICE Lecture - Transient Analysis
• Select Trace – Add trace and the Add Traces window will appear.
• Select or type the names of one or more waveforms to view
• Voltages in the list are all node voltages, so to find the resistor voltage V(B,C) was
entered (positive terminal is listed first).
PSPICE Lecture - Transient Analysis
7) Add text and mark points
text
Toggle Cursor
On/Off
Cursor
Select Waveform
for Cursor
Cursor value –
currently for V(B,C)
Mark
Point
10
PSPICE Lecture - Transient Analysis
• Capacitor voltage – Charges from the initial value (20V) to 100V as expected
• Resistor voltage – Decays from its initial voltage (100 – 20 = 80V) to zero
• Source voltage – Constant 100V
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PSPICE Lecture - Transient Analysis
12
8) Other graphing features
There are many other graphing features which may be demonstrated in class or you may
try on your own. Features include:
• Two cursors: PSPICE has two cursors that can be added to determine values of
waveforms at different points.
• Controlling two cursors: The left-mouse button controls Cursor 1 and the rightmouse button controls Cursor2 (coarse adjustments). The cursors can be moved for
fine adjustments with the arrow keys (Cursor1) or Shift + arrow keys (Cursor2).
• Marking Points: Use Plot - Label – Mark or the toolbar.
• Saving graphs: Use Window – Display Control. Last graph is saved here
automatically.
• Printing graphs: Use Window – Copy to Clipboard. This was used to create the
graph on the previous slide. Note that the black background is changed to white.
• Graphing expressions: Note the functions in the Add Trace window, such as *, /,
abs(), sin(), exp(), etc. You could, for example, graph V(C)*I(C1) or abs(I(C1)).
• Trace Properties: Right-click on a trace to change its color, line width, etc.
• Other: You can zoom in and out, use linear or log scales, turn off minor gridlines,
etc.
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PSPICE Lecture - Transient Analysis
Charging and discharging a capacitor using VPULSE
Suppose that the switch in the circuit below moves back and forth between A
and B (for at least 5Tau in each position). The result is that the capacitor will
charge and discharge repeatedly.
A
100 V
+
_
1 k
B
1 F
+
VC
_
VC
100 V
0V
Switch
moves
to A
5Tau
Switch
moves
to B
10Tau
Switch
moves
to A
15Tau
Switch
moves
to B
20Tau
Switch
moves
to A
t
14
PSPICE Lecture - Transient Analysis
How is a switch moved back and forth in PSPICE?
This is simulated by using a pulse waveform.
VPulse
Connect
100 V
to the
RC circuit
Connect
0V
to the
RC circuit
100 V
0V
5Tau
10Tau
A
100 V
+
_
B
1 k
1 F
15Tau
20Tau
Equivalent
+ circuits
VC
_
0 to100V
Pulse
Waveform
t
1 k
1 F
+
VC
_
15
PSPICE Lecture - Transient Analysis
Part VPULSE in PSPICE
VPULSE is a part in the Source Library. It has various properties as defined
below:
V1 = First voltage
V2 = Second voltage
TD = Time Delay (time before pulse starts). It is OK to use TD = 0.
TR = Rise Time (time to go from V1 to V2). TR cannot be 0.
TF = Fall Time (time to go from V2 to V1). TF cannot be 0.
PW = Pulse Width (time when output = V2)
PER = Period
Illustration:
VPulse
TF
V2
TR
PW
V1
TD
PER
t
PSPICE Lecture - Transient Analysis
PSPICE Example using part VPULSE
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PSPICE Lecture - Transient Analysis
Example: Use a transient analysis to graph the capacitor voltage in the circuit
below. Assume that the switch moves to position A at t = 0 and then moves
back and forth between A and B every 5Tau to repeatedly charge and discharge
the capacitor. Graph VC as it charges and discharges 3 times.
A
100 V
+
_
B
1 k
1 F
+
VC
_
18
PSPICE Lecture - Transient Analysis
Solution:
1. Create the Project and draw the circuit: Create a project and draw the
circuit shown below.
Notes:
• Use part VPULSE from the
Source Library
• 5Tau = 5RC = 5ms, so use
PW = 5ms (the capacitor
charges here)
• 5RC is also needed for the
capacitor to discharge, so
use PER = 10ms (time to
charge and discharge)
• TR and TF cannot be 0, so
make them very small
compared to PER. Note
that 1ns is only one tenmillionth of PER.
• The initial condition for the
capacitor must be set to 0V.
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PSPICE Lecture - Transient Analysis
2. Create a new Simulation Profile and determine the length of the analysis:
In order to charge and discharge the capacitor 3 times, the analysis should last
for 30Tau (or 30ms) as illustrated below.
VC
100 V
0V
5Tau
10Tau
15Tau
20Tau
25Tau
30Tau
t
PSPICE Lecture - Transient Analysis
3. Simulate the circuit and graph the results:
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PSPICE Lecture - Transient Analysis
Parametric Analysis in
PSPICE
A parametric analysis is
used to vary a second
parameter to generate a
series of curves. Many
types of parametric
analysis are possible in
PSPICE.
Examples:
• Top Example: Vary
voltage and current
• Bottom Example: Vary
time and resistance
Vary current (parametric analysis
Vary voltage (DC Sweep)
Vary R (parametric analysis
Vary time (Transient analysis
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PSPICE Lecture - Transient Analysis
Example:
Graph VC versus time as the capacitor charges for R = 10k, 20k, 30k,
40k and 50k.
t=0
R
100 V
+
_
+
1 F
VC
_
Solution:
• A transient analysis can be used to vary time from 0 to 5Tau.
• Use the largest value of Tau, so 5Tau = 5(50k)(1F) = 250 ms
• A parametric analysis can be used to vary R from 10k  to 50k 
PSPICE Lecture - Transient Analysis
1. Create the Project and draw the circuit: Create a project and draw the
circuit shown below.
Use the following steps to vary a resistor:
a) Use a variable resistor part, R_var
b) Change the value of R_var to a name in braces, such as {Rvalue}
c) Change the property SET to 1 for R_var (be sure to display it also)
d) Add a part named PARAM from the Special Library
e) Add a property (column) to PARAM with the same name as the resistor
value in braces – Rvalue in this cases
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PSPICE Lecture - Transient Analysis
2. Create a new Simulation Profile
• Select Time Domain (Transient) for the Analysis type.
• Enter 250 ms for the value for Run to time:
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PSPICE Lecture - Transient Analysis
• Check the box labeled Parametric Sweep. (Note that the window is the
same one used with a DC Sweep analysis.)
• Select Global parameter
• Enter Rvalue for the Parameter name
• Enter the Start value, End value, and Increment.
• Select OK.
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PSPICE Lecture - Transient Analysis
3. Simulate the circuit and graph the results:
(Select OK when the Available Sections
window appears to select data for all 5
curves.)
PSPICE Lecture - Transient Analysis
27
Analyzing 2nd-order circuits in PSPICE
There is not much difference between analyzing 2nd-order circuits and
analyzing 1st-order circuits. A transient analysis is still used. One difference is
in determining the length of the analysis.
1st-order circuit:
• Length of transient analysis = 5Tau
• Find Tau = ReqC or L/Req
• Or find Tau from the expression x(t) = B + Ae-t/Tau
• Example: If v(t) = 10 – 10e-500t, then Tau = 1/500 = 2ms, so 5Tau = 10ms
2nd-order circuit:
• Recall that the natural response has three forms: overdamped, criticallydamped, and underdamped. Since an overdamped response has two
exponential terms, use the one with the largest Tau (the dominant root).
• In each case assume that the exponential terms have the form e-t/Tau and again
use: Length of transient analysis = 5Tau
• Example: If v(t) = e-500t[20cos(50t) + 30sin(50t)]
(underdamped), then
Tau = 1/500 = 2ms, so 5Tau = 10ms
28
PSPICE Lecture - Transient Analysis
Example: Graph VC versus time until the capacitor reaches steady state.
t=0
1 mH
12.62
20 V
+
_
+
1.57 F
VC
_
Solution: First determine the length of the analysis.
This is a series RLC circuit so:
 
R
2L

12.62
 6310,
w
2(1E - 3)
The characteri stic equation
2
o

1
1

LC
 6.369  10
(1E - 3)(1.57E - 6)
has the form s  2 s  w o  0 or s  1 2610s  6.369  10  0
2
2
The complex
roots are s1, s2    j   - 6310  j48873
The response
will decay in 5Tau 
So the transient
analysis
8
should
5


5
 0.7924ms
2
 0.8ms
6310
be performed
from 0 to 0.8ms
8
PSPICE Lecture - Transient Analysis
1. Create the Project and draw the circuit: Create a project and draw the
circuit shown below.
Notes:
• Set the initial condition to 0 for the capacitor and the inductor (IC = 0).
• Label the nodes. The node voltage V(C) is the capacitor voltage.
2. Create the simulation profile
• Perform a transient analysis from 0 to 0.8 ms
3. Analyze the circuit and graph the capacitor voltage
• See the following slide
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PSPICE Lecture - Transient Analysis
Note that the capacitor voltage is as expected: It has an initial
voltage of 0V, a final voltage of 20V, and it is underdamped.
Two other quantities are useful to show on this graph: rise time and
% overshoot. They are defined on the following slides.
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PSPICE Lecture - Transient Analysis
Rise Time (tr) – the time for a waveform to go from 10% of its final value
to 90% of its final value. Rise time can be used with any order circuit and
any type of response.
Example:
10V
9V
1V
0V
tr
t
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PSPICE Lecture - Transient Analysis
% Overshoot – This term is only used with underdamped circuits. It is a
measure of how far the waveform shoots past its final value before settling on
the final value. It is defined (for a voltage) as:
Example:
% Overshoot

V max - V final
 100 %
V final
v(t)
13V
10V
0V
t
For this example
:
% Overshoot

13 - 10
10
 100  30%
PSPICE Lecture - Transient Analysis
Add rise time and % overshoot to the previous graph:
A cursor was used to mark 3 points at:
• 2V (10% of the 20V final value)
• 18V (90% of the 20V final value)
• The max value (use the cursor peak tool)
33
```