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SPICE
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Simulation Program with Integrated-Circuit Emphasis,
Originated in Berkeley, 1972
Commercial versions are $$, limited free demos are
available (OrCAD).
LTspice is freeware offered by Linear Technology, a
producer of analog IC's: op-amps, regulators, ADCs.
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Includes 1100 models for their devices and 100's of
switch-mode supply circuit models
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Not node or component limited
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External models and subcircuits may be added
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Includes waveform viewer
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Limitations? No automated optimization (Monte Carlo
analysis? See example file MonteCarlo.asc)
SPICE Demonstration - LTspice
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What is LTspice and what can you do with it?
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Circuit simulation: extendable parts libraries; built-in
diode and transistor behavioral models
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Draw a circuit (schematic capture)
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Create a BOM
Where to get a free copy of Ltspice?
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Go to http://www.linear.com/LTspice
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Left click on “Download LTspice IV”
How to learn to use it: LTspice IV Getting Started Guide
SPICE Text Listing
SPICE originally required a text based input. Remember punch cards anyone?
Now, the graphical capture front end automatically creates the necessary node
and command lists. Listing here is from the simple linear supply design.
Each circuit node in your design is automatically numbered and can be renamed.
Example syntax for transistor: Qxxx Collector Base Emitter [Substrate Node] model
* C:\Users\Steve\Desktop\SPICE Demonstration\SimpleSupply.asc
Q1 N001 N003 Vout 0 2N3055
R1 N003 N002 {10k-Pres}
R4 0 N003 {Pres}
R2 N001 N002 820
R3 Vout 0 {Rload}
C3 Vout 0 470µF
V1 N001 N004 20
D1 0 N002 BZX84C15L
I1 Vout 0 PULSE(1mA 50mA 10ms 10ns 10ns 100ms 500ms 1)
C2 Vout N001 .02pf
V2 N004 0 SINE(0 2 120 0 0 0 20)
.model D D
.lib C:\PROGRA~2\LTC\LTSPIC~1\lib\cmp\standard.dio
.model NPN NPN
.model PNP PNP
.lib C:\PROGRA~2\LTC\LTSPIC~1\lib\cmp\standard.bjt
.PARAM Rload=970
.dc I1 10mA 100mA 2mA
;tran 0 200ms 0
.step param Pres 1k 9k 1k
* .step temp -40 80 15
.param Pres=5k
.backanno
.end
SPICE Prefixes
and Suffixes
From Ltspice Help
Leading
Character
Type of line
*
Comment
C
Capacitor
D
Diode
E
Voltage dependent voltage source
F
Current dependent current source
G
Voltage dependent current source
H
Current dependent voltage source
I
Independent current source
J
JFET transistor
Suffix
Multiplier
T
1e12
K
Mutual inductance
G
1e9
L
Inductor
Meg
1e6
M
MOSFET transistor
O
Lossy transmission line
K
1e3
Q
Bipolar transistor
mil
25.4e-6
R
Resistor
S
Voltage controlled switch
m
1e-3
T
Lossless transmission line
u(or μ)
1e-6
U
Uniform RC-line
n
1e-9
V
Independent voltage source
X
Subcircuit Invocation
p
1e-12
+
f
1e-15
A continuation of the previous line. The "+" is
removed and the remainder of the line is
considered part of the prior line.
Power Supply Example
20V
2N3055
60Vceo
10A
15V
*My assumptions in Blue
Simulation Types
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Transient Analysis (.tran)
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User sets starting conditions and runs the simulation for a
specified time.
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Output graphs show voltages/currents as a function of time
DC Analysis (.dc)
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Shows the DC operating point while a voltage or current source
is stepped through the specified range.
AC Analysis (.ac)
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A voltage or current symbol is the source for the circuit exitation
specified as a frequency range.
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Output graphs are a log scale (dB) voltage or current function
of frequency.
There is also “Noise”, “DC Operating Point”, and “DC Transfer Function”
LTspice Simulation
Demo capture of the following design: SimpleSupply.asc
To simplify, the transformer and rectifier are replaced with a voltage source V1.
Represent a potentiometer with two resistors and use of a “parameter”.
What makes a good power supply?
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An ideal supply is a voltage source with zero Output
Resistance
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The output voltage should be stable as Temperature changes
The output voltage should be stable if the Output Current
changes suddenly.
The output voltage should be independent of Input Power
Fluctuations
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Are all supply components operating within their limits for
voltage, current, and power dissipation?
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This really is true for any design...
Output Resistance?
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Use .DC Analysis with a current source
to vary the load
Simple Supply Output Resistance over Adjustment Range
40
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Attach 2 cursors to the output voltage
trace and measure the change in
voltage and current.
35
30
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Up/Down cursor keys moves between
traces. Right click on cursor to display
step information.
Rout = deltaV / delta I
ohms
25
Output Resistance (ohms)
20
15
10
5
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Test at multiple output voltage settings
(“.step” with parameters)
0
0
1
2
3
4
5
6
7
8
9
10
Potentiometer Setting {Pres} *1000 ohms
Why is the output resistance worse (higher) in the middle of the pot travel?
Temperature Effect on Output?
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Use .temp or “.step temp” with
.DC Analysis to see how the
output voltage changes with
temperature
From -40C to +80C, the output
shifts about 0.4V, or about 3.3mV
per degree C.
What is causing this?
(hint:PN junctions)
-40F
+80C
-40C
-4
Stable with Output Current Changes?
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Add a Current Source in “Pulse” mode to
vary load current
Use .tran Analysis
Curves show the effect of a short (90ms)
spike of 50mA in output current.
Aside from voltage droop due to non-zero
output resistance, no sign of ringing or
other instability.
Input Source Rejection?
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We know that the AC source
(transformer and rectifier)
will produce a 120Hz ripple
on the input to the series
transistor.
39mVp-p
Add a second voltage
source to simulate 120 Hz
ripple, set to 4Vp-p (just a
guess).
Use .tran to measure effect
on output. (.AC analysis
could be used, too).
Worst rejection 40dB (top)
high output voltage and
large load current (50mA).
Best rejection 65dB (bottom)
low output voltage and low
load current (1mA).
2.2mVp-p
Waveform Viewer
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Click any node to display voltage waveform (cursor is a red voltage probe)
Click and hold any node (red probe appears) then drag to another node and release when
black probe appears. Displays voltage difference between the selected nodes.
Hover over any symbol lead until current probe appears and Click. Current waveform is
displayed.
Hold Alt key and hover over a component. Click the thermometer symbol for power waveform
display.
On Waveform Display
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Cursor position appears in bottom frame.
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Drag a box and hold – Box limits appear in bottom frame.
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Drag a box and click – display zooms.
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Left click on node name – select # of cursors
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Right click on node name – cursor is attached to waveform and info box appears.
Drag cursors as desired.
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Hold Ctrl key and right click node name – Average value for displayed range is
calculated and appears.
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The FFT of any signal can be displayed: under VIEW menu
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Many more... The help is actually rather helpful!
Modified Circuit #1
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Reduce resistance “seen”
by Q1 Base by adding Q2
emitter follower.
Output resistance now
<1.5 ohms (was 37 ohms)
Still stable with output
current change.
Input source rejection is
much better: now 55dB
worst case vs. 40dB, but...
Modified Supply Output Resistance over Adjustment Range
Larger temperature
sensitivity: 5.1mV/C vs.
3.3mV/C
Likely worth the trade-off!
1.5
1
ohms
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Output Resistance (ohms)
0.5
0
0
1
2
3
4
5
6
7
8
9 10
Potentiometer Setting {Pres} *1000 ohms
What about the Transformer and Rectifier??
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The simulation runs faster as demonstrated with fewer
components.
Transformer model parameters would have to be measured
for accuracy - not given in datasheets.
Nevertheless, to model a transformer:
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draw multiple inductors, set inductance and series
resistance. Note: inductance ratio is square of turns ratio.
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Link inductors with Mutual Inductance SPICE directive,
eg. “K L1 L2 0.98” links L1 and L2 with a 0.98 coupling
coefficient.
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Note the appearance of phasing dots.
See Model: Transformer and Rectifier.asc
One Last Improved Version
Circuit Source: Bob
Pease, Troubleshooting
Analog Circuits, 1991
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Output resistance is 72
micro-ohms!
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Stable with output current
change +50mA.
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Input source rejection is better: now 58dB worst case
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Temperature sensitivity virtually undetectable: 5mV TOTAL!
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R1 sets the output current limit, ~300mA.
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Lack of output capacitor protects circuit under test - low discharge energy
.AC Frequency Analysis
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To demonstrate .AC analysis, the following model shows 2 independent
filters
Source: Hayward et al. Experimental
Methods in RF DESIGN, 2009 pg 8.14
Last Word
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LTspice is a very useful tool.
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See “LTC/LTspiceIV/examples/Educational” folder for
useful examples
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Most Linear Technology components provide “test
fixtures”, ready to run macromodels.
Like any tool, results are only as good as the inputs (models).
In SPICE, wires have zero resistance, capacitors, inductors,
and resistors are ideal.
Real components (even passives) have stray capacitance and
inductance, as well as non-linear behavior.
Any circuit design must be built and tested on the bench to
verify function.