LTSpice Tutorial: Part 3
What we have learned
so far enables us to simulate most designs. This
LTSpice Tutorial digs deeper into circuit analysis with
FIG 1 shows the jig of
the LTC3878 external FET buck converter with a
resistive load of 80mOhms. It can be downloaded
Running the simulation
shows the output settles at 1.2V. Right click on the
plot pane and select Add Plot Pane. In the schematic
window hold down the ALT key and left click over the
resistor Rload. This will plot the instantaneous
power dissipation in the plot window as shown in FIG
2 with the units on the y axis.
If it appears in the
incorrect plot pane, left click on the plot icon and
drag it to the correct pane.
In the plot window,
hold down the CTRL key and left click over the plot
icon (circled above in FIG 2) to bring up details of
the average power dissipation, averaged over the
time span of the plot. Selecting a section of the
plot zooms in on the waveform and repeating CTRL left
click shows the average power of that subsection of
LTSpice Tutorial 3: Generating the
To generate an
efficiency report, from the menu bar select Simulate
-> Edit Simulation cmd and select 'Stop simulating
if steady state is detected'. Rerun the simulation.
Then click on View -> Efficiency Report -> Show on
Schematic. The efficiency report will appear on the
schematic, as shown in FIG 3.
Note: the efficiency
report can only be generated when there is only one
voltage source (which is assumed to be the input)
and either one current source or a load called Rload
(which is assumed to be the load).
Repeating the above
steps removes the efficiency report from the
It is also worthwhile
returning to Simulate -> Edit Simulation cmd and
unchecking the box 'Stop Simulating if steady state
LTSpice Tutorial 3: Using Maths
functions to Calculate Efficiency
There is no reason why
you cannot use the maths functions in LTSpice to
examine efficiency and indeed this is an effective
way of measuring the efficiency of a multi output
A behavioral voltage
source gives an output voltage according to a
mathematical function. If we set up a behavioral
voltage source to produce a voltage equal to the
product of the input voltage and input current, we
can then plot the input power over time. Likewise we can set
up a behavioral voltage source to produce a voltage
equal to the output powers of each converter. We can
then set up a behavioral voltage source to produce a
voltage equal to the ratio of output power to input
power that then gives us a real time efficiency
Consider the circuit
in FIG 3a. This is a simple circuit consisting of
two 5V linear regulators driven from a 10V input,
each providing a load current of 10mA. B1 is a
behavioral voltage source whose output voltage is
equal to Vin * Iin. B5 and B2 produce voltages (P1
and P2) equal to the output
powers of each regulator. B3 outputs a voltage equal
to the function (100*(P1+P2)/Pin) - in other words
Setting up behavioral
voltage sources can be tricky. It is best to
construct the main circuit first, run the
simulation, probe the desired voltages and currents
and see what LTSpice calls each voltage plot. The behavioral
components need to correspond with these plot names.
The circuit of FIG 3a
can be downloaded here:
Behavioral Voltage Sources
LTSpice Tutorial 3: Simulating a
We are now going to
add a transient load to the output via a switch.
icon and select 'sw' and place the switch on the
schematic. Add the extra load components as shown in
FIG 4, including a PULSE voltage source transitioning
from 0V to 5V at 1.7ms of duration 1ms to control
The switch now needs
to be defined. The best way of defining a switch is
to go into the help files (by hitting <F1>) and
searching for 'switch', then selecting Voltage
Controlled Switch. This will bring up the dialogue
shown in FIG 5
Highlight the line
beginning '.model' and copy it. Back in the
schematic editor, press the Spice Directive button
and paste the text in as a Spice directive. Change
the Spice directive to:
.model MySwitch SW(Ron=1m
to characterise the
switch with an ON resistance of 1mOhm, an OFF
resistance of 1MOhm and a threshold voltage of 1V.
We don't need the other characteristics.
Place the text
anywhere on the schematic.
To relate the switch
to the .model we have just defined, right click on
the switch symbol in the schematic window. In the
Value field, enter MySwitch to relate the .model
definition with the switch, as shown in FIG 6
Run the simulation.
The output voltage and current through the extra
resistor should look like FIG 7.
Now the above
simulation has been conducted with ideal components.
This is normally quite acceptable until it comes to
measuring the impact of the effective series
resistance (ESR) of the output capacitor, C2. The
circuit is designed to support a 15A load, so large
current surges will be flowing from the inductor to
the output capacitor and any ESR in C2 will manifest
itself as ripple on the output voltage. With a 1.2V
output, this ripple could quite easily violate the
Right clicking over
capacitor C2 brings up the parameter table for the
capacitor. We can either enter an ESR of our choice
(a typical ESR of a tantalum capacitor is about
0.5Ohms), or we can press the Select Capacitor
button to select one of the library capacitors in
LTSpice. LTSpice contains libraries of many passive
and active components and these can be selected by
right clicking over the component of choice and
browsing the libraries.
Running the simulation
with an ESR of 0.5 Ohms shows the effect on the
Right clicking over
most passive components allows us to add parasitic
elements or select from the component tables already
stored in LTSpice.
On a note of design, if the ESR is a problem, adding
several capacitors in parallel reduces the effective
ESR. A simulation with 6x100uF capacitors, each with
an ESR of 0.5Ohms reduced the ripple from 329mV to
In the same way we can
add a parasitic ESR to a capacitor, we can also add
a series resistance to an inductor, just by right
clicking over the inductor and entering a value in
the Series Resistance box.
It is easy to lose
track of what parasitics have been included in the
circuit, so pressing <CTRL> <ALT><SHIFT> H
highlights on the schematic what components have
parasitic elements. Press <F9> to undo.
We are now going to
look at simulating a transformer based design.
Download the circuit in FIG 9:
LTC3872 flyback converter
transformer can be made up of 2 inductors, together
with a SPICE directive coupling the inductors.
Insert an inductor of
50uH in the primary and 2uH in the secondary. To
make the two inductors into a transformer, we need
to tell LTSpice that they are linked via a mutual
inductance, K. Insert a SPICE directive using the
button and insert
K1 L1 L2 1
This creates a mutual
inductance, K1, between inductors L1 and L2 of value
1 (they have perfect coupling).
You will notice that
the dots have been added to the transformer. Make
sure they have the correct polarity for a flyback
transformer. Change the value of the primary
inductor to 50uH and the secondary inductor to 2uH.
You should now have a
circuit similar to FIG 10
It is worth noting
that the ratio of inductances on a transformer is
equal to the square of the turns ratio. Thus
the transformer above has an inductance ratio of
25:1, giving a turns ratio of 5:1.
Also, if n inductors
of the same value and wound on the same core are
placed in series, the combined inductance is n2
x the value of a single inductor.
If the transformer
datasheet specifies a leakage inductance, this can
be modelled as an additional primary inductor, but
this inductor is not included in the SPICE directive
detailing the mutual inductance. Thus any energy in
this leakage inductance is not coupled into the
secondary winding during the flyback cycle and
manifests itself as ringing.
Want to know more?
Tutorial: Part 4