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ECE 241L * Intro to EE Lab
Lab 5 : DC to DC Power Conversion
Objective: To simulate DC to DC power converters using LTSpice and examine performance.
Equipment: Computer with LTSpice and Matlab installed.
Introduction:
Batteries are a convenient source for providing DC power at a specific nominal voltage
level. To provide voltages at various levels the circuits shown below may be used. These circuits
are called DC-to-DC converters; basically transforming an input DC voltage level to another DC
level (at very high efficiency, i.e. there are minimal power losses).
Fig. 1. Converter #1
Fig. 2. Converter #2
Fig. 3. Converter #3
Circuit description:
Each of the three circuits contains exactly the same elements: an inductor, a capacitor, the
input voltage source (which may be a battery), a load resistance, a voltage controlled switch
driven by a pulse generator and a diode. Each circuit provides a different function of voltage
transformation, which you will discover below.
The resistance in the circuit simply models a load, that is, the device to which we would
like to deliver power to, and is thus generally referred to as the load resistance. A diode is a
semiconductor device which functions as a switch. It is ON when the voltage across it is of one
polarity (known as being forward biased) and OFF in the opposite polarity (known as being
reversed biased). Current through a diode can only flow in one direction.
The other switch in the circuits is a voltage controlled switch. The state of the switch
(that is, whether it is ON or OFF) is determined by the voltage across a pair of control terminals.
If this voltage is above a certain threshold level 𝑉𝑡 , the switch will be ON, otherwise it is OFF.
The switch itself is modeled within LTSpice as a bi-valued resistance. The resistance when the
switch is ON is denoted by 𝑅𝑜𝑛 and will need to varied from its default along with that of 𝑉𝑡 .
The final element to be discussed is the voltage pulse generator. This device is simply a
voltage source which produces a periodic rectangular output voltage waveform as shown below.
The period of the waveform is denoted 𝑇, and the length of the pulse is denoted 𝑇𝑜𝑛 . The
magnitude of the pulse height is denoted 𝑉𝑚 , going from 0 volts to 𝑉𝑚 volts.
𝑇
𝑉𝑚
𝑇𝑜𝑛
Fig. 4. Periodic pulse waveform used to drive the voltage controlled switch
The ratio of the ON time to the period is called the duty ratio, D, and may range from zero to
one.
𝐷=
𝑇𝑜𝑛
𝑇
The output voltage level of each of the converters is controlled by value of the duty ratio. A
straightforward analysis of the converters leads to the following steady state input to output
voltage conversion ratios:
A:
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛
=𝐷
B:
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛
1
= 1−𝐷
C:
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛
𝐷
= − 1−𝐷
Task:
For each of the converters:
1) Construct the circuit in LTSpice paying attention to the following points:
a) Use an ideal diode.
b) Specify the pulse source to have a magnitude of 5V and period of 20𝜇𝑠. The duty
ratio will be adjusted by you a number of times, as detailed below.
c) A couple of the default property values of the voltage controlled switch need to be
changed: set 𝑅𝑜𝑛 = 0.0001𝛺 and 𝑉𝑡 = 2.5𝑉. These changes will be reflected in the
model statement for the switch. These changes are necessary for the circuits to
operate properly.
2) Set up LTSpice to perform a transient simulation. Since we want the converter output
voltage to reach a steady state, you will need to simulate for a sufficiently long time for
this to happen. You can start with a simulation time of the order of 150𝑚𝑠 and adjust as
necessary. You can determine when steady state has been reached by observing the
output voltage plot.
3) Our aim is to determine the steady state output voltage for a range of five or more duty
ratio values ranging from 0.1 to 0.9. Note that the duty ratio specification can be changed
by adjusting the 𝑇𝑜𝑛 length of the pulse source. For example, to achieve a duty ratio of
𝐷 = 0.1 set 𝑇𝑜𝑛 =2µs or for 𝐷 = 0.5 set 𝑇𝑜𝑛 =10µs. (Of course, this assumes that 𝑇=20µs,
as required).
4) Tabulate for your five or more duty ratio values the voltage conversion ratios,
𝑉𝑜𝑢𝑡
𝑉𝑖𝑛
. Be
sure to make clear which converter circuit your results came from.
5) Plot your results using Matlab ( x-axis: duty ratio, y-axis: input to output voltage
conversion ratio).
6) Determine which of the three functions given above (A, B or C) model the conversion
ratio and plot it on the same graph as your simulated points.
7) Write a conclusion for your report.