Download ECE 3155 Experiment V DC Power Supplies

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ECE 3155
Experiment V
DC Power Supplies
Rev. lpt Nov 2012
Introduction
One of the most important uses of diodes is in rectifier circuits, where they are used to convert
zero-mean waves, like sinusoids, to waves with a net dc component. A filtering circuit is used to
convert this waveform to one that has a dc component primarily. Thus, ac-excited dc power
supplies normally consist of a transformer, a rectifier bank, and a filter. The transformer adjusts
the voltage from the ac line to the appropriate voltage and provides isolation.
A power supply has been assembled for you, and is available from the laboratory TAs. The
circuit is shown in Figure 1. The filter is simply a shunt capacitor that charges to the peak
voltage applied to it. Switch SW1 allows you to connect and disconnect the transformer from the
ac power line and thus performs the function of an on/off switch. Switch SW2 enables you to see
the full wave rectified output of the transformer/bridge rectifier by disconnecting the filter
capacitor.
A brief summary of the theory behind the power supply follows. This theory assumes that the
transformer is nearly ideal with little internal resistance, that the diodes have a constant voltage
drop in the forward conduction mode of Vf = 1[V], and that the output peak ripple of the power
supply with the capacitor filter is small compared to the average value, or dc component, of the
output voltage.
ECE 3155 Experiment V – DC Power Supplies
Figure 1. DC Power Supply Module available in the lab. The capacitance C = 3000[µF], and the resistance R = 1[kΩ]. Note that RL is
supplied externally; it is not part of the power supply module. The terminals labeled “ac” on the module are connected to VS. The
terminals labeled “DC” are connected to Vout.
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ECE 3155 Experiment V – DC Power Supplies
Operation of the Power Supply The transformer in the power supply (see Figure 1) changes
the primary voltage to a secondary voltage, with a voltage ratio given by vP/vS = TR, where vP is
the RMS value of the primary winding voltage, vS is the RMS value of the secondary winding
voltage, and TR is the transformer turns ratio. Normally the transformer ratio is given, and with
knowledge of the primary voltage we can calculate the secondary voltage. In this case, we will
measure the transformer ratio early in the laboratory, and then use it throughout the rest of the
exercise.
The transformer output (the secondary voltage) serves as the input to the bridge rectifier, whose
output is a full wave rectified signal (if no capacitor is used) with peak amplitude
(1)
where vS_peak is the peak of the secondary sinusoidal voltage and Vf is the forward voltage drop
across a rectifier diode with a fairly large current passing through it. As mentioned earlier, this
voltage drop is conventionally assumed to be 1[V] for a silicon rectifier in this current range.
When the capacitor is in the circuit, it is charged to this peak voltage. It is charged only at those
times when the peak voltage of the rectifier exceeds that of the parallel combinations of the
capacitor and load. The rest of the time the transformer rectifier is disconnected from the
capacitor load, since the diodes are all reverse-biased. During this time only the capacitor is
furnishing power to the load.
If we suppose that the average current absorbed by the load is approximately constant, and that
the capacitor charging time is small compared to the time the load is absorbing current from the
capacitor, an excellent approximation for the output ripple and power supply average voltage
output can be derived from the current-voltage relationship for a capacitor. This relationship is
(2)
which can be written
(3)
We now substitute
(4)
where VDC is the average (dc) output voltage of the power supply. Next, we set
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ECE 3155 Experiment V – DC Power Supplies
(5)
where f is the line frequency. In that case, dvC can be thought of as the peak-to-peak ripple
voltage. This ripple voltage is usually called Vr, and can be found from
(6)
Your text takes a slightly different approach, but reaches the same conclusion.
Figures of Merit We now introduce some terminology that is relatively common in discussing
and characterizing dc power supplies. These definitions are useful because as we might expect,
the power supply does not behave ideally; the output voltage is not constant, but is in fact a
function of several variables.
The load regulation factor FL is a figure of merit that describes the change of output voltage as a
function of load current. Usually it is stated in percent, and is calculated as
(7)
In this equation, VDC is the dc output voltage, and the indications “no load” and “full load” refer
to measurements of VDC without and with a load. Note that this figure of merit would be zero for
an ideal power supply.
Another figure of merit for power supplies is the line regulation factor, FLINE, which describes
the change of dc output voltage resulting from a change of ac input voltage. Since power
supplies are usually expected to give the same dc output no matter what the line voltage is, it is
zero for an ideal supply. It is usually specified over the maximum range of expected ac line
voltage variation, and is given by
(8)
Here, ∆VDC is the change of the dc output voltage resulting from a change in line voltage ∆vLINE.
Voltage Regulator An electronically regulated power supply is an ordinary
transformer/rectifier/filter power supply (such as that in Figure 1) with an integrated circuit
called a voltage regulator connected to the output. The voltage regulator detects the power
supply output voltage and maintains it approximately constant despite a varying output current.
It operates by maintaining a large voltage drop between the input and output of the regulator, and
varying this voltage drop to compensate for changing input voltage. Electronic regulation of a
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ECE 3155 Experiment V – DC Power Supplies
non-regulated power supply is a simple task, and there exists a large selection of integrated
circuit voltage regulators for a wide range of output voltages and currents. After selecting a
voltage regulator with the appropriate current range and the desired output voltage, there are two
items of concern. First, the input voltage to the series regulator must always exceed the regulator
output voltage by an amount called the Drop-Out Voltage VDROP_OUT. For the power supply you
are experimenting with, this means
(9)
The second item of concern is that of total power dissipation. Since the input current to the
regulator is nearly equal to the load current, the power dissipation by the regulator is equal to the
average voltage drop across the regulator times the load current. This specifies the power
dissipation limits of the regulator and its heat sink requirements.
For regulated supplies one defines the Ripple Rejection Factor (RRF) as the ratio of output ripple
voltage to input ripple voltage. It is commonly expressed in dB. In other words, with a regulator,
the amount of ripple voltage should decrease. The RRF is a measure of how much decrease is
achieved.
WARNING! WARNING!
There are several problems that can arise during this exercise. Some of these can cause injuries,
although usually minor ones. Make sure that you know what you are doing at all times. If you do
not know what you are doing, ask for help.
WARNING # 1: The series voltage regulators can become very hot. If they become too hot,
they can burn you, or stop working, or both. It is relatively easy to show that they will dissipate a
power approximately equal to the product of the current they provide to the load times the drop
between their input and output. For the values specified in this experiment, this can be a large
amount of power. Take care not to touch the regulator when it is running. You should plan to
either connect a heat sink to the regulator before you start, or arrange to have it on only for very
short periods of time during measurements.
WARNING # 2: It is very easy to blow fuses in the ammeter in this experiment. The fuses in our
lab ammeters are expensive and are not available locally. Even if they were cheap and plentiful,
you should not blow a large number of fuses anyway, so please be careful. The typical way in
which the fuses blow is that the decade resistance box, which is used as the load resistor seen in
Figure 1, is set to zero, which shorts out the ammeter and blows the fuse immediately. Therefore,
do not set the decade box to zero!
A good working rule is to have only one person operate the decade box. Start with a nonzero
value on the decade box. Another good rule is to monitor the output voltage with the
oscilloscope, so that you can watch what is happening. Finally, many problems arise when the
ammeter is not correctly set for the type of measurement you are making:
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ECE 3155 Experiment V – DC Power Supplies
•
The dc component of a zero-mean sinusoid will be approximately zero for all decade box
settings. If you want to measure the ac component, choose the ac measurement on the
ammeter.
•
The ac component of the rectified signal will be much smaller than the dc component.
When measuring the dc component of the current, it is important to set the ammeter to
the dc mode.
Finally, it should be pointed out that many students find this to be a long experiment. Adjust the
time that you set aside to perform it accordingly.
Components Required
•
•
•
•
•
power supply module (supplied by TA): See Figure 1 for connections to ac and DC.
resistance substitution box (at most of the lab benches)
adjustable ac source (large box at a limited number of benches)
7812 voltage regulator
0.1[µF] capacitor (2)
Pre-Lab
There is no Pre-Lab for this exercise. However, be sure to read the introduction carefully,
especially the warnings!
Procedure
Be sure to use graph paper to construct all the plots indicated below. Make sure your plots are
neat and complete with axis labels and titles. Staple or otherwise connect your plots to this
handout when you turn it in.
1. Obtain a power supply module from the lab instructor. Connect its input to the adjustable ac
source, and adjust its voltage input to 110[VRMS]. Use extra care in this step, since these voltages
are high enough to be dangerous. You may feel free to assume that the meter on the adjustable ac
source is accurate, so that you do not need to measure it directly. This assumption will not affect
your results in this experiment. Measure the secondary voltage, and calculate the TR. Record
your results here.
For this step, make your measurement at the “ac” connections on the power supply module.
Also, you may find it necessary to set the oscilloscope coupling to DC to get the correct reading.
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ECE 3155 Experiment V – DC Power Supplies
2. Use the capacitor switch (SW2) to remove the filter capacitor from the circuit (“out”). After
reading the warnings in the introduction, connect a power decade resistance box to the power
supply, along with a series ammeter to measure current. Adjust the resistance to about 500[Ω]
before you connect it. Observe the full wave rectified output from the unit, and sketch it. Attach
your plot to this report. Be careful to plot the magnitudes and time accurately.
3. Reconnect the filter capacitor using switch SW2 (“in”). Adjust RL so that iL = 0.5[A]. Observe
vOUT(t) and sketch it on the same plot you constructed in step 2.
4. Vary iL from 0 to 0.5[A] and plot VDC as a function of iL. Also, plot the peak output ripple
voltage as a function of iL. On these same plots, sketch the theoretical plots that would result
from the equations derived in the introduction. Compare these results and comment on their
relationship.
5. Vary vLINE (the ac input voltage) from 85[V] to 125[V] with RL = 500[Ω], and measure VDC.
Plot the results.
6. Consider the full load current to be 0.5[A], and calculate the load regulation factor for your
power supply. Consult the introduction for the definition of the load regulation factor.
7. Given vLINE variations in the range 85[V] < vLINE < 125[V], calculate the line regulation factor.
Again, consult the introduction for the definition of the line regulation factor.
8. Set up the circuit shown in Figure 2. The 7812 is an IC series regulator with 1[A] load current
capacity, and a nominal 12[VDC] output voltage. Its VDROP_OUT is about 2[V]. The 0.1[µF]
capacitors are recommended by the manufacturer. They limit the high-frequency impedance of
the input and output circuits so that the regulator remains stable (i.e., does not oscillate).
9. Adjust vLINE to 110[VRMS] if it is not already at that setting. Measure the voltage at the input
and output of the voltage regulator, vIN_REG(t) and vOUT_REG(t), as a function of iL. Plot the dc and
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ECE 3155 Experiment V – DC Power Supplies
ac (peak) values of these quantities. (Note: vOUT_REG(t), the peak ac output ripple, will be a very
small value. Take special care to measure it accurately).
10. Adjust iL = 0.25[A] for vLINE = 110[VRMS]. Then vary vLINE from 125[VRMS] downward, while
measuring vOUT_REG(t) with the oscilloscope. Plot the ac and dc components of vOUT_REG(t). Stop
when you note a sharp decrease in vOUT_REG(t).
11. The VDC and vOUT_REG(t) peak quantities plotted in Step 9 may be compared with those
plotted in Step 4. The VDC curves from Step 9 should be considerably flatter and of much smaller
amplitude for the ac component. Compute the load regulation factor for this power supply.
12. Calculate the ripple rejection factor for the 7812 regulator.
13. Calculate the line regulation factor for the regulated supply.
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ECE 3155 Experiment V – DC Power Supplies
Figure 2. Regulated dc power supply consisting of the power supply of Figure 1 and a series voltage regulator.
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ECE 3155 Experiment V – DC Power Supplies
Questions
1. Why is a transformer used in a power supply?
2. When one refers to “isolation” with respect to a transformer, what does it mean?
3. Why is the output ripple voltage of a capacitor filtered power supply linear in output current?
4. What is the theoretical line regulation factor?
5. If the input line voltage decreases beyond a certain point the series regulator will stop
regulating. Why? Give your answer in terms of what you know at this point about the regulator.
It is not expected that you will do additional research on the regulator.
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