Power Supplies

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Power Supplies
Did you know that there are at least 4 diodes in
almost every piece of electronics that you own?
Want to know why?
Homework Barnaal 7, pg 139, Barnaal 8, pg 140
Lab simple power supplies Read pg.289-309
(bipolar transistors and amplifiers), HH handout
2
Review PN-junction
If the PN junction is reverse biased, the
depletion region widens building a large electric
field barrier across the junction However if the
depletion zone is forward biased (meaning a
voltage is applied) with the p-type positive
relative to the n-type, the depletion region
shrinks and carriers pass through the junction.
3
Review diode characteristics
Diode law
This is where the Zener diode operates
This is the barrier potential of the PN
junction (material dependent)
4
AC? DC
Power is delivered to homes and industries as
alternating current, or AC
Advantages of AC (from http//www.howstuffworks.co
m/power.htm) 1. AC suffers from significantly
less line loss than DC when transmitted over long
distances because it is easy to step it up to
extremely high voltages. 2. Large electrical
generators happen to generate AC naturally, so
conversion to DC would involve an extra
step. 3. Transformers must have alternating
current to operate, and the power distribution
grid depends on transformers. 4. It is easy to
convert AC to DC but expensive to convert DC to
AC, so if you were going to pick one or the other
AC would be the better choice of the two.
In order to use electronics powered by direct
current, DC, it is necessary to change the
negative portion of the alternating current
input. Otherwise the supply on average has a DC
or average value of zero. Since this involves
treating the negative and positive portions
differently a diode seems perfect for this
purpose.
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Half - wave rectifier
The circuit can be used as part of a power supply
(only positive portions of the AC signal)
Remember the small voltage drop due to the
barrier potential of the PN-junction.
This is the symbol for a transformer, which is
two coils coupled by an alternating magnetic
field. It is commonly used to change AC voltage.
Note that there is a waste of voltage, as the
negative portions result in no current, which
means no power out.
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Application battery charger
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Full - wave bridge rectifier
This circuit results in a waveform that has both
negative and positive portions of the AC signal
and a peak value of Vpp - 2 Vdiode. To see how
it works lets trace the current path
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Full - wave bridge rectifier (2)
On part of the cycle when the top of the
secondary is positive, current flows through the
two red diodes, with current flowing down
through the load. On part of the cycle when the
top of the secondary is negative, current flows
through the two green diodes, with current still
flowing down through the load. The drop across
the load is two diode drops less than the voltage
on the secondary coil.
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Full - wave bridge rectifier (3)
This output has an average value that is not
equal to zero, but is less than the peak value.
In order to get a current out that has a larger
voltage we need to store current for parts of
the cycle where the voltage is low. It would
also be nice if there wasnt the changes in
voltage, even at 60 Hz. Another way to think of
this is that we would like to get rid of the
time-varying part, keep the DC part we need a
LOW PASS FILTER or a simple CAPACITOR FILTER!
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Filtering to obtain DC
Half wave rectified case
Smooth out the peaks with a simple capacitor
filter
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Peak to peak ripple voltage

• Half wave rectified case

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Filter capacitor in full - wave bridge
Again, the capacitor charges up during part of
the cycle where the bridge is at a higher
voltage, until it reaches the value of the peak,
Vp-2Vdiode. This reduces ripple, the AC
portion of the waveform. With a load resistor,
the capacitor can discharge and there is some
ripple as shown by the red line. (If there is
no load then the constant blue line results.)
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Filter capacitor (2)
Any current drawn by the load resistance will
discharge the capacitor during the portions of
the cycle that the green line is less than the
blue, resulting in a near exponential decay. As
the voltage on the bridge increases it will
eventually exceed that of the discharging
capacitor, and start charging it as well as
providing current to the load.
For the full wave rectifier the discharging takes
place every 1/120 seconds and allows the ripple
ratio to be reduced by a factor of two because
the capacitor has only half as long to discharge.
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Summary filtering
Also other filtering schemes like LC-filters
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Power supply design
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Power supplies
For most electronic devices it is necessary to
provide a stable source of DC power. The ripple
remaining after filtering may be in excess of
what is tolerable. Batteries often serve this
function but they are limited in the amount of
energy available (alkaline will last a day at 100
mA, a car battery 2 hours at 1 A). Two other
problems

• Line voltage variations If the power company
  voltage fluctuates by 10, for example, this
  fluctuation also appears in the transformer
  output and ultimately in the DC voltage.
• Load current variations When the load resistance
  changes, changes occur in the current drawn from
  the transformer and through the diodes and
  filter. Because of resistance in these elements,
  increased current results in a drop in the dc
  voltage and also typically results in an increase
  in ripple.

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Unregulated DC (2)
Variations in the input AC can be in the form of
surges or sags. A two year study by Bell
Laboratories showed that most locations will
experience approximately 25 power line
disturbances/year, many sagging below 96 volts.
This is a result of power loading, and you
probably have noticed light bulbs flicker or dim.
A two year study by IEEE members Martzloff and
Hahn completed in 1970 shows that surges and
impulse voltage spikes can occur as frequently as
twice per hour in a typical residence with peak
values of 1500V to 2500V. All these variations
in voltage (they are all somewhat smoothed by the
capacitor) result in changes in the output
voltage.
Electronic circuits that greatly reduce the
fluctuations in dc voltage due to ripple, line
voltage variations, and load current variations
are called Voltage Regulators
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Electronic voltage regulators
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Three - terminal regulators
(Barnaal pg.132)
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Three-terminal regulators (2)
An example are the 7800 series fixed positive
voltage regulators, which come as a three pin
package and can be connected with a few optional
capacitors to make a regulated supply.
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Adjustable 3-terminal regulators
There are only a handful of voltages that fixed
regulators are made for. In addition if you want
to change the voltage you need to rebuild the
supply. An alternative is an adjustable supply
where you change a potentiometer (variable
resistor) to get a voltage out. The value of
VREF for a LM117-series is 1.25 V. The current
through the adjustment connection IADJ is less
than 100 mA. A typical value for R1 is 240 W.
The actual value of the output voltage is
normally tweaked with the adjustable resistor
since there are variations from chip-to-chip of
the actual values of VREF and IADJ.
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Adjustable 3-terminal regulators (2)
The rest of the parameters that you need for
design are listed in the datasheets for the
regulator. Typically a designer uses a reference
such as Horowitz and Hill to help with selection
once the performance criteria are chosen. More
recently manufacturers provide web info that
helps with design selection in their product line
(see www.national.com). Ultimately in either
case the actual circuit is based on material in
the datasheets.
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Summary regulated dc power supply

• Quality of the regulation specified according to
• Ripple regulation
• Line regulation
• Load regulation