Download Lecture 9: Limiting and Clamping Diode Circuits. Voltage Doubler

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project

Document related concepts

Immunity-aware programming wikipedia , lookup

Ground loop (electricity) wikipedia , lookup

Three-phase electric power wikipedia , lookup

Spark-gap transmitter wikipedia , lookup

Variable-frequency drive wikipedia , lookup

Electrical ballast wikipedia , lookup

Flexible electronics wikipedia , lookup

History of electric power transmission wikipedia , lookup

Power inverter wikipedia , lookup

Pulse-width modulation wikipedia , lookup

Islanding wikipedia , lookup

Electrical substation wikipedia , lookup

Ohm's law wikipedia , lookup

Transistor wikipedia , lookup

Integrating ADC wikipedia , lookup

Capacitor wikipedia , lookup

Metadyne wikipedia , lookup

Current source wikipedia , lookup

Triode wikipedia , lookup

Power electronics wikipedia , lookup

Alternating current wikipedia , lookup

Stray voltage wikipedia , lookup

Schmitt trigger wikipedia , lookup

Power MOSFET wikipedia , lookup

Semiconductor device wikipedia , lookup

Voltage regulator wikipedia , lookup

Voltage optimisation wikipedia , lookup

Resistive opto-isolator wikipedia , lookup

Switched-mode power supply wikipedia , lookup

Surge protector wikipedia , lookup

Mains electricity wikipedia , lookup

Buck converter wikipedia , lookup

Rectifier wikipedia , lookup

Network analysis (electrical circuits) wikipedia , lookup

Diode wikipedia , lookup

Opto-isolator wikipedia , lookup

Transcript
Whites, EE 320
Lecture 9
Page 1 of 8
Lecture 9: Limiting and Clamping Diode
Circuits. Voltage Doubler. Special Diode Types.
We’ll finish up our discussion of diodes in this lecture by
consider a few more applications. We’ll discuss limiting and
clamping circuits for diodes as well as voltage doubling circuits.
Voltage Limiting Circuits
These types of circuits are used to “cap” voltages between preset
limits. These are useful as voltage protection circuitry or as
signal “conditioning.”
Examples of such circuits are shown in text Figure 4.33:
© 2016 Keith W. Whites
Whites, EE 320
Lecture 9
Page 2 of 8
A simple signal conditioning example is a circuit with the
following transfer function:
Then one would see this output voltage vO for this particular
input voltage vI:
8
5
vI
vO
6
1
t
-3
A circuit with ideal diodes can be designed to realize the above
transfer function from a combination of the concepts shown
above in Fig. 4.33:
R
+
Ideal
vI
5V
-
+
Ideal vO
-
Whites, EE 320
Lecture 9
Page 3 of 8
Clamped Capacitor Circuits
An idealized circuit of this type in shown below:
(Fig. 4.34b)
There are three important things to note about this circuit:
1. The ideal D keeps vO  0 .
2. C charges only when vI  0 . Without a load, there is no
other path for current.
3. The vC polarity is positive as shown above.
With these insights, let’s look at a specific example to illustrate
the operation of this circuit. Consider this input voltage:
(Fig. 4.34a)
The capacitor C in Fig. 4.34b will eventually charge completely
so that vC = +6 V. In that case, the lowest output voltage will be
“clamped” to zero. The output voltage will appear as:
Whites, EE 320
Lecture 9
Page 4 of 8
(Fig. 4.34c)
Hence, this is called a clamped capacitor circuit. Without the
diode present in this circuit, the capacitor would not retain any
net charge per period so it would never “charge up” to 6 V.
Note that here we are looking at the steady state response. It
may take a few periods for the capacitor to completely charge.
We’re not looking at the transient response.
There are two applications of the clamped capacitor circuit
discussed in the text.
(a) Pulse width modulation detector. PWM is used for
motor speed control, for example. The width of the pulse
contains the information.
To demodulate the signal, one AC couples to give zero
time average voltage (i.e., 0 VDC). The signal is then
passed through a clamped capacitor circuit to give a
Whites, EE 320
Lecture 9
Page 5 of 8
well-defined DC component, then through a low pass
filter to extract the DC.
This DC voltage is the time average value, which
changes depending on the width of the pulses (if the
period is constant, as assumed).
(b) Combined clamped capacitor with peak rectifier. This
is also called a voltage doubler circuit.
+
C1
Vpcos( t)
D1
-
D2
+
vD1 C2
-
+
vO
-
Clamped
capacitor
Half-cycle peak
rectifier
Ignoring the transient behavior when the input voltage is
first applied, vD1 is:
2Vp
vD1
0
t
This voltage is fed to a half-cycle peak rectifier (with
R   ) yielding the output voltage:
vO
2Vp
0
t
Whites, EE 320
Lecture 9
Page 6 of 8
It’s obvious now why this is called a voltage doubler
circuit.
Special Diode Types
1. Schottky barrier diode. Often just called a “Schottky
diode.” (Used in Laboratory #1 and in the NorCal 40A in EE
322.)
These are formed from a metal and an n-doped
semiconductor. The big difference from a silicon diode is a
smaller forward-bias voltage drop of approximately 0.2 V.
Also, because all conduction current in a Schottky diode is
carried by majority carriers (electrons) there is little to no
junction capacitance due to the absence of minority carrier
charge accumulation in the vicinity of the depletion region.
Because of this, one would expect the switching speeds of the
Schottky diodes to be faster than silicon diodes, for example
2. Varactor. A reversed biased diode acting as a voltagecontrolled capacitance. (Used in the NorCal 40A in EE 322.)
Whites, EE 320
Lecture 9
Page 7 of 8
To understand the operation of the varactor, recall that in the
pn junction:
This separated charge region acts as a capacitance. As shown
in the text, the junction capacitance can be expressed as
C j0
Cj 
(Similar to 3.49),(1)
1  VR V0
It is readily apparent from this equation that as VR changes,
so does Cj. (This model is used in Spice.)
+++
+++
+++
+++
+++
+++
+++
3. Photodiodes. This is a reversed biased pn junction
illuminated by light:
When the pn junction is exposed to incident light in the
correct frequency band(s), the incident photons can break
covalent bonds in the depletion region thus generating
electron-hole pairs. These are swept away from the junction
Whites, EE 320
Lecture 9
Page 8 of 8
by the electric field in the depletion region with e- to the n
region and holes to the p region.
Thus a reverse bias current has been generated. This is called
a photocurrent.
+
+
+
+
+
+
+
+
+
4. Light Emitting Diode (LED). This is the reverse of the
photodiode. In the LED, a pn junction is forward biased:
When electron-hole recombination occurs, light can be given
off in certain types of semiconductors such as GaAs.