Transcript Applications of Semiconductor devices – Switches.

Applications of
Semiconductor
devices – Switches.
Electrical and Electronic
Principles
© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.
The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a part
of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course
codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year
undergraduate programme.
The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing
the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond.
Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This
course has been designed to provide you with knowledge, skills and practical experience encountered in everyday
engineering environments.
Contents
 Mechanical Switches- Relays
 Switching Using a Thyristor- DC Supply
 Switching Using a Thyristor- DAC Supply
 Opto-isolator
 Credits
In addition to the resource below, there are supporting documents which should be used in combination with this
resource. Please see:
Green D C, Higher Electrical Principles, Longman 1998
Hughes E , Electrical & Electronic, Pearson Education 2002
Hambly A , Electronics 2nd Edition, Pearson Education 2000
Storey N, A Systems Approach, Addison-Wesley, 1998
Applications of Semiconductor
Devices- Switches
Mechanical Switches - Relays
Conventional mechanical switches have the
following characteristics:
Excellent ON resistance
Excellent OFF resistance
High drive capability
less than 0.01 typical  0
open circuit  
depending upon switch 100s
volts and 10s amps
Input output isolation
no electrical connection
between input and output
slow tenths of seconds at
best.
Speed of switching
The last parameter makes mechanical switches of no use
in computer circuitry or in power control.
Semiconductor switches are an alternative but are
limited in some of their parameters:
Poor values for:
ON resistance
OFF resistance
High drive capability
Input output isolation
What is in the favour of such devices is speed of
switching. These can be switched at 100 MHz  GHz.
Applications of Semiconductor
Devices- Switches
Consider the set up below.
Vs = 9V
Ic
L
O
A
D
Rc = 4.5k
Vout
Rb
Vin
Ib
2N2222
To determine the quality
of the ON/OFF
switching we need to use
the characteristics and
plot a load line.
Vbe
intersect on the Vce axis is at Vs = 9V
intersect on the Ic axis is at Vs/Load = 2mA
Applications of Semiconductor
Devices- Switches
2N2222 Characteristic
2.5
Ib = 10 A
Ib = 8 A
Ic (mA)
2
Ib = 6 A
1.5
1
Ib = 4 A
0.5
Ib = 2 A
Ib = 0 A
0
0
1
2
3
4
5
6
7
8
9
10
Vce (volts)
Let us assume the input can be switched between 0V and 5V
For 5V we want 10A
When the input = 0V Ib = 0A Vce = 9v Ic = .01mA R = 900k
When the input = 5V Ib = 10A Vce = 0.5v Ic = 1.9mA R = 263
This shows that the switch is far from ideal but there is
a factor of nearly 3500 between ON and OFF.
This device can only switch a couple of 10s of volts at a
few milli amps. Larger transistors can do better.
There is no electrical isolation between input and output
but switching is rapid.
FETs can be used in a similar way to switch devices ON
and OFF and on the whole have similar characteristics
and values.
Applications of Semiconductor
Devices- Switches
Switching using a Thyristor.
D.C. Supply
Ia
L
O
A
D
Rb
Vin
Vs
Vout
With no input the thyristor will
be switched off – Ia will be very
close to 0A and all of Vs will
appear across the thyristor.
If the thyristor is fired by
applying a voltage on Vin then
Vout drops to very near ground
potential and the load will
control Ia.
If Vin is removed then the
thyristor will remain on, keeping
the load powered up.
Uses – this is a latching device
and can be used for applications
such as alarms etc.
A.C. Supply.
If the D.C. is replaced by an A.C. supply then when the
thyristor is fired the load will see the positive half cycle.
When the input is removed then the thyristor will remain
on until the supply drops to 0V – i.e. the end of the
positive half cycle. We therefore have a half wave
rectifying semiconductor switch.
The real power of the thyristor for controlling AC comes
from the periodic switching of the device at different
points during the positive half cycle. This has the effect
of supplying the load with different amounts of the half
cycle. See below.
Applications of Semiconductor
Devices- Switches
Complete Half
Cycle
Thyristor
switched on early
in the half cycle
Thyristor
switched on late
in the half cycle
The sine wave areas on the second two graphs represent
the power that would be switched to the load.
Control the switching point, and you control the load
power.
The control of the switching point is achieved by using a
phase shift circuit as shown:
Vin
Vout
Vin
Vout
The amount of phase lag between the two voltages
depends upon the values of the resistor and the
capacitor.
When R is small  0 the phase lag will be small  0
When R is large   the phase lag will be large  90
If the input to the R.C. network is derived from the AC
driving the thyristor then if the output voltage is used to
fire the thyristor then we will have 100% of the positive
half wave when the angle is 0 and 50% of the half cycle
when the angle is 90.
Applications of Semiconductor
Devices- Switches
See below
90 phase lag.
Load
voltage
Supply
voltage
Firing
voltage
This gives us control over only half of the wave. In order to
have total control over the full half-wave we must use a
centre tap transformer – as shown below:
R
C
L
O
A
D
Rs
D
The centre
tap
transformer
will generate
two sine
waves 180
of phase
with each
other.
When R is small the firing voltage is in phase with the
supply and the thyristor fires as soon as the positive half
cycle begins. Load receives maximum power
When R is large the firing angle approaches 180 and the
thyristor is turned on just as the positive half cycle
ends. Load receives zero power.
To control the whole of the cycle – positive and negative
cycles we use two thyristors back to back. This is called a
triac.
Symbol
Structure
©André Karwath aka Aka’ available via
http://en.wikipedia.org/wiki/File:Optoisolator_%28aka%29.jpg under a
Creative Commons Attribution-Share
Alike 2.5 Generic license.
Opto-isolator
In electronics, an opto-isolator (or optical
isolator, optocoupler or photocoupler) is a
device that uses a short optical transmission
path to transfer a signal between elements
of a circuit, typically a transmitter and a
receiver, while keeping them electrically isolated — since the signal
goes from an electrical signal to an optical signal back to an electrical
signal, electrical contact along the path is broken.
A common implementation involves an LED and a light sensor,
separated so that light may travel across a barrier but electrical
current may not. When an electrical signal is applied to the input of
the opto-isolator, its LED lights, its light sensor then activates, and
a corresponding electrical signal is generated at the output.
Applications of Semiconductor
Devices- Switches
Unlike a transformer, the opto-isolator
allows for DC coupling and generally
provides significant protection from
serious over voltage conditions in one
circuit affecting the other.
With a photodiode as the detector, the output current is proportional
to the amount of incident light supplied by the emitter. The diode can
be used in a photovoltaic mode or a photoconductive mode.
In photovoltaic mode, the diode acts like a current source in parallel
with a forward-biased diode. The output current and voltage are
dependent on the load impedance and light intensity.
In photoconductive mode, the diode is connected to a supply voltage,
and the magnitude of the current conducted is directly proportional
to the intensity of light.
Applications of Semiconductor
Devices- Switches
An opto-isolator can also be constructed using a small incandescent
lamp in place of the LED; such a device, because the lamp has a much
slower response time than an LED, will filter out noise or half-wave
power in the input signal. In so doing, it will also filter out any audio- or
higher-frequency signals in the input. It has the further disadvantage,
of course, (an overwhelming disadvantage in most applications) that
incandescent lamps have finite life spans. Thus, such an unconventional
device is of extremely limited usefulness, suitable only for applications
such as science projects.
The optical path may be air or a dielectric waveguide. The transmitting
and receiving elements of an optical isolator may be contained within a
single compact module, for mounting, for example, on a circuit board; in
this case, the module is often called an optoisolator or opto-isolator.
Applications of Semiconductor
Devices- Switches
The photosensor may be a
photocell, phototransistor, or an
optically triggered SCR or
Triac. Occasionally, this device
will in turn operate a power
relay or contactor.
Applications
Among other applications, opto-isolators can help cut down on ground
loops and block voltage spikes.
One of the requirements of the MIDI (Musical Instrument Digital
Interface) standard is that input connections must be opto-isolated.
Applications of Semiconductor
Devices- Switches
Opto-isolators are used to protect hospital patients from accidental
electric shock. Patients with IV's in their bodies are particularly
susceptible, sometimes succumbing to 'carpet shock.'
They are used to isolate low-current control or signal circuitry from
transients generated or transmitted by power supply and high-current
control circuits. The latter are used within motor and machine control
function blocks.
The classical computer mouse is a common application, using infrared
emitter led's and phototransistors to form optocouplers. They are
used to translate the mousewheel velocity into digital motion
information. The principle of operation does not require to utilize
infrared light, though this frequency range is somehow resistant
against interference with visible light.
Applications of Semiconductor
Devices- Switches