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 2002Hambly A , Electronics 2nd Edition, Pearson Education 2000Storey N, A Systems Approach, Addison-Wesley, 1998

Applications of Semiconductor Devices- Switches

Mechanical Switches - Relays

Conventional mechanical switches have the following characteristics:

The last parameter makes mechanical switches of no use in computer circuitry or in power control.

Excellent ON resistance less than 0.01 typical 0

Excellent OFF resistance open circuit High drive capability depending upon switch 100s

volts and 10s amps

Input output isolation no electrical connection between input and output

Speed of switching slow tenths of seconds at best.

Semiconductor switches are an alternative but are limited in some of their parameters: Poor values for:

ON resistanceOFF resistanceHigh drive capabilityInput output isolation

What is in the favour of such devices is speed of switching. These can be switched at 100 MHz GHz.


Consider the set up below.

Vs = 9V

Rb

Ic

Ib

Vout

Vbe

LOAD

Vin

2N2222

Rc = 4.5k

To determine the quality of the ON/OFF switching we need to use the characteristics and plot a load line.

intersect on the Vce axis is at Vs = 9Vintersect on the Ic axis is at Vs/Load = 2mA


2N2222 Characteristic

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7 8 9 10

Vce (volts)

Ic (m

A)

Ib = 10 A

Ib = 8 A

Ib = 6 A

Ib = 4 A

Ib = 2 A

Ib = 0 A

Let us assume the input can be switched between 0V and 5VFor 5V we want 10AWhen the input = 0V Ib = 0A Vce = 9v Ic = .01mA R = 900kWhen the input = 5V Ib = 10A 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.


Switching using a Thyristor.

D.C. Supply

Rb

Ia

Vout

LOAD

Vin

Vs

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:

VinVout

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 0When 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.


See below90 phase lag.

Firing voltage

Supplyvoltage

Load 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:

L O A D

R

C

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 powerWhen 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.

Structure

Symbol

Opto-isolator

In electronics, an opto-isolator (or opticalisolator, 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.

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*The text on slides 16 – 20 is adapted from http://en.wikipedia.org/wiki/Opto-isolator and is available under a Creative Commons Attribution-ShareAlike License


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.



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.



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.

ApplicationsAmong 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.



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.




This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

© 2009 University of Wales Newport

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applications of semiconductor devices- switches

Education

semiconductor switches

complete half cycle

positive half cycle

load power

speed of switching

switching point

vin vout

periodic switching