University of Nottingham

Electromechanical devices MM2EMD

Lecture 7 – Transistors - Switching high voltage things on with a low voltage

Dr. Roderick MacKenzie [email protected] Summer 2015

@rcimackenzie Released under Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

2 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

3 Roderick MacKenzie MM2EMD Electromechanical devices Think back to lecture 1.

Smart Electronic Simple Electrical Circuits (low Circuits (high voltage) voltage) d r a a W

e

d s

. e J . n S o J

e v a D •This lecture is making low voltage electronics control high voltage electrical devices such as motors. 4 Roderick MacKenzie MM2EMD Electromechanical devices Think about an AND gate chip

And gate Biro

•Look at the tiny thin pins which are used to carry current in and out of the chip.

•These pins can supply 25 mA @ 5V at the most. 5

Roderick MacKenzie MM2EMD Electromechanical devices But why is this? i

•If we take the top off the chip h p o

with acid. G

● Look how much small the actual chip is and look at

the tiny bond wires (25 mA @ 5V max!!!) 6

Roderick MacKenzie MM2EMD Electromechanical devices Now think about this motor.

•It needs 10 Amps at 500 V to run.

d r a a W

e d

. J . S

•It will also need cable as

J B

M thick as my finger to carry C the current. 7

Roderick MacKenzie MM2EMD Electromechanical devices Using chips to run large voltage/current loads.

•Now imagine trying to run this motor which needs 500V @ 10 Amps from this tiny chip chat can only deliver 5V @ 25 mA.

8

Roderick MacKenzie MM2EMD Electromechanical devices Using chips to run large voltage/current loads.

•It would not work – the chip would just get hot and melt because it can not deliver enough voltage/current.

9

Roderick MacKenzie MM2EMD Electromechanical devices Voltage/current amplifier

•So if we want to use our chip to run a motor, we need some type of voltage/current amplifier component.

500V/10 A

5V/25 mA

Amplifier

(Notice this is different from a transformer) 10 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

11 Roderick MacKenzie MM2EMD Electromechanical devices One such component is the relay

•A relay is a an electromechanical switch

An electromagnet Switch

•And this is how it works..... 12 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

Relay

+ + - -

5 V/25 mA 500V / 10 A •Low voltage side •High voltage side

13 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•When the 5 V supply is turned on current flows in the low voltage side of the relay. This current could come from a chip. Relay

+ + - -

5 V/25 mA 500V / 10 A

14 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•The electromagnet becomes energized

Relay

+ + - -

5 V/25 mA 500V / 10 A

15 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•The magnetic field pulls the switch shut.

Relay

+ + - -

5 V/25 mA 500V / 10 A

16 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•Current flows in the high voltage circuit.

Relay

+ + - -

5 V/25 mA 500V / 10 A

17 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•And the motor turns

Relay

+ + - -

5 V/25 mA 500V / 10 A

18 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•When the 5V supply is turned off...

Relay

+ + - -

5 V/25 mA 500V / 10 A

19 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•When the 5V supply is turned off...

Relay

+ + - -

5 V/25 mA 500V / 10 A

20 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•The magnetic field disappears and the motor turns off.

Relay

+ + - -

5 V/25 mA 500V / 10 A

21 Roderick MacKenzie MM2EMD Electromechanical devices Relay example

•The magnetic field disappears and the motor turns off.

Relay

+ -

5 V/25 mA 500V / 10 A

22 Roderick MacKenzie MM2EMD Electromechanical devices A real relay looks like this:

23 Roderick MacKenzie MM2EMD Electromechanical devices Advantages of using relays for switching things on and off

•You get physical isolation of both circuits. Important when you are dealing with very high voltages.

•Relays were the primary way to turn high voltages on/off before the advent of the transistor in the 1960s – they not so bad at this.

24 Roderick MacKenzie MM2EMD Electromechanical devices The drawbacks of using relays for switching

•You have to keep that coil energized if you want to keep your device on.

•This uses a lot of power.

•They are mechanical devices and therefore slow.

•They are full of copper and cost a lot (a few pounds.)

•They are big. 25 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

26 Roderick MacKenzie MM2EMD Electromechanical devices Transistors

•You can think of a transistor as being just like a relay except they have only one control wire.

Relay Transistor

Control Input Input

Control

Control Output Output

27

Roderick MacKenzie MM2EMD Electromechanical devices You can think of a transistor like this.... The Transistor Man

Output (Emitter)

Input (collector)

Control (base) After Horowitz and Hill 28 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The Transistor Man

Output (Emitter)

Input (collector)

Control After Horowitz and Hill (base) 29 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The Transistor Man

Output (Emitter)

Input (collector)

Control (base) After Horowitz and Hill30 Roderick MacKenzie MM2EMD Electromechanical devices Transistors

Collector Collector Collector

Base Base Base

Emitter Emitter Emitter

1. No base 2. Small base 3. Big base current – no current – small current – big output current output current output current 31

Roderick MacKenzie MM2EMD Electromechanical devices Transistors

•Transistors look like this

•Little black beads (although sometimes silver boxes)

•They always have three legs.

•Invented in June 30th 1948

o t s i s n a r John Bardeen, William T Shockley and Walter Brattain 32 Roderick MacKenzie MM2EMD Electromechanical devices Advantages of Transistors

•No moving parts so they won't break – very long lifetime.

•Don't need as much control current to operate as a relay.

•Cheep as chips!

o t s i s n a r T

33 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – Different types of transistors

•There are lots of different types of transistors – too many to learn about in this course.

•I have decided to teach you about the three of the most useful ones.

34 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – Different types of transistors

•There are lots of different types of transistors – too many to learn about in this course.

•I have decided to teach you about the three of the most useful ones.

35 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

36 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•It's called an NPN transistor because it's made of three layers of material, one with negative charge, one with positive charge and one with negative charge.

Collector (input) Collector - n-type Base Base p-type (control) + - n-type Emitter

Emitter (output) 37 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•This is how you would use one in 20V a circuit...... Light bulb.

C B

1V E

0V 38 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•Put a small current into the 20V Base.

C B

1V E

0V 39 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•The transistor switches on 20V

C B

1V E

0V 40 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•A large current flows between 20V the Collector and Emitter.

C B

1V E

0V 41 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•When you stop feeding the 20V base current, no more current flows between the Collector and the Emitter.

C B

1V E

0V 42 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•And the light 20V bulb will turn off.

C B

1V E

0V 43 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•The base current could come 20V from a chip!

•Now we can power any sized device using a decision made form a tiny chip we want. C B

E

0V

44 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT)

•In summary: 20V

The NPN Bipolar Junction Transistor will turn ON when a positive voltage is supplied to the Base. C B

1V E

0V

45 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

46 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

Emitter Emitter

p-type + Base Base - n-type Collector + p-type

Collector

47 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•The PNP transistor is just like 20V the NPN transistor. E •Except it turns ON when the B input is switched OFF by connecting it to 0V. C

•And switched OFF when the input is connected to a positive voltage.

0V 48 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•If you connect the base to 0V 20V allowing a small current to flow out of the transitor. E B

C

0V 49 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•This turns the transistor on 20V

E B

C

0V 50 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•Then a large current is turned 20V on between the emitter and collector. E B •Thus your light bulb will turn on. C

0V 51 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•When the switch is opened... 20V

E B

C

0V 52 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•Remove the base current and 20V the transistor will turn off. E B

C

0V 53 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The PNP Bipolar Junction Transistor (BJT)

•In summary the the PNP 20V transistor will turn ON when 0V is applied to the input. E B •Which is the opposite of how the NPN transistor works. C

0V 54 Roderick MacKenzie MM2EMD Electromechanical devices NPN/PNP Bipolar Junction Transistors summary

•We have two transistors the NPN that turns ON when a positive voltage is applied

•And then PNP which turns ON when 0V is applied. 20V NPN 20V PNP B E

C B C

1V E 0V 0V 55 Roderick MacKenzie MM2EMD Electromechanical devices Advantages and disadvantages of BJT transitors

•Advantages •They can turn on and off very quickly – good for audio amplifiers and (Radio Frequency) RF.

•Low cost.

•Disadvantages •They continually need a base current to keep them turned on.

•Not so good for switching high currents i.e. running motors.

56 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

57 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•Good for high currents i.e. running motors.

•MOSFETs consists of two contacts which are made + of a material with a positive charge. 20 V

•Between these to contacts + is an insulator – no charge can flow through this layer. 58 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•On the edge of our MOSFET we attach two metal plates.

•These act like a + capacitor.

20 V +

59 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•We attach these to an external circuit and turn it on.... + 1 V + -

20 V +

60 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•Charge starts to flow into the capacitor. + 1 V + -

20 V +

61 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•The plates charge up.....

-

- + + - 1 V - + - + - + - + - + - 20 V - + -

62 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•Notice we now have lots of positive charge on the surface of the insulator. -

- + + - 1 V - + - + - + - + - + - 20 V - + -

63 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•And have a continuous layer of charge from one terminal of the MOSFET to the other. -

•The transistor can now - + + - conduct current. 1 V - + - + - + - + - + - 20 V - + -

64 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

-

- + + - 1 V - + - + - + - + - + - 20 V - + -

65 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•If we open the switch the charge will gradually leave the device (eventually). -

- + + - 1 V - + - + - + - + - + - 20 V - + -

66 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•We attach these to an external circuit and turn it on.... + 1 V + -

20 V +

67 Roderick MacKenzie MM2EMD Electromechanical devices MOSFET (Mattel Oxide Semiconductor Field Effect Transistor)

•MOSFETs are very good for driving motors. + 1 V + -

20 V +

68 Roderick MacKenzie MM2EMD Electromechanical devices The problem with MOSFETs

•The problem with MOSFETs is that they are in effect a capacitor

- - +

- + - + + - + - + - - + - - + - + - + - + - + - + - + + + - - +

- + - + - -

69

Roderick MacKenzie MM2EMD Electromechanical devices The problem with MOSFETs

•And capacitors take a long time to charge up.

•Especially if you are driving it from a tiny chip that can't supply much current. - + - + - + - + - + - + - + + - + - + -

•This means they will turn off and on really really slowly in fact too slowly to do anything useful...... 70 Roderick MacKenzie MM2EMD Electromechanical devices Circuit symbol for a MOSFET

•Notice it looks a bit Input like a capacitor

Control wire

Output

71 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Turning things on and off

•Relays •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary 72 Roderick MacKenzie MM2EMD Electromechanical devices Question:

•Q: What type of transistors turn on and off very quickly??

73 Roderick MacKenzie MM2EMD Electromechanical devices Question:

•Q: What type of transistors turn on and off very quickly??

•A: Bipolar Junction transistor

74 Roderick MacKenzie MM2EMD Electromechanical devices Two types of transistor

Bipolar Junction MOSFET: Slow to turn on transistors: Fast / not very and off/ But good for good for high currents motors. PNP - - + + - + - + - - + - + + - - + NPN •May be we can combine these - transistors to make an optimum + circuit to switch high current - devices on and off...... 75 Roderick MacKenzie MM2EMD Electromechanical devices Yes, we can and the circuit looks like this.... a push pull pair circuit...

Load Push pull pair.

NPN

MOSFET transistor PNP - notice the plate.

76 Roderick MacKenzie MM2EMD Electromechanical0 V devices Push pull pair – turning the MOSFET ON

•If we attach a positive voltage to the base of an NPN transistor it will turn ON.

•If we attach a positive voltage to the base of a PNPV transistor it will turn OFF. +

NPN ON

1 V OFF PNP

Roderick MacKenzie MM2EMD Electromechanical0 V devices77 Push pull pair – turning the MOSFET ON

•This will allow charge to flow through the NPN transistor, to the gate of the MOSFET V +

NPN ON

1 V OFF PNP

78 Roderick MacKenzie MM2EMD Electromechanical0 V devices Push pull pair – turning the MOSFET ON

•And the MOSFET will turn ON!

V +

NPN ON

ON

1 V OFF PNP

79 Roderick MacKenzie MM2EMD Electromechanical0 V devices Push pull pair – turning the MOSFET OFF

•If we attach a 0 Volts to an NPN transistor it will turn OFF.

•If we attach 0 Volts to a PNP transistor it will turnV ON. +

NPN OFF

ON

ON PNP

Roderick MacKenzie MM2EMD Electromechanical0 V devices80 Push pull pair – turning the MOSFET OFF

V +

NPN OFF

ON

1 V ON PNP

Roderick MacKenzie MM2EMD Electromechanical0 V devices81 Push pull pair – turning the MOSFET OFF

V +

NPN OFF

OFF

1 V ON PNP 0 V 82 Roderick MacKenzie MM2EMD Electromechanical0 V devices82 Notice...

•The current to charge and discharge our MOSFET does not come from the chip it comes directly from the power supply. V •Thus the speed of + charging is not limited by the current the chip can supply.

0 V 83 Roderick MacKenzie MM2EMD Electromechanical devices Push-pull pair Push pull pair •This now means that we can now use any digital circuit such as a microcomputer to turn of big loads such as a motor FAST. h c s o b s a c u L

84 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

85 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture

•No recap of last lecture :)

•Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs

•Push pull pairs to drive MOSFETs

•One last thing

•Summary

86 Roderick MacKenzie MM2EMD Electromechanical devices