1. MOSFETs
AIM:
To understand how MOSFETs can be used as transducer drivers
PRIOR KNOWLEDGE:
Output transducers, Current in circuits, Calculating resistor values, calculating power, silicon diodes

2. MOSFET Basics A MOSFET has 3 leads called the GATE, DRAIN and SOURCE
A MOSFET behaves like a switch
A voltage applied to the gate from some other circuit can be used to control a more powerful output transducer.
The device should be called an “n-channel MOSFET” to be completely correct.
Current comes out of the SOURCE
Current flows down the DRAIN
DRAIN
SOURCE
GATE
NOTE: Other symbols may have an arrow on the source

3. MOSFET Basics (2) Some common MOSFETs are: BUZ71A 2N7000 IRF510
NOTE: MOSFETs are static sensitive and so should always be handled with care. The metal tag is connected to the SOURCE

4. MOSFET Basics (3)
Large DRAIN current (Id)
When there is no GATE voltage the MOSFET does not conduct – it is OFF
Voltage between GATE and SOURCE allows current to flow from DRAIN to SOURCE
When the GATE-SOURCE (Vgs) voltage is greater than about 4V the MOSFET conducts and current (Id) can flow from DRAIN to SOURCE – it is ON
Vgs
NO current flows into the GATE
Even when it is turned ON, there is still NO current in the GATE, the MOSFET only works on Voltage
0 v

5. Using MOSFETs When used as a transducer driver:
The DRAIN is connected to the load e.g. the bulb
The SOURCE is connected to 0 v
The GATE is connected to the control circuit e.g. a logic circuit
When GATE voltage is about 4V, the bulb is ON
12 v
The 12v bulb is being controlled by the MOSFET, the current in the bulb flows through the MOSFET
The MOSFET conducts when the voltage between the GATE and the SOURCE is big enough
0 v

6. Using MOSFETs (2)
When used as a transducer driver with electric motors, relays or any device containing a coil a protection diode must be used
When the motor turns OFF a large backwards voltage is produced. Called “back emf”
The back emf can destroy the MOSFET
The diode limits the back emf to 0.7V which is a safe voltage and so protects the MOSFET
12 v M
Diode in reverse BIas
0 v

7. Bipolar transistors vs MOSFETs
MOSFETs require a VOLTAGE at the GATE to allow them to conduct
Bipolar transistors require a CURRENT flowing into the BASE to allow them to conduct
No current flows into the GATE
MOSFETs have a very high input resistance
Current flows into the BASE
The base resistance is not very high
Very easy to use – no calculations required
Need a base resistor.
Calculations of base resistor and gain are needed to ensure correct operation
Require about 4 v to turn them on
Cannot be used with low voltage battery operated circuits
Need 0.7V to turn them on and so can be used with low voltage battery operated circuits
Easily damaged by static
Quite robust and not easily damaged

8. Summary MOSFETs have a GATE, SOURCE and DRAIN
When a voltage is applied between the GATE and the SOURCE a large current can flow between DRAIN and SOURCE
No current flows into the base of a MOSFET
Protection diodes must be used when switching loads with a coil
Maximum ratings of current, voltage and power must be researched using online datasheets and these values are different for each transistor
Transistors can get hot if they dissipate too much power and so a heatsink might be necessary where high power output transducers are used

9. Questions
What are the advantages of using MOSFETs as transducer drivers compared to other alternatives?
Why must MOSFETs be handled with care?
Why might MOSFETs be un-suitable for some battery powered circuits?
How much current flows in to the Gate of a MOSFET?
Do MOSFETs control the Current, Voltage or Power supplied to an output transducer?

10. Answers
MOSFETs are very easy to use – no extra components or calculations are required
They are static sensitive and can be damaged by the static charge on clothes or on the skin when being handled
The Gate voltage needs to be above about 4V to turn the MOSFET on but some battery powered devices operate at less than 4V (e.g. 2 x AA battery is only 3V)
None, or at least very little and so practically none
The Current and therefore the Power. The power supply used determines the Voltage, the MOSFET simply switches the supply voltage ON or OFF and allows current to flow.