MOSFETs AIM To understand how MOSFETs can be

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 www. pfnicholls. com

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 flows down the DRAIN GATE Current comes out of the SOURCE NOTE: Other symbols may have an arrow on the source

MOSFET Basics (2) Some common MOSFETs are: Ø BUZ 71 A Ø 2 N 7000 Ø IRF 510 NOTE: MOSFETs are static sensitive and so should always be handled with care. The metal tag is connected to the SOURCE

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

Using MOSFETs When used as a transducer driver: o The DRAIN is connected to the load e. g. the bulb o The SOURCE is connected to 0 V o The GATE is connected to the control circuit e. g. a logic circuit o When GATE – SOURCE voltage is above 3 V, the bulb is ON 12 V The 12 V 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

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” 12 V M Diode in reverse Bias The back EMF can destroy the MOSFET The diode limits the back EMF to 12. 7 V which is a safe voltage and so protects the MOSFET 0 V

The MOSFET Equation The current that flows into the Drain (ID) is related to the Gate. Source Voltage (VGS), the threshold voltage (VTH) and a property of the MOSFET, called the transconductance (g. M), that describes how well it conducts Previously VTH was assumed to be 3 V but can be more or less The circuit shown can be used to investigate the MOSFET equation The potentiometer allows the VGS to be varied and the ammeter measures the Drain current, ID

The MOSFET Equation When the Gate-Source voltage (VGS) is less than the threshold voltage (VTH) the MOSFET does not conduct When the Gate-Source voltage is above threshold voltage, the Drain current increases linearly The rate at which the Drain current increases is determined by the Transconductance (g. M) The Transconductance is a property of the individual MOSFET and is measured in Siemens (S) or milli. Siemens (m. S).

The MOSFET Equation, which is used to determine the Drain current when the MOSFET is conducting is: ID = g. M (VGS − VTH) Example: The threshold voltage of a MOSFET is 3. 0 V and the Transconductance is 0. 4 S (400 m. S). What is the Drain current when the Gate-Source voltage is 4. 5 V? Solution: ID = 0. 4 × (4. 5 − 3. 0) ID = 0. 4 × 1. 5 = 0. 6 A If no other components are limiting the current, the Drain current flowing through the MOSFET will be 0. 6 amps.

How VDS depends on VGS The circuit shown can be used to investigate the transfer characteristics of a MOSFET The potentiometer allows the Gate-Source voltage to be varied Region 1: When the Gate-Source voltage is less than the threshold voltage no current flows through the MOSFET The potential difference across the load is zero (V = I x R and I = 0 therefore V = 0) and the Drain-Source voltage equals the supply voltage

How VDS depends on VGS (sometimes called Vin) and VDS (sometimes called Vout) are measured using voltmeters. Region 2: When the Gate-Source voltage is above threshold voltage, current flows through the load resistor VDS = Vsupply − Vload The Drain-Source voltage is not close to zero and the Drain current is not zero so the MOSFET dissipates power and may get hot or be damaged.

How VDS depends on VGS The transfer characteristics of the MOSFET describe how the Drain-Source Voltage VDS depends on the Gate-Source Voltage VGS Region 3: When Gate-Source voltage is well above threshold voltage the maximum current is limited by the load resistor. All of the supply voltage is dropped across the load resistor and the Drain-Source voltage is (almost) zero. When the MOSFET is used as a transducer driver, VGS must be high enough such that VDS is close to zero.

How ID depends on VGS The circuit shown can be used to investigate the transfer characteristics of a MOSFET and is very similar to the circuit used to investigate the MOSFET equation. Region 1: VGS is less than the threshold voltage, the MOSFET does not conduct and ID is zero

How ID depends on VGS A voltmeter measures VGS and an ammeter in series with a load resistor measures ID The addition of the load resistor limits the maximum Drain current Region 2: VGS > VTH and the MOSFET conducts The Drain current depends on the transconductance of the MOSFET as described by the MOSFET equation and this part of the transfer characteristic is linear The greater the transconductance, the steeper the line.

How ID depends on VGS The transfer characteristics of the MOSFET describe how the Drain Current ID depends on the Gate. Source voltage VGS The linear section of the graph is described by the MOSFET equation Region 3: VGS is high enough to make ID large enough to make the potential difference across the load resistor the same as the supply voltage. VDS is (very close to) zero and the Drain current cannot be any greater because it is limited by the load resistor

Bipolar transistors vs MOSFET BIPOLAR TRANSISTOR 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 3 V to turn them on Cannot be used with low voltage battery operated circuits Need 0. 7 V 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

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 gate of a MOSFET • Protection diodes must be used when switching loads such as electric motors and relays (with a coil or inductance) • The MOSFET equation describes the relationship between Drain current (ID) and the gate – source voltage (VGS) • Maximum ratings of current, voltage and power must be researched using online datasheets and these values are different for each MOSFET • MOSFETS can get hot if they dissipate too much power and so a heatsink might be necessary where high power output transducers are used

Questions 1. What are the advantages of using MOSFETs as transducer drivers compared to other alternatives? 2. Why must MOSFETs be handled with care? 3. Why might MOSFETs be un-suitable for some battery powered circuits? 4. How much current flows in to the Gate of a MOSFET? 5. What is the MOSFET equation? 6. Do MOSFETs control the Current, Voltage or Power supplied to an output transducer?

Answers 1. MOSFETs are very easy to use – no extra components or calculations are required 2. They are static sensitive and can be damaged by the static charge on clothes or on the skin when being handled 3. The Gate voltage needs to be above about 3 V to turn the MOSFET on but some battery powered devices operate at less than 3 V (e. g. a AA battery is only 1. 5 V) 4. None, or at least very little and so practically none 5. ID = g. M (VGS – VTH) where VTH is nominally 3 V 6. 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.