MOSFET as a Switch
The MOSFET as a Switch
We saw previously, that the N-channel, Enhancement-mode MOSFET operates using a positive input
voltage and has an extremely high input resistance (almost infinite) making it possible to interface with nearly
any logic gate or driver capable of producing a positive output. Also, due to this very high input
(Gate) resistance we can parallel together many different MOSFETs until we achieve the
current handling limit required. While connecting together various MOSFETs may enable us to switch high currents
or high voltage loads, doing so becomes expensive and impractical in both components and circuit board space. To
overcome this problem Power Field Effect Transistors or Power FET's were developed.
We now know that there are two main differences between field effect transistors, depletion-mode
only for JFET's and both enhancement-mode and depletion-mode for MOSFETs. In this tutorial we will look at using the
Enhancement-mode MOSFET as a Switch as these transistors require a positive gate voltage to turn "ON" and
a zero voltage to turn "OFF" making them easily understood as switches and also easy to interface with logic gates.
The operation of the enhancement-mode MOSFET can best be described using its I-V characteristics curves
shown below. When the input voltage, ( VIN ) to the gate of the transistor
is zero, the MOSFET conducts virtually no current and the output voltage, ( VOUT )
is equal to the supply voltage VDD. So the MOSFET is "fully-OFF" and in its "cut-off"
MOSFET Characteristics Curves
The minimum ON-state gate voltage required to ensure that the MOSFET remains fully-ON when carrying
the selected drain current can be determined from the V-I transfer curves above. When VIN
is HIGH or equal to VDD, the MOSFET Q-point moves to point A along the load line. The drain
current ID increases to its maximum value due to a reduction in the channel resistance.
ID becomes a constant value independent of VDD, and is
dependent only on VGS. Therefore, the transistor behaves like a closed switch but the channel
ON-resistance does not reduce fully to zero due to its RDS(on) value, but gets very small.
Likewise, when VIN is LOW or reduced to zero, the MOSFET Q-point moves
from point A to point B along the load line. The channel resistance is very high so the transistor acts like an open circuit and
no current flows through the channel. So if the gate voltage of the MOSFET toggles between two values, HIGH and LOW the MOSFET
will behave as a "single-pole single-throw" (SPST) solid state switch and this action is defined as:
1. Cut-off Region
Here the operating conditions of the transistor are zero input gate voltage ( VIN ),
zero drain current ID and output voltage VDS = VDD
Therefore the MOSFET is switched "Fully-OFF".
- The input and Gate are grounded ( 0v )
- Gate-source voltage less than threshold voltage VGS < VTH
- MOSFET is "fully-OFF" ( Cut-off region )
- No Drain current flows ( ID = 0 )
- VOUT = VDS = VDD = "1"
- MOSFET operates as an "open switch"
Then we can define the "cut-off region" or "OFF mode" of a MOSFET switch as being, gate voltage,
VGS < VTH and ID = 0.
For a P-channel Enhancement MOSFET, the Gate potential must be more positive with respect to the Source.
2. Saturation Region
In the saturation or linear region, the transistor will be biased so that the maximum amount of gate voltage is applied
to the device which results in the channel resistance RDS(on) being as small as possible with maximum
drain current flowing through the MOSFET switch. Therefore the MOSFET is switched "Fully-ON".
- The input and Gate are connected to VDD
- Gate-source voltage is much greater than threshold voltage VGS > VTH
- MOSFET is "fully-ON" ( saturation region )
- Max Drain current flows ( ID = VDD / RL )
- VDS = 0V (ideal saturation)
- Min channel resistance RDS(on) < 0.1Ω
- VOUT = VDS = 0.2V ( RDS.ID )
- MOSFET operates as a "closed switch"
Then we can define the "saturation region" or "ON mode" of a MOSFET switch as gate-source voltage,
VGS > VTH and ID = Maximum.
For a P-channel Enhancement MOSFET, the Gate potential must be more negative with respect to the Source.
By applying a suitable drive voltage to the gate of an FET, the resistance of the drain-source channel,
RDS(on) can be varied from an "OFF-resistance" of many hundreds of kΩ's, effectively
an open circuit, to an "ON-resistance" of less than 1Ω, effectively a short circuit. We can also drive the MOSFET to turn
"ON" faster or slower, or pass high or low currents. This ability to turn the power MOSFET "ON" and "OFF" allows the
device to be used as a very efficient switch with switching speeds much faster than standard bipolar junction transistors.
An example of using the MOSFET as a switch
In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch
a simple lamp "ON" and "OFF" (could also be an LED). The gate input voltage VGS
is taken to an appropriate positive voltage level to turn the device and therefore the lamp either fully "ON",
( VGS = +ve ) or at a zero voltage level that turns
the device fully "OFF", ( VGS = 0 ).
If the resistive load of the lamp was to be replaced
by an inductive load such as a coil, solenoid or relay a "flywheel diode" would be required in parallel
with the load to protect the MOSFET from any self generated back-emf.
Above shows a very simple circuit for switching a resistive load such as a lamp or LED. But when using
power MOSFETs to switch either inductive or capacitive loads some form of protection is required to prevent the MOSFET
device from becoming damaged. Driving an inductive load has the opposite effect from driving a capacitive load. For example,
a capacitor without an electrical charge is a short circuit, resulting in a high "inrush" of current and when we remove the
voltage from an inductive load we have a large reverse voltage build up as the magnetic field collapses, resulting in an
induced back-emf in the windings of the inductor.
For the power MOSFET to operate as an analogue switching device, it needs to be switched between its
"Cut-off Region" where VGS = 0 and its "Saturation Region" were
VGS(on) = +ve. The power dissipated in the MOSFET
( PD ) depends upon the current flowing through the channel
ID at saturation and also the "ON-resistance" of the channel given as
RDS(on). For example.
Lets assume that the lamp is rated at 6v, 24W and is fully "ON", the standard MOSFET has a channel
"ON-resistance" ( RDS(on) ) value of 0.1ohms. Calculate the power dissipated
in the MOSFET switch.
The current flowing through the lamp is calculated as:
Then the power dissipated in the MOSFET will be given as:
You may think, well so what!, but when using the MOSFET as a switch to control DC motors or high
inrush current devices the "ON" channel resistance ( RDS(on) ) is very
important. For example, MOSFETs that control DC motors, are subjected to a high in-rush current when the motor first begins
to rotate, because the motors starting current is only limited by the very low resistance value of the motors windings.
Then a high RDS(on) channel resistance value would simply result
in large amounts of power being dissipated and wasted within the MOSFET itself resulting in an excessive temperature rise,
which if not controlled could result in the MOSFET becoming very hot and damaged due to a thermal overload.
A lower value RDS(on) on the other hand, is also a desirable parameter as it
helps to reduce the channels effective saturation voltage
( VDS(sat) = ID x RDS(on) )
across the MOSFET. Power MOSFETs generally have a RDS(on) value of less than 0.01Ω.
One of the main limitation of a MOSFET is the maximum current it can handle. So the RDS(on)
parameter is an important guide to the switching efficiency of the MOSFET and is simply the ratio of
VDS / ID when the transistor is turned "ON". When using a MOSFET or
any type of field effect transistor for that matter as a solid-state switching device it is always advisable to select ones that
have a very low RDS(on) value or at least mount them onto a suitable heatsink to help reduce
any thermal runaway and damage. Power MOSFETs used as a switch generally have surge-current protection built into their design,
but for high-current applications the bipolar junction transistor is a better choice.
Power MOSFET Motor Control
Because of the extremely high input or gate resistance that the MOSFET has, its very fast switching speeds and
the ease at which they can be driven makes them ideal to interface with op-amps or standard logic gates. However, care must be taken
to ensure that the gate-source input voltage is correctly chosen because when using the MOSFET as a switch the device
must obtain a low RDS(on) channel resistance in proportion to this input gate voltage.
Low threshold type power MOSFETs may not switch "ON" until a least 3V or 4V has been applied to its gate
and if the output from the logic gate is only +5V logic it may be insufficient to fully drive the MOSFET into saturation.
Using lower threshold MOSFETs designed for interfacing with TTL and CMOS logic gates that have thresholds as low as 1.5V
to 2.0V are available.
Power MOSFETs can be used to control the movement of DC motors or brushless stepper motors directly from computer
logic or by using pulse-width modulation (PWM) type controllers. As a DC motor offers high starting torque and which is also proportional
to the armature current, MOSFET switches along with a PWM can be used as a very good speed controller that would provide smooth and
quiet motor operation.
Simple Power MOSFET Motor Controller
As the motor load is inductive, a simple flywheel diode is connected across the inductive load to
dissipate any back emf generated by the motor when the MOSFET turns it "OFF". A clamping network formed by a zener diode in
series with the diode can also be used to allow for faster switching and better control of the peak reverse voltage and
For added security an additional silicon or zener diode D1 can also
be placed across the channel of a MOSFET switch when using inductive loads, such as motors, relys, solenoids, etc, for
suppressing overvoltage switching transients and noise giving extra protection to the MOSFET switch if required. Resistor
R2 is used as a pull-down resistor to help pull the TTL output voltage down to 0V
when the MOSFET is switched "OFF".
P-channel MOSFET Switch
Thus far we have looked at the N-channel MOSFET as a switch were the MOSFET is placed between
P-channel MOSFET Switch
the load and the ground. This also allows the gate drive or switching signal to be referenced to ground (low-side switching).
But in some applications we require the use of P-channel enhancement-mode MOSFET were the load is connected
directly to ground. In this instance the MOSFET switch is connected between the load and the positive supply rail (high-side
switching) as we do with PNP transistors.
In a P-channel device the conventional flow of drain current is in the negative direction so a negative gate-source
voltage is applied to switch the transistor "ON". This is achieved because the P-channel MOSFET is "upside down" with its source
terminal tied to the positive supply +VDD. Then when the switch goes LOW, the MOSFET turns
"ON" and when the switch goes HIGH the MOSFET turns "OFF".
This upside down connection of a P-channel enhancement mode MOSFET switch allows us to connect it in series
with a N-channel enhancement mode MOSFET to produce a complementary or CMOS switching device as shown across a dual supply.
Complementary MOSFET Motor Controller
The two MOSFETs are configured to produce a bi-directional switch from a dual supply with the motor connected
between the common drain connection and ground reference. When the input is LOW the P-channel MOSFET is switched-ON as its gate-source
junction is negatively biased so the motor rotates in one direction. Only the positive +VDD
supply rail is used to drive the motor.
When the input is HIGH, the P-channel device switches-OFF and the N-channel device switches-ON as its gate-source
junction is positively biased. The motor now rotates in the opposite direction because the motors terminal voltage has been reversed
as it is now supplied by the negative -VDD supply rail. Then the P-channel MOSFET is used to
switch the positive supply to the motor for forward direction (high-side switching) while the N-channel MOSFET is used to switch
the negative supply to the motor for reverse direction (low-side switching).
There are a variety of configurations for driving the two MOSFETs with many different applications. Both
the P-channel and the N-channel devices can be driven by a single gate drive IC as shown. However, to avoid cross conduction with
both MOSFETs conducting at the same time across the two polarities of the dual supply, fast switching devices are required to
provide some time difference between them turning "OFF" and the other turning "ON". One way to overcome this problem is to drive
both MOSFETs gates separately. This then produces a third option of "STOP" to the motor when both MOSFETs are "OFF".
Complementary MOSFET Motor Control Table
|MOSFET 1||MOSFET 2||Motor Function|
|OFF||OFF||Motor Stopped (OFF)|
|ON||OFF||Motor Rotates Forward|
|OFF||ON||Motor Rotates Reverse|
Please note that it is important that no other combination of inputs are allowed at the same time as
this may cause the power supply to be shorted out, as both MOSFETs, FET1 and
FET2 could be switched "ON" at the same time, ( fuse = bang! ).