Darlington Transistor Pair

The Darlington Transistor named after its inventor, Sidney Darlington is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together. The Emitter of one transistor is connected to the Base of the other to produce a more sensitive transistor with a much larger current gain being useful in applications where current amplification or switching is required.

Darlington Transistor pairs can be made from two individually connected Bipolar Transistors or a one single device commercially made in a single package with the standard: Base, Emitter and Collector connecting leads and are available in a wide variety of case styles and voltage (and current) ratings in both NPN and PNP versions.

As we saw in our Transistor As A Switch tutorial, as well as being used as an amplifier, the bipolar junction transistor, (BJT) can be made to operate as an ON-OFF switch as shown.

Bipolar Transistor as a Switch

When the base of the NPN transistor is grounded (0 volts) and no base current, Ib flows, no current flows from the emitter to the collector and the transistor is therefore switched "OFF". If the base is forward biased by more than 0.7 volts, a current will flow from the emitter to the collector and the transistor is said to be switched "ON". When operated in these two modes, the transistor operates as a switch.

The problem here is that the transistors base needs to be switched between zero and some large, positive value for the transistor to become saturated at which point an increased base current, Ibflows into the device resulting in collector current Ic becoming large while Vce is small. Then we can see that a small current on the base can control a much larger current flowing between the collector and the emitter.

The ratio of collector current to base current ( β ) is known as the current gain of the transistor. A typical value of β for a standard bipolar transistor may be in the range of 50 to 200 and varies even between transistors of the same part number. In some cases where the current gain of a single transistor is too low to directly drive a load, one way to increase the gain is to use a Darlington pair.

A Darlington Transistor configuration, also known as a "Darlington pair" or "super-alpha circuit", consist of two NPN or PNP transistors connected together so that the emitter current of the first transistor TR1 becomes the base current of the second transistor TR2. Then transistor TR1 is connected as an emitter follower and TR2 as a common emitter amplifier as shown below.

Also note that in this Darlington pair configuration, the collector current of the slave or control transistor, TR1 is "in-phase" with that of the master switching transistor TR2.

Basic Darlington Transistor Configuration

Using the NPN Darlington pair as the example, the collectors of two transistors are connected together, and the emitter of TR1 drives the base of TR2. This configuration achieves β multiplication because for a base current ib, the collector current is β.ib where the current gain is greater than one, or unity and this is defined as:

But the base current, IB2 is equal to transistor TR1 emitter current, IE1 as the emitter of TR1 is connected to the base of TR2. Therefore:

Then substituting in the first equation:

Where β1 and β2 are the gains of the individual transistors.

This means that the overall current gain, β is given by the gain of the first transistor multiplied by the gain of the second transistor as the current gains of the two transistors multiply. In other words, a pair of bipolar transistors combined together to make a single Darlington transistor pair can be regarded as a single transistor with a very high value of β and consequently a high input resistance.

Darlington Transistor Example No1

Two NPN transistors are connected together in the form of a Darlington Pair to switch a 12V 75W halogen lamp. If the forward current gain of the first transistor is 25 and the forward current gain of the second transistor is 80. Ignoring any voltage drops across the two transistors, calculate the maximum base current required to switch the lamp fully-ON.

Firstly, the current drawn by the lamp will be equal to the collector current of the second transistor, then:

Using the equation above, the base current is given as:

Then we can see that a very small base current of only 3.0mA, such as that supplied by a digital logic gate or the output port of a micro-controller, can be used to switch the 75 Watt lamp "ON" and "OFF".

If two identical bipolar transistors are used to make a single Darlington device then β1 is equal to β2and the overall current gain will be given as:

Generally the value of β2 is much greater than that of 2β, in which case it can be ignored to simplify the maths a little. Then the final equation for two identical transistors configured as a Darlington pair can be written as:

Identical Darlington Transistors

Then we can see that for two identical transistors, β2 is used instead of β acting like one big transistor with a huge amount of gain. Darlington transistor pairs with current gains of more than a thousand with maximum collector currents of several amperes are easily available. For example: the NPN TIP120 and its PNP equivalent the TIP125.

The advantage of using an arrangement such as this, is that the switching transistor is much more sensitive as only a tiny base current is required to switch a much larger load current as the typical gain of a Darlington configuration can be over 1,000 whereas normally a single transistor stage produces a gain of about 50 to 200.

Then we can see that a darlington pair with a gain of 1,000:1, could switch an output current of 1 ampere in the collector-emitter circuit with an input base current of just 1mA. This then makes darlington transistors ideal for interfacing with relays, lamps and motors to low power microcontroller, computer or logic controllers as shown.

Darlington Transistor Applications

The base of the Darlington transistor is sufficiently sensitive to respond to any small input current from a switch or directly from a TTL or 5V CMOS logic gate. The maximum collector current Ic(max)for any Darlington pair is the same as that for the main switching transistor, TR2 so can be used to operate relays, DC motors, solenoids and lamps, etc.

One of the main disadvantage of a Darlington transistor pair is the minimum voltage drop between the base and emitter when fully saturated. Unlike a single transistor which has a saturated voltage drop of between 0.3v and 0.7v when fully-ON, a Darlington device has twice the base-emitter voltage drop (1.2 V instead of 0.6 V) as the base-emitter voltage drop is the sum of the base-emitter diode drops of the two individual transistors which can be between 0.6v to 1.5v depending on the current through the transistor.

This high base-emitter voltage drop means that the Darlington transistor can get hotter than a normal bipolar transistor for a given load current and therefore requires good heat sinking. Also, Darlington transistors have slower ON-OFF response times as it takes longer for the slave transistorTR1 to turn the master transistor TR1 either fully-ON or fully-OFF.

To overcome the slow response, increased voltage drop and thermal disadvantages of a standardDarlington Transistor device, complementary NPN and PNP transistors can be used in the same cascaded arrangement to produce another type of Darlington transistor called a Sziklai Configuration.

Sziklai Transistor Pair

The Sziklai Darlington Pair, named after its Hungarian inventor George Sziklai, is a complementary or compound Darlington device that consists of separate NPN and PNPcomplementary transistors connected together as shown below.

This cascaded combination of NPN and PNP transistors has the advantage that the Sziklai pair performs the same basic function of a Darlington pair except that it only requires 0.6v for it to turn-ON and like the standard Darlington configuration, the current gain is equal to β2 for equally matched transistors or is given by the product of the two current gains for unmatched individual transistors.

Sziklai Darlington Transistor Configuration

We can see that the base-emitter voltage drop of the Sziklai device is equal to the diode drop of a single transistor in the signal path. However, the Sziklai configuration can not saturate to less than one whole diode drop, i.e. 0.7v instead of the usual 0.2v.

Also, as with the Darlington pair, the Sziklai pair have slower response times than a single transistor. Sziklai pair complementary transistors are commonly used in push-pull and class AB audio amplifier output stages allowing for one polarity of output transistor only. Both the Darlington and Sziklai transistor pairs are available in both NPN and PNP configurations.

Darlington Transistor IC's

In most electronics applications it is sufficient for the controlling circuit to switch a DC output voltage or current "ON" or "OFF" directly as some output devices such as LED's or displays only require a few milliamps to operate at low DC voltages and can therefore be driven directly by the output of a standard logic gate.

However as we have seen above, sometimes more power is required to operate the output device such as a DC motor than can be supplied by an ordinary logic gate or micro-controller. If the digital logic device cannot supply sufficient current then additional circuitry will be required to drive the device.

One such commonly used Darlington transistor chip is the ULN2003 array. The family of darlington arrays consist of the ULN2002A, ULN2003A and the ULN2004A which are all high voltage, high current darlington arrays each containing seven open collector darlington pairs within a single IC package.

Each channel of the array is rated at 500mA and can withstand peak currents of up to 600mA making it ideal for controlling small motors or lamps or the gates and bases of high power semiconductors. Additional suppression diodes are included for inductive load driving and the inputs are pinned opposite the outputs to simplify the connections and board layout.

The ULN2003A Darlington Transistor Array

The ULN2003A is a inexpensive unipolar darlington transistor array with high efficiency and low power consumption making it useful for driving a wide range of loads including solenoids, relays DC Motor's and LED displays or filament lamps. The ULN2003A contains seven darlington transistor pairs each with an input pin on the left and an output pin opposite it on the right as shown.

ULN2003A Darlington Transistor Array

The ULN2003A Darlington driver has an extremely high input impedance and current gain which can be driven directly from either a TTL or +5V CMOS logic gate. For +15V CMOS logic use the ULN2004A and for higher switching voltages up to 100V it is better to use the SN75468 Darlington array.

When an input (pins 1 to 7) is driven "HIGH" the corresponding output will switch "LOW" sinking current. Likewise, when the input is driven "LOW" the corresponding output switches to a high impedance state. This high impedance "OFF" state blocks load current and reduces leakage current through the device improving efficiency.

Pin 8, (GND) is connected to the loads ground or 0 volts, while pin 9 (Vcc) connects to the loads supply. Then any load needs to be connected between +Vcc and an output pin, pins 10 to 16. For inductive loads such as motors, relays and solenoids, etc, pin 9 should always be connected to Vcc.

The ULN2003A is capable of switching 500mA (0.5A) per channel but if more switching current capability is required then both the Darlington pairs inputs and outputs can be paralleled together for higher current capability. For example, input pins 1 and 2 connected together and output pins 16 and 15 connected together to switch the load.

Darlington Transistor Summary

The Darlington Transistor is a high power semiconductor device with individual current and voltage ratings many times higher than a conventional small signal junction transistors.

The DC current gain values for standard high power NPN or PNP transistors are relatively low, as low as 20 or even less, compared to small signal switching transistors. This means that large base currents are required to switch a given load.

The Darlington arrangement uses two transistors back to back, one of which is the main current carrying transistor, while the other being a much smaller "switching" transistor provides the base current to drive the main transistor. As a result, a smaller base current can be used to switch a much larger load current as the DC current gains of the two transistors are multiplied together. Then the two transistor combination can be regarded as one single transistor with a very high value of β and consequently a high input resistance.

As well as standard PNP and NPN Darlington transistor pairs, complementary Sziklai Darlington transistors are also available which consist of separate matching NPN and PNP complementary transistors connected together within the same Darlington pair to improve efficiency. Also Darlington arrays such as the ULN2003A are available which allow high power or inductive loads such as lamps, solenoids and motors to be safely driven by microprocessor and micro-controller devices in robotic and mechatronic type applications.

We can summarise this transistors tutorial section as follows:

• The Bipolar Junction Transistor (BJT) is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased.
• There are two main types of bipolar junction transistors, the NPN and the PNP transistor.
• Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow.
• The arrow in a transistor symbol represents conventional current flow.
• The most common transistor connection is the Common Emitter (CE) configuration but Common Base (CB) and Common Collector (CC) are also available.
• Requires a Biasing voltage for AC amplifier operation.
• The Base-Emitter junction is always forward biased whereas the Collector-Base junction is always reverse biased.
• The standard equation for currents flowing in a transistor is given as:  IE = IB + IC
• The Collector or output characteristics curves can be used to find either Ib, Ic or β to which a load line can be constructed to determine a suitable operating point, Q with variations in base current determining the operating range.
• A transistor can also be used as an electronic switch between its saturation and cut-off regions to control devices such as lamps, motors and solenoids etc.
• Inductive loads such as DC motors, relays and solenoids require a reverse biased "Flywheel" diode placed across the load. This helps prevent any induced back emf's generated when the load is switched "OFF" from damaging the transistor.
• The NPN transistor requires the Base to be more positive than the Emitter while the PNP type requires that the Emitter is more positive than the Base.

Field Effect Transistor Tutorial

• Field Effect Transistors, or FET's are "Voltage Operated Devices" and can be divided into two main types: Junction-gate devices called JFET's and Insulated-gate devices called IGFET´sor more commonly known as MOSFETs.
• Insulated-gate devices can also be sub-divided into Enhancement types and Depletion types. All forms are available in both N-channel and P-channel versions.
• FET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches.
• The input impedance of the MOSFET is even higher than that of the JFET due to the insulating oxide layer and therefore static electricity can easily damage MOSFET devices so care needs to be taken when handling them.
• When no voltage is applied to the gate of an enhancement FET the transistor is in the "OFF" state similar to an "open switch".
• The depletion FET is inherently conductive and in the "ON" state when no voltage is applied to the gate similar to a "closed switch".
• FET's have much higher current gains compared to bipolar junction transistors.
• The most common FET connection is the Common Source (CS) configuration but Common Gate (CG) and Common Drain (CD) configurations are also available.
• MOSFETS can be used as ideal switches due to their very high channel "OFF" resistance, low "ON" resistance.
• To turn the N-channel JFET transistor "OFF", a negative voltage must be applied to the gate.
• To turn the P-channel JFET transistor "OFF", a positive voltage must be applied to the gate.
• N-channel depletion MOSFETs are in the "OFF" state when a negative voltage is applied to the gate to create the depletion region.
• P-channel depletion MOSFETs, are in the "OFF" state when a positive voltage is applied to the gate to create the depletion region.
• N-channel enhancement MOSFETs are in the "ON" state when a "+ve" (positive) voltage is applied to the gate.
• P-channel enhancement MOSFETs are in the "ON" state when "-ve" (negative) voltage is applied to the gate.

The Field Effect Transistor Chart

Biasing of the Gate for both the junction field effect transistor, (JFET) and the metal oxide semiconductor field effect transistor, (MOSFET) configurations are given as:

Differences between a FET and a Bipolar Transistor

Field Effect Transistors can be used to replace normal Bipolar Junction Transistors in electronic circuits and a simple comparison between FET's and Transistors stating both their advantages and their disadvantages is given below.

Below is a list of complementary bipolar transistors which can be used for the general–purpose switching of low-current relays, driving LED's and lamps, and for amplifier and oscillator applications.

Complementary NPN and PNP Transistors

We saw previously, that the N-channel, Enhancement-mode MOSFET (e-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 in parallel 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 where 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, or e-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 "OFF" operating within its "cut-off" region.

MOSFET Characteristics Curves

The minimum ON-state gate voltage required to ensure that the MOSFET remains "ON" when carrying the selected drain current can be determined from the v-i transfer curves above. When VINis HIGH or equal to VDD, the MOSFET Q-point moves to point A along the load line. The drain currentID 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 itsRDS(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 currentID and output voltage VDS = VDD. Therefore for an enhancement type MOSFET the conductive channel is closed and the device is switched "OFF".

Cut-off Characteristics

• The input and Gate are grounded ( 0v ) • Gate-source voltage less than threshold voltage VGS < VTH • MOSFET is "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" when using an e-MOSFET as a 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 for the enhancement type MOSFET the conductive channel is open and the device is switched "ON".

Saturation Characteristics

• The input and Gate are connected to VDD • Gate-source voltage is much greater than threshold voltage VGS > VTH • MOSFET is "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 due to RDS(on) • MOSFET operates as a low resistance "closed switch"

Then we can define the saturation region or "ON mode" when using an e-MOSFET as a 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.

When using the MOSFET as a switch we can 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 load either "ON", (VGS = +ve ) or at a zero voltage level that turns the device "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.

Then we can summarise the switching characteristics of both the N-channel and P-channel type MOSFETS in the following table.

Note that unlike the N-channel MOSFET whose gate terminal must be made more positive (attracting electrons) than the source to allow current to flow through the channel, the conduction through the P-channel MOSFET is due to the flow of holes. That is the gate terminal of a P-channel MOSFET must be made more negative than the source and will only stop conducting (cut-off) until the gate is more positive than the source.

So for the enhancement type power MOSFET to operate as an analogue switching device, it needs to be switched between its "Cut-off Region" where VGS = 0 (or VGS = -ve) 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 asRDS(on). For example.

MOSFET as a Switch Example No1

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 switching device.

The current flowing through the lamp is calculated as:

Then the power dissipated in the MOSFET will be given as:

You may be sat there thinking, well so what!, but when using the MOSFET as a switch to control DC motors or electrical loads with high inrush currents the "ON" Channel resistance ( RDS(on) ) between the drain and the source 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.

As the basic power relationship is: P = I2R, 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 and will therefore operate at a cooler temperature. Power MOSFETs generally have a RDS(on) value of less than 0.01Ωwhich allows them to run cooler, extending their operational life span.

One of the main limitations when using a MOSFET as a switching device is the maximum drain current it can handle. So the RDS(on) parameter is an important guide to the switching efficiency of the MOSFET and is simply given as the ratio of VDS / ID when the transistor is switched "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 drop-out time.

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, relays, solenoids, etc, for suppressing over voltage 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 the load and the ground. This also allows for the MOSFET's gate drive or switching signal to be referenced to ground (low-side switching).

P-channel MOSFET Switch

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

Please note it is important that there are no other combination of inputs allowed at the same time as this may cause the power supply to become shorted out, as both MOSFETS, FET1 and FET2 could be switched "ON" together resulting in: ( fuse = bang! ), be warned.