©2002 Fairchild Semiconductor Corporation

Application Note 7503 Rev. A1

turns off, unlike the thyristor family of power semiconductors,

which must be either externally or naturally (internally) com-

mutated.

The on-state voltage drop or resistance characteristic of a

conductivity-modulated FET is markedly different from that

of a standard power MOSFET, and is similar to that of a

thyristor family member, the SCR. There is an offset voltage

component (typically 0.6V) due to the p-n junction on the

drain side, and a somewhat nonlinear resistive component,

both of which are in series between the drain and source

terminals. This series arrangement results in a highly

nonlinear equivalent resistance, unlike the linear resistive

The structure of the conductivity-modulated FET operates

during its turn on just as a standard FET does, hence its

turn-on speed is very similar to that of a standard FET. With

its high input impedance and its short propagation delay, the

turn-on transition of the conductivity-modulated FET, as well

as the standard power FET, is easily controlled by the gate

driving circuit. This characteristic allows the designer the

ability to control EMI and RPI generation easily. With other

power semiconductors, it may be necessary to employ elab-

orate circuit schemes to limit rapidly rising in-rush currents.

A significant characteristic that must be considered in power

switching applications is that of turn-off speed. The internal

action that makes the conductivity-modulated FET such a

silicon-efficient device also makes it an inherently slower

device during turn-off. The injection of the minority carriers

during the on-state conduction of current results in these

carriers being present at the moment of turn-off. Without any

way of removing these carriers by external means, they must

recombine within the structure itself before the device can

revert to its fully off-state condition. The quantity of these

carriers and how fast they

can deplete themselves

determines the turn-off switching speed of the conductivity-

modulated

FET.

This

process

of

recombination

is

considerably slower than the simple discontinuance of

majority carrier flow by which the standard power FET turns

off. Hence, again, the conductivity-modulated FET is an

inherently slower device. Its turn-off speed lies somewhere

between the performance of a thyristor and that of a bipolar

transistor.

The

final

characteristic

that

makes

the

conductivity-

modulated FET different from a conventional FET is the

variance

of

on-state

voltage

with

temperature.

The

characteristic of the conductivity-modulated FET is similar to

FET has a positive temperature coefficient such that on high-

system designer must take this characteristic into consider-

ation when the heat sink is being designed for the system.

It is these similarities and differences that make the conduc-

tivity-modulated FET a unique member of the family of

power-semiconductor switching devices. Applications of this

alternative power switching device invariably make use of

one or more of its unique characteristics.

Applications

Automotive Ignition

An application that can take advantage of the low drive-

power capability of the conductivity-modulated FET is the

electronic automotive ignition system. In Figure 2, the control

IC takes the signal from the pickup coil located in the

distributor and regulates the current through the ignition coil.

At the proper time, the IC removes base drive from the

bipolar transistor, which all systems currently employ as their

coil driver. This removal of base drive allows the transistor to

shut off which, in turn, causes a rapid decrease in the

ignition-coil

primary

current.

As

the

primary

current

decreases to zero, the energy stored in the field surrounding

the primary is transferred to the secondary coil. The

secondary coil, consisting of many more turns than the

primary, transforms this energy into a higher voltage,

resulting in a spark being generated in the cylinder. The

control IC determines when this spark occurs, so as to

derive usable power. With the use of a bipolar transistor, it is

estimated that approximately two-thirds of the power

dissipation that occurs in the control IC is the result of the

need to be able to drive the required base current of the

ignition output transistor. The high-impedance input of the

conductivity-modulated FET virtually eliminates the base-

current drive dissipation of the control IC.

With improved silicon usage, the conductivity-modulated

FET brings to power semiconductor switching devices the

die size necessary to attain the required voltage and current-

handling capabilities of the electronic ignition. This smaller-

sized die makes possible smaller modules, whether they be

hybrid or standard PC-based systems, than those currently

implemented with bipolar-transistor technology.

Brushless DC Motors

Another emerging application that can make use of

conductivity-modulated FETs is the emerging field of

brushless DC motors. In this class of application, the solid-

state devices are used to electronically switch the voltage to

the multiplicity of windings that are employed. The motor

consists of an armature that has a number of N and S poles

consisting of high-strength permanent magnets. The stator

is made up of the multiplicity of windings that were

TABLE 1. CONDUCTIVITY-MODULATED FET

CHARACTERISTICS

Voltage Gated

Small gate power required. Similar

to standard power MOSFET.

Turn Off

When gate drive is removed...

Unlike an SCR!

Nonlinear On-State

Voltage drop

Like that of an SCR.

Turn On Speed

Fast! Comparable to a standard

power MOSFET.

Turn-Off Speed

Slow! Comparable to a bipolar

transistor.

Temperature Independent

On-State Voltage Drop

Unlike the typical 2x variation of a

power MOSFET.

Application Note 7503