For power
semiconductor deviceSemiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide. Semiconductor devices have replaced thermionic devices in most applications...
(such as BJT,
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
,
thyristorThe thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased .Some sources define silicon controlled...
or IGBT) a
safe operating area (SOA) is defined as the
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented in transistor
datasheetA datasheet is a document summarizing the performance and other characteristics of a component , a sub-system or software in sufficient detail to be used by a design engineer to design the component into a system...
s as a graph with V
CE (collector-emitter voltage) on the
abscissaIn mathematics, abscissa refers to the first element of an ordered pair.It is the first of the two terms by which a point is referred to, in a system of fixed rectilinear coordinate axes. The abscissa is also known as the "x" coordinate of a point...
and I
CE (collector-emitter current) on the
ordinateIn mathematics, the ordinate of an ordered pair is its second component, i.e. it is y.It is the second of the two terms by which a point is referred to, in a system of fixed rectilinear coordinate axes...
; the safe 'area' referring to the area under the curve.
For power
semiconductor deviceSemiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide. Semiconductor devices have replaced thermionic devices in most applications...
(such as BJT,
MOSFETThe metal–oxide–semiconductor field-effect transistor is a device used to amplify or switch electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925...
,
thyristorThe thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased .Some sources define silicon controlled...
or IGBT) a
safe operating area (SOA) is defined as the
voltageVoltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...
and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented in transistor
datasheetA datasheet is a document summarizing the performance and other characteristics of a component , a sub-system or software in sufficient detail to be used by a design engineer to design the component into a system...
s as a graph with V
CE (collector-emitter voltage) on the
abscissaIn mathematics, abscissa refers to the first element of an ordered pair.It is the first of the two terms by which a point is referred to, in a system of fixed rectilinear coordinate axes. The abscissa is also known as the "x" coordinate of a point...
and I
CE (collector-emitter current) on the
ordinateIn mathematics, the ordinate of an ordered pair is its second component, i.e. it is y.It is the second of the two terms by which a point is referred to, in a system of fixed rectilinear coordinate axes...
; the safe 'area' referring to the area under the curve. The SOA specification combines the various limitations of the device — maximum voltage, current, power, junction temperature, secondary breakdown — into one curve, allowing simplified design of protection circuitry.
Often, in addition to the continuous rating, separate SOA curves are provided for short duration conditions (1 ms pulse, 10 ms pulse etc.).
SOA specifications are incredibly useful to the design engineer working on power circuits such as
amplifierGenerally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of...
s and
power suppliesPower supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU...
as they allow him/her to quickly determine the conditions under which a design will catch fire, allowing the design of appropriate protection circuitry and/or selection of a more capable device. SOA curves are also important in the design of
foldbackFoldback may refer to:In technology:*Foldback , a speaker used to direct sound to performers or a current-limit device in power amplifiers.*Foldback *Foldback Companies:*Foldback Records...
circuits.
The most common form of SOA protection used with BJTs senses the collector-emitter current with a low-value series resistor; the voltage across this resistor is applied to a small auxiliary transistor that progressively 'steals' base current from the power device as it passes excess collector current.
This approach is effective but not bullet-proof. In practice it is very difficult to design a protection circuit that will work under all conditions, and it is left up to the design engineer to weigh the likely fault conditions against the complexity and cost of the protection.
Types of safe operating area
- Reverse bias safe operating area (or RBSOA) is SOA when turning the device into the off-state. The RBSOA may be different from generic SOA, for example for IGBTs, upper half corner of the RBSOA is progressively cut out which reduces the RBSOA as the rate of change of the collector to emitter voltage across the device, dVce/dt, is increased.
- Forward bias safe operating area (or FBSOA) is SOA when turning the device into the on-state.