Safe operating area
Encyclopedia
For power semiconductor device
s (such as BJT, MOSFET
, thyristor
or IGBT), the safe operating area (SOA) is defined as the voltage
and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented in transistor datasheet
s as a graph with VCE (collector-emitter voltage) on the abscissa
and ICE (collector-emitter current) on the ordinate
; 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, second breakdown — into one curve, allowing simplified design of protection circuitry.
Often, in addition to the continuous rating, separate SOA curves are plotted for short duration pulse conditions (1 ms pulse, 10 ms pulse, etc.).
The safe operating area curve is a graphical representation of the power handling capability of the device under various conditions. The SOA curve takes into account the wirebond current carrying capability, transistor junction temperature, internal power dissipation and secondary breakdown limitations.
s, the borders of the SOA are straight lines:
SOA specifications are useful to the design engineer working on power circuits such as amplifier
s and power supplies
as they allow quick assessment of the limits of device performance, the design of appropriate protection circuitry, or selection of a more capable device. SOA curves are also important in the design of foldback
circuits.
The second breakdown is an irreversible failure mode in bipolar power transistors. In a power transistor with a large junction area, under certain conditions of current and voltage, the current concentrates in a single area of the base-emitter junction. This current contraction effect causes local heating and destruction of the transistor. Second breakdown can occur both with forward and reverse base drive. Except at low collector-emitter voltages, the second breakdown limit restricts the collector current more than the steady-state power dissipation of the device. Power MOSFETs do not exhibit secondary breakdown, and their safe operating area is limited only by maximum current (the capacity of the bonding wires), maximum power dissipation and maximum voltage. However, Power MOSFETs have parasitic PN and BJT elements within the structure, which can cause more complex localized failure modes resembling Secondary Breakdown.
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.
Semiconductor device
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...
s (such as BJT, MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
, thyristor
Thyristor
A 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 trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
or IGBT), the safe operating area (SOA) is defined as the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented in transistor datasheet
Datasheet
thumb|A floppy disk controller datasheet.A datasheet, data sheet, or spec sheet is a document summarizing the performance and other technical characteristics of a product, machine, component , material, a subsystem or software in sufficient detail to be used by a design engineer to integrate the...
s as a graph with VCE (collector-emitter voltage) on the abscissa
Abscissa
In mathematics, abscissa refers to that element of an ordered pair which is plotted on the horizontal axis of a two-dimensional Cartesian coordinate system, as opposed to the ordinate...
and ICE (collector-emitter current) on the ordinate
Ordinate
In mathematics, ordinate refers to that element of an ordered pair which is plotted on the vertical axis of a two-dimensional Cartesian coordinate system, as opposed to the abscissa...
; 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, second breakdown — into one curve, allowing simplified design of protection circuitry.
Often, in addition to the continuous rating, separate SOA curves are plotted for short duration pulse conditions (1 ms pulse, 10 ms pulse, etc.).
The safe operating area curve is a graphical representation of the power handling capability of the device under various conditions. The SOA curve takes into account the wirebond current carrying capability, transistor junction temperature, internal power dissipation and secondary breakdown limitations.
Limits of the safe operating area
Where both current and voltage are plotted on logarithmic scaleLogarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...
s, the borders of the SOA are straight lines:
- IC = ICmax — current limit
- VCE = VCEmax — voltage limit
- IC VCE = Pmax — dissipation limit, thermal breakdown
- IC VCEα = const — this is the limit given by the second breakdown (bipolar junction transistors only)
SOA specifications are useful to the design engineer working on power circuits such as amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s and power supplies
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
as they allow quick assessment of the limits of device performance, the design of appropriate protection circuitry, or selection of a more capable device. SOA curves are also important in the design of foldback
Foldback (power supply design)
Foldback is a design feature that functions as follows. When overcurrent is drawn by the load, the supply reduces output voltage and current to well below the current limit. This approach has been used in power supplies and power amplifiers....
circuits.
Second breakdown
For a device that makes use of the second breakdown effect see Avalanche transistorAvalanche transistor
An avalanche transistor is a bipolar junction transistor designed for operation in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector to emitter breakdown voltage, called avalanche breakdown region ...
The second breakdown is an irreversible failure mode in bipolar power transistors. In a power transistor with a large junction area, under certain conditions of current and voltage, the current concentrates in a single area of the base-emitter junction. This current contraction effect causes local heating and destruction of the transistor. Second breakdown can occur both with forward and reverse base drive. Except at low collector-emitter voltages, the second breakdown limit restricts the collector current more than the steady-state power dissipation of the device. Power MOSFETs do not exhibit secondary breakdown, and their safe operating area is limited only by maximum current (the capacity of the bonding wires), maximum power dissipation and maximum voltage. However, Power MOSFETs have parasitic PN and BJT elements within the structure, which can cause more complex localized failure modes resembling Secondary Breakdown.
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.
Protection
The most common form of SOA protection used with bipolar junction transistors 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.