For
power semiconductor device
A power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics (for example in a switch-mode power supply). Such a device is also called a power device or, when used in an integrated circuit, a power IC ...
s (such as
BJT
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...
,
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
,
thyristor
A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
or
IGBT
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...
), the safe operating area (SOA) is defined as the
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
and
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
conditions over which the device can be expected to operate without self-damage.
SOA is usually presented in transistor
datasheet
A datasheet, data sheet, or spec sheet is a document that summarizes the performance and other characteristics of a product, machine, component (e.g., an electronic component), material, subsystem (e.g., a power supply), or software in suffici ...
s as a graph with V
CE (collector-emitter voltage) on the
abscissa
In common usage, the abscissa refers to the (''x'') coordinate and the ordinate refers to the (''y'') coordinate of a standard two-dimensional graph.
The distance of a point from the y-axis, scaled with the x-axis, is called abscissa or x coo ...
and I
CE (collector-emitter current) on the
ordinate
In common usage, the abscissa refers to the (''x'') coordinate and the ordinate refers to the (''y'') coordinate of a standard two-dimensional graph.
The distance of a point from the y-axis, scaled with the x-axis, is called abscissa or x c ...
; 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 Junction temperature, short for transistor junction temperature, is the highest operating temperature of the actual semiconductor in an electronic device. In operation, it is higher than case temperature and the temperature of the part's exterior. T ...
, secondary breakdown — into one curve, allowing simplified design of protection circuitry.
![BDV66C limits](https://upload.wikimedia.org/wikipedia/commons/f/f9/BDV66C_limits.png)
Often, in addition to the continuous rating, separate SOA curves are also 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 wire bond 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 scale
A logarithmic scale (or log scale) is a way of displaying numerical data over a very wide range of values in a compact way—typically the largest numbers in the data are hundreds or even thousands of times larger than the smallest numbers. Such a ...
s, the borders of the SOA are straight lines:
# I
C = I
Cmax — current limit
# V
CE = V
CEmax — voltage limit
# I
C V
CE = Pmax — dissipation limit, thermal breakdown
# I
C V
CEα = const — this is the limit given by the secondary breakdown (bipolar junction transistors only)
SOA specifications are useful to the design engineer working on power circuits such as
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
s and
power supplies
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a res ...
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.
Secondary breakdown
''For a device that makes use of the secondary breakdown effect see
Avalanche 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 regio ...
''
Secondary breakdown is a 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 small spot of the base-emitter junction. This causes local heating, progressing into a short between collector and emitter. This often leads to the destruction of the transistor. Secondary breakdown can occur both with forward and reverse base drive. Except at low collector-emitter voltages, the secondary breakdown limit restricts the collector current more than the steady-state power dissipation of the device. Older power MOSFETs did not exhibit secondary breakdown, with their safe operating area being limited only by maximum current (the capacity of the bonding wires), maximum power dissipation and maximum voltage. This has changed in more recent devices as detailed in the next section. However, power MOSFETs have parasitic PN and BJT elements within the structure, which can cause more complex localized failure modes resembling secondary breakdown.
MOSFET thermal runaway in linear mode
In their early history, MOSFETs became known for their absence of secondary breakdown. This benefit was due to the fact that ON-resistance increases with increasing temperature, so that part of the MOSFET which is running hotter (e.g. due to irregularities in the die-attachment, etc.) will carry a lower current density, tending to even out any temperature variation and prevent hot spots. Recently, MOSFETs with very high transconductance, optimised for switching operation, have become available. When operated in linear mode, especially at high drain-source voltages and low drain currents, the gate-source voltage tends to be very close to the threshold voltage. Unfortunately the threshold voltage decreases as temperature increases, so that if there are any slight temperature variations across the chip, then the hotter regions will tend to carry more current than the cooler regions when Vgs is very close to Vth. This can lead to thermal runaway and the destruction of the MOSFET even when it is operating within its Vds, Id and Pd ratings. Some (usually expensive) MOSFETs are specified for operation in the linear region and include DC SOA diagrams, e.g. IXYS IXTK8N150L.
Reverse bias safe operating area
Transistors require some time to turn off, due to effects such as minority carrier storage time and capacitance. While turning off, they may be damaged depending on how the load responds (especially with poorly
snubbed inductive loads). The reverse bias safe operating area (or RBSOA) is the SOA during the brief time before turning the device into the off state—during the short time when the base current bias is reversed. As long as the collector voltage and collector current stay within the RBSOA during the entire turnoff, the transistor will be undamaged. Typically the RBSOA will be specified for a variety of turn-off conditions, such as shorting the base to the emitter, but also faster turn-off protocols where the base-emitter voltage bias is reversed.
The RBSOA shows distinct dependencies compared to the normal SOA. For example in
IGBT
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...
s the high-current, high-voltage corner of the RBSOA is cut out when the collector voltage increases too quickly.
[M. H. Rashid ,'' Power electronics handbook'', Academic Press, 2001, , pp 108-109] Since the RBSOA is associated with a very brief turn-off process, it is not constrained by the continuous power dissipation limit.
The ordinary safe operating area (when the device is in the on state) may be referred to as the Forward bias safe operating area (or FBSOA) when it is possible to confuse it with the RBSOA.
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.
Another style of protection is to measure the temperature of the outside of the transistor, as an estimate of junction temperature, and reduce drive to the device or switch it off if the temperature is too high. If multiple transistors are used in parallel, only a few need to be monitored for case temperature to protect all parallel devices.
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.
See also
*
Derating
In electronics, derating (or derating) is the operation of a device at less than its rated maximum capability to prolong its life. Typical examples include operations below the maximum power rating, current rating, or voltage rating.
In electron ...
References
* H. A. Schafft, J. C. French, ''Secondary Breakdown in Transistors'', IRE Trans. Electron Devices ED-9, 129-136 (1962)
online* Michaël Bairanzade, ''Understanding Power Transistors Breakdown Parameters'', OnSemi application node AN1628/
onlineApex technical document on operating power opamps within SOA
{{DEFAULTSORT:Safe Operating Area
Power electronics
Electronic engineering