Safe operating area: Difference between revisions
Wtshymanski (talk | contribs) show an example SOA diagram |
Wtshymanski (talk | contribs) copyedit; more on secondary breakdown and reference |
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SOA is usually presented in transistor [[datasheet]]s as a graph with V<sub>CE</sub> (collector-emitter voltage) on the [[abscissa]] and I<sub>CE</sub> (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|secondary breakdown]] — into one curve, allowing simplified design of protection circuitry. |
SOA is usually presented in transistor [[datasheet]]s as a graph with V<sub>CE</sub> (collector-emitter voltage) on the [[abscissa]] and I<sub>CE</sub> (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|secondary breakdown]] — into one curve, allowing simplified design of protection circuitry. |
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[[File:BDV66C limits.png|thumb| |
[[File:BDV66C limits.png|thumb|right|Illustration of safe operating area of a bipolar power transistor. Any combination of collector current and voltage below the line can be tolerated by the transistor.]] |
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Often, in addition to the continuous rating, separate SOA curves are provided for short duration conditions (1 ms pulse, 10 ms pulse etc.). |
Often, in addition to the continuous rating, separate SOA curves are provided for short duration pulse conditions (1 ms pulse, 10 ms pulse, etc.). |
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The [[safe operating area]] (SOA) of a power [[transistor]] is a subset of the parameter combinations of collector emitter voltage and collector current, which the device can sustain without damage. In a double logarithmic diagram the borders of the SOA are straight lines: |
The [[safe operating area]] (SOA) of a power [[transistor]] is a subset of the parameter combinations of collector emitter voltage and collector current, which the device can sustain without damage. In a double logarithmic diagram the borders of the SOA are straight lines: |
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==Second breakdown== |
==Second breakdown== |
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The '''second breakdown''' is an irreversible failure mode in power |
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. <ref> L.W. Turner,(ed), ''Electronics Engineer's Reference Book'', 4th ed. Newnes-Butterworth, London 1976 ISBN 0 408 00168, pages 8-45 and 8-46 </ref> 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. <ref> [[Paul Horowitz]] and Winfield Hill, ''The Art of Electronics 2nd Ed. '' Cambridge University Press, Cambridge, 1989 ISBN 0521370957 page 321 </ref> |
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==Types of safe operating area== |
==Types of safe operating area== |
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Revision as of 22:05, 3 January 2010
For power semiconductor device (such as BJT, MOSFET, thyristor or IGBT) a safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage.[1]
SOA is usually presented in transistor datasheets 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, 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 pulse conditions (1 ms pulse, 10 ms pulse, etc.).
The safe operating area (SOA) of a power transistor is a subset of the parameter combinations of collector emitter voltage and collector current, which the device can sustain without damage. In a double logarithmic diagram 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
SOA specifications are incredibly useful to the design engineer working on power circuits such as amplifiers and power supplies 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 foldback 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.
Second breakdown
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. [2] 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. [3]
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.[4]
- Forward bias safe operating area (or FBSOA) is SOA when turning the device into the on-state.
See also
References
- ^ Tim Williams ,The circuit designer's companion 2nd ed.,Butterworth-Heinemann, 2004 ISBN 0750663707, pp.129-130
- ^ L.W. Turner,(ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN 0 408 00168, pages 8-45 and 8-46
- ^ Paul Horowitz and Winfield Hill, The Art of Electronics 2nd Ed. Cambridge University Press, Cambridge, 1989 ISBN 0521370957 page 321
- ^ M. H. Rashid , Power electronics handbook, Academic Press, 2001, ISBN 0125816502, pp 108-109