SOAR diagram

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The SOAR diagram ( S afe O perating Ar ea) indicates the safe working area for power transistors , triacs and thyristors .

backgrounds

In the analog and switching operation of power semiconductors, it is not sufficient simply not to exceed the maximum average power loss of the component. The following effects also occur:

• the power loss during the switching process can be quite a multiple, which is why there are pulse-time-dependent SOA curves. They can depend on the thermal impedance and thus on the junction temperature or they are associated with secondary breakdown effects.
• In particular, bipolar transistors and IGBTs can suffer a so-called second breakthrough even below the maximum power loss in the static state , which is caused by inhomogeneous current distribution on the active semiconductor surface (temperature-dependent material parameters)
• in MOSFET and IGBT there are parasitic structures that "ignite" (latch) in the event of a too rapid voltage rise after power conduction
• with thyristors and triacs lead to rapid voltage increases, especially after the current-carrying state for reignition (release time)
• Thyristors can be destroyed by an excessively rapid current increase (possibly also with an excessively high ignition current) due to inhomogeneous current distribution on the semiconductor wafer

The SOA information is often divided into forward (FBSOA) and reverse bias (RBSOA) in order to specify the control status of the transistor for which they apply: even if the control connection carries negative current, is at zero potential or negative potential ( reverse bias ) (Base of a bipolar transistor, gate of an IGBT), restrictions may apply.

All these effects are component-specific and should therefore be found in the data sheets as diagrams or values. They have been obtained from the manufacturer through test circuits and durability tests.

The safe functioning of a power semiconductor is, however, also guaranteed by user-specific conditions, which can otherwise lead to failures. These include, for example, parasitic oscillations ( ringing ) or the restart of MOSFETs due to excessive voltage rise speed at the drain due to the Miller effect . They cannot be specified by the manufacturer, but they give instructions for use.

Examples

Typical safe working area of ​​a bipolar transistor

Safe Operating Area of ​​the pnp Darlington power transistor BDV66C: the transistor can only be operated safely within the area below the boundary lines.

The SOA diagram of a bipolar transistor is often limited by four straight lines with a logarithmic axis division.

They are:

1. maximum collector current ≥ ${\displaystyle I_{c,\mathrm {max} }}$
2. maximum power dissipation ${\displaystyle P_{v,\mathrm {max} }}$
3. Second breakthrough (local hot spots)
4. maximum collector-emitter voltage ${\displaystyle U_{CE0}}$

Measures for safe operation

user

• Limitation of current and voltage rise rates through
  • special wiring of the control connections (base or gate), feedback from a current or voltage measurement
  • Inductivities in the load line of thyristors, connected with capacitive (see snubber ) wiring of the switching output of MOSFET, IGBT and thyristors
• Pay attention to the outflow of minority charge carriers (body diode of MOSFET, basis of bipolar transistors)
• Choice of suitable components, for example
  • snubberless thyristors
  • IGBT for high frequencies
  • MOSFET with an integrated fast-releasing diode or Schottky structure

Component manufacturer

• special semiconductor technologies depending on the application
• Analysis of the behavior of parasitic component structures, changes in doping or geometry
• Notes for the user