Safe operating area

The SOAR diagram ( Safe Operating Area ) indicates the safe operating area for power transistors, triacs and thyristors.

Background

In analog and switched-mode power semiconductors it is not enough merely not to exceed the maximum average power dissipation of the component. There are in addition the following effects:

  • The power loss can be as long as the shift operation is a multiple, which is why there is time-dependent pulse SOA curves. You can on the thermal impedance and thus depend on the junction temperature or they are associated with secondary breakdown effects.
  • Especially bipolar transistors and IGBT can suffer a so-called secondary breakdown, caused by inhomogeneous current distribution on the active semiconductor surface and below the maximum power dissipation in the static state (temperature-dependent material parameters )
  • In MOSFETs and IGBTs, there are parasitic structures, the " light " in the case of a too rapid voltage increase after current line ( latch )
  • Thyristors and Triacs lead to rapid voltage increases, especially after the current-carrying state to re-ignition ( recovery time )
  • Thyristors can be destroyed on the semiconductor wafer by too rapid current rise (possibly at too high trigger current ) due to inhomogeneous current distribution

The SOA information is often divided into forward ( FBSOA ) and reverse bias ( typical) to specify for which control state of the transistor apply it: even if the control connection carries negative current at zero potential or negative potential (reverse bias) (base of a bipolar transistor, the gate of an IGBT ) can, restrictions apply.

All these effects are device- specific and should therefore be found in the data sheets as a diagram or value information. They have been obtained from the manufacturer by test circuits and life tests.

The secure operation of a power semiconductor is however beyond ensured by user-specific conditions that may otherwise lead to failure. These include parasitic oscillations ( ringing ) or switch-on of the MOSFET due to excessive voltage slew rate at the drain due to the Miller capacitance. You can not be specified by the manufacturer, however, be those application notes.

Examples

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

They are:

Measures for safe operation

User

  • Limitation of current and voltage slew rates by special wiring of the control terminals ( base or gate ), feedback from a current or voltage measurement
  • Inductances in the load line in thyristors, connected with capacitive (see snubber ) wiring of switch output according MOSFET, IGBT and SCR
  • Snubberless thyristors
  • IGBT for high frequencies
  • MOSFET with integrated quickly become free diode or Schottkystruktur

Component manufacturers

  • Special semiconductor Techno Logo Gien depending on the application
  • Analysis of the behavior of parasitic component structures, change of doping or geometry
  • User Information
  • Power Electronics
Darlington transistor
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