Power electronics is one area where engineers and designers can have exacting reliability, safety, and performance standards. The components, placement, layout, and circuit designs all need to be developed to ensure maximum reliability. One of the most prominent components in high-power systems is MOSFETs, which function as high-power switching elements for DC power delivery or pulsed power delivery.
There are a few important specifications determining the operational characteristics and reliability of MOSFETs, but greater analysis is needed to select an appropriate MOSFET for a power system. If your operating specifications are pushing towards the absolute maximum current, voltage, and temperature ratings, then you should determine the safe operating area for your MOSFETs.
What Defines the Safe Operating Area?
All MOSFETs have some absolute maximum specifications that should not be exceeded, but these do not fully capture the range of operating capabilities for a MOSFET, both in AC and in DC. The safe operating area is a more comprehensive metric for understanding MOSFET operation as it captures a range of possible operating conditions where a MOSFET can be damaged.
In short, the safe operating area of a MOSFET is defined by a set of voltage, current, and temperature values at which the MOSFET simultaneously operates. These can be thought of as absolute maxima, but with some tradeoffs that are normally captured in a curve on a graph. An example is shown below.
Should this particular MOSFET be exposed to conditions outside the safe operating area, it can quickly fail. One of the well-known advantages of MOSFETs is the absence of secondary breakdown, a failure mechanism which is associated with BJTs. As some MOSFETs have scaled smaller, they now exhibit some secondary breakdown. However, for larger power electronics systems, the MOSFETs will also be physically larger and secondary breakdown will still be absent.
Explaining the Safe Operating Area
The safe operating area shows a range of absolute maximum current and voltage values that must not be exceeded. The curves in a safe operating area diagram are shown for specific operating conditions, delineated by:
- Driving method, including:
- Pulsed driving (specific peak current and frequency)
- DC driving, as a function of applied DC voltage
- AC (sinusoidal) driving, as a function of applied peak-to-peak voltage and any DC offset
- Steady-state temperature
- ON-state resistance
The example below shows the safe operating area for DC driving compared to pulsed driving at fixed frequency. As the pulse duration decreases, the maximum allowed current for a given drain-source voltage increases. At high enough drain-source voltage, the MOSFET will have a maximum allowed voltage beyond which the MOSFET will break down and fail regardless of current. This is why we have the vertical line at V(DS) = 30 V.
In DC driving, the safe operating area boundary will move closer to the origin if operating temperature is higher. In AC driving (with DC offset) or pulse driving, the safe operating area boundary will be roughly a function of the average current; reducing the average current moves the boundary away from the origin. For pulse driving, this dependence on average current means the pulse frequency and duration (duty cycle) can be adjusted to set the MOSFET into the safe operating area for a given temperature.
What Drives a MOSFET Beyond its Safe Operating Area?
Even if you determine a safe operating area for your MOSFET based on simulations, or the datasheet for your component documents a safe operating area, there are still some factors that can cause the system to operate outside this area:
- Unexpected high temperature or temperature cycling
- Load line/operating point located outside the safe operating area
- Incorrect pulse driving characteristics
- Sudden power surges or load impedance changes
The above list does not capture every possible failure mechanism, but it covers many possibilities that can cause a MOSFET to be damaged or destroyed. If the risk factors for a system are known at the outset of the design, the MOSFET specifications and operating parameters can be selected to provide sufficient derating.
Derating Provides Additional Safety
Once the safe operating area is determined based on driving/switching characteristics and temperature, it’s important to derate the safe operating area. Derating provides additional headroom in which the system can operate. The idea in derating is simple: apply sufficient headroom above your operating specs to ensure that the MOSFET is always in the safe operating area.
If you have a SPICE model for your MOSFET that includes the package temperature coefficient, you can determine a safe operating area for your MOSFET circuit directly from operating point simulations. This involves the following simulation process for a circuit with a MOSFET:
- Define the excitation source (DC, pulse driving, etc.)
- Vary the driving parameters and determine the MOSFET junction temperature
- Compare the driving parameters and temperature against absolute maximum values, or against a safe operating area diagram in a datasheet
Once you have determined your MOSFET safe operating area and you’re ready to design your power circuits, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.