The Safe Operating Area (SOA) is the safe operating range of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It is visible in the MOSFET's characteristic curve, defined by the area enclosed by curves with Vds (horizontal axis) and Ids (vertical axis). When a MOSFET is operating normally, the voltage and current should not exceed the limits of the SOA. This concept is important for evaluating the safety of MOSFET operation, especially in applications like hot swapping, motor driving, and switch-mode power supplies.

If the MOSFET's instantaneous power exceeds the SOA region during switching, it can lead to dangerous situations.

The SOA consists of five limiting lines, including the Rds(on) limit line, current limit line, power limit line, thermal stability limit line, and breakdown voltage limit line. These lines represent different parameters and performance limits, ensuring that the MOSFET operates within a safe range.

a. Rds(on) limit line: This line represents the ON-state resistance of the MOSFET when fully conducting, operating in the ohmic region. The value presented in datasheets is a range, not entirely consistent with the Rds(on) inferred from the SOA, which is a constant shown in the linear SOA curve provided by manufacturers, related mainly to Vgs and temperature.

b. Current limit line: This usually refers to the maximum pulse peak current Idm of the MOSFET, determined by the device's packaging.

c. Power limit line: This power generates a stable junction temperature Tj of 150°C in thermal equilibrium, where Tc = 25°C. This is calculated based on the maximum power the device is allowed to dissipate.

d. Thermal stability limit line: The voltage and current should not exceed this line during operation, otherwise, it may cause thermal instability and damage to the MOSFET. This is discussed in detail later.

The breakdown voltage limit line indicates the voltage withstand capability of the MOSFET, corresponding to the Vds(max) in the specifications.

So, how does the thermal stability limit line occur?

With a fixed Vds, the voltage of Vgs remains unchanged, and different Vgs values result in different ID currents.

For example, at 25°C, IDS=IDS(A).

When we raise the temperature to 150°C, at point B in the figure, IDS increases, IDS=IDS(B). This means that as the temperature rises, the IDS current increases, forming a positive temperature coefficient (forming a positive feedback loop), which is the red area. Conversely, the blue area represents a negative temperature coefficient.

From this, we can see that thermal instability is related to temperature and is divided into two types: positive temperature coefficient & negative temperature coefficient.

What is the relationship?

When a certain area on the silicon chip is hotter than others and is in the positive temperature coefficient area, its current will also be higher than other areas. As the current increases, more heat is generated, the temperature becomes higher, and ultimately leads to thermal runaway.

So why are there several lines in an SOA curve?

This is because they correspond to different pulse time limits. The longer the pulse duration, the greater the electrical and thermal stress the MOSFET will endure, making it more prone to damage.

Therefore, the shorter the duration, the higher the voltage and current it can withstand.

Finally, how do you determine if a MOSFET is operating in the SOA?

1. By measuring the voltage and current waveforms of the MOSFET to determine if they exceed the set limits.

2. Use the oscilloscope's multiplication function to obtain the energy waveform of the pulse signal and its duration △t.

3. Find the power peak value P(max) and record the voltage Ucross and current Icross data at this time.

4. According to the SOA curve and pulse duration △t, calculate the allowable current Isoa corresponding to the voltage Ucross.

5. When the MOSFET is operating normally, measure its case temperature and use the derating formula to calculate the adjusted current value under temperature influence: I(derating) = Isoa × temperature adjustment factor.

6. If the measured current Icross is less than the calculated adjusted current I(derating), the MOSFET is operating in the safe SOA region; if it is greater, it means it has exceeded the safe range.

In engineering practice, designers need to carefully consider the operating conditions of MOSFETs in the circuit to ensure they operate within the SOA. This typically involves calculating and evaluating parameters such as voltage, current pulses, and peak power in the circuit, so that suitable devices can be selected and operating conditions determined at the design stage.

Additionally, for high-power, high-frequency, or pulse applications, it is particularly important to pay attention to the SOA characteristics of MOSFETs and select appropriate device models and operating conditions based on specific application scenarios. In practical applications, a combination of experimental verification and simulation analysis is often used to ensure that MOSFETs operate reliably within the SOA.