Calculating Safe Operating Area of Mosfet: P^2 t Stress Explained

In summary, the datasheet for a device that drives a Mosfet includes a calculation for stress, represented by P^2 t, which is the accumulated heat over time or the integrated heat effect. This value is essentially constant for intervals of less than 100ms and is related to the Safe Operating Area curve in a Mosfet datasheet. If the stress is 20W^2 sec, it means that the Mosfet can withstand 20 watts for 1 second. The units come from two separate effects, P t and P, which are modeled multiplicatively. The value of P^2 t is confusing because it is a combination of both the integrated heat effect and the instantaneous power delivered to the load. It is
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Saw a calculation of stress in a datasheet (page14) here -
http://tinyurl.com/43465sv
(The device drives a Mosfet.)
How do you relate the P^2 t stress calculation in the datasheet to the Safe operating area curve in a Mosfet datasheet.
If the Stress is 20W^2 sec, then how do you interpret it? The Mosfet should be able to withstand 20watts for 1sec.
The watt square is confusing me. Why watt square*t? why not joules.
 
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Anyone?
 
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The first section on the same page says what it is but you have to apply engineering knowledge to infer where it comes from.

The units come from two separate effects:

  • P t which results in units of energy delivered as heat that modulates to heat transfer effectiveness - this is the "accumulated heat over time" or integrated heat effect
  • P which is the instantaneous power delivered to the load and dissipated as heat

These combined as function: Damage(Pshort-circuit, Eheat) which are modeled multiplicatively, hence P2t

This value is "essentially constant for intervals of less than 100ms for any given device type" so basically you have thermal equilibrium because you aren't generating heat fast enough to exceed the heat sink capacity of the package. Above that you start to have a positive feedback loop on heat that accelerates the effect of the short circuit power (P).
 
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1. What is the purpose of calculating the safe operating area of a MOSFET?

The safe operating area (SOA) of a MOSFET refers to the maximum voltage and current limits within which the device can operate without being damaged. Calculating the SOA helps to ensure that the MOSFET is operated within its safe limits, preventing potential failures and ensuring its longevity.

2. How is the safe operating area of a MOSFET calculated?

The safe operating area is calculated by using the P^2 t method, which takes into account the power dissipation and duration of operating at that power. The formula is P^2 t = (V^2/R) x t, where P is power, V is voltage, R is resistance, and t is time. This method helps to determine the maximum power and duration that the MOSFET can handle without exceeding its safe limits.

3. What is P^2 t stress and why is it important?

P^2 t stress is the amount of heat generated by the MOSFET while operating at a certain power level for a specific amount of time. It is crucial to consider P^2 t stress when calculating the safe operating area because excessive heat can cause the MOSFET to fail. By understanding and managing P^2 t stress, one can ensure that the MOSFET is operated within its safe limits and avoid potential failures.

4. What factors can affect the safe operating area of a MOSFET?

Several factors can affect the safe operating area of a MOSFET, such as temperature, voltage, current, and duration of operation. Higher temperatures can decrease the safe operating area as it increases the P^2 t stress. Similarly, higher voltages and currents can also decrease the safe operating area, as they may cause the MOSFET to exceed its maximum ratings. The duration of operation also plays a role, as prolonged operation at high power levels can cause thermal stress on the MOSFET.

5. How can one ensure that the MOSFET is operated within its safe operating area?

The best way to ensure that the MOSFET is operated within its safe operating area is by accurately calculating the P^2 t stress and comparing it to the maximum ratings of the device. Additionally, using proper heat sinks and cooling systems can help to manage the P^2 t stress and keep the MOSFET operating within its safe limits. It is also important to regularly monitor the temperature, voltage, and current levels during operation to ensure that they are within the safe limits of the MOSFET.

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