AN4337 datasheet(10/14 Pages) STMICROELECTRONICS

10 / 14 page

IAR and EAS electrical thermal approach

AN4337

10/14

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from the application, it is rather improbable that a MOSFET device will fail for EAS single

pulse avalanche energy.

This approach suggests that during an avalanche phenomenon, the IAR parameter should

be addressed rather than the EAS one.

3.2

IAR electrical evaluations

IAR parameter defines the maximum avalanche current without any bipolar latching

phenomenon. This parameter doesn't depend on the avalanche energy, which means the

MOSFET device is safe if the avalanche energy is lower than the EAS datasheet

specification and the avalanche current is lower than IAR absolute maximum rating; vice

versa, the MOSFET is certainly safe if the maximum avalanche current is lower than IAR.

This last assertion is validated by the result of Equation 15; in fact, any leakage inductance

in a real flyback converter can have the value of 5.8 mH.

As already mentioned, this suggests that the avalanche current parameter needs to be

monitored more than avalanche energy.

The worst case scenario during the avalanche phenomenon is when the ferromagnetic core

of the flyback transformer becomes saturated. Due to the uncontrolled saturation effect, the

current peak may be very high and hence dangerously close to the IAR specification.

Below is a waveform with typical saturation phenomenon during the avalanche.

Figure 6. Saturation phenomenon during the avalanche

In this condition, two methods exist to increase the safety margin in terms of IAR avalanche

current.

The first one is to optimize the driving and the network circuit in order to avoid the avalanche

phenomenon. The snubber increase, a different clamp circuit and/or input capacitance

increase can satisfy this requirement.

Here some examples:

AM15968v1

Saturation effect

Vbreakdown

AN4337 Datasheet(PDF) 10 Page - STMicroelectronics