![]() For a given total drain current, cells in older technologies are more likely to operate beyond the ZTC point, where operation is thermally stable, since the current per cell is higher. Older trench (or planar) technologies show a higher R DSon, due to the wider cell pitch and thus lower cell density. Burn marks will appear near the center of the die and close to the die bonding structure, as documented in AN11243 Failure signature of electrical overstress on power MOSFETs (1.3 Linear mode operation).Īlso, cell density influences the shape of the SOA. This process eventually leads to thermal runaway and the destruction of the MOSFET as a three-terminal short. Below the ZTC point, if a small region is at a higher temperature than the rest of the die, it will draw more current and dissipate more power becoming even hotter. This phenomenon is also known as Spirito effect, and it is caused by the uneven distribution of current across the silicon die. The reduction in performance can be quite severe: in this case for a V DS of 20 V, the maximum current the MOSFET can handle goes from a theoretical 60 A down to around 15 A (75% less). In this case Z th ≠ Z th(j-mb) and therefore the limit cannot be found using an electrical model. The dashed red line indicates the real performance of the device. This limit can be found using an RC thermal model, like the Cauer model shown in Fig. The theoretical limit, in the dashed blue line, is calculated using Equation 4, where Z th = Z th(j-mb). ![]() 6 (the inflexion point is located at 5 V for a 1 ms pulse). On the SOA graph, thermal instability is indicated by a two-slopes line and an additional inflexion point, as shown in Fig. As shown in Equation 4, it depends on the thermal impedance of the MOSFET (Z th), maximum junction temperature (T j(max) = 175 ˚C) and mounting base temperature (T mb).įor a more in-depth description of the SOA limits see AN11158 Understanding power MOSFET data sheet parameters (3.1 Safe Operating Area (SOA) curves).įrom Equation 4 it follows that the limit increases (more power can be dissipated) as the time pulse decreases. Finally, the dashed red line indicates the Spirito region, where thermal instability occurs. This limit is verified experimentally by keeping V DS constant while the current is pulsed for a given duration. The red line shows the limit during linear mode operation.The blue line shows the avalanche limit (at the maximum rated voltage, before avalanche occurs).The green line indicates the limit imposed by the package.The yellow line corresponds to the limit in R DSon mode.3 shows how the SOA graph can be subdivided depending on the MOSFET’s working region (a time pulse of 1 ms is considered).
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