Safe operating area of a MOSFET
Safe operating area of a MOSFET:
The similarity of the output characteristics of a MOSFET with that of a BJT should be apparent. Both of them have three distinct modes of operation, namely, (i)cut off, (ii) active and (iii) ohmic (saturation for BJT) modes. However, there are some important differences as well.
- Unlike BJT a power MOSFET does not undergo second break down.
- The primary break down voltage of a MOSFET remains same in the cut off and in the active modes. This should be contrasted with three different break down voltages (VSUS, VCEO & VCBO) of a BJT.
- The ON state resistance of a MOSFET in the ohmic region has positive temperature coefficient which allows paralleling of MOSFET without any special arrangement for current sharing. On the other hand, vCE (sat) of a BJT has negative temperature coefficient making parallel connection of BJTs more complicated.
- As in the case of the FBSOA of a BJT the SOA of a MOSFET is plotted on a log-log graph. On the top, the SOA is restricted by the absolute maximum permissible value of the drain current (IDM) which should not be exceeded even under pulsed operating condition. To the left, operating restriction arise due to the nonzero value of rDS(ON) corresponding to vGS = vGS(Max).
- To the right, the first operating restriction is due to the limit on the maximum permissible junction temperature rise which depends on the power dissipation inside the MOSFET. This limit is different for DC (continuous) and pulsed operation of different pulse widths.
- As in the case of a BJT the pulsed safe operating areas are useful for shaping the switching trajectory of a MOSFET. A MOSFET does not undergo “second break down” and no corresponding operating limit appears on the SOA. The final operation limit to the extreme right of the SOA arises due to the maximum permissible drain source voltage (VDSS) which is decided by the avalanche break down voltage of the drain -body p-n junction.
- This is an instantaneous limit. There is no distinction between the forward biased and the reverse biased SOAs for the MOSFET. They are identical. Due to the presence of the anti-parallel “body diode”, a MOSFET cannot block any reverse voltage. The body diode, however, can carry an RMS current equal to IDM.
- It also has a substantial surge current carrying capacity. When reverse biased it can block a voltage equal to VDSS. For safe operation of a MOSFET, the maximum limit on the gate source voltage (VGS (Max)) must be observed.
- Exceeding this voltage limit will cause dielectric break down of the thin gate oxide layer and permanent failure of the device.
- It should be noted that even static charge inadvertently put on the gate oxide by careless handling may destroy it.
- The device user should ground himself before handling any MOSFET to avoid any static charge related problem.