Silicon power MOSFETs include an intrinsic body diode between the drain and source terminals. With the MOSFET in the off state, the body diode blocks current flow in the reverse direction and conducts current when the diode is forward biased.
The MOSFET breakdown voltage, BVDSS, is the maximum voltage that can safely be applied from drain-to-source without driving the device into avalanche and is also the maximum body diode reverse bias voltage. The body diode forward voltage, VSD, specified in the data sheet is the drop from source-to-drain at the defined current and is typically on the order of 0.8 V to 1.0 V.
Often, engineers ask if the body diode can support the same maximum drain current specified in the MOSFET data sheet. In theory, the body diode can be expected to since the body diode is the same current path through the device. However, the limiting factor becomes the power dissipation. When the MOSFET is on, the conduction loss is ID2 x RDS(on). When the FET is off and the body diode conducts, the conduction loss is ISD x VSD. As explained in the Understanding MOSFET data sheets, Part 3 - Continuous current ratings blog, the maximum power dissipation specified in the FET data sheet is calculated as follows:
Where RθJX is either the thermal resistance from junction-to-case, RθJC, or from junction-to-ambient, RθJA and TX is either the case temperature, TC, or the ambient temperature, TA. Solving for ID, the maximum drain current specified in the data sheet at TX = 25°C when the FET is on:
This equation can be extended to calculate the maximum continuous at 25°C current for the body diode:
Solving for ISD, the maximum body diode current when the FET is off:
The CSD19532Q5B, 100-V N-channel MOSFET is used as an example. First, a quick review of the maximum ratings from the first page of the data sheet. Note that the maximum body diode current is not specified in the FET data sheet. Also, there are multiple specifications for maximum drain current as explained in the previously referenced blog. For this example, the maximum body diode current is calculated for an ambient temperature, TA = 25°C, using the typical RθJA = 40°C/W as follows:
The CSD19532Q5B maximum continuous drain current:
Maximum RDS(on) at TJ = 150°C can be calculated using the maximum specified RDS(on) = 4.9 mΩ at VGS = 10 V and multiplying it by the normalizing factor shown in Figure 8 of the device data sheet:
To calculate the maximum diode current, use the maximum forward voltage specified in the data sheet as follows:
Similarly, the silicon limited maximum continuous body diode current at TC = 25°C can be calculated using the junction-to-case thermal impedance, RθJC = 0.8°C/W, as shown in the following equations.
Note that this calculation assumes an ideal heat sink is used to hold the case temperature at 25°C. This case is impossible in a real application with an actual heat sink and 156 W of power dissipation in a 5x6mm package.
At elevated temperatures, these calculations yield lower currents. For example, substituting TA = 75°C in the previous calculation results in lower power dissipation and reduced body current capability, provides the following calculation results:
MOSFET data sheets specify the maximum drain current. A common question is, can the intrinsic body diode carry the same amount of current? As this application brief demonstrates, the current carrying capability of the body diode can be calculated, is limited by power dissipation, and is typically less than the maximum drain current specified in the data sheet.