How to get MOSFET correct RDSon_{ }value is the pre-requisite to compute the MOSFET conduction loss or static power dissipation. RDSon is the drain to source on-state resistance of the MOSFET. The term “correct” is relative to the target result. Supposing the target is to compute the maximum power dissipation, then the correct RDSon value must be the worst-case value. On the other hand, if the target is to derive the realistic system efficiency, then the correct RDSon value must be the typical value. Read the article How to Compute MOSFET Conduction Loss to appreciate the application of the correct RDSon value.

**How to Get MOSFET Correct RDSon Value from the Datasheet**

The RDSon value is provided in the datasheet. To be more realistic here, let us consider IPP040N06N MOSFET from Infineon Technologies. Infineon is offering great solutions for your designs and products. Visit them at https://www.infineon.com/.

**Aiming for Maximum RDSon**

In the datasheet, under Product Summary, there is a specified maximum RDSon value (see below).

Considering our objective is how to get MOSFET correct RDSon value for worst case application, like to derive the maximum static power dissipation or conduction loss, is this the value that we are looking for?

R_{DSon} is a MOSFET complicated parameter. Let us explore further the datasheet. In “6 Typ. drain-source on resistance”, there is a graph of drain current (I_{D}) versus R_{DSon} with different V_{GS} values. The curve values are derived from a typical junction temperature of 25’C.

From the graph, the highest R_{DSon} is happening at the lowest V_{GS} (V_{GS} is the voltage applied in the gate and source terminals). Therefore, the Product Summary value is not yet the maximum value of the R_{DSon}.

Let us assume here (for the sake of explaining clearly on how to get MOSFET correct RDSon) an applied V_{GS} to the MOSFET circuit of 6V with a +/-1V swing. This means that the V_{GS} could go as low as 5V. With this, the worst case R_{DSon} should be derived from the 5V curve.

Since the R_{DSon} is a function of the drain current, there is a need to get the drain current. Let us assume that the drain current is 40A, and from the 5V curve, the corresponding R_{DSon} is 7 milliohms. Therefore, 7 milliohms must be the maximum R_{DSon} that we should be using.

Furthermore, R_{DSon} will vary with temperature. Under “9 Drain-source on-state resistance”, RDSon is plotted to a graph with the junction temperature. Though the graph is plotted with I_{D} = 80A and V_{GS} = 10V, still it is a good reference for discussion on how R_{DSon} affected with temperature. If you have access to the curve that corresponds for V_{GS} = 5V and drain current of 40A, then you can further see the increase of the R_{DSon} value at a certain temperature above 25’C. For example, the worst-case junction temperature could reach 100’C (this will be happening at the maximum ambient temperatures), then get the R_{DSon} at 100’C.

**How to Get MOSFET Correct R**_{DSon} Value for Typical Application

_{DSon}Value for Typical Application

Typical R_{DSon} value as discussed is the preferred to get the realistic system efficiency. In our example above, the V_{GS} is 6V with a swing of +/-1V. Since this time, we are looking for the typical value of R_{Dson}, let us only consider the 6V since this is the typical level of the V_{GS}. So, from “6 Typ. drain-source on resistance”, the corresponding R_{DSon} at the drain current of 40A is 4.75 milliohm.

Moreover, for typical R_{Dson}, it is important to get the equivalent typical junction temperature. This happens at the typical ambient temperature. The typical ambient temperature is usually 25’C. This value will give an equivalent junction temperature as the junction temperature is a function of the actual power dissipation plus the ambient temperature. Supposing the equivalent typical junction temperature is 30’C, look from the graph (T_{J} versus R_{DSon}) the equivalent R_{DSon}.

**Summary**

These are the key takeaways regarding how to get MOSFET correct R_{DSon} value.

1. R_{DSon} value could be worst-case or typical.

2. Worst case R_{DSon} value could be used for calculating the maximum static power dissipation.

3. Typical R_{DSon} is more preferrable to use in getting the system efficiency.

4. R_{DSon} is a function of drain current.

5. R_{DSon} is also a function of temperature.

6. In computing the worst case R_{DSon} value, the minimum V_{GS }must be considered as well as the maximum junction temperature.