How to Use MOSFET RDSon Data from the Datasheet

In this article I will discuss the proper way on how to use MOSFET RDSon data provided by datasheets. RDSon is a channel on-state resistance of MOSFET. This is provided in the data sheet taken from different conditions like below table.

 

How to Use MOSFET RDSon Data from Datasheet

 

There is also a graph between RDSon or normalized RDSon value and temperature like below. Normalized RDSon is actually a temperature coefficient of the drain on state resistance (RDSon). A dedicated article for this is here.

 

How to Use MOSFET RDSon Graph

 

Some datasheet also provides RDSon versus drain current that is taken at nominal ambient temperature like below

 

How to Use MOSFET RDSon Graph between junction temperature and drain current

 

What RDSon I should select? Well, you can go for the worst case for quick design analysis. However, if you want to know how to use MOSFET RDSon data that is more realistic, continue on reading below.

 

How to Use MOSFET RDSon Data that Gives Realistic Result

As mentioned above, you may consider directly the worst case scenario for easier analysis. However, the result is not that realistic with respect to your needs. For instance, the worst case RDSon using below table are 2.8 ohms and 9.25 ohms.

 

How to Use MOSFET RDSon Data

 

To compute for the drain current, the worst case is the smallest value since it will give the highest drain current. In solving for the power dissipation, the worst case is the highest value.

The more realistic approach is to consider a single RDSon corresponding to a particular condition. For instance, the design maximum operating temperature is 60’C ambient; then get the corresponding RDSon for this.

 

Example #1 on How to Use MOSFET RDSon Data

Design requirements:

Ambient temperature range: maximum 60’C

Drain current: 100mA

Applied VGS: 4.5V

MOSFET P/N: 2N7002

For 2N7002, I see the graph below. It seems perfect for the requirement.

 

MOSFET RDSon Graph

 

From the graph, I get 3.5 ohms. However, this is only taken at a junction temperature (Tj) of 25’C. Our target is to get the RDSon at 60’C ambient. From the datasheet, I also see this graph.

 

MOSFET Normalized RDSon

 

This is a normalized RDSon with respect to junction temperature. The value we derived above which is 3.5 ohms will be multiplied by a factor that is found on this graph.

My objective is to get the RDSon at 60’C ambient; can I use the information at the junction temperature of 60’C on the above graph? It’s better not. There is a relation between junction and ambient temperatures as given by below equation.

 

MOSFET thermal rise

 

What we can get from the above equation is the temperature difference between the junction and ambient (Tj-Ta). We know the drain current which is given at 100mA. Based from this drain current we will compute for the power dissipation PD using below equation

 

mosfet power dissipation

 

In order to get the power dissipation, we need the RDSon. In this equation, we are going to use the typical RDSon value we get earlier that is 3.5 ohms. So, the power dissipation is

 

mosfet power dissipation equation

 

And the temperature difference is

mosfet thermal rise equation

(Note: for 2N7002, the Rth(j-a) is 350K/W)

Finally, we get the delta temperature between junction and ambient. So, for an ambient of 60’C, the equivalent junction temperature is 60’C+12.25’C = 72.25’C. The multiplier we need is 1.2 based from the graph below.

 

pic 7

 

Therefore, the RDSon to be used at an ambient temperature of 60’C is

 

eq 5

 

Example #2 on How to Use MOSFET RDSon Data

Ambient temperature range: maximum 100’C

 

MOSFET switch circuit

 

In this second example, I will show you the way on how to use MOSFET RDSon data for STN3N45K3 N-channel MOSFET from ST Microelectronics.

Based on the STN3N45K3 datasheet

 

How to use MOSFET RDSon Table

MOSFET RDSon Curve

 

Our circuit uses MCU pin voltage to the gate. MCU pin is can be 3.3V or 5V. The table above specified RDSon value that is taken at VGS = 10V and ID = 0.6A.  The graph also shows only a curve with VGS = 10V. If you want to, you can ask another curve from the manufacturer specific for your needs for more exact analysis. In my case, I just make use of these informations.

First, we will estimate the drain current of our circuit. The drain current is opposed by Rload and the RDSon of the MOSFET. For this estimation, we can use the typical RDSon from the table above which is 3.3 ohms. So, the drain current is

 

mosfet drain current equation

 

If you want, you may not consider the RDSon in the estimation of the drain current. If so, the computed drain current is just equal to 0.4A.

From the drain current versus RDSon graph, the RDSon is around 3.08 ohms when the drain current is around 375mA as shown below.

 

pic 11

 

This value is just for junction temperature of 25’C as datasheet said. Our goal is to get the RDSon at 100’C ambient temperature.

Again, we need to solve for the temperature difference between junction and ambient as we did on the first example.

 

eq 7

 

Where Rth(j-a) is given below

 

Mosfet thermal resistance

 

So, for 100’C ambient, the equivalent junction temperature will be 100’C+16.4’C = 116.4’C.

Based from the normalized RDSon curve, the multiplier is 1.9.

 

 

pic 13

 

Therefore, the RDSon at 100’C ambient temperature is

 

eq 8

 

Example #3 on How to Use MOSFET RDSon Data

In this last example on how to use MOSFET RDSon data, I will use a power MOSFET in a circuit below. The requirement is to get the MOSFET RDSon at 100’C case temperature.

 

MOSFET low side driver

 

First we need to get the drain current using the typical RDSon. The typical RDSon of this device based on datasheet is very small which is 0.001 ohm; it can be neglected in solving for the drain current.

 

pic 15

 

From the drain current versus RDSon with VGS of 10V, the typical RDSon is 0.8milliohm as shown below.

 

pic 16

 

The RDSon we get above is typical. So, we need to use the normalized RDSon table below to get the target RDSon.

 

pic 17

 

However, the specified temperature for the target RDSon is 100’C case temperature but the graph above uses junction temperature. Like we did in the previous examples, we will derive the junction temperature corresponds to the specified case temperature. We can use below relation

 

eq 9

 

(Note: Tj-Tc is the temperature difference between junction and case, Rth(j-c) is the thermal resistance junction to case)

The MOSFET circuit above is a normal on/off switch so we just consider only the RDSon loss in the power dissipation. If the above circuit is use in switching converter that continuously switching on and off many times in a high frequency rate, we need to consider the switching losses. So

 

pic 18

 

Where Rth(j-c) is

 

pic 19

 

The difference between the junction and case temperatures is only 1.6’C. So, for 100’C case, the equivalent junction temperature is 100’C+1.6’C = 101.6’C.

Now, we can use the normalized RDSon graph to get the required RDSon.

 

pic 20

 

Therefore, the RDSon to use for 100’C case temperature is

 

eq 10

 

Summarizing What We Did

What I demonstrated is how to use MOSFET RDSon data from the datasheet using the detailed approach. There are lot of assumptions as well during the process. In some cases, I am not doing this long method instead go directly for the worst case for easier analysis. If I pass on worst case, I pass to all conditions. However, there are some scenarios that I need the more realistic value of RDSon for instance to compute the efficiency of the circuit. Based on experienced, using the worst case RDSon to efficiency calculation will give huge error compared to the measured efficiency. The long and detailed approach gives the closer result. If you don’t after for this detailed calculation (for example only aiming for stress), you can simply use the ready given worst case RDSon value. You may not mind to derive the difference between the junction and case or ambient temperatures as well if you have huge margin in your design and this simplifies the calculation.

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