Interpreting MOSFET ID/Vds curve for a MOSFET Switch

In summary, when choosing a logic-level MOSFET as a switch for an IC working at 3.3V, it is important to ensure that the MOSFET can provide a low enough voltage at the drain for the IC to read as low. This can be determined by looking at the drain current vs. V(DS) graph in the datasheet of the MOSFET and using the Rds(on) value at a gate voltage of 2.5V and a drain current of 500mA to calculate the maximum possible Vds. This value can then be adjusted for temperature using Figure 3 in the datasheet.
  • #1
saad87
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0
Hello,

I'm considering using a logic-level MOSFET as a switch in a design. The drain of the MOSFET will be connected to an IC. The IC works at 3.3V and the maximum voltage at which it's guaranteed to read a low is just 0.8V.

Now, I think every MOSFET will able to satisfy this but I'm just trying to understand how can I tell? From looking at the datasheet of http://www.fairchildsemi.com/ds/FD%2FFDG6317NZ.pdf" , the drain current vs. V(DS) graph seems relevant.

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Let's assume I need a current of 500mA through the MOSFET. If I look at the above graph, it suggests that when Id = 500mA, the drain to source voltage should be around about 0.25V, for a gate voltage of 3V. Is my analysis correct?
 
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  • #2
saad87 said:
Is my analysis correct?

Yes, but keep in mind those curves are for a "typical" NFET at "typical" temperatures (see the heading for the graphs).

For a one off prototype this will very likely be fine. But if you were going to build many of these then you would following the process below.

The nominal curve let's you know you'll be in the saturated region. This means you can determine Vds by Id*Rds(on). So for a worst case you can use the Rds(on) @ Vgs=2.5V,Id=0.6 line item spec so for Id=500mA I would use Vds(max)=0.55Ω*0.5A=0.275V.

You can then use Figure 3 to margin for temperature, if applicable. So if a Tj of 100C was possible then you would multiply this result by 1.275 for a new Vds(max) of 0.35V over process and temperature.
 

1. What is a MOSFET switch?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) switch is a type of electronic switch that uses a voltage applied to its gate terminal to control the flow of current between its source and drain terminals. It is commonly used in electronic circuits for switching and amplification purposes.

2. How do I interpret the ID/Vds curve for a MOSFET switch?

The ID/Vds curve is a graphical representation of the relationship between the drain current (ID) and the drain-to-source voltage (Vds) for a MOSFET switch. It helps to determine the operating characteristics and performance of the switch. The curve typically has three regions: the cutoff region, the linear region, and the saturation region. The cutoff region is when the switch is off, the linear region is when the switch is partially on, and the saturation region is when the switch is fully on. By analyzing the curve, you can determine the switch's threshold voltage, on-resistance, and maximum current handling capability.

3. What affects the shape of the ID/Vds curve for a MOSFET switch?

The shape of the ID/Vds curve is affected by the physical properties of the MOSFET, such as the channel length, channel width, and gate oxide thickness. It is also influenced by the operating conditions, including the gate voltage, temperature, and source-drain voltage. Changes in these factors can cause the curve to shift or change shape.

4. How does the ID/Vds curve differ for different types of MOSFET switches?

The ID/Vds curve can vary for different types of MOSFET switches, such as enhancement-mode and depletion-mode MOSFETs. In enhancement-mode MOSFETs, the curve is typically shifted to the right, indicating a higher threshold voltage, while in depletion-mode MOSFETs, the curve is shifted to the left, indicating a lower threshold voltage. Additionally, the saturation region of the curve may differ, depending on the MOSFET's design and intended use.

5. What are some potential issues when interpreting the ID/Vds curve for a MOSFET switch?

One potential issue when interpreting the ID/Vds curve is that it may not be accurate for all operating conditions. The curve is typically generated under specific conditions, and changes in temperature or other factors may affect its accuracy. Another issue is that the curve may not be applicable for all types of MOSFETs. It is essential to understand the specifications and characteristics of the specific MOSFET switch you are analyzing to properly interpret its ID/Vds curve.

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