Let’s start with the shorthand notation of the NMOS and the PMOS FETs in enhancement mode. In particular, some of the parts of the symbol represent the physical layout of that particular type of FET. If you’re not comfortable with these terms, then this tutorial is not going to make much sense. The 48-V system incorporates low-side hot-swap control and pass-MOSFET +12-V systems use a high-side controller and pass-MOSFET. It’s so important, we actually created a tutorial talking about it and recommend you go check it out if you’re not confident. Hot-Swap Topologies The two system-power levels commonly found in high-availability systems, 48 V and +12 V, use different configurations for hot-swap protection. However, we will review the IEEE standard symbols and that should help in the vast majority of situations.īefore we jump into it, it’s important to understand the difference between NMOS and PMOS FETs as well as the difference between depletion and enhancement mode FETs. There are also different requirements when dealing with discrete MOSFETs or MOSFETs in an integrated circuit. While we feel this will be very helpful, there are enough personal and company variations that we can’t guarantee that there will still occasionally be confusion. In this tutorial, we will help you know the difference between the different MOSFET symbols that you come across. Retrieved April 30, 2017.There are well over a dozen different MOSFET schematic symbols in circulation and, between the different symbols that represent the same thing and the many different types of MOSFETs to be represented, this can become incredibly confusing. R DS(on) (T) = R DS(on) x (25☌) x (T/300) 2.3, where T is absolute temperature. R DS(on) is a function of temperature as defined by the following formula: R DS(on) increases with increasing temperature (this is also known as a positive temperature coefficient.) This is because of the mobility of the hole and electron decrease with increasing temperature. The latter can be identified as R WCML, or the total of the bond wire resistance, contact resistance, and the resistance of the lead frame. (Source: AN-9010 MOSFET Basics by ON Semi)īesides these inherent structural contributors to R DS(on), imperfect contact between the source and drain metal and even the wiring that connects the die to the leads on the package can also contribute to R DS(on). A MOSFET vertical structure, showing the total resistances that make up RDS(on). See Figure 2 of a vertical structure of a MOSFET and a series of resistances in series forms the total R DS(on) in the path of current flow from source to drain. However, in low voltage MOSFETs it can have a large effect on R DS(on). R S is the resistance of the substrate itself and can be ignored in high-voltage MOSFETs. R D is the drift region resistance and the most important factor in high-voltage MOSFETs. R J is the resistance of an area called the JFET region. R A is the resistance of an area called the accumulation region. R N is the source region’s diffusion resistance. R DS(on), the total resistance in the path from source to drain, is made up of a series of resistances that traverses the path of current flow. The inversion layer becomes the conductive path (or channel) of the MOSFET between drain and source. When V GS reaches the threshold voltage V GS(th), an inversion layer forms that enables current flow. Current flows between the n-channels when a gate-to-source voltage (V GS) is applied, otherwise the MOSFET behaves like a resistor. Example applications for power MOSFETs include Switched Mode Power Supplies (SMPS), motor control, automotive, and in any application where a heavy-duty electronic switch is needed, such as a driver. MOSFETs make perfect switching devices and are often used in power applications. The voltage applied to the Gate determines if current will flow between the Source and Drain terminals. All things being equal, the lower the R DS(on), the better. R DS(on) stands for “drain-source on resistance,” or the total resistance between the drain and source in a Metal Oxide Field Effect Transistor, or MOSFET when the MOSFET is “on.” R DS(on) is the basis for a maximum current rating of the MOSFET and is also associated with current loss.
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