ON Resistance
What is ON Resistance in Drain to Source?
The resistance value between the Drain and Source of a MOSFET during operation (ON) is called the ON Resistance (RDS(ON)). RDS(ON) is measured by evaluating the voltage drop from drain to source under specific conditions. The smaller this value is, the lower the (power) loss.
Electrical Characteristics Related to ON Resistance
In the case of a transistor, the power consumed is expressed by multiplying the Collector saturation voltage (VCE(sat)) by the Collector current (IC).
(Collector Loss PC) = (Collector Saturation Voltage VCE(sat)) x (Collector Current IC)In contrast, the power consumption of a MOSFET involves the ON Resistance (RDS(on)) between the Drain and Source.
As a result, the power consumed by the MOSFET (PD) is expressed as the ON Resistance multiplied by the square of the Drain current (ID).
This power is dissipated as heat. In general, the ON Resistance of a MOSFET is on the order of Ω or less, and the power consumed is typically lower than a conventional transistor. In other words, the amount of heat generated is less, making it easier to implement heat dissipation measures.
As shown on the graph at left (above), ON Resistance decreases as the Gate-Source voltage increases. The same Gate-Source voltage also changes with current.
Therefore, to calculate power loss it is necessary to use the ON Resistance after taking into consideration the Gate-Source voltage and Drain current. As can be seen from the graph at right (above), the ON Resistance also varies with the temperature, so care must be taken. According to Ohm's Law, a lower RDS(ON) results in greater power efficiency, as less power is lost due to resistance.
ON Resistance Comparison and Positive Temperature Coefficient
Generally, the larger the chip size (surface area) of the MOSFET the smaller the ON resistance. The figure below compares the lowest level of ON Resistance for various compact packages. As the package size increases, the possible chip size also increases, resulting in lower ON resistance. ROHM offers a broad lineup of low ON-resistance products in a variety of package sizes. When selecting, please keep in mind that the larger the package size the lower the ON resistance.
It is important to note that RDS(ON) has a positive temperature coefficient, meaning it increases with rising temperature.
Click on the links below for the parametric search page for each package
DFN0604 (0.6x0.4mm)
DFN1006 (1.0x0.6mm)
DFN2020 (2.0x2.0mm)
It is important to note that the threshold voltage (VGS(th)) is critical for ensuring proper MOSFET operation in these packages. The gate-to-source voltage must be sufficiently exceeded to properly measure the device's on-state resistance (RDS(on)).
