How to Select Power MOSFET
Image credit: Advanced Power Electronics Corp. | RS Components | Mouser Electronics
Power MOSFETs are specialized metal oxide semiconductor field-effect transistors (MOSFET) designed to handle significant power levels.
Although early power MOSFET manufacturing used several different designs, the Vertical Diffused MOS (VDMOS) — also known as the Double-diffused MOS (DMOS) — emerged as the predominate design. The terms "vertical" and "diffusion" are adequate to describe the construction of a power MOSFET. "Vertical" refers to the fact that, unlike traditional (or "lateral") MOSFETs, a power MOSFET's source terminal is above its drain terminal, resulting in a primarily vertical current. By contrast, a lateral MOSFET's source and drain terminals are on opposite sides of the device, with the gate situated on the n-doped channel. "Diffusion" refers to the device's manufacturing process: the p-type and n-type wells that sit on the n-type substrate are created using a double-diffusion process.
A VDMOS diagram, showing its vertical construction and N+ and P+ wells.
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Despite the predominance of VDMOS, some lateral power MOSFETs are still used in specialized applications.
Power MOSFET operation is slightly different than that of a lateral e-MOSFET. In lateral MOSFETs, planar construction results in current and breakdown voltage values which are functions of the channel dimensions, rendering the device relatively inefficient. In contrast, the vertical construction of power MOSFETs results in voltage ratings which are functions of the doping and thickness of the n-type well, while current rating remains a function of channel width. Power MOSFETs are therefore able to sustain high voltages and high currents without increasing the size of the device.
The video below provides an excellent introduction to transistors, semiconductors, doping, and, towards the end of the video, power MOSFET.
Video credit: DesignWorldVideo
Power MOSFETs are used in many applications necessitating power semiconductors. Compared to insulated-gate bipolar transistors (IGBT) and thyristors, power MOSFETs feature very high switching speeds and improved efficiency at low voltages. Power MOSFETs are the most common low-voltage (less than 200 V) switch in use, and can be found in power supplies, DC-DC converters, and controllers.
Operating Principles and Specifications
Like all MOSFETs, power MOSFETs switch and regulate a current that flows between the source (S) and drain (contacts) by varying voltage at the gate (G) terminal. By applying a voltage to the gate, a channel is created between the gate and the source, allowing current to flow.
By increasing the gate-source voltage (VGS), the channel becomes larger and the drain current (ID) increases. The relationship between the gate voltage and drain voltage is dependent upon the following equation:
ID = drain current
K = device constant
VGS = gate voltage
VT = threshold voltage
Threshold voltage (VT) refers to the minimum voltage necessary to create a channel. By visually plotting a device's drain current against its various gate-source voltages we can determine the device's transfer characteristics. A graphical representation of a typical n-channel power MOSFET's transfer characteristics is shown below.
Image credit: Circuitstoday
When selecting power MOSFET products, two important specifications to consider include drain saturation current and gate-source cutoff voltage.
Drain saturation current (IDSS) is a measure of drain current saturation, which occurs when the drain-source voltage equals the gate-source voltage. When a MOSFET's drain current reaches a maximum value it remains there despite any increases in the drain-source voltage; this extra voltage is accommodated by a depletion layer located at the drain end of the gate. This condition is known as drain current saturation, and is represented by IDSS as a maximum current value.
Gate-source cutoff voltage (VGS(Off)) is the value of the gate-source voltage (VGS) which results in a drain current (ID) value of close to zero.
Power MOSFETs can be manufactured and tested using standards developed by a diverse range of societies and associations. Examples include:
ASTM F1096 (Measuring MOSFET saturated threshold voltage)
BS IEC 60747-8-4 (MOSFETs for power switching applications)
JEDEC JESD 24 (Power MOSFETs)
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