Power Management 101: Power MOSFET Charactertics

May 15, 2009 1:49 PM

What are the important power MOSFET characteristics?

To understand the planar and trench MOSFET characteristics, check several parameters critical to their performance:

• Blocking voltage (BVDSS)
• Maximum single pulse avalanche energy (EAS)
• On-resistance (RDS(ON))
• Maximum junction temperature (TJ(max))
• Continuous drain current (ID)
• Safe operating area (SOA)
• Gate charge (QG)
• Threshold Voltage (VGS(th))
• Body-Diode Forward Voltage (VSD)
• Maximum Allowable Power Dissipation (PD)
• Thermal Resistance, Junction-to-Case (Rθjc)
• dv/dt capability

The following figures are plots of the Vishay SiE848DF that is an N-Channel, 30 V trench power MOSFET housed in a PolarPAK® package. The MOSFET is package-limited at 60A and 25°C.

What is the blocking voltage?

Blocking voltage, BVDSS, is the maximum voltage that can be applied to the MOSFET. When driving an inductive load, this includes the applied voltage plus any inductively induced voltage. With inductive loads, the voltage across the MOSFET can actually be twice the applied voltage.

What are the avalanche characteristics of a MOSFET?

This determines how much energy the MOSFET can withstand under avalanche conditions. Avalanche occurs if the maximum drain-to-source voltage is exceeded and current rushes through the device. The higher the avalanche value the more rugged the device. The avalanche condition can cause two possible failure modes that can destroy a MOSFET. The most destructive is "bipolar latching" that occurs if the device current causes a voltage drop across its internal device resistance, resulting in transistor action and latching of the parasitic bipolar structure of the MOSFET. A second failure mode is thermal, which occurs if the avalanche condition raises the device temperature above its maximum junction temperature.

Trench technology offers an avalanche capability approaching industry-leading planar technology. To ensure satisfactory performance, devices in this technology can be fully characterized for single pulse avalanche energy (EAS) up to their maximum junction temperature. The higher the EAS, the more rugged the device. Some devices are rated in terms of EAR, the repetitive avalanche energy.

Trench technology provides the desirable characteristics of low on-resistance sometimes at the expense of high avalanche energy. Trench power MOSFET technology provides 15% lower device on-resistance per unit area than existing benchmark planar technologies but usually at the cost of higher charge. And, the trench technology allows 10% lower on-resistance temperature coefficient.

What is Power MOSFET On-Resistance, RDS(on)

For both planar and Trench MOSFETs, on-resistance is important because it determines the power loss and heating of the power semiconductor. The lower the on-resistance the lower the device power loss and the cooler it will operate. This is particularly important in applications where the nominal operating temperatures usually exceed 125°C. Low on-resistance drastically reduces heat-sinking requirements in many applications, which lowers parts count and assembly costs. In many applications, the low on-resistance also eliminates the need to parallel MOSFETs for low on-resistance, which leads to improved reliability and lower overall system cost than previous MOSFET generations.

RDS(on) decreases with increasing cell density. The cell density has increased over the years from around half a million per square inch in 1980 to around eight million for planar MOSFETs and around 12 million and higher for trench technology.

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