June 2006

Rev 1

1/30

AN2386

Application note

How to achieve the threshold voltage thermal coefficient

of the MOSFET acting on design parameters

Introduction

Today, the MOSFET devices are used mainly as switches in electronic circuits. In such

operational conditions, the MOSFET device works in switch on and switch off modes.

However, in some applications, as in audio amplifiers or air conditioning, the MOSFET

works in a linear zone. The MOSFET works in a linear zone when either it is subject to a

high voltage, or a high current passes through the device. As it is well known in literature,

during the linear zone operation mode the MOSFET could fail if a thermal run-away occurs.

The failure conditions depend on either of the internal structure of MOSFET or of the

package used. The threshold voltage thermal coefficient (TVTC) is one of the big elements

that could bring the MOSFET to fail. TVTC is achieved deriving the MOSFET threshold

voltage against the temperature. TVTC is a negative coefficient because of when the

temperature increases the threshold voltage decreases. When TVTC increases in absolute

value, the MOSFET becomes thermally instable and a failure could occur. Therefore, in

order to understand if a MOSFET device can be used in an application working in linear

zone in safety conditions, a device with a low TVTC value must be considered and, thus, it is

important to achieve a theoretical expression for it.

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