January 2015

DocID025572 Rev 1

1/19

AN4407

Application note

Advantage of the use of an added driver source lead in discrete

Power MOSFETs

Antonino Gaito, Marc Laudani, Massimo Nania, Cristiano Gianluca Stella

Introduction

In modern power supply design, more and more attention is given to the electrical efficiency

of an overall system and to the junction temperature of semiconductor devices handling all

the power to be converted into a usable form.

Among all the semiconductor devices, transistors are by far the most important category;

almost all of them are three pin devices (MOSFET, BJT, IGBT) and, as opposed to diodes,

they have a driving section which makes them more sensitive to issues related to the

interaction between power to handle and input signal. Even if this document focuses on

Power MOSFETs, in some cases the results could be extended to other power transistors

depending on the current level to be switched and on the switching speed. The aim of this

document is to illustrate the limitation related to a 3-pin device and detail the advantages of

using a fourth driving source pin, also known as Kelvin source, besides the traditional power

source.

In a 3-pin device, during every switching (turn-on or turn-off) cycle, the stray inductance of

the source wire bonding coupled with the slope of the current being interrupted always

The effect of this opposing signal is to slow down the switching cycle, which in turn

increases the cycle by cycle switching loss. The introduction of the Kelvin source pin allows

separating the path of the power from that of the driving signal and to refer the driving

network to the Kelvin source, where no current is supposed to flow. In this situation, it is

possible to obtain a driving signal which is immune to any disturbance deriving from the

verylarge and fast current variation flowing though the power path. This results in the

reduction of the overall power loss in the transistor, and consequently a lower operating

temperature and potentially more reliable and longer lasting power systems.

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