AN4407 Datasheet, PDF (1/19 Pages) STMicroelectronics – Advantage of the use of an added driver source lead in discrete Power MOSFETs

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
generates a voltage signal opposite to the driving signal (V_GS) of a MOSFET, in our case.
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.
January 2015
DocID025572 Rev 1
1/19
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