AN605 Vishay Semiconductors, AN605 Datasheet

AN605

Manufacturer Part Number

AN605

Description

Manufacturer

Vishay Semiconductors

Datasheet

1.AN605.pdf (4 pages)

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Jess Brown, Guy Moxey

INTRODUCTION

Power MOSFETs have become the standard choice as the

main switching device for low-voltage (<200 V) switchmode

power-supply (SMPS) converter applications. However using

manufacturers’ datasheets to choose or size the correct

device for a specific circuit topology is becoming increasingly

difficult. The main criteria for MOSFET selection are the power

loss associated with the MOSFET (related to the overall

efficiency of the SMPS) and the power-dissipation capability

of the MOSFET (related to the maximum junction temperature

and thermal performance of the package). This application

note focuses on the basic characteristics and understanding

of the MOSFET.

There are several factors which affect the gate

MOSFET, and it is necessary to understand the fundamental

basis of the device structure before the MOSFET behavior can

be explained. This application note details the basic structure

of the Trench MOSFET structure, identifying the parasitic

components and defining related

describes how and why the parasitic parameters occur.

With a large variety of topologies, switching speeds, load

currents, and output voltages

impossible to identify a generic MOSFET that offers the best

performance across the wide range of circuit conditions. In

some circumstances

dominate, and in others it is the switching losses of the

transient current and voltage waveforms, or

associated with driving the gate of the device. It also has been

shown

dominant loss.

INTRODUCING THE FIGURE OF MERIT FOR

MOSFET SELECTION

To add to this confusion,

MOSFET

conditions, diminishing designers’ ability to compare like for

like. Therefore, the only true method of making the correct

MOSFET choice is to compare a selection of devices within

the circuit in which the MOSFET will be used.

There are methods available that, though sometimes difficult

to implement, enable the designer to compare MOSFETs that

appear suited for a given application. One method

evaluating MOSFETs is according to “figure of merit.” In its

simplest form, the figure of merit compares gate charge (Q

against r

certain device technology, which is effectively scalable to

Document Number: 71933

08-Sep-03

1,2

DS(on)

that the input and output capacitances can be the

parameters at different static and dynamic

Understanding MOSFET Characteristics Associated

. The result of this multiplication relates to a

the on-resistance (r

device manufacturers specify

available, it has become

terminology. It also

With The Figure of Merit

Power MOSFET Basics:

DS(on)

the losses

) losses

of the

for

)

achieve the required r

comparing devices is the “Baliga high-frequency figure of

merit,”

switching loss will be associated with the charging and

discharging of the input capacitance (C

uses the “new high-frequency figure of merit,” NHFFOM

which assumes that the dominant switching loss is due to the

charging and discharging of the output capacitance (C

The latter two methods are geared towards the applications in

which the MOSFETs will be implemented. However, these

methods only allow like-for-like comparisons; they do not

enable the user to determine that a device with one figure of

merit is necessarily better than a different device with another.

Figure 1 shows the Q

Vishay Siliconix’s range of 30-V SO-8 n-channel MOSFETs.

The Si4888DY, for example, may be better in certain switching

applications than the Si4842DY, but it is not possible to use this

graph—or other graphs using more complex

merit—to determine objectively the best device for a specific

application.

DS(on)

FIGURE 1. Typical figure of merit for Vishay Siliconix n-channel,

IEEE Electron Device Letters, Vol. 10, No. 10, October 1989, “Power

Semiconductor Device Figure of Merit for High Frequency applications,”

B. Jayant Baliga.

Proc. of 1995 Int. Sym. on Power Semiconductor Devices and ICs,

Hokohama, “New Power Device Figure of Merit for High-Frequency

Applications,” IL-Jung Kim, Satoshi Mastumoto, Tatsuo Sakai, and

Toshiaka Yachi.

0.015

0.014

0.013

0.012

0.011

0.010

0.009

0.008

0.005

0.006

0.007

the higher the gate charge will be. A similar method for

BHFFOM

30-V SO-8 MOSFETs

Si4886

Si4888

, which assumes that the

Si4842

S Siliconix V

DS(on)

Si4880

x r

Gate Charge (nC)

DS(on)

Si4872

or Q

Si4822

figure of merit for a sample of

Vishay Siliconix

. However, the lower the

= 4.5 V

Si4874

Si4420

iss

Si4442

). A third method

Si4430

www.vishay.com

AN605

figures of

dominant

oss

Related parts for AN605

AN605 Summary of contents

Page 1

... B. Jayant Baliga. 2. Proc. of 1995 Int. Sym. on Power Semiconductor Devices and ICs Hokohama, “New Power Device Figure of Merit for High-Frequency Applications,” IL-Jung Kim, Satoshi Mastumoto, Tatsuo Sakai, and Toshiaka Yachi. AN605 Vishay Siliconix However, the lower the DS(on which assumes that the ) ...

Page 2

... AN605 Vishay Siliconix MOSFET STRUCTURE Table 1 identifies the common definitions of the majority of the MOSFET parameters and parasitics found in a Trench MOSFET. TABLE 1 Definitions of MOSFET Parameters Á Á Á Á Á Á Á Á Á Á Á Á Á Á Á Á Á Á ...

Page 3

... FIGURE 4. Simple equivalent circuit for a n-channel MOSFET, Vishay Siliconix Source Gate Interconnect Interconnect N+ + Channel P-Body Spreading Epi Substrate Drain Interconnect b) Source C gs Body Gate Gate Drain Drain Gate Source Drain npn Source showing the parasitic capacitance, npn transistor and R resistor. b www.vishay.com AN605 ...

Page 4

... AN605 Vishay Siliconix DG(on) Turn-Off C DG(off) Turn- FIGURE 5. Gate-drain capacitance over the drain-source voltage range However, it should be noted that because the equivalent circuit is much more complex than a resistor and three capacitors, these capacitances may only be used for understanding the behavior of the switching transients. ...

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