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VISHAY SILICONIX

Power MOSFETsDevice Application Note AN608A

Power MOSFET Basics: Understanding Gate Charge

and Using it to Assess Switching Performance www.vishay.com

APPLICATION NOTE

Revision: 16-Feb-161Document Number: 73217

For technical questions, contact: pmostechsupport@vishay.com

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000

INTRODUCTION

This note is part of a series of application notes that define the fundamental behavior of MOSFETs, both as standalone devices and as switching devices implemented in a Switch

Mode Power Supply (SMPS). Vishay Application Note

AN-605 [1] provides a basic description of the MOSFET and the terminology behind the device, including definitions and physical structure. AN-850 [2] provides a broad, physical description of the switching process. This application note goes into more detail on the switching behavior of the MOSFET when used in a practical application circuit and attempts to enable the reader / designer to choose the right device for the application using the minimum available information from the datasheet. The note goes through several methods of assessing the switching performance of the MOSFET and compares these methods against practical results. Several definitions used within the text are drawn from application note AN-605.

SWITCHING THE MOSFET IN ISOLATION

Using Capacitance

To get a fundamental understanding of the switching behavior of a MOSFET, it is best first to consider the device in isolation and without any external influences. Under these conditions, an equivalent circuit of the MOSFET gate is illustrated in Fig. 1, where the gate consists of an internal gate resistance (R g ), and two input capacitors (C gs and C gd With this simple equivalent circuit it is possible to obtain the output voltage response for a step gate voltage.

The voltage V

GS is the actual voltage at the gate of the device, and it is this point that should be considered when analyzing the switching behavior. Throughout this note upper case subscripts will refer to rated or applied voltages and currents, while measured or time varying quantities will be designated by lower case subscripts. For example the variable gate drain capacitance is designated as C gd while its corresponding charge will be designated as Q GD

If a step input is applied at V

GS , then the following holds true:

Fig. 1 - An Equivalent MOSFET Gate Circuit

Showing Just C

gs , C gd , and R g (1) (2) (3)

Since V

DS is fixed (4) therefore (5) and (6) V DS C gd C gs I gd I gs R g I g V GS V gs I g V GS - V gs R g I g I gs + I gd I gs C gs x dV gs dt-------------= I gd C gd x dV gs V DS dt----------------------------------= = C gd x dV gs dt------------- I g V GS - V gs R g ------------------------I gs + I gd I g C gs dV gs dt------------- + C gd dV gs dt-------------= C gs + C gd =dV gs dt------------- dV gs V GS - V gs --------------------------dt R g x C gs + C gd

Power MOSFET Basics: Understanding Gate Charge

and Using it to Assess Switching Performance

Device Application Note AN608A

www.vishay.com

Vishay Siliconix

APPLICATION NOTE

Revision: 16-Feb-162Document Number: 73217

For technical questions, contact: pmostechsupport@vishay.com

THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000

giving (7) (8) at t = 0 s, V GS = 0 V, solving for K, (9)

We define two parameters R

G and C iss to simplify the equations. R G is the effective total gate resistance defined as the sum of internal gate resistance R g of the MOSFET and any external resistance R gext that is part of the gate drive circuitry. C iss is the effective input capacitance of the

MOSFET as seen by the gate drive circuit.

R G = R g + R gext and C iss = C gs + C gd Rewriting equation (9) with effective values of gate resistance and capacitance In most cases the parameter of importance is not the actual gate voltage but the time taken to reach it. (10) While the RC circuit of Fig. 1 is rather simple, when the MOSFET is considered with additional parasitics, it becomes increasingly difficult to manipulate these equations manually. Therefore a method of analyzing a practical circuit is required. If the second order or parasitic components are ignored, then it is possible to come up with analytical solutions for formulas for the turn-on and turn-off time periods of the MOSFET. These are given in equations (11) through to (16) and the resulting waveforms are shown in Fig. 2 and Fig. 3. These equations are based on those developed in [3], V TH is the MOSFET threshold voltage, and V gp is the gate plateau voltage.

Fig. 2 - Turn-On Transient of the MOSFET

(11) (12) and (13)

This gives accurate t

1 and t 2 when using datasheet values, but the time period t 3 is difficult to calculate since C gd changes with V ds . During t 3 , gate voltage V gs is constant at V gp and all of the gate current goes to discharge C gd from V DS to almost zero. The drain source voltage across the MOSFET when conducting full load current is considered negligible compared to V DS voltage across the MOSFET when it is off. Using the same principles for turn-off, the formulas for the switching transients are given below: (14) (15) (16) -In V GS - V gs t R g x C gs + C gd V gs V GS - K x e -t R g x C gs + C gd V gs V GS 1 - e -t R g x C gs + C gd e -t R G C iss 1 - V gs V GS tR G C iss x 1 1 - V gs V GS ln= t 1 V gs V TH V ds Vquotesdbs_dbs33.pdfusesText_39

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