© Semiconductor Components Industries, LLC, 2016

February, 2016 − Rev. 2

1

Publication Order Number:

AND9083/D

AND9083/D

MOSFET Gate-Charge

Origin and its Applications

Introduction

Engineers often estimate switching time based on total

drive resistances and gate charge or capacitance. Since

capacitance is non-linear, gate charge is an easier parameter

for estimating switching behavior. However, the MOSFET

switching time estimated from datasheet parameters does not

normally match what the oscilloscope shows. This is due to

differences between the parameters taken from the datasheet

and the application conditions. For example, in Figure 1 the

gate charge of NTD5805N was characterized at two

different conditions and results varied greatly. If datasheet

values are characterized at conditions different from the

user, the differences will introduce error in the estimation.

This article will explain how to better estimate gate charge

from datasheets and their applications. For simplicity in this

article, power MOSFET NTD5805N’s datasheet [1] is used

with circuit conditions of 32 V and 30 A.

Figure 1. NTD5805N Gate-to-Source Voltage vs.

Total Charge

0

1

2

3

4

5

6

7

8

9

10

0

5

10

15

20

25

30

35

Inductive Switching

In switch-mode power supplies, MOSFETs switch

inductive loads. Figure 2 shows a basic buck circuit with high

side MOSFET turn on transition. Before the high side

MOSFET is turned on, inductor current is flowing through the

is broken down into three regions (Figure 3). These regions

will be individually explained. Figure 4 shows the transition

through these regions in terms of output characteristics. Gate

charge can be derived from the non-linear capacitance

32 V

30 A

0 à 10 V

+

−

Figure 2. Inductive Switching

−

+

AB

C

30

20

10

0

0

1

4

6

3

9

10

2

5

7

8

0

15

25

10

5

20

30

35

Figure 3. Gate-to-Source Voltage

and Switching vs. Total Charge

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APPLICATION NOTE