AND9083/D
MOSFET Gate-Charge
Origin and its Applications
Introduction
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APPLICATION NOTE
32 V
+
ID
0 à 10 V
+
−
VDS
−
VGS
10
9
8
ID = 30 A, VDS = 5 V
30 A
7
6
5
ID = 5 A, VDS = 30 V
4
Figure 2. Inductive Switching
3
2
QSW
35
VGS, GATE−TO−SOURCE VOLTAGE (V)
10
1
0
0
5
10
15
20
25
30
35
QG, TOTAL GATE CHARGE (nC)
Figure 1. NTD5805N Gate-to-Source Voltage vs.
Total Charge
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
low side MOSFET’s body diode (VBD). The turn-on transition
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
curves, which are fully characterized at a range of VDS
(VGS = 0 V) and VGS (VDS = 0 V) as shown in Figure 5.
© Semiconductor Components Industries, LLC, 2016
February, 2016 − Rev. 2
9
8
ID
VDS
30
QG
7
6
20
5
VGP
15
4
3
2
25
10
VTH
A
1
0
B
C
5
0
0
ID, DRAIN CURRENT (A),
VDS, DRAIN−SOURCE VOLTAGE (V)
VGS, GATE−TO−SOURCE VOLTAGE (V)
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.
10
20
30
QG, TOTAL GATE CHARGE (nC)
Figure 3. Gate-to-Source Voltage
and Switching vs. Total Charge
1
Publication Order Number:
AND9083/D
AND9083/D
VGS = 5.5 V − 10 V
100
3000
VGS = 5.2 V
80
2500
C, CAPACITANCE (pF)
ID, DRAIN CURRENT (A)
90
5V
70
60
4.5 V
50
40
C
4.2 V
VGP
B
30
4V
20
1
2
3
30
1000
A
Coss
B
3.5 V
0
C
1500
500
A
10
0
Ciss
2000
Crss
0
10
40
5
0
VGS
VDS, DRAIN−TO−SOURCE VOLTAGE (V)
5
10
15
20
25
30
35
40
VDS
GATE−TO−SOURCE OR DRAIN−TO−SOURCE VOLTAGE (V)
Figure 4. On-Region Characteristics for Different
Gate-to-Source Voltages
Figure 5. Capacitance Variation
Region A: MOSFET QGS
Region C: MOSFET Remaining Total Gate Charge
This is the region where gate-to-source voltage (VGS)
rises from 0 V to its plateau voltage (VGP). When the gate
rises from 0 V to its threshold voltage (VTH), the MOSFET
is still off with no drain current (ID) flow and drain-to-source
voltage (VDS) remains clamped. Once gate voltage reaches
VTH, the MOSFET starts conducting and ID rises. Its VDS is
still clamped to VDD + VBD until all inductor current is
being supplied by the MOSFET. In this region, gate current
is used to charge the input capacitance (Ciss) with its VDS
being clamped. Since voltage across gate-to-drain changes
from VDD to VDD – VGP, charge is stored from the input
capacitance curve at that range. It can be approximated by
Equation 1.
This is the region where the MOSFET enters into ohmic
mode operation as seen in the ID−VDS curve (Figure 4). VGS
rises from VGP to driver supply voltage (VGDR). Both ID and
VDS remain relatively constant. ID is still clamped by the
inductor current. As VGS increases, the channel
(VDS = I * RDS(ON)) continue to be more enhanced and VDS
dropped slightly. The charge needed is shown as region C in
Figure 5 and can be calculated by Equation 3.
Q GS ^
ŕ
V DD
V DD * V GP Ciss(V DS) @ dV
QC ^
DD * V GP
GP
GP
Ciss(V GS) @ dV
(eq. 3)
It was explained above how different sections of gate
charge are formed. Circuit conditions determine gate charge
boundaries between regions A, B and C (Figure 6). The
range is set by VDD and VGDR. VGP can be found from
ID−VDS curves at inductor current (ID) and supply voltage
(VDD). With these three voltages found, gate charge equals
to area under those capacitance regions. An example is
shown in Table 1 employing methodology described the
same circuit conditions as characterization data in Figure 1
using only simple estimations. Total gate charge (QGTOT) is
the total amount charge stored by the MOSFET on its gate
up to the driver voltage. Switching gate charge (QSW) is the
amount charge needed to complete ID and VDS transitions.
(eq. 1)
This is the region where VGS is held at VGP and remains
flat. ID clamps to inductor current and VDS clamping effect
is gone, MOSFET’s VDS starts to drop. It can be seen from
ID−VDS curve (Figure 4) that VGS remains relatively
constant at fixed ID with varying VDS. This is the origin of
the flat plateau seen on the gate charge curve. During this
region, the gate current is used to charge the reverse transfer
capacitance (Crss). VDS is decreasing from VDD + VBD to
ID * RDS(ON). Thus the voltage across Crss (gate-to-drain
capacitance) changes from {(VDD + VBD) − VGP} to
{(ID * RDS(ON)) − VGP}. The polarity of voltage is reversed.
Charge (Equation 2) needed for this transition is shown as
the area under region B capacitance curve of Figure 5.
ŕ V0
V
)ŕ
0
GDR
Getting the Gate Charge for Different Conditions
Region B: MOSFET QGD
Q GD ^
ŕ VV
Crss(V DS) @ dV )
(eq. 2)
Crss(V GS) @ dV
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AND9083/D
GATE−TO−SOURCE OR DRAIN−TO−SOURCE VOLTAGE (V)
VGS
VDS
3000
C, CAPACITANCE (pF)
2500
2000
C
1500
A
1000
500
B
0
VGDR
VGP(ID)
VDD − VGP(ID)
VDD
Figure 6. Circuit Parameters Effects
Table 1. ESTIMATION OF GATE CHARGE BASED ON METHOD DESCRIBED
Parameters
VDD = 30 V, ID = 5 A
VDD = 5 V, ID = 30 A
Refer to
VGP
3.6 V
4.2 V
ID – VDS Curve
Region A − Charge
3.6 V * 1.7 nF ≈ 6.1 nC
4.2 V * 1.9 nC ≈ 8.0 nC
Region B − Charge
(30 V – 3.6 V) * 0.2 nF +
3.6 V * 1.1 nF ≈ 9.2 nC
(5 V – 4.2 V) * 0.4 nF +
3.6 V * 1.1 nF ≈ 4.9 nC
Region C − Charge
(10 V – 3.6 V) * 2.7 nF ≈ 18 nC
(10 V – 4.2 V) * 2.7 nF ≈ 15.95 nC
QGTOT
33 nC
29 nC
Capacitance Curve
Sum A, B & C
VTH
2.7 V
2.7 V
Datasheet Value
QSW
(3.6 V – 2.7 V) / 3.6 V * 6.1 nF +
9.2 nC ≈ 11 nC
(4.2 V – 2.7 V) / 4.2 V * 8.0 nF +
4.9 nC ≈ 7.8 nC
QA(after VTH) + QB
32 V
Resistive Switching
LED and heating coil are examples of resistive switching.
The main difference between inductive and resistive
switching is that there is no clamping of drain current
involved. Before reaching its threshold voltage, the FET is
off. When the MOSFET starts to turn-on in the saturation
region, VDS is dependent on resistive load and voltage
supply. Once the FET is in ohmic mode, the MOSFET and
the load become a simple resistor divider. There is no flat
plateau region as both VDS and ID are changing resulting in
increasing VGS (Figure 9 region E). Fortunately, QSW and
QGTOT are unchanged from inductive switching.
32 / 30 W
+
0 à 10 V
+
−
ID
VGS
Figure 7. Resistive Switching
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3
VDS
−
AND9083/D
QSW
35
ID
VDS
8
30
25
7
6
20
5
4
15
3
10
2
VTH
F
E
1
0
5
D
0
0
10
20
100
90
ID, DRAIN CURRENT (A)
VGS, GATE−TO−SOURCE VOLTAGE (V)
9
ID, DRAIN CURRENT (A),
VDS, DRAIN−SOURCE VOLTAGE (V)
10
80
70
60
50
40
F
30
E
20
10
0
30
D
0
QG, TOTAL GATE CHARGE (nC)
10
20
30
40
VDS, DRAIN−TO−SOURCE VOLTAGE (V)
Figure 8. Gate-to-Source Voltage and Switching
vs. Total Charge (Resistive Switching)
Figure 9. On-Region Characteristics for Different
Gate-to-Source Voltages
Gate Charge Applications
charge (QSW) is the amount of current the gate driver needed
to supply to complete the switching transitions of drain
voltage and current. Gate charge loss (PQG) is power
dissipated due to charging and discharging of the gate.
One important aspect of MOSFET applications is the
power losses. There are several power loss components.
Conduction loss is power dissipated in the resistive element
(RDSON) of the channel. Switching loss (PSW) is power
dissipated in switching current and voltage. Switching gate
P QG + Q GTOT@VGDR @ V GDR @ F SW
(eq. 4)
Q SW + Q GS(afterVth) ) Q GD
(eq. 5)
T SW(ON) + Q SWń
ǒ
Ǔ
ǒ
Ǔ
V GDR * V GP
V GP
, T SW(OFF) + Q SWń
R DR ) R G
R DR ) R G
(eq. 6)
P SW(inductive) + 0.5 @ V DD @ I D @ ǒT SW(ON) ) T SW(OFF)Ǔ @ F SW
(eq. 7)
P SW(resistive) + 0.25 @ V DD @ I D @ ǒT SW(ON) ) T SW(OFF)Ǔ @ F SW
(eq. 8)
inductive switching or short-circuit performance can also be
evaluated.
Derivations above do not apply to zero voltage switching
applications. For example in synchronous rectification,
MOSFET has a negative diode voltage drop across VDS
(body diode conduction) before it is turned on. They can still
be derived from the capacitances (VGS side) and ID−VDS
curve using the same idea.
40
QGTOT@10 VGS
GATE CHARGE (nC)
35
Conclusion
With different circuit conditions, it has been shown how
datasheet gate charge parameters changes. Only simple
mathematics is needed in getting the right gate charge.
The origins of gate charge are analytically explained.
Through understanding of MOSFET gate charge, more
accurate estimations can be made in designing for different
circuit conditions (Figure 10). Trade offs are evaluated in
selecting gate drive schemes. A lower gate drive voltage
would save some energy but must be balanced between
higher on-resistance. Using methods described by D. Lee in
[2], extreme operating conditions like repetitive unclamped
30
25
ID = 1 A
ID = 50 A
20
15
QSW
10
5
0
0
10
20
30
VDS, DRAIN−TO−SOURCE VOLTAGE (V)
Figure 10. NTD5805N Gate Charge at Various
Conditions
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4
40
AND9083/D
APPENDIX A: ESTIMATION WITHOUT CAPACITANCE-vs-VGS CURVE
3000
C, CAPACITANCE (pF)
2500
Ciss
2000
1500
1000
Coss
500
0
10
5
Vgs
Crss
0
5
10
Vds
15
20
25
30
35
40
GATE−TO−SOURCE OR DRAIN−TO−SOURCE VOLTAGE (V)
Figure 11. NTD5805N Capacitance Curves
Since most of the MOSFETs datasheet are without
Capacitance-vs-VGS curve (shaded part of the Figure 11),
estimation will have to be made based on the available
information. The missing Capacitance-vs-VGS curves will
concern region B and region C.
Figure 12. Circuit Parameters Effects
For region C, we can estimate the gate charge after
VGP(ID) due to its constant capacitance.
For region B, we can assume VGP(ID) are relative constant
in modern trench MOSFET devices. Due to the high trench
density (high transconductance), a large change in drain
current, ID, only resulted in small increase in gate plateau
voltage, VGP(ID).
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5
AND9083/D
For example using 40 V NTMFS5C442NL,
220
VDS = 3 V
160
200
3.8 V
180
160
ID, DRAIN CURRENT (A)
ID, DRAIN CURRENT (A)
180
4.0 V
10 V to 4.5 V
3.6 V
140
3.4 V
120
3.2 V
100
80
3.0 V
60
2.8 V
40
140
120
100
80
TJ = 25°C
60
40
TJ = 125°C
20
20
0
TJ = −55°C
0
0
0.5
1.0
1.5
2.0
2.5
3.0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
VDS, DRAIN−TO−SOURCE VOLTAGE (V)
VGS, GATE−TO−SOURCE VOLTAGE (V)
Figure A. On-Region Characteristics
Figure B. Transfer Characteristics
4.0
Figure 13. NTMFS5C442NL Datasheet Curves
for every 15 A increase or decrease in drain current. We can
conclude that VGP for modern trench MOSFET devices are
relative constant due to high transconductance.
From Figure A and B of NTMFS5C442NL, we can see
that when Gate-to-Source voltage, VGS, changed from 3.0 V
to 3.2 V the drain current, ID, increase by 30 A. Therefore,
it implied gate plateau VGP change by approximately 0.1 V
Table 2. NTMFS5C442NL DATASHEET PARAMETERS
Parameter
Symbol
Test Condition
Min
Typ
Max
Total Gate Charge
QG(TOT)
VGS = 4.5 V, VDS = 32 V, ID = 50 A
−
23
−
Total Gate Charge
QG(TOT)
VGS = 10 V, VDS = 32 V, ID = 50 A
−
50
−
Threshold Gate Charge
QG(TH)
−
5.0
−
−
9.8
−
−
6.7
−
−
3.1
−
Gate-to-Source Charge
QGS
Gate-to-Drain Charge
QGD
Plateau Voltage
VGP
VGS = 4.5 V, VDS = 32 V, ID = 50 A
Figure 14. NTMFS5C442NL Capacitance Curves with Datasheet Test Conditions
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6
Unit
nC
V
AND9083/D
Region A = QGS = 9.8 nC (estimated from curve = 3.1 V * 3100 pF = 9.6 nC)
Region B = QGD = 6.7 nC
Region C = QGTOT – QGS – QGD = 33.5 nC
Calculate for Different Test Conditions
For example at VGS = 6 V, VDS = 20 V, ID = 20 A:
Figure 15. NTMFS5C442NL Capacitance Curves with New Test Conditions
Region A = 3.1 V * 3100 pF = 9.6 nC
Region B = 6.7 nC – (12 V * 100 pF) = 5.5 nC
Region C = 33.5 nC / (10 V – 3.1 V) * (6 V – 3.1 V) = 14.1 nC
Figure 16. Graphic Representation of Change in Above NTMFS5C442NL Estimation
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AND9083/D
Figure 17. Gate Charge Comparison between Test Conditions
The change in gate change can be seen in Figure 16 with
new test condition in shaded regions.
REFERENCES
[1] ON Semiconductor, “Power MOSFET 40 V
NTD5805N Datasheet”,
http://www.onsemi.com/pub_link/Collateral/
NTD5805N−D.PDF
[2] ON Semiconductor, “MOSFET Transient Junction
Temperature Under Repetitive UIS/Short-Circuit
Conditions”,
http://www.onsemi.com/pub_link/Collateral/
AND9042−D.PDF
[3] ON Semiconductor, “Power MOSFET 40 V
NTMFS5C442NL Datasheet”,
http://www.onsemi.com/pub_link/Collateral/
NTMFS5C442NL-D.PDF
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