EiceDRIVER™ - Gate resistor for power devices

EiceDRIVER™ - Gate resistor for power devices
Application Note AN2015-06
EiceDRIVER™
Gate resistor for power devices
About this document
Scope and purpose
This application note explains the role of the gate resistor, and it points out what are the most important issues that
should be considered when selecting the gate resistors of power devices.
Intended audience
Power electronics engineers who want to design reliable and efficient gate driving circuits
Table of Contents
About this document....................................................................................................................... 1
Table of Contents ........................................................................................................................... 1
1
Scope ........................................................................................................................... 2
2
Gate resistor basics ........................................................................................................ 3
3
3.1
3.2
3.3
3.4
3.5
3.6
What should be considered for gate resistance when design a gate driver circuit.................. 4
Gate charging/discharging peak current ............................................................................................... 4
Power dissipation.................................................................................................................................... 5
Switching speed and electromagnetic compatibility ............................................................................ 6
Gate ringing ............................................................................................................................................. 7
Parasitic turn-on...................................................................................................................................... 8
Typical gate circuit ................................................................................................................................ 10
4
4.1
4.2
The tricks to quickly select a proper gate resistor ............................................................ 11
Power rating of gate resistors ............................................................................................................... 11
Rule of Thumb ....................................................................................................................................... 11
5
References ................................................................................................................... 12
Revision History ............................................................................................................................ 13
Application Note AN2015-06 Please read the Important Notice and Warnings at the end of this document
www.infineon.com
<Revision 1.0>
2015-12-17
EiceDRIVER™
Gate resistor for power devices
Scope
1
Scope
This application note explains the most common related issues for the identification of suitable gate resistors for
power electronic devices. It will depict the physical effects of the gate circuits and clarify the related equations.
Please note, that this application note cannot give recommendations for each and every power transistor or power
module. However, it will highlight which considerations are important and the effect they can have on the selection
of the gate resistors for discrete power transistors as well as for power modules.
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
Gate resistor basics
2

Gate resistor basics
The gate resistor is located between the output of the driver and the input of the power device. It is commonly
labelled RG as depicted in Figure 1.
+5V
+15V
VDC
VCC2
CVCC2
RDY
CGC
DESAT
IG_charging
/FLT
IGC
T1
IG
IDC
OUT
IG_discharging RG
RST
To control
logic
+5V
IGE
CGE
VCC
CVCC1
GND2
GND
IN-
CLAMP
VEE2
CVEE2
-8V
RGint
IN+
1ED020I12-F2
Figure 1
Basic gate driver circuit
The on/off status of the gate controlled power devices, for instance IGBT or power MOSFET, is defined by the gate
voltage. The change of this gate voltage influences the charging and discharging of the power device’s gate
capacitor, which consist of the gate-collector capacitor CGC and the gate-emitter capacitor CGE when an IGBT is
taken as example for this application note. The charging of the gate capacitor by IG_charging will turn the power device
on and the discharging of the gate capacitor by IG_discharging of the gate capacitor will turn it off.
The current, charging and discharging the gate, is limited by the gate resistor RG. This will influence the switching
speed of the power device. Besides this, there are also several other influences from the gate resistor:





Limit peak gate current to protect the driver output stage
Dissipate power in the gate loop
Electromagnetic interference
Prevent gate ringing
Avoid parasitic turn-on by carefully choosing gate resistance
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
3
What should be considered for gate resistance when design
a gate driver circuit
The gate resistor as a physical entity has to be sufficient for its purpose in a multitude of aspects. This section
focuses on the gate resistors abilities to define the peak current, its influence to the power dissipation, switching
speed and the correlated switching losses. Furthermore Electromagnetic Compatibility, gate ringing and parasitic
turn-on which are also affected by gate resistance are also explained. Finally a typical gate circuit is introduced.
3.1
Gate charging/discharging peak current
The gate driver IC’s output stage has the maximum allowed peak current values I OH_PEAK for turn-on and IOL_PEAK for
turn-off which are often specified within the driver IC’s datasheet. When the output current of the driver IC is higher
than this peak current value, the output stage could be damaged. Thus it is important to limit the output current for
safety reason. Figure 2 depicts an example and helps to understand it in detail.
VCC2
RDSon
VDC
IG_charging
CGC
IGC
RGon
T1
IDC
RGoff
RDSoff
IGE
IG_discharging
VEE2
Figure 2
CGE
RGint
Gate charging/discharging diagram
Here the RDSon depicts the resistance of the driver IC’s output stage during turn-on and RDSoff depicts the resistance
during turn-off respectively, both are driver IC internal parameters. The resistor R Gon represents the turn-on gate
resistance and RGoff represents the turn-off gate resistance. During IGBT T1 turn-on, the gate charging current
IG_charging is driven by the driver IC’s supply VCC2 through RDSon and RGon to the gate of the IGBT. During IGBT turnoff, the gate discharging current IG_discharging flows from the IGBT’s gate through RGoff and RDSoff to the power ground
of the driver IC. Following this charging/discharging path, the peak current value can be derived by equations (1)
and (2)
𝐼G_charging_peak =
𝑉𝐶𝐶2 − 𝑉𝐸𝐸2
𝑅DSon + 𝑅Gon
𝑎𝑛𝑑 𝐼G_discharging_peak =
𝑉𝐶𝐶2 − 𝑉𝐸𝐸2
.
𝑅DSoff + 𝑅Goff
(1)
(2)
This charging/discharging current must be lower than the peak current allowed for the driver IC’s output stage to
guarantee the safe operation.
𝐼G_charging_peak < 𝐼OH_PEAK
Application Note AN2015-06
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(3)
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
𝐼G_discharging_peak < 𝐼OL_PEAK
3.2
(4)
Power dissipation
The gate resistor has a very important function – help to limit the power dissipation in the driver IC’s output stage. A
higher resistance reduces the current, thus reducing the power to be dissipated in the driver IC.
According to Figure 2, during turn-on the power dissipation at the internal resistance RDSon can be calculated as
𝑃G_charging =
1
𝑅DSon
𝑄G ∙ (𝑉𝐶𝐶2 − 𝑉𝐸𝐸2) ∙ 𝑓SW ∙
.
2
𝑅DSon + 𝑅Gon
(5)
During turn-off, the power dissipation at the internal resistance R DSoff can be calculated as
1
𝑅DSoff
𝑃G_discharging = 𝑄G ∙ (𝑉𝐶𝐶2 − 𝑉𝐸𝐸2) ∙ 𝑓SW ∙
.
2
𝑅DSoff + 𝑅Goff
(6)
Here, fSW is the switching frequency and VCC2 – VEE2 is the gate voltage step between charging and discharging of
the gate capacitance. The value of the charge QG to be considered depends on the gate-emitter voltage VGE and
an accurate approximation can be done using the gate charge curve [1] as given in Figure 3.
Figure 3
Gate charge diagram
In order to conduct quick calculation, some datasheets [2] directly specify the gate charge Q G under the condition
VGE = -15V…+15V. In this case, when the turn-on voltage from the driver IC is +15V and turn-off voltage is -15V,
this value can be directly used in equation (5) and (6). In case the turn-off voltage VGE = 0V or VGE = -8V, the
respective value QG’ can be approximateed as given in equations (7) and (8),
Q G ′|VGE =0..15V = 0.62 ∙ Q G
Application Note AN2015-06
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(7)
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
Q G ′|VGE =−8..15V = 0.75 ∙ Q G .
(8)
The overall power dissipation of the driver IC must remain below the maximum allowed power dissipation value
PD_max, leading to the damand
PG_charging + PG_discharging + PQ < PD_max .
(9)
In this equation, PQ depicts the quiescent power of the driver IC. Combining equation (5), (6) and (9), it can be
easily seen that higher values for the gate resistances R Gon and RGoff will reduce the power dissipation at the driver
IC’s output stage, so that it will help to guarantee a safe operation of the component.
3.3
Switching speed and electromagnetic compatibility
As described in section 3.2, higher gate resistance values can be beneficial from the driver IC’s loss-situation and
thermal point of view. Furthermore, the changing of the gate resistance will influence the switching speed of the
power device, which will have an impact in two directions: efficiency and noise.
Figure 4
Typical switching losses as function of gate resistor for Infineon IKW25T120 [3]
Generally speaking, with higher gate resistance value, the switching speed of the power device will decrease. This
means the switching loss will be increased as illustrated in Figure 4 as an example.
This in turn will finally influence the efficiency. In contrast, when a lower gate resistance is applied, the switching
will become faster which can reduce the switching loss. Meanwhile, the noise induced by di/dt and dv/dt will
increase with higher switching speed. Since the electromagnetic compatibility (EMC) regulation must be fulfilled,
the gate resistance value needs to be chosen carefully.
To get more information about this topic, please refer to the Infineon paper: Balancing losses and noise considerations for choosing the gate resistor [4].
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
3.4
Gate ringing
In power electronic applications, gate ringing is an often observed phenomenon due to parasitics along the gate
loop. As described in Figure 5, OUT is the output of the driver IC, which is used to drive a power device. I G is the
gate current along the gate loop. LP and CGE are the parasitics along the gate loop which form the resonant tank.
With excitation, oscillation could appear.
VDC
VCC2
CGC
T1
OUT
LP
RG
LP
CGE
IG
LP
Figure 5
LP
Gate loop with parasitics
For the applications which use power MOSFETs, this gate ringing effect normally becomes more severe, especially
for parallel driving since more resonance tanks are formed. To avoid the ringing effect, one important measure is to
limit the parasitic inductance LP by layout optimization:


Shorten the trace length to physically keep the gate loop as small as possible which can normally reduce the
parasitic inductance by 1nH/mm.
Connect Kelvin emitter/source when it is available on power device
In some designs, this cannot be achieved due to the geometry. In this case, use of a larger gate resistance value
could be a workaround, which will increase the damping effect and finally suppress the ringing effect. Designers
should be aware that, as already mentioned in section 3.3, increasing the gate resistance value will decrease the
efficiency. Also, considering signal integrity, too high gate resistance value will decouple the gate signal which
comes from the output stage of the driver. This will lead to longer delay times or even operation failure. Therefore,
the value of the gate resistor should be chosen carefully.
Besides the value choosing, the layout of the gate resistor is also important. If it is physically possible, the gate
resistor should be put as close as possible to the gate of the power device.
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
3.5
Parasitic turn-on
Parasitic turn-on is a common phenomenon in many situations. This is a physical effect due to the high dv/dt
combined with the parasitic Miller capacitance.
1ED020I12-F2
VCC2
CGC
T1
RDSon
IDIS
dvCE/dt
RG
RDSoff
CGE
VEE2
Figure 6
Parasitic tun-on
As sketched in Figure 6, high dvCE/dt across the power device T1 will generate a displacement current I DIS through
the Miller capacitor CGC. This is often triggered by turning on the paired device in a half bridge topology. Through
the gate resistor RG, this displacement current IDIS will generate a voltage drop across the gate resistor and will lift
up the gate voltage of the power device. This can be calculated according to equation (10) and (11)
IDIS = CGC ∙
dvCE
dt
VG = IDIS ∙ (R G + R DSoff ).
(10)
(11)
RDSoff depicts the resistance of the driver IC’s pulling down device. Here, the pulling down device is on when the
power device T1 is off. VG is the voltage which appears at the gate of the power device. When V G is higher than the
threshold voltage of the power device T1, the parasitic turn-on happens. This will influence the safety of the system
as bridge shoot-through could appear.
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Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
This parasitic turn-on effect could become more severe when the temperature increases, since the gate threshold
voltage of the power device is normally negatively proportional to temperature. Figure 7 gives an example based on
the datasheet of Infineon’s IGBT IKW25T120 [3].
Figure 7
Gate-emitter threshold voltage as function of junction temperature [3]
One measure to limit this parasitic turn-on effect is to reduce the gate resistance value according to equation (11).
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
What should be considered for gate resistance when design a gate driver circuit
3.6
Typical gate circuit
In many applications, separated turn-on and turn-off gate resistors are used as seen in Figure 8. The series
connected fast recovery diode DGoff is used to keep the gate resistance during turn-off lower than the gate
resistance during turn-on. This is done because the turn-off delay time is often longer than the turn-on delay time
for some of the power devices. It can also help to prevent a capacitive turn-on via the Miller capacitance as
explained in section 3.5. On the other hand, if the value chosen for R Goff is too low, it could lead to large voltage
overshoot across power devices due to too high di/dt during turn-off. So it is always a trade-off between switching
speed and robustness. As a rule of thumb it can be assumed that R Gon=2RGoff.
T1
D1
VCC2
DGoff
Vds
RGoff
T2
OUT
VCC2
RGon
D2
VGate
GND2
Figure 8
Basic gate circuit
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
The tricks to quickly select a proper gate resistor
4
The tricks to quickly select a proper gate resistor
Despite the various influences that are found in choosing a proper gate resistor, considering some basic hints can
help the designer to get a quick start. This section focuses on the power rating of gate resistors and a thumb rule
for quick selection.
4.1
Power rating of gate resistors
Driving the gate is the typical pulse operation. The gate resistor needs to sustain the gate charging and discharging
pulse current repetitively. To select a proper gate resistor, the pulse power capability needs to be checked. As
describe in section 3.1, the peak pulse current during gate charging/discharging can be calculated according to
equation (1) and (2). Then, the pulse power can be derived from equations (12) and (13)
𝑃G_charing_peak = 𝐼 2 G_charging_peak ∙ 𝑅G
(12)
𝑃G_discharing_peak = 𝐼 2 G_discharging_peak ∙ 𝑅G .
(13)
This calculated pulse power must be in the range of the maximum allowed continuous pulse load, as displayed in
Figure 9 as an example, to guarantee a safe and reliable operation.
Figure 9
4.2
Maximum pulse load, continuous pules, Vishay thin film resistor [5]
Rule of Thumb
To quickly find a balance point between stability and efficiency, the gate resistance value between nominal value
specified in the datasheet and twice of this nominal value can be considered as a point to start from. This is only in
the case if there is no time to check through all related issues which are explained in the above chapters. To be
sure the gate resistor really fits into the specified application, individual test must be applied based on the real
system.
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
References
5
References
[1] Infineon Technologies: Application Note; Industrial IGBT Modules Explanation of Technical Information;
Infineon Technologies, Germany
[2] Infineon Technologies: Datasheet; DF200R12W1H3_B27; Infineon Technologies, Germany
[3] Infineon Technologies: Datasheet; IKW25T120; Infineon Technologies, Germany
[4] Infineon Technologies: Paper; Balancing losses and noise - considerations for choosing the gate resistor;
Infineon Technologies, Germany
[5] Vishay: Datasheet; MCA1206AT; Vishay, USA
Application Note AN2015-06
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EiceDRIVER™
Gate resistor for power devices
Revision History
Revision History
Major changes since the last revision
Page or Reference
Description of change
Application Note AN2015-06
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Edition 2015-12-17
Published by
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81726 Munich, Germany
ifx1owners.
© 2015 Infineon Technologies AG.
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Application Note AN2015-06
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