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Application Note AN-6076
Design and Application Guide of Bootstrap Circuit for
High-Voltage Gate-Drive IC
The purpose of this paper is to demonstrate a systematic
approach to design high-performance bootstrap gate drive
circuits for high-frequency, high-power, and high-efficiency
switching applications using a power MOSFET and IGBT.
It should be of interest to power electronics engineers at all
levels of experience. In the most of switching applications,
efficiency focuses on switching losses that are mainly dependent on switching speed. Therefore, the switching characteristics are very important in most of the high-power switching
applications presented in this paper. One of the most widely
used methods to supply power to the high-side gate drive circuitry of the high-voltage gate-drive IC is the bootstrap
power supply. This bootstrap power supply technique has the
advantage of being simple and low cost. However, it has
some limitations, on time of duty-cycle is limited by the
requirement to refresh the charge in the bootstrap capacitor
and serious problems occur when the negative voltage is presented at the source of the switching device. The most popular bootstrap circuit solutions are analyzed; including the
effects of parasitic elements, the bootstrap resistor, and
capacitor; on the charge of the floating supply application.
2. High-Speed Gate-Driver Circuitry
2.1 Bootstrap Gate-Drive Technique
The focus of this topic is the bootstrap gate-drive circuit
requirements of the power MOSFET and IGBT in various
switching-mode power-conversion applications. Where
input voltage levels prohibit the use of direct-gate drive circuits for high-side N-channel power MOSFET or IGBT, the
principle of bootstrap gate-drive technique can be considered. This method is utilized as a gate drive and accompanying bias circuit, both referenced to the source of the main
switching device. Both the driver and bias circuit swing
between the two input voltage rails together with the source
of the device. However, the driver and its floating bias can
be implemented by low-voltage circuit elements since the
input voltage is never applied across their components. The
driver and the ground referenced control signal are linked by
a level shift circuit that must tolerate the high-voltage difference and considerable capacitive switching currents between
the floating high-side and ground-referenced low-side circuits. The high-voltage gate-drive ICs are differentiated by
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
unique level-shift design. To maintain high efficiency and
manageable power dissipation, the level-shifters should not
draw any current during the on-time of the main switch.
A widely used technique for these applications is called
pulsed latch level translators, shown in Figure 1.
VB
UVLO
PULSE GENERATOR
IN
R
NOISE
CANCELLER
S
R
Q
Shoot-through current
compensated gate driver
1. Introduction
HO
VS
Figure 1. Level-Shifter in High-Side Drive IC
2.2 Bootstrap Drive Circuit Operation
The bootstrap circuit is useful in a high-voltage gate driver
and operates as follows. When the VS goes below the IC
supply voltage VDD or is pulled down to ground (the lowside switch is turned on and the high-side switch is turned
off), the bootstrap capacitor, CBOOT, charges through the
bootstrap resistor, RBOOT, and bootstrap diode, DBOOT, from
the VDD power supply, as shown in Figure 2. This is provided by VBS when VS is pulled to a higher voltage by the
high-side switch, the VBS supply floats and the bootstrap
diode reverses bias and blocks the rail voltage (the low-side
switch is turned off and high-side switch is turned on) from
the IC supply voltage, VDD.
RBOOT
DBOOT
DC SUPPLY
Bootstrap charge current path
Bootstrap discharge current path
VB
RG1
VDD
HO
Q1
ILOAD
CBOOT
VDD
VS
LOAD
RG2
COM
Q2
LO
Figure 2. Bootstrap Power Supply Circuit
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AN-6076
APPLICATION NOTE
2.3 Drawback of Bootstrap Circuitry
2.4 Cause of Negative Voltage on VS Pin
The bootstrap circuit has the advantage of being simple and
low cost, but has some limitations.
A well-known event that triggers VS go below COM
(ground) is the forward biasing of the low-side freewheeling
diode, as shown in Figure 5.
Duty-cycle and on time is limited by the requirement to
refresh the charge in the bootstrap capacitor, CBOOT.
Major issues may appear during commutation, just before
the freewheeling diode starts clamping.
The biggest difficulty with this circuit is that the negative
voltage present at the source of the switching device during
turn-off causes load current to suddenly flow in the low-side
freewheeling diode, as shown in Figure 3.
In this case, the inductive parasitic elements, LS1 and LS2,
may push VS below COM, more than as described above or
normal steady-state condition.
This negative voltage can be trouble for the gate driver’s output stage because it directly affects the source VS pin of the
driver or PWM control IC and might pull some of the internal circuitry significantly below ground, as shown in Figure
4. The other problem caused by the negative voltage transient is the possibility to develop an over-voltage condition
across the bootstrap capacitor.
The amplitude of negative voltage is proportional to the parasitic inductances and the turn-off speed, di/dt, of the switching device; as determined by the gate drive resistor, RGATE,
and input capacitance, Ciss, of switching device.
It is sum of Cgs and Cgd, called Miller capacitance.
VCC
The bootstrap capacitor, CBOOT, is peak charged by the bootstrap diode, DBOOT, from VDD the power source.
DBOOT
VDC
Since the VDD power source is referenced to ground, the
maximum voltage that can build on the bootstrap capacitor is
the sum of VDD and the amplitude of the negative voltage at
the source terminal.
INPUT
IN
VB
CBOOT
HVIC
VDD
HO
Q1
A
B
RGATE
CDRV
VS
C
C
LS2
DC SUPPLY
RBOOT
iLOAD
LS1
GND
DBOOT
iFree
GND
- VS
COUT
VOUT
D1
VB
RG1
VDD
HIN
HO
LIN
LIN
Figure 5. Step-Down Converter Applications
Q1 High Side OFF
CBOOT
HIN
Ls1
iLoad
VS
Figure 6 shows the waveforms of the high-side, N-channel
MOSFET during turn-off.
ifree
Ls2
CIN
RG2
COM
LO
Q2Freewheeling Path
Figure 3. Half-Bridge Application Circuits
A-Point
HIN
B-Point
VBS
VDC+VGS,Miller
t
C-Point
VDC
VS -COM
Recovery Time
t
-VS
VGS=B-C Point
Freewheeling
Figure 4. VS Waveforms During Turn-off
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
Figure 6. Waveforms During Turn-off
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2
AN-6076
APPLICATION NOTE
2.5 Effects in the Undershoot Spike on VS Pin
2.6 Consideration of Latch-up Problem
If undershoot exceeds the absolute maximum rating specified in the datasheet, the gate drive IC suffers damage or the
gate drive IC temporarily latches in its current state.
The most integrated high-voltage gate-drive ICs have parasitic diodes, which, in forward or reverse break-down, may
cause parasitic SCR latch-up. The ultimate outcome of latchup often defies prediction and can range from temporary
erratic operation to total device failure. The gate-drive IC
may also be damaged indirectly by a chain of events following initial overstress. For example, latch-up could conceivably result in both output drivers assuming a HIGH state,
causing cross-conduction followed by switch failure and,
finally, catastrophic damage to the gate-drive IC. This failure
mode should be considered a possible root-cause, if power
transistors and/or gate-drive IC are destroyed in the application. The following theoretical extremes can be used to help
explain the relationships between excessive VS undershoot
and the resulting latch-up mechanism.
Figure 7 shows the high-side output not changed by input
signal, which represents latch-up condition where external,
main, high- and low-side switches are in short-circuit condition in half-bridge topology.
INPUT
In the first case, an "ideal bootstrap circuit" is used in which
VDD is driven from a zero-ohm supply with an ideal diode
feed VB, as shown in Figure 9. When the high current flowing through freewheeling diode, VS voltage is below ground
level by high di/dt. This time, latch-up risk appears since
internal parasitic diode, DBS of the gate driver ultimately
enters conduction from VS to VB, causing the undershoot
voltage to sum with VDD, causing the bootstrap capacitor to
overcharge, as shown Figure 10.
OUTPUT
Latch-Up Problem
For example, if VDD=15V, then VS undershoot in excess of
10V forces the floating supply above 25V, risking breakdown in diode DBS and subsequent latch-up.
Figure 7. Waveforms in Case of Latch-up
If undershoot exceeds the absolute maximum rating specified in the datasheet, the gate drive IC does not suffer damage. However, the high-side output does not respond to input
transitions while in undershoot condition as shown in Figure
8. In this situation, the level shifter of the high-side gate
driver suffers from a lack of the operating voltage headroom.
This should be noted, but proves trivial in most applications,
as the high-side is not usually required to change state immediately following a switching event.
VDD
VB
DBS
COM
VS
Gate Driver
Figure 9. Case 1: Ideal Bootstrap Circuits
INPUT
VB
VS
OUTPUT
Signal Missing Problem
HIGH VBS
Figure 8. Waveforms in Case of Signal Missing
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
GND
Figure 10. VB and VS Waveforms of Case 1
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3
AN-6076
APPLICATION NOTE
2.7 Effect of Parasitic Inductances
Suppose that the bootstrap supply is replaced with the ideal
floating supply, as shown in Figure 11, such that VBS is fixed
under all circumstances. Note that using a low impedance
auxiliary supply in place of a bootstrap circuit can approach
this situation. This time, latch-up risk appears if VS undershoot exceeds the VBS maximum specified in datasheet,
since parasitic diode DBCOM ultimately enters conduction
from COM to VB, as shown in Figure 12.
VCC
VCC
The amplitude of negative voltage is:
VS − COM = −(VRBOOT + VFDBOOT ) − (LS1 + LS 2 ) di dt
To reduce the slope of current flowing in the parasitic inductances to minimize the derivative terms in Equation 1.
For example, if a 10 ampere, 25V gate driver with 100nH
parasitic inductance switches in 50ns, the negative voltage
spike between VS and ground is 20V.
VB
3. Design Procedure of Bootstrap
Components
DBCOM
COM
(1)
3.1 Select the Bootstrap Capacitor
VS
The bootstrap capacitor (CBOOT) is charged every time the
low-side driver is on and the output pin is below the supply
voltage (VDD) of the gate driver. The bootstrap capacitor is
discharged only when the high-side switch is turned on. This
bootstrap capacitor is the supply voltage (VBS) for the high
circuit section. The first parameter to take into account is the
maximum voltage drop that we have to guarantee when the
high-side switch is in on state. The maximum allowable voltage drop (VBOOT) depends on the minimum gate drive voltage (for the high-side switch) to maintain. If VGSMIN is the
minimum gate-source voltage, the capacitor drop must be:
Gate Driver
Figure 11. Case 2: Ideal Floating Supply
VB
VS
VB Below COM
GND
Δ V BOOT = V DD − V F − VGSMIN
(2)
where:
Figure 12. VB and VS Waveforms of Case 2
VDD = Supply voltage of gate driver [V]; and
A practical circuit is likely to fall somewhere between these
two extremes, resulting in both a small increase of VBS and
some VB droop below VDD, as shown in Figure 13.
VF = Bootstrap diode forward voltage drop [V]
The value of bootstrap capacitor is calculated by:
CBOOT =
VB
QTOTAL
ΔVBOOT
(3)
VS
where QTOTAL is the total amount of the charge supplied by
the capacitor.
VB close to COM
The total charge supplied by the bootstrap capacitor is calculated by equation 4.:
GND
Increased VBS
QTOTAL = QGATE + (ILKCAP + ILKGS + IQBS + ILK + ILKDIODE) ⋅ tON + QLS
Figure 13. Typical Response of VB and VS
(4)
where:
Exactly which of the two extremes is prevalent can be
checked as follows. If the VS pins undershoot spike has a
time length that is on order of tenths of nanoseconds; the
bootstrap capacitor, CBOOT, can become overcharged and the
high-side gate-driver circuit has damage by over-voltage
stress because it exceeds an absolute maximum voltage
(VBSMAX) specified in datasheet. Design to a bootstrap circuit, that does not exceed the absolute maximum rating of
high-side gate driver.
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
QGATE = Total gate charge;
ILKGS = Switch gate-source leakage current;
ILKCAP = Bootstrap capacitor leakage current;
IQBS = Bootstrap circuit quiescent current;
ILK = Bootstrap circuit leakage current;
QLS = Charge required by the internal level shifter, which is
set to 3nC for all HV gate drivers;
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AN-6076
APPLICATION NOTE
The voltage drop due to the external diode is nearly 0.7V.
Assume the capacitor charging time is equal to the high-side
on-time (duty cycle 50%). According to different bootstrap
capacitor values, the following equation applies:
tON = High-side switch on time; and
ILKDIODED = Bootstrap diode leakage current.
The capacitor leakage current is important only if an electrolytic capacitor is used; otherwise, this can be neglected.
Q TOTAL
ΔV BOOT = --------------------C BOOT
(8)
3.2 Select the Bootstrap Resistor
100nF ⇒ ΔVBOOT = 1.05 V
When the external bootstrap resistor is used, the resistance,
RBOOT, introduces an additional voltage drop:
V RBOOT
I
• R BOOT
= CHARGE
t CHARGE
150nF ⇒ ΔV BOOT = 0.7 V
(5)
220nF ⇒ ΔV BOOT = 0.48 V
where:
ICHARGE = Bootstrap capacitor charging current;
RBOOT = Bootstrap resistance; and
tCHARGE = Bootstrap capacitor charging time (the low-side
turn-on time).
570nF ⇒ ΔV BOOT = 0.18 V
Suggested values are within the range of 100nF ~ 570nF, but
the right value must be selected according to the application
in which the device is used. When the capacitor value is too
large, the bootstrap charging time slows and the low-side on
time might be not long enough to reach the bootstrap voltage.
Do not exceed the ohms (typically 5~10Ω) that increase the
VBS time constant. This voltage drop of bootstrap diode
must be taken into account when the maximum allowable
voltage drop (VBOOT) is calculated. If this drop is too high or
the circuit topology does not allow a sufficient charging
time, a fast recovery or ultra-fast recovery diode can be used.
4. Consideration of Bootstrap
Application Circuits
4.1 Bootstrap Startup Circuit
For example: Evaluate the bootstrap capacitor value when
the external bootstrap diode used.
The bootstrap circuit is useful in high-voltage gate driver, as
shown in Figure 1. However, it has a initial startup and limited charging a bootstrap capacitor problem when the source
of the main MOSFET (Q1) and the negative bias node of
bootstrap capacitor (CBOOT) are sitting at the output voltage.
Bootstrap diode (DBOOT) might be reverse biased at startup
and main MOSFET (Q1) has a insufficient turn-off time for
the bootstrap capacitor to maintain a required charge, as
shown in Figure 1.
Gate Drive IC = FAN7382 (Fairchild)
Switching Device = FCP20N60 (Fairchild)
Bootstrap Diode = UF4007
VDD = 15V
QGATE = 98nC (Maximum)
ILKGS = 100nA (Maximum)
ILKCAP = 0 (Ceramic Capacitor)
IQBS = 120µA (Maximum)
ILK = 50µA (Maximum)
QLS = 3nC
TON = 25µs (Duty=50% at fs=20KHz)
ILKDIODE = 10nA
If the maximum allowable voltage drop on the bootstrap
capacitor is 1.0V during the high side switch on state, the
minimum capacitor value is calculated by Equation 3.
In certain applications, like in battery chargers, the output
voltage might be present before input power is applied to the
converter. Delivering the initial charge to the bootstrap
capacitor (CBOOT) might not be possible, depending on the
potential difference between the supply voltage (VDD) and
output voltage (VOUT) levels. Assuming there is enough
voltage differential between input voltage (VDC) and output
voltage (VOUT), a circuit comprised of startup resistor
(RSTART), startup diode (DSTART), and Zener diode (DZ) can
solve the problem, as shown in Figure 14. In this startup circuit, startup diode DSTART serves as a second bootstrap
diode used for charging the bootstrap capacitor (CBOOT) at
power up. Bootstrap capacitor (CBOOT) is charged to the
Zener diode of DZ, which is supposed to be higher than the
driver's supply voltage (VDD) during normal operation. The
charge current of the bootstrap capacitor and the Zener current are limited by the startup resistor. For best efficiency,
the value of startup resistor should be selected to limit the
current to a low value, since the bootstrap path through the
startup diode is permanently in the circuit.
QTotal = (98 × 10−9 ) + {(100 × 10−9 + 120 × 10−6 + 50 × 10−6
+ 10 × 10−9 ) × (25 × 10−6 )} + (3 × 10−9 )
(6)
−9
= 105.2 × 10 [C]
The value of bootstrap capacitor is calculated as follows:
C BOOT =
QTOTAL
105 .2 × 10 −9
=
≅ 105 [nF ]
ΔV BOOT
1
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
(7)
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5
AN-6076
APPLICATION NOTE
VDD
For example, if RBOOT=10, CBOOT=1µF, and D=10%; the
time constant is calculated in following equation:
VDC
D BOOT
RBOOT
VDD
DSTART
–6
R BOOT ⋅ C BOOT
10 ⋅ 1
τ = ---------------------------------------- = ------------------ = 100 [ μs ]
D
0.1
VB
CBOOT
INPUT
RSTART
DZ
HO
HIN
Q1
Even with a reasonably large bootstrap capacitor and resistor, the time constant may be large. This method can mitigate
the problem. Unfortunately, the series resistor does not provide a foolproof solution against an over voltage and it
slows down the recharge process of the bootstrap capacitor.
RGATE
L
VS
COM
(10)
D
COUT
VOUT
Figure 14. Simple Bootstrap Startup Circuit
4.3 Resistor Between VS and VOUT
4.2 Resistor in Series with Bootstrap Diode
In the second option, the bootstrap circuit includes a small
resistor, RVS, between VS and VOUT, as shown in Figure 16.
Suggested values for RVS are in the range of some ohms.
In the first option, the bootstrap circuit includes a small
resistor, RBOOT, in series with bootstrap diode, as shown in
Figure 15. The bootstrap resistor, RBOOT, provides current
limit only during a bootstrap charging period which represents when the VS goes below the IC supply voltage, VDD,
or is pulled down to ground (the low-side switch is turned on
and the high-side switch is turned off). The bootstrap capacitor, CBOOT, charge through the bootstrap resistor, RBOOT,
and diode, DBOOT, from the VDD power supply. The bootstrap diode must have a break-down voltage (BV) larger than
VDC and a fast recovery time to minimize the amount charge
fed back from the bootstrap capacitor to VCC power supply.
IN
VCC
CDRV
GND
VB
CBOOT
Q1
HO
RGATE
GND
VOUT
RVS
VS
L1
COUT
D1
Figure 16. Adding RVS in Bootstrap Circuit
The RVS works as, not only bootstrap resistor, but also turnon and turn-off resistors, as shown in Figure 17. The bootstrap resistor, turn-on, and turn-off resistors are calculated by
the following equations:
VB
HVIC
CDRV
IN
HVIC
IN
VCC
DBOOT
DBOOT
RBOOT
IN
RBOOT
VDC
VCC
VDC
VCC
Q1
CBOOT
HO
RGATE
L1
VS
D1
COUT
VOUT
Figure 15. Adding a Series Resistor with DBOOT
This method has the advantage of being simple for limiting
the current when the bootstrap capacitor is initially charged,
but it has some limitations. Duty-cycle is limited by the
requirement to refresh the charge in the bootstrap capacitor,
CBOOT, and there are startup problems. Do not exceed the
ohms (typically 5~10Ω) that would increase the VBS time
constant. The minimum on-time for charging the bootstrap
capacitor or for refreshing its charge must be verified against
this time constant. The time constant depends on the values
of bootstrap resistance, capacitance, and duty cycle of
switching device calculated in following equation:
R BOOT∗ = R BOOT + RVS
(11)
R ON∗ = R GATE + R VS
(12)
R OFF∗ = RGATE + R VS
(13)
VCC
RBOOT
DBOOT
IBCHG
VB
IN
IN
ITURN-ON
CBOOT
RGATE
CDRV
L1
RVS
ITURN-OFF
(9)
where RBOOT is the bootstrap resistor; CBOOT is the bootstrap capacitor; and D is the duty cycle.
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
VOUT
VS
GND
R BOOT ⋅ C BOOT
τ = ---------------------------------------- [ s ]
D
Q1
HO
VCC
D1
COUT
Figure 17. Current Paths of Turn-on and Turn-off
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6
AN-6076
APPLICATION NOTE
5. Choose Current Capability HVIC
4.4 Clamping Diode for VS and Relocation
Gate Resistor
The approximate maximum gate charge QG that can be
switched in the indicated time for each driver current rating
is calculated in Table 1:
In the third option, the bootstrap relocates a gate resistor
between VS and VOUT and adds a low forward-voltage drop
Schottky diode from ground to VS, as shown in Figure 18.
The difference between VB and VS should be kept inside the
absolute maximum specification in the datasheet and must
be satisfied by the following equation:
V B − V S < V BS _ abs max
Table 1. Example HVIC Current-Drive Capability
Switching Time (tSW_ON/OFF)
Needed
Current
Rating
(14)
VDC
VCC
DBOOT
IN
IN
VB
HVIC
VCC
CDRV
1.
HO
VOUT
RGATE
GND
D1
COUT
V B – V S < V BS, ABSMAX
133nC
9A
600nC
300nC
For a single 4A, parallel the two channels of a dual 2A!
QG
tsw _ on / off
(16)
3. tSW_ON/OFF is how fast the MOSFET should be switched.
If unknown, start with 2% of the switching period tSW:
0.02
t SWON, OFF = 0.02 × tSW = ----------f SW
(17)
If channel (V-I) switching loss is dominated by one switching transition (turn-on or turn-off), size the driver for that
transition. For clamped inductive switching (the usual case),
channel switching loss for each transition is estimated as:
(15)
VDC
E SW = 0.5V DS × I D × t SW Joules
Q1
HVIC
CBOOT
HO
VOUT
4. The approximate current drive capability of gate driver
may be calculated like below
L1
VS
DZ
D1
(18)
where VDS and ID are maximum values during the switching
interval.
VB
RGATE
GND
267nC
If the actual gate-drive voltage VGS is different from the test
condition in the specifications table, use the VGS vs. QG
curve instead. Multiply the datasheet value by the number
of MOSFETs in parallel.
DBOOT
VDD
4A
2. The maximum gate charge, QG, is read from the MOSFET
datasheet.
The fourth options includes relocating a gate resistor
between VS and VOUT and a clamp device should be positioned between ground and VS, as shown in Figure 19, where
a Zener diode and a 600V diode are placed. The Zener voltage must be sized according to the following rule:
CDRV
67nC
IG.AV.SW =
The gate resistor sets the turn-on and turn-off speeds in the
MOSFET and provides current limiting for the Schottky
diode during the negative voltage transient of the source terminal of the main switch. In additional, the bootstrap capacitor is protected against over voltage by the two diodes
connected to the ends of CBOOT. The only potential hazard
by this circuit is that the charging current of the bootstrap
capacitor must go through gate resistor. The time constant of
CBOOT and RGATE slows the recharge process, which might
be a limiting factor as the PWM duty cycle.
IN
133nC
1. Needed gate driver current ratings depend on what gate
charge QG must be moved in switching time tSW-ON/OFF
(because average gate current during switching is IG):
4.5 Relocated Gate Resistor; Double Purpose
IN
2A
For example, a switching time of 100ns is:
1% of the converter switching period at 100KHz;
3% of the converter switching period at 300KHz; etc.
Figure 18. Clamping Structure
VCC
Maximum Gate Charge (QG,MAX)
L1
VS
DSCHT
50ns
Note:
Q1
CBOOT
100ns
(1) Sourcing Current Capability (Turn-on)
COUT
D2
QG
I SOURCE ≥ 1.5 × ------------------t SW, ON
(19)
Figure 19. Clamping Structure with Zener Diode
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
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7
AN-6076
APPLICATION NOTE
6.1 Sizing the Turn-On Gate Resistor
(2) Sinking Current Capability (Turn-off)
QG
I SINK ≥ 1.5 × ---------------------tSW, OFF
Turn-on gate resistor, Rg(ON), can be chosen to obtain the
desired switching time by using switching time, tsw. To
determine a value of resistor using the switching time, supply voltage, VDD (or VBS), equivalent on resistance
(RDRV(ON)) of the gate driver, and switching device parameters (Qgs, Qgd, and Vgs(th)) are needed.
(20)
where:
QG = MOSFET gate charge at VGS = VDD;
tSW_ON/OFF = MOSFET switch turn-on / turn-off time; and
1.5 = empirically determined factor (influenced by delay
through the driver input stages and parasitic elements).
The switching time is defined as the time spent to reach the
end of the plateau voltage (a total Qgd + Qgd has been provided to the MOSFET gate), as shown in Figure 21.
The turn-on gate resistor calculated as follows:
6. Gate Resistor Design Procedure
The switching speed of the output transistor can be controlled by values of turn-on and turn-off gate resistors controlling the turn-on and turn-off current of gate driver. This
section describes basic rules for values of the gate resistors
to obtain the desired switching time and speed by introducing the equivalent output resistor of the gate driver. Figure
20 shows the equivalent circuit of gate driver and current
flow paths during the turn-on and turn-off, including a gate
driver and switching devices.
Q gs + Q gd
I g ( avr ) = ------------------------t SW
(21)
V DD + V gs
R TOTAL = Rg ( ON ) + R DRV ( ON ) = --------------------------I g ( avr )
(22)
where Rg(ON) is the gate on resistance and RDRV(ON) is the
driver equivalent on resistance.
6.2 Output Voltage Slope
VDC
Turn-on gate resistor Rg(ON) can be determined by control
output slope (dVOUT/dt). While the output voltage has a nonlinear behavior, the maximum output slope can be approximated by:
HVIC
Turn-On
VB
ON
R GATE
DRI VER
VBS
RDRV(ON)
Cgd
2
1
Cgs
OFF
dVOUT
dt
Inserting the expression yielding Ig(avr) and rearranging:
VDD
Turn-Off
OFF
DRI VER
ON
V DD – V gs ( th )
RTOTAL = -----------------------------------------dV OUT
C gd ( off ) ⋅ -----------------dt
dVOUT
dt
1
Cgd
RG( ON)
LO
Cds
2
RDRV(OFF
(24)
Cgs
where Cgd(off) is the Miller effect capacitor, specified as Crss
in the datasheet.
RG( OFF)
)
(23)
V OUT
VS
VDD
dV OUT
Ig ( avr )
------------------ = ------------------dt
Cgd ( off )
HO
GND
Figure 20. Gate Driver Equivalent Circuit
6.3 Sizing the Turn-Off Gate Resistor
Figure 21 shows the gate-charge transfer characteristics of
switching device during turn-on and turn-off.
The worst case in sizing the turn-off resistor is when the
drain of the MOSFET in turn-off state is forced to commutate by external events.
In this case, dV/dt of the output node induces a parasitic current through Cgd flowing in RG(OFF) and RDRV(OFF), as
shown in Figure 22
The following describes how to size the turn-off resistor
when the output dv/dt is caused by the companion MOSFET
turning-on, as shown in Figure 22.
For this reason, the off-resistance must be sized according to
the application worst case. The following equation relates
the MOSFET gate threshold voltage to the drain dv/dt:
Figure 21. Gate Charge Transfer Characteristics
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
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APPLICATION NOTE
VDC
RTotal =
HVIC
VB
2
DRIVER
R GATE
Cgd
RDRV ( ON ) =
HO
1
Cgs
OFF
dVOUT
dt
VS
VDD
iLOAD
Load
Turn- Off
DRIVER
RG(ON)
Cds
Cgs
RDRV(OFF
ON
RDRV ( OFF ) =
RG(OFF)
)
GND
R g(off) ≤
Figure 22. Current Paths: Low-Side Switch Turned Off,
High-Side Switch Turned On
V gs ( th ) ≥ {( R g ( OFF ) + R DRV
( OFF )
dV out
dt
(25)
Vgs ( th )
− R( drv )
dV
C gd ⋅ out
dt
The static losses are due to the quiescent currents from the
voltage supplies VDD and ground in low-side driver and the
leakage current in the level shifting stage in high-side driver,
which are dependent on the voltage supplied on the VS pin
and proportional to the duty cycle when only the high-side
power device is turned on.
Qgs=13.5nC, Qgd=36nC, Cgd=95pF, VGS(th) =5V,
The dynamic losses are defined as follows: In the low-side
driver, the dynamic losses are due to two different sources.
One is due to whenever a load capacitor is charged or discharged through a gate resistor, half of energy that goes into
the capacitance is dissipated in the resistor. The losses in the
gate drive resistance, internal and external to the gate driver,
and the switching loss of the internal CMOS circuitry. Also,
the dynamic losses of the high-side driver have two different
sources. One is due to the level-shifting circuit and one due
to the charging and discharging of the capacitance of the
high side. The static losses are neglected here because the
total IC power dissipation is mainly dynamic losses of gate
drive IC and can be estimated as:
VGS(th)MIN =3V
6.4.1 Turn-On Gate Resistance
1) If the desired switching time is 500ns at VDD=15V, the
average gate charge current is calculated as:
RTotal =
t SW
VDD − Vgs (th )
RDRV (ON ) =
36nC + 13.5nC
= 99[mA]
500ns
(27)
15 − 5
= 101[Ω]
99mA
(28)
VDD
15V
=
≈ 43[Ω]
I SOURCE 350mA
(29)
I g ( avr )
=
(33)
The total power dissipation is the sum of the gate driver
losses and the bootstrap diode losses. The gate driver losses
are comprised of the static and dynamic losses related to the
switching frequency, output load capacitance on high- and
low-side drivers, and supply voltage, VDD.
(26)
Determine the turn-on and off gate resistors using the Fairchild MOSFET with FCP20N60 and gate driver with
FAN7382. The power MOSFET of FCP20N60 parameters
are as follows:
=
Vgs ( th ) min
3
− R( drv ) =
− 23 = 8.6
dVout
95 × 10 −12 ×109
C gd ⋅
dt
7.1 Gate Driver Power Dissipation
6.4 Design Example
Qgs + Qgd
(32)
7. Power Dissipation Considerations
Rearranging the equation yields:
I g ( avr ) =
VDD
15V
=
≈ 23[Ω]
I SINK 650mA
) × ig }
= {( R g ( OFF ) + R ( drv ) ) × C gd
R g(off) ≤
(31)
6.4.2 Turn-Off Gate Resistance
If dVout/dt=1V/ns, the turn-off gate resistor is calculated as:
Cgd
LO
VDD
15V
=
≈ 43[Ω]
I SOURCE 350mA
The turn-on resistance value is about 62Ω.
OFF
VDD
(30)
RDRV(ON)
ON
VBS
Turn-On
VDD − VGS ( th )
15 − 5
=
= 105[Ω]
dVOUT 95 ×10 −12 ×109
C gd ( off ) ⋅
dt
PDGATE = 2 × C L × f s × VDD [W ]
2
The turn-on resistance value is about 58Ω.
Figure 23 shows the calculated gate driver power dissipation
versus frequency and load capacitance at VDD=15V. This
plot can be used to approximate the power losses due to thegate driver.
2) If dVout/dt=1V/ns at VDD=15V, the total gate resistor is
as calculated as:
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
(34)
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APPLICATION NOTE
8. General Guidelines
At VDD = 15V
8.1 Printed Circuit Board Layout
1
The layout for minimized parasitic inductances is as follows:
CLOAD=4400PF
• Direct tracks between switches with no loops or deviation.
• Avoid interconnect links. These can add significant
inductance.
• Reduce the effect of lead-inductance by lowering package
height above the PCB.
• Consider co-locating both power switches to reduce track
length.
• Placement and routing for decoupling capacitor and gate
resistors as close as possible to gate drive IC.
• The bootstrap diode as close as possible to bootstrap
capacitor.
Power [W]
CLOAD=2200PF
CLOAD=1000PF
0.1
CLOAD=470PF
0.01
0.1
1
10
100
1000
Switching frequency [kHz]
Figure 23. Gate Driver Total Power Dissipation
8.2 Bootstrap Components
The bootstrap circuit power dissipation is the sum of the
bootstrap diode losses and the bootstrap resistor losses if any
exist. The bootstrap diode loss is the sum of the forward bias
power loss that occurs while charging the bootstrap capacitor
and the reverse bias power loss that occurs during reverse
recovery. Since each of these events happens once per cycle,
the diode power loss is proportional to switching frequency.
Larger capacitive loads require more current to recharge the
bootstrap capacitor, resulting in more losses.
The bootstrap resistor (RBOOT) must be considered in sizing
the bootstrap resistance and the current developed during initial bootstrap charge. If the resistor is needed in series with
the bootstrap diode, verify that VB does not fall below COM
(ground), especially during startup and extremes of frequency and duty cycle.
The bootstrap capacitor (CBOOT) uses a low-ESR capacitor,
such as ceramic capacitor. The capacitor from VDD to COM
supports both the low-side driver and bootstrap recharge. A
value at least ten times higher than the bootstrap capacitor is
recommended.
Higher input voltages (VDC) to the half-bridge result in
higher reverse recovery losses. The total IC power dissipation can be estimated by summing the gate driver losses with
the bootstrap diode losses, except bootstrap resistor losses.
The bootstrap diode must use a lower forward voltage drop
and switching time as soon as possible for fast recovery,
such as ultra-fast.
If the bootstrap diode is within the gate driver, add an external diode in parallel with the internal bootstrap diode
because the diode losses can be significant. The external
diode must be placed close to the gate driver to reduce parasitic series inductance and significantly lower forward voltage drop.
7.2 Package Thermal Resistance
The circuit designer must provide:
• Estimate power dissipation of gate driver package
• The maximum operating junction temperature TJ,
MAX,OPR, e.g., 120°C for these drivers if derated to 80%
of TJ,MAX =150°C.
• Maximum operating lead temperature TL,MAX,OPR,
approximately equal to the maximum PCB temperature
underneath the driver, e.g., 100°C.
• Maximum allowable junction-to-lead thermal resistance
is calculated by:
θJL,max =
TJ ,max − TL,max
PPKG
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
(35)
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AN-6076
APPLICATION NOTE
Table 2. Summary of High-Side Gate Drive Circuitry
Method
Basic Circuit
Advantages & Limitations
High-Side Gate Drivers for P-Channel
VCC
Can be implemented if the maximum input voltage is
less than the gate-to-source break down voltage of
the device.
VCC
PWM
Controller
Direct Drive
Q1
RGATE
OUT
VOUT
L1
GND
VOUT
COUT
D1
V DC
VCC
VCC
RPULL
PWM
Controller
Open Collector
Simple method, but is not suitable for driving
MOSFET directly in a high-speed application.
Q1
RGATE
OUT
VOUT
L1
GND
VOUT
COUT
D1
VDC
R1
VCC
Q1
RGATE
Level-Shifted Drive
PWM
Controller
VOUT
R2
VCC
Suitable for high-speed application and works
seamlessly with regular PWM controller.
L1
R BASE
OUT
QINV
D1
COUT
VOUT
GND
High-Side Gate Drivers for N-Channel
Easiest high-side application the MOSFEF and can
be driven directly by the PWM controller or by a
ground referenced driver, but it must meet two
conditions, as follows:
VDC
V CC
VCC
PWM
Controller
Direct Drive
Q1
RGATE
OUT
VOUT
GND
L1
D SCHT
D1
COUT
and V DC < V CC − V GS , Miller
VDC
VCC
VCC
Floating
Supply
HO
Floating Supply
Gate Drive
V CC < V GS , MAX
VOUT
Q1
RGATE
Opto
PWM
Controller
VOUT
L1
RGATE
LO
Q2
COUT
VOUT
Cost impact of isolated supply is significant. Optocoupler tends to be relatively expensive, limited in
bandwidth, and noise sensitive.
GND
VDC
VCC
PWM
Controller
T1
VCC
Transformer
Coupled Drive
Q1
RGATE
VOUT
CBLOCK
L1
OUT1
Q2
R GATE
OUT2
COUT
VOUT
Gives full gate control for an indefinite period of time,
but is somewhat limited in switching performance.
This can be improved with added complexity.
GND
VDC
VCC
VCC
Charge Pump
Drive
PWM
Controller
Q1
OUT
VOUT
L1
GND
COUT
D1
VOUT
V DC
VCC
DBOOT
IN
IN
VCC
CDRV
GND
Q1
CBOOT
HO
RGATE
L1
VS
D1
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
Simple and inexpensive with limitations; such as,
the duty cycle and on-time are both constrained by
the need to refresh the bootstrap capacitor.
Requires level shift, with the associated difficulties.
VB
HVIC
Bootstrap Drive
The turn-on times tend to be long for switching
applications. Inefficiencies in the voltage
multiplication circuit may require more than low
stages of pumping.
COUT
V OUT
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APPLICATION NOTE
Consideration Points of Bootstrap Circuit Problem
A-Point
VCC
VBS
DBOOT
INPUT
IN
VB
HVIC
HO
VBS= (VCC -VFBD ) - (-VS)
A
Recovery Time
Q1
B
VGS=B-C Point
RGATE
CDRV
iLOAD
LS1
GND
VDC
C-Point
CBOOT
VCC
VDC+VGS,Miller
B-Point
VDC
VS
C
C
LS2
GND
- VS
iFree
COUT
D1
Negative voltage transient
at high-side switch turn-off.
If VS goes significantly below
ground, the gate driver can
have serious troubles.
Latch-up,
propagation signal
missing and overvoltage across the
bootstrap
capactor
The amplitude of the negative voltage is proportional
parasitic inductances and the turn-off speed (di/dt) of
the switching device, Q1, which is determined by gate
resistor, RGATE, and input capacitance, Ciss.
Remedies of Bootstrap Circuit Problem
© 2008 Fairchild Semiconductor Corporation
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APPLICATION NOTE
DISCLAIMER
FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS
HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE
APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS
PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
LIFE SUPPORT POLICY
FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used
herein:
1. Life support devices or systems are devices or systems which,
(a) are intended for surgical implant into the body, or
(b) support or sustain life, or
(c) whose failure to perform when properly used in accordance
with instructions for use provided in the labeling, can be reason
ably expected to result in significant injury to the user.
© 2008 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/30/08
2. A critical component is any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
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