Fairchild FAN3268T Application review and comparative evaluation of low-side gate driver Datasheet

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AN-6069
Application Review and Comparative Evaluation
of Low-Side Gate Drivers
Summary
Power MOSFETs require a gate drive circuit to translate the
on/off signals from an analog or digital controller into the
power signals necessary to control the MOSFET. This paper
provides details of MOSFET switching action in
applications with clamped inductive load, when used as a
secondary synchronous rectifier, and driving pulse/gate
drive transformers. Potential driver solutions, including
discrete and integrated driver designs, are discussed.
MOSFET driver datasheet current ratings are examined and
circuits are presented to assist with evaluating the
performance of drivers on the lab bench.
Introduction
In many low-to-medium power applications, a low-side
(ground referenced) MOSFET is driven by the output pin of
a PWM control IC to switch an inductive load. This solution
is acceptable if the PWM output circuitry can drive the
MOSFET with acceptable switching times without
dissipating excessive power. As the system power
requirements grow, the number of switches and associated
drive circuitry increases. As control circuit complexity
increases, it is becoming more common for IC
manufacturers to omit onboard drivers because of grounding
and noise problems.
Synchronous rectifiers (SRs) are increasingly used to replace
standard rectifiers when high efficiency and increased power
density are important. It is common for isolated power
stages delivering tens of amps to parallel two or more lowresistance MOSFETs in each rectifying leg, and these
devices require current pulses reaching several amps to
switch the devices in the sub-100ns timeframe desired.
External drivers can provide these high-current pulses and a
means to implement timing to eliminate shoot-through and
optimize efficiency to control the SR operation. In addition,
drivers can translate logic control voltages to the most
effective MOSFET drive level.
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
Low-side drivers are also used to drive transformers, which
provide isolated MOSFET gate drive circuits or
communication across the power supply isolation boundary.
In these applications, a driver is required to handle concerns
specific to transformer drive, discussed later.
Low-side drivers may seem a mundane topic; several papers
have been written on the subject. Though often presented as
an ideal voltage source that can source or sink current
determined by the circuit’s series impedance, the current
available from a driver is, in fact, limited by the discrete or
integrated circuit design. This note reviews the basic
requirements of drivers from an application viewpoint, then
investigates methods for testing and evaluating the current
capability of drivers on the lab bench.
Clamped Inductive Switching
The simplified boost converter in Figure 1 provides the
schematic for a typical power circuit with a clamped
inductive load. When the MOSFET Q is turned on, the input
voltage VIN is applied across inductor L and the current
ramps up in a linear fashion to store energy in the inductor.
When the MOSFET turns off, the inductor current flows
through diode D1 and delivers energy to COUT and RLOAD at
voltage VDC. The inductor is assumed large enough to
maintain current constant during the switching interval.
VIN
VDC
L
D1
VDD
RLOAD
CBYP
COUT
RG
Q
VOUT
IG
Figure 1.
Simplified Boost Converter
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AN-6069
APPLICATION NOTE
The circuit waveforms for a MOSFET turning on into a
clamped inductive load are illustrated in Figure 2.
During interval t1, IG increases quickly and charges the
combination of CGS and CGD to the gate threshold voltage
VTH through the path shown in Figure 3(a). In this interval,
the MOSFET carries no inductor current.
As interval t2 begins, the MOSFET starts to conduct current
in the linear mode as:
VDD
VGS
ID = gm ( VGS − VTH )
VPL
through the current paths shown in Figure 3(b). The parallel
combination of CGD and CGS are charged from the threshold
voltage to a plateau level given by
VTH
IL
VPL =
IDS
VO
IPK
t 2 = t IDS,rise =
IPL
t1
Figure 2.
ID
+ VTH
gm
(2)
as the drain current rises from zero to IL. QGS2 is the charge
needed during this transition and can be determined from the
MOSFET datasheet characteristic curves, as illustrated in
the application example presented later in this section. QGS2
allows calculation of the time required for this transition as:
VDS
IG
(1)
t2
t3
t4
QGS 2
IG
(3)
Throughout t2, VDS remains at VOUT, clamped by diode D.
At the end of t2, the MOSFET conducts the full IL current
and the diode commutates.
time
MOSFET Turn on with Inductive Load
As interval t3 commences, the gate current flows through
CGD and the MOSFET channel as shown in Figure 3(c). All
of IG is used to discharge CGD as VGS remains at VPL, and
VDS begins to fall with a time period given by:
Figure 3 indicates the gate current paths active during the
individual intervals of the MOSFET turn -on process.
t3 = t VDS,fall =
Q GD
IG
(4)
In interval t4, IG flows through a combination of CGS, CGD,
and the decreasing channel resistance RDS, as shown in
Figure 3(d). During t4, the gate-source voltage rises from
the plateau level to VDD. This allows determination of the
total gate charge QG,T required to turn on the MOSFET.
As the drain current rises during t2 and VDS falls during t3,
the MOSFET has simultaneous high voltage across it and
high current flowing through it, so the instantaneous power
can be very high. An equation relating IG to the switching
loss during the turn on interval is:
⎛Q
Q ⎞
⎛ V ×I
⎞
PSW ,ON = ⎜ IN LOAD ⎟(fSW )⎜⎜ GS2 + GD ⎟⎟
2
IG,t 3 ⎠
⎝
⎠
⎝ IG,t 2
Figure 3.
Current Paths During MOSFET Turn on
This equation shows the importance of the magnitude of IG
in relation to the switching losses. Unfortunately, there are
no formal equations to calculate the current available from a
given driver as the output voltage swings throughout its
range. Empirical methods can determine the value of IG at
different driver output voltage levels and are presented in
the section “Evaluating Drivers on the Bench” below.
RG represents the series combination of the MOSFET
internal gate resistance along with any series gate resistor.
RHI represents the driver’s internal resistance whose
effective value changes throughout the switching interval.
As shown below, the driver current, IG, is determined by
combining information presented in references [1] and [2].
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
(5)
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AN-6069
APPLICATION NOTE
For a practical example, the gate-source voltage versus total
gate charge is reproduced from the Fairchild FCP20N60
power MOSFET datasheet in Figure 4. The curve was
produced using a test circuit that drives the gate of the
Device Under Test (DUT) with a small current source of
3mA. In this example, the gate charge needed to reach the
threshold voltage of 3V is approximately 7nC. The charge
required during interval t2, QGS2, is found to be 14nC – 7nC
= 7nC. In interval t3, the value of QGD is found to be QGD =
46nC – 14nC = 32nC. In this typical case, the effect of QGD
on the switching loss is more significant than the
contribution resulting from QGS2.
VDD
VGS
VPL
VTH
IL
IDS
FCP20N60
Vo
VDS
IG
-IPL
time
-IPK
t1
t2
Figure 4.
t3
t5
t4
Figure 5.
VGS vs. Qg for FCP20N60
(6)
where fsw is the switching frequency of the power stage.
With the average current requirement known, the input
power drawn from the VDD bias supply can be found as:
Pdr = VDD ⋅ IDD = VDD ⋅ QG ⋅ fSW
t7
t8
MOSFET Turn Off with Inductive Load
In the t5 interval, IG rises to discharge VGS from VDD to the
plateau level defined by (2). In the t6 interval, VGS remains
at the plateau voltage while VDS rises to the off state voltage.
The t6 interval lasts for a time approximated by:
With VGS at final drive level, the value for QG,total is known.
To find the average current required from the bias supply:
IDD = QG ⋅ fSW
t6
t 6 = t VDS,rise =
Q GD
IG
(8)
In interval t7, the drain current IDS falls from the value of IL
to 0 while VGS falls from VPL to VTH. This time interval is
given by:
(7)
The circuit waveforms and current paths during inductive
load turn off are similar to those for turn on, but taken in a
reverse order. For brevity, the circuit waveforms are
indicated in Figure 5, but the current paths are not shown.
t7 = t IDS,fall =
Q GS,2
IG
(9)
In the t8 interval, VGS is discharged from the threshold
voltage to zero.
An equation relating IG to the switching loss during the turn
off interval is given as:
⎛Q
Q
⎛ V ×I
⎞
PSW .OFF = ⎜ IN LOAD ⎟ ⋅ (f SW ) ⋅ ⎜ GD + GS 2
⎜
2
IG , t 7
⎝
⎠
⎝ IG , t 6
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
⎞
⎟
⎟
⎠
(10)
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AN-6069
APPLICATION NOTE
Synchronous Rectifier Operation
t off =
A MOSFET operated as a synchronous rectifier (SR)
experiences a switching interval significantly different from
the case of a clamped inductive load. Figure 6 shows a
simplified forward converter power stage with a
synchronous rectifier QSR in place of the freewheel diode.
QQ,SR
(11)
IG
where QQSR is defined in reference [3] to be:
QQ,SR = (CGS + CGD,SR ) ⋅ VDD
(12)
Also in reference [3], CGS,SR is estimated as:
CGD,SR = 2 ⋅ CRSS,SPEC ⋅
VDS,SPEC
0.5 ⋅ VDD
(13)
From standard MOSFET nomenclature:
CGS = CISS − CRSS
Figure 6.
In Figure 7(b), the MOSFET is fully off, IL flows through
the body diode, and the VSEC polarity has not changed.
When VSEC changes polarity, as shown in Figure 7(c),
current flows from VSEC to recover the body diode stored
charge and the diode commutates. In Figure 7(d), the body
diode has been fully recovered and VDS rises quickly. The
high dV/dT on the MOSFET drain can cause a capacitive
current to flow through the CDS/CGS voltage divider, so a
driver with strong current sink capability is essential to hold
the gate voltage below the threshold voltage.
Simplified Forward Converter
In this example, an SR signal generated by the control
circuit crosses the isolation boundary to keep the
synchronous rectifier QSR on while Q1 is off. However, the
SR signal should command QSR to turn off before Q1 turns
on to apply positive voltage to the transformer. Figure 7
shows four intervals used to illustrate the turn-off sequence
of the synchronous rectifier.
VDC
VSEC
CGD
IG
DBD
CDS
RG
D
Transformer Drive Applications
In power converters such as a half-bridge, full-bridge, twoswitch forward converters; and active clamp forward
converters there are high-side switches or a combination of
high/low switches that must be controlled. If galvanic
isolation is not needed between the control and the power
switches, the MOSFETs may be driven with a
semiconductor half-bridge gate driver, but the inherent
propagation delay must be considered in the design. For
circuits that need isolation or can benefit from short
propagation delays, the gate drive transformer should be
considered as a potential solution.
DBD
CGS
CGS
RLOW
RLOW
S
S
(a)
(b)
VDC
VDC
VSEC
IL
VSEC
IL
+ D
IG
CGD
CDS
RG
IL
CGD
RDS
RG
-
+
D
In the synchronous rectifier application, IG does not affect
switching losses as it did in the clamped inductive load
application. However, the paralleled MOSFETS used in SR
applications require high-current pulses to switch
effectively, and high current drivers are often located in
close proximity.
VDC
VSEC
IL
-
+
+
D
CGD
CDS
DBD
RG
CGS
In a related application, it is often necessary to provide highspeed communication between the primary and secondary
sides of an isolated converter. This can be accomplished
using technologies such as opto-isolators with digital outputs
or magnetic pulse transformers. These pulse transformers
are similar to the gate drive transformer, but they are only
required to transmit logic signals instead of delivering the
high-current pulses to turn a power MOSFET on and off.
DBD
CGS
RLOW
RLOW
S
S
(c)
Figure 7.
(d)
SR MOSFET Turn Off
Prior to turn off, the MOSFET conducts load current IL
through the resistive channel RDS and the drain-to-source
voltage is negative. In Figure 7(a) the output of the driver is
low and the combination of CGD and CGS are discharged in
parallel in a time interval given by:
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
(14)
The simplified circuit of Figure 8 is used to illustrate the
basic operation of a low-side driver and pulse transformer
used in a communication circuit. The transformer is shown
as ideal transformer with turns ratio NP:NS = 1:1 in parallel
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AN-6069
APPLICATION NOTE
applied directly to the primary winding of T1, the
transformer would saturate and not be able to transmit useful
information. To prevent this, coupling capacitor CC is
inserted in series with the primary winding to block the DC
voltage while passing the AC portion of the VOUT signal.
Transformers designed for pulse and gate drive applications
usually specify a voltage-time product the device can
withstand without saturating the transformer.
with magnetizing inductance LMAG. In both cases, the DC
blocking capacitor CC is large enough so that its voltage is
approximately constant.
Figure 8.
In many cases, the same transformer could be used as either
a pulse transformer operation or a gate drive transformer. In
0, the major difference between the two applications is in the
current waveforms. With a constant drive voltage and
magnetizing inductance, LMAG, the magnetizing current IMAG
is the same in both circuits. In the pulse transformer
waveforms shown in 0(a), the resistor current IR follows the
secondary voltage VS, and the driver supplies a current that
is the sum of these two components. In the MOSFET gate
drive waveforms shown in 0(b),
the gate current IG is positive pulses at turn on and negative
pulses at turn off. As in the first example, the driver
supplies a current that is the sum of these two components,
but the waveform has a larger RMS value due to the highcurrent pulses.
Simplified Pulse Transformer Circuit
In Figure 9, the circuit is modified so that the resistor is
replaced by the gate-to-source terminal of a MOSFET
located on the high side of a bridge circuit.
+Bulk
T1
NP:NS
VDD
IDR
+
IG
CC
IMAG
VOUT
IN
Figure 9.
-
+
VP
LMAG
-
+
VS
It is important to examine the direction of current flow
between driver and transformer for the examples of 0. When
VOUT swings high as shown Figure 11(a), one might expect
the driver to immediately source current. However, the
magnetizing current is negative and, if the load current is not
larger than the magnetizing current, the driver must sink
current until IDR goes positive. The opposite situation exists
in Figure 11(b), when VOUT goes from high to low and the
driver must source current when expected to operate as a
current sink. Figure 11(c) shows additional diodes providing
a current path if the driver cannot sink current when VOUT is
high or source current when VOUT is low, as found in drivers
with a bipolar output stage.
-
Simplified Gate Drive Transformer Circuit
0(a) shows the operational waveforms for the pulse
transformer circuit, while 0(b) shows operation in a gate
drive application.
VOUT
VOUT
IDR
IDR
Gate Transformer
Pulse Transformer
(a)
(a)
(b)
Figure 10. (a) Pulse Transformer Waveforms and
(b) Gate Drive Transformer Waveforms
The output of the driver swings from 0V to VDD producing a
DC component equal to VDD x duty cycle. If this voltage is
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
(b)
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AN-6069
APPLICATION NOTE
Figure 13. Improved Gate Drive Transformer Circuit
The PNP transistor added at the gate of the MOSFET is
turned on when the secondary voltage goes negative to
speed up the turn-off time of the MOSFET.
(c)
Figure 11. Current Flow and Diode Clamp Circuit for
Transformer Driver
Reference [3], “Design and Application Guide for High
Speed MOSFET Gate Drive Circuits,” offers further
information on transformer-coupled gate drives and should
be consulted for detailed design methodology beyond the
scope of the present topic.
If the transformer is designed with low leakage inductance,
the propagation delays through the transformer can be less
than 50ns. The GT03 series of transformers from ICE
Components[4] is an example of devices with leakage
inductance of a few hundred nanoHenries. This is achieved
by using tightly coupled windings on a small ferrite core.
Discrete or Integrated Drivers
External drivers can be designed using discrete transistors or
integrated circuit solutions that come as predesigned blocks.
To select a solution, designers must evaluate the competing
size, features, cost, and the overall range of applications to
be covered. Regardless of the driver selection, there are
some common requirements. Integrated or discrete-design
drivers need a local bypass capacitor to supply the high
current pulses delivered during the switching intervals and
might include a resistor between the driver and the PWM
supply VDD. In general, drivers have the greatest impact
when located close to the MOSFET gate-source connections
to minimize parasitic inductance and resistance effects.
In the previous transformer examples, the positive and
negative peaks vary with duty cycle, while the secondary
voltage VS swings around zero volts. In a pulse transformer
application, the pulses might feed circuits that cannot accept
the negative-going pulses. The circuit in Figure 12
incorporates a clamp circuit consisting of a second coupling
capacitor CCS and a diode that restores the DC level of the
secondary voltage.
Discrete solutions can be designed using bipolar transistors,
as shown in Figure 14. The NPN/PNP totem pole features a
non-inverting configuration driven by the PWM output. This
circuit prevents shoot-through in the bipolar stage because
only one of the totem pole devices can be forward biased at
a time. In the bipolar common emitter configuration, the
driving signal must have fast edges to provide fast
switching, and it should be noted that the MOSFET gate is
not ohmically connected to the rail when high or low.
Figure 12. Pulse Transformer with DC Restore Circuit
Series resistor RS serves to damp the initial transient at
startup when CCS is initially uncharged, and is often a
discrete resistor in addition to the internal driver impedance.
From classical RLC circuit theory, a value of RS for critical
damping is approximately:
RS = 2 ⋅
LMAG
CCC
(15)
where LMAG is the magnetizing inductance of the
transformer.
Figure 13 shows a gate drive application circuit that utilizes
the DC restore circuit of the previous example with some
additional modifications.
Figure 14. Discrete Bipolar Transistor Drive Circuit
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
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AN-6069
APPLICATION NOTE
Common methods used for driver datasheet current ratings:
The PMOS/NMOS version shown in Figure 15 has a natural
inversion and would require an inverter to follow the PWM
signal polarity. This circuit offers rail-to-rail operation, but
shoot-through is a problem that must be considered in design
because both devices can conduct when the common gate
node voltage is in the middle part of the VDD range.
•
•
•
•
Peak current available from device, usually at initial
turn on at maximum VDD
Current available with the output clamped at a specific
voltage, often around VDD/2
Current available with low value resistance to rails
(perhaps 0.5Ω, even short circuit)
Current measured with a current probe
Integrated MOSFET drivers are commonly available in one
of three technologies: primarily MOSFET, bipolar, or a
combination of the two, often referred to as “compound”
devices. The MOSFET and bipolar versions are similar to
the discrete solutions previously mentioned, while the
compound design combines features from both technologies.
For low-side drivers built with a MOS output state (PMOS
high side and NMOS low side, similar to the discrete circuit
illustrated in Figure 15), the datasheet current rating is
generally specified as the peak current available from the
part, often specified with VDD near the maximum rating of
the part. Figure 16 shows the output current and voltage for
a 4A driver using test methods detailed in the section
“Evaluating Drivers on the Bench” below. This testing
shows that the internal circuitry limits the peak output
current to a value near the rated 4A with no external resistor.
Figure 15. Discrete PMOS/NMOS Drive Circuit
Using the discrete driver approach leads to a higher
component count that requires more PCB board space and
more assembly and test time. The higher component count
can lead to more procurement costs and reliability concerns.
If the input signal comes from a logic circuit or a lowvoltage PWM, the discrete driver requires additional
circuitry to translate from logic levels to power drive levels.
Integrated circuit drivers offer significant benefits in
addition to large pulse current capability. New integrated
dual drivers in 3x3mm packages and single drivers in
2x2mm packages include a thermal pad for heat removal.
These devices require less board space than discrete
solutions, while offering enhanced thermal performance, so
they are well-suited for the most dense power designs.
Features integrated into the device, such as an enable
function and UVLO, create ease of use and reduce
component-level design. It has been common practice to
offer drivers with TTL-compatible input thresholds that can
accept inputs ranging from logic-level signals up to the VDD
range of the device. Drivers utilizing CMOS input
thresholds (2/3 VDD = high, 1/3VDD = low) can help alleviate
noise issues or set more accurate timing delays at the input
of the driver.
Figure 16. PMOS/NMOS Driver VOUT and IOUT
The PMOS/NMOS drivers usually specify the driver output
resistance when it is sinking or sourcing a specified current,
such as 100mA. It is interesting to note that the MOS-type
driver does not attain the RO,high or RO,low resistance values
immediately when the device begins switching. For
example, 4A drivers commonly specify a value for RO,high or
RO,low from 1 – 2Ω. If the devices reached this low resistance
value instantaneously, the peak currents would be more than
7A with VDD = 15V.
Driver Datasheet Current Ratings
Driver datasheet current ratings and test conditions can lead
to confusion. Many consider the gate driver to be a near
ideal voltage source that can instantly deliver current as
determined by the circuit series resistance. This is not
necessarily true. Usually, the current available from a driver
is limited by the internal circuit design, regardless of the
semiconductor technology used. This self-limiting nature
should not be confused with self-protecting; if a driver
output is shorted high or low, the device is likely to fail.
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
In compound devices, bipolar and MOSFET devices are
combined in a parallel configuration, such as the one shown
in Figure 17, where the power output devices are shaded.
The bipolar transistors are able to deliver high sink and
source current, while the output voltage swings through the
middle of the output range. The PMOS and NMOS operate
in parallel with the bipolar devices to pull the output voltage
to the positive or negative rail as required.
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APPLICATION NOTE
according to specific circuit layout and ground structure,
reference [6] gives an approximate value of 10nH/inch
(4nH/cm) for microstrip on FR-4 with the trace exposed to
air on one side. This provides an estimate that can be used
with the circuit capacitance to calculate a damping resistor
when needed.
It is difficult to compare competing devices using only
datasheets, which offer information produced using different
test conditions. Competing technologies used in integrated
circuit solutions further complicate device comparison. In
the following paragraphs, several circuits that can be used to
test and compare drivers on the bench are presented.
Figure 19 shows a circuit that can be used to test the pulsed
current source capability of a driver by clamping VOUT to a
level equal to VDSCH + VDZEN when the output is high. To
minimize power dissipation, the input is driven with a 200ns
positive-going pulse (for non-inverting driver) with a 2%
duty cycle. In this circuit, the positive-going voltage across
RCS is used to monitor the current sourced out of the driver.
To change the value of the output clamping voltage, the
voltage rating of DZEN must be changed.
Figure 17. Compound Driver Output Stage
For compound drivers, the output current is often specified
with the output voltage at a specified voltage, such as VDD/2,
to highlight the current that is available during the Miller
plateau region of the VGS waveform. In tests performed
using the methods described in section “Evaluating Drivers
on the Bench” below, the peak output current is generally
higher than the current specified at VDD/2. Figure 18 shows
the sink current capability of a 4A compound driver
(FAN3224C) to be 4.76A, while the output is at 6.1V after
reaching a peak just under 6A. A compound driver rated at
4A might deliver a higher peak current than a comparably
rated PMOS/NMOS driver. This type of information is
practically impossible to obtain from the driver datasheets,
so specific test methods are required.
VDD
VPULSE
VOUT
CBYP
DSCH
DZEN
+
VCS
RCS
-
INPUT at 10V/div
Figure 19. Current Source Test Circuit with
Clamped VOUT
Figure 20 shows a circuit used to test the pulsed current sink
capability of a driver with the output voltage clamped at a
level VADJ-VDSCH. Here, the input is driven with a 200ns
negative-going pulse (for a non-inverting driver) with a 2%
duty cycle. In this circuit, the negative-going voltage across
RCS is used to monitor the current that the driver is sinking.
IOUT at 2A/div
VOUT at 5V/div
Time = 200ns/div
Figure 18. Compound Driver Current Sink Waveform
Evaluating Drivers on the Bench
Real-world driver comparisons are difficult to perform in the
lab because the fast signal ramp rates cause complex
interactions between the inductive and capacitive circuit
components. These fast edge rates can introduce overshoots
and undershoots of several volts. Some examples to help
quantify this effect in power circuits can be found in
reference [5]. Although the parasitic inductance varies
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
Figure 20. Current Sink Test Circuit with Clamped VOUT
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APPLICATION NOTE
In both these circuits, there is a voltage transient that may
last for 50-100ns as the current increases to the limits of the
driver. A compact layout using surface mount components
keeps the loop area small to minimize parasitic inductance.
The two previous circuits require a unique surface mount
layout. It is possible to evaluate driver current capability by
connecting a relatively large capacitive load on the output of
a driver with the simple circuit shown in Figure 21.
Figure 22. Compound Driver Current Source
Waveforms
Figure 22 shows the leading spike across the inductance
introduced by the wire loop inserted in the circuit to enable
use of a current probe. If the wire loop is removed and the
0.1µF surface mount capacitor is installed in a layout with
minimal parasitic inductance, the waveforms shown in
Figure 23 are obtained. In short intervals where the voltage
waveform is approximately linear, the basic relation is:
Figure 21. "Large" Load Test Circuit
⎞
⎛ dV
I = CLOAD ⋅ ⎜ OUT ⎟
dT
⎠
⎝
For a starting point, CLOAD is chosen to be 100 times larger
than the load used for rise and fall time measurements and
the input is driven with a 1kHz square wave. On typical
datasheets, 2A drivers are specified with 1nF load for the
rise and fall time specifications, so CLOAD would be selected
to be 0.1µF. This relatively large load prevents the output
from changing rapidly, allowing the driver output current to
reach its internal limiting value. A current probe, IPRB, can
be used to monitor the output current along with the output
voltage VOUT on an oscilloscope. This enables plotting the
output current available at the corresponding output voltage.
Bench comparisons have shown that the current
measurement obtained using this method agrees closely with
that obtained using the clamp circuits in Figure 19 and
Figure 20. In addition, the slower current rise and fall times
allow the current measurements to be made comfortably
within the bandwidth limits of a current probe.
can be applied to provide an estimate of the current.
Figure 23. Compound Driver Current Estimation
Figure 22 shows the waveforms obtained using the test
circuit shown in Figure 21 to evaluate a 2A sink / 1.5A
source driver (FAN3227C) with a compound output stage.
When the driver input, VIN, goes high, there is a transient
glitch on the VOUT trace as the output current quickly
increases to 3A through the inductance of the current probe
loop. After approximately 70ns, the current has reached its
peak value and the voltage spike across the parasitic
inductances vanishes. With VOUT = 6V, the output current is
measured as 1.5A (source current).
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
(16)
The oscillogram in Figure 23 allows calculation of current
during the cursor interval as:
⎛ 1.131V ⎞
I = 0.1µF ⋅ ⎜
⎟ = 2.8 A
⎝ 40.6ns ⎠
(17)
providing close agreement with the peak value seen in the
IOUT trace in Figure 22. A similar calculation around VOUT =
6V provides a current estimation of 1.5A, nearly identical to
the result obtained with direct current measurement using a
current probe. The close agreement between the current
measurement techniques using the large load helps develop
confidence in the results obtained.
www.fairchildsemi.com
9
AN-6069
APPLICATION NOTE
Conclusion
There is not a simple unified method to characterize the
output current sink and source capability of the many types
of drivers available. The test circuits presented in this note
can be used to investigate the VOUT vs. IOUT capability of
discrete and integrated circuit drivers, enabling evaluation
and comparison of drivers for a range of applications.
Low-side drivers are used to drive power MOSFETs in
applications including clamped inductive load switching,
synchronous rectifier circuits, and pulse/gate transformer
drive circuits. The relationship of gate drive current to the
MOSFET switching and transition intervals has been
detailed during the prominent MOSFET switching intervals.
Potential driver solutions; including discrete components,
integrated PMOS/NMOS, and compound drivers, were
examined. Some of the non-ideal characteristics of the
various driver circuits were highlighted.
References
[1]
[2]
[3]
[4]
[5]
[6]
2006 Fairchild Power Seminar Topic, “Understanding Modern Power MOSFETs,” available on the fairchildsemi.com website at the link:
http://www.fairchildsemi.com/powerseminar/pdf/understanding_modern_power_mOSFETs.pdf
Oh, K. S., “MOSFET Basics”, July, 2000, available as AN9010 from the fairchildsemi.com website.
Balogh, L. “Design and Application Guide for High Speed MOSFET Gate Drive Circuits,” Power Supply Design Seminar SEM-1400, Topic 2,
Texas Instruments Literature No. SLUP169.
ICE Components Gate Drive Transformer Datasheet “GT03.pdf” dated 10/06, available from www.icecomponents.com.
2006 Fairchild Power Seminar Topic, “Practical Power Application Issues for High Power Systems,” available on the fairchildsemi.com website at the
link: http://www.fairchildsemi.com/powerseminar/pdf/practical_power_high_power_systems.pdf
Johnson, H. Dr, “High-Speed Digital Design On-Line Newsletter,” Vol. 3 Issue 8, www.sigcon.com/Pubs/news/3_8.htm
Author
Mark Dennis was born in Troy, NC, and received the Bachelor of Engineering degree from Duke University in 1983. After
graduation he has worked in industries encompassing power electronics applications such as offline and DC to DC power
supply design for telecom and computer systems, high voltage supplies for electrostatic precipitators, and online UPS
systems. For over eight years Mark has been working in the semiconductor industry and he is employed by Fairchild
Semiconductor as a Staff Engineer working in High Power Systems.
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
www.fairchildsemi.com
10
AN-6069
APPLICATION NOTE
Related Parts
Type
Single 1A
Part
Number
FAN3111C
(1)
Gate Drive
(Sink/Src)
+1.1A / -0.9A
Input
Threshold
CMOS
(2)
Logic
Package
Single Channel of Dual-Input/Single-Output
SOT23-5, MLP6
Single 1A
FAN3111E
+1.1A / -0.9A
External
Single Non-Inverting Channel with External Reference
SOT23-5, MLP6
Single 2A
FAN3100C
+2.5A / -1.8A
CMOS
Single Channel of Two-Input/One-Output
SOT23-5, MLP6
Single 2A
FAN3100T
+2.5A / -1.8A
TTL
Single Channel of Two-Input/One-Output
SOT23-5, MLP6
Dual 2A
FAN3216T
+2.4A / -1.6A
TTL
Dual Inverting Channels
SOIC8
Dual 2A
FAN3217T
+2.4A / -1.6A
TTL
Dual Non-Inverting Channels
SOIC8
Dual 2A
FAN3226C
+2.4A / -1.6A
CMOS
Dual Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 2A
FAN3226T
+2.4A / -1.6A
TTL
Dual Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 2A
FAN3227C
+2.4A / -1.6A
CMOS
Dual Non-Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 2A
FAN3227T
+2.4A / -1.6A
TTL
Dual Non-Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 2A
FAN3228C
+2.4A / -1.6A
CMOS
Dual Channels of Two-Input/One-Output, Pin Config.1
SOIC8, MLP8
Dual 2A
FAN3228T
+2.4A / -1.6A
TTL
Dual Channels of Two-Input/One-Output, Pin Config.1
SOIC8, MLP8
Dual 2A
FAN3229C
+2.4A / -1.6A
CMOS
Dual Channels of Two-Input/One-Output, Pin Config.2
SOIC8, MLP8
Dual 2A
FAN3229T
+2.4A / -1.6A
TTL
Dual Channels of Two-Input/One-Output, Pin Config.2
SOIC8, MLP8
Dual 2A
FAN3268T
+2.4A / -1.6A
TTL
20V Non-Inverting Channel (NMOS) and Inverting Channel
SOIC8
(PMOS) + Dual Enables
Dual 2A
FAN3278T
+2.4A / -1.6A
TTL
30V Non-Inverting Channel (NMOS) and Inverting Channel
SOIC8
(PMOS) + Dual Enables
Dual 4A
FAN3213T
+2.5A / -1.8A
TTL
Dual Inverting Channels
SOIC8
Dual 4A
FAN3214T
+2.5A / -1.8A
TTL
Dual Non-Inverting Channels
SOIC8
Dual 4A
FAN3223C
+4.3A / -2.8A
CMOS
Dual Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 4A
FAN3223T
+4.3A / -2.8A
TTL
Dual Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 4A
FAN3224C
+4.3A / -2.8A
CMOS
Dual Non-Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 4A
FAN3224T
+4.3A / -2.8A
TTL
Dual Non-Inverting Channels + Dual Enable
SOIC8, MLP8
Dual 4A
FAN3225C
+4.3A / -2.8A
CMOS
Dual Channels of Two-Input/One-Output
SOIC8, MLP8
Dual 4A
FAN3225T
+4.3A / -2.8A
TTL
Dual Channels of Two-Input/One-Output
SOIC8, MLP8
Single 9A
FAN3121C
+9.7A / -7.1A
CMOS
Single Inverting Channel + Enable
SOIC8, MLP8
Single 9A
FAN3121T
+9.7A / -7.1A
TTL
Single Inverting Channel + Enable
SOIC8, MLP8
Single 9A
FAN3122T
+9.7A / -7.1A
CMOS
Single Non-Inverting Channel + Enable
SOIC8, MLP8
Single 9A
FAN3122C
+9.7A / -7.1A
TTL
Single Non-Inverting Channel + Enable
SOIC8, MLP8
Notes:
1. Typical currents with OUTx at 6V and VDD=12V.
2. Thresholds proportional to an externally supplied reference voltage.
To review the datasheets for the above low-side gate drivers, visit Fairchild Semiconductor’s website at:
http://www.fairchildsemi.com/sitesearch/fsc.jsp?command=eq&attr1=AAAFamily&attr2=Low-Side+Drivers
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
www.fairchildsemi.com
11
AN-6069
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 reasonably expected to
result in significant injury to the user.
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
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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12