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Texas Instruments Understanding Peak IOH and IOL Currents Application notes
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Understanding Peak Source and Sink Current Parameters
Mateo Begue, High Power Drivers
Gate drivers are often confused as continuous current
sources because of the IOH and IOL specifications in the
datasheet. For example, designers looking at the
UCC5320SC might read the parameters 4.3-A source
and 4.4-A sink and mistakenly believe these devices
are capable of providing these currents continuously.
Gate drivers do not need to provide constant current
because they only have to source/sink current when
switching the gate of the MOSFET or IGBT. Refer to
Figure 1 for the turn-on waveforms.
In order to understand the IOH and IOL specifications,
we need to look at the pull-up and pull-down structures
inside the device. The output stage of a gate driver
typically comes in some variation of Figure 2.
UCC5320SC is offered in a split output pinout that
gives designers more control of the rise and fall times
without adding extra components like schottky diodes.
UVLO2
Level
Shifting
and
Control
Logic
VCC2
ROH
RNMOS
OUTH
OUTL
ROL
VEE2
Figure 2. Gate Driver Output Stage
Under a no load condition, IOH is determined by VCC2
and the parallel combination of RNMOS and ROH while IOL
is set by VCC2 and ROL. RNMOS helps the pull-up
structure deliver the peak current with a brief boost in
peak-sourcing current during the Miller plateau region
shown as interval 3 in Figure 1. This is done by turning
on the N-channel MOSFET during a narrow instant
when the output is changing states from low to high.
When driving MOSFETs and IGBTs high, the external
gate resistor RON and the transistor's internal gate
resistance RGFET_Int , reduce the peak output current as
shown in Equation 1.
IOH
RGFET _ Int
·
¸
¸
¹
(1)
Likewise, the peak sink current is limited by the
external gate resistor ROFF in series with ROL and
RGFET_int and is determined by Equation 2
§
·
VCC2
IOL min ¨ 4.4 A,
¸
¨
ROL R OFF RGFET _ Int ¸¹
©
(2)
Understanding Peak Source and Sink Current Parameters Mateo Begue, High Power
1
Figure 1. MOSFET Turn-On Time Intervals
SLLA387 – March 2018
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§
VCC2
min ¨ 4.3 A,
¨
R
||
R
NMOS
OH RON
©
Copyright © 2018, Texas Instruments Incorporated
Drivers
www.ti.com
This TI TechNote will use the isolated single-channel
gate driver, UCC5320SC and a 100-nF capacitive load
to demonstrate different techniques to determine the
peak drive current. The first method calculates the
expected peak currents based on Equation 1 and
Equation 2. Use these equations to estimate the peak
drive current when selecting a gate driver for your
system.
In order to simulate driving a MOSFET or IGBT before
installing it onto the PCB, select a load capacitor that
is equivalent to the switch's input capacitance, CISS.
Determine the input capacitance by looking up the
required gate charge from the MOSFET or IGBT's
datasheet at the drive voltage condition.
The results of the three presented techniques are
shown in Table 1. Even with the 0.1-Ω sense resistor
in series with the capacitor, Equation 1 predicts 4.30-A
sourcing current. Equation 3 uses the largest
measured dV/dt value in the linear region of the gate
drive waveform which gives an estimated 4.53-A. In
this same linear region, the voltage across the sense
resistor is measured in Figure 4 and Ohm's law is
used to determine peak IOH at 4.29-A.
A second technique uses this CISS value and the dV/dt
of the switching waveform to determine the source or
sink current. Figure 3 measures the dV/dt using
cursors set to a fixed 35-ns interval and swept across
the rising edge in order to find the peak dV/dt. As a
guideline, set the oscilloscope's cursors to a time
interval, Δt of approximately 10% of the rise time to
determine the current through the load capacitor.
Figure 4. Voltage Across Series Sense Resistor
The first method is a good starting point when
selecting a gate driver but it is not an actual measured
value. The second method relies on the engineer to
accurately measure the highest dV/dt by using a fixed
Δt and sweeping it across the waveform. Lastly, the
voltage measured across the 0.1-Ω sense resistor will
give the engineer a value calculated from
measurement of the peak drive current using Figure 4
and Ohm's law. The key to the third measurement
technique is to select a small valued sense resistor to
prevent any limitations in the peak output current. All
presented methods are acceptable approximations of
a gate driver's peak output current.
Figure 3. Measuring Peak dV/dt Across Load
Capacitor
Use the measured peak dV/dt and load capacitor
value along with Equation 3 to calculate the peak
current.
dV
IC C
dt
To reiterate, IOH and IOL are not continuous DC values.
The peak current charges or discharges CISS in an
instant and then reduces in value as the switch begins
to turn on.
A third method inserts a 0.1-Ω sense resistor between
the capacitor and ground to calculate IOH or IOL.
Figure 4 shows the voltage waveform across the
sense resistor, VSENSE and its measurement coincides
with the highest dV/dt value of the Vcap waveform.
2
Table 1. Measurement Comparison
(3)
Theoretical vs.
Measured
Method
Result
Theoretical
Equation 1:
IOH = min[4.30A, 4.44A]
4.30A
Calculated from
Measurement
Equation 3:
IC= 102nF(44.4MV/s)
4.53A
Calculated from
Measurement
Ohm's Law:
IOH = 438mV/102mΩ
4.29A
Understanding Peak Source and Sink Current Parameters Mateo Begue, High Power
Drivers
Copyright © 2018, Texas Instruments Incorporated
SLLA387 – March 2018
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