conduct at the same time.
transistor turns off when the
optocoupler LED turns on; this
type of design, however, requires
additional fail-safe circuitry to
turn off the power transistor if an
over-current condition is
detected. The timing illustrated in
Figure 17 assumes that the power
transistor turns on when the
optocoupler LED turns on.
time is the sum of the maximum
difference in turn-on delay plus
the maximum difference in turn-
off delay,
Extremely large currents will flow
if there is any overlap in their
conduction during switching
transitions, potentially damaging
the transistors and even the sur-
rounding circuitry. This “shoot-
through” current is eliminated by
delaying the turn-on of one
transistor (Q2) long enough to
ensure that the opposing
transistor (Q1) has completely
turned off. This delay introduces a
small amount of “dead time” at
the output of the inverter during
which both transistors are off
during switching transitions.
Minimizing this dead time is an
important design goal for an
inverter designer.
[(tPLHmax-tPLHmin)+(tPHLmax-tPHLmin)].
This expression can be
rearranged to obtain
[(tPLHmax-tPHLmin)-(tPHLmin-tPHLmax)],
and further rearranged to obtain
The LED signal to turn on Q2
should be delayed enough so that
an optocoupler with the very
fastest turn-on propagation delay
(tPHLmin) will never turn on before
an optocoupler with the very
[(tPLH-tPHL max
)
-(tPLH-tPHL)min],
which is the maximum minus the
slowest turn-off propagation delay minimum data sheet values of
(tPLHmax) turns off. To ensure this, (tPLH-tPHL). The difference
the turn-on of the optocoupler
should be delayed by an amount
no less than (tPLHmax - tPHLmin),
which also happens to be the
maximum data sheet value for the
propagation delay difference
specification, (tPLH - tPHL). The
HCPL-4504/0454 and
HCNW4504 specify a maximum
(tPLH - tPHL) of 1.3 µs over an
operating temperature range
of 0-70°C.
between the maximum and
minimum values depends directly
on the total spread in propagation
delays and sets the limit on how
good the worst-case dead time
can be for a given design.
Therefore, optocouplers with tight
propagation delay specifications
(and not just shorter delays or
lower pulse-width distortion) can
achieve short dead times in power
inverters. The HCPL-4504/0454
and HCNW4504 specify a
The amount of turn-on delay
needed depends on the propaga-
tion delay characteristics of the
optocoupler, as well as the
characteristics of the transistor
base/gate drive circuit. Consider-
ing only the delay characteristics
of the optocoupler (the charac-
teristics of the base/gate drive
circuit can be analyzed in the
same way), it is important to
know the minimum and maximum
Although (tPLH-tPHL max
)
tells the
minimum (tPLH - tPHL) of -0.7 µs
over an operating temperature
range of 0-70°C, resulting in a
turn-on (tPHL) and turn-off (tPLH
propagation delay specifications,
preferably over the desired
)
designer how much delay is
needed to prevent shoot-through
current, it is insufficient to tell the maximum dead time of 2.0 µs
operating temperature range. The
importance of these specifications
is illustrated in Figure 17. The
waveforms labeled “LED1”,
designer how much dead time a
design will have. Assuming that
the optocoupler turn-on delay is
when the LED turn-on delay is
equal to (tPLH-tPHL max, or 1.3 µs.
)
exactly equal to (tPLH - tPHL max
)
,
It is important to maintain
accurate LED turn-on delays
because delays shorter than
“LED2”, “OUT1”, and “OUT2” are
the input and output voltages of
the optocoupler circuits driving
Q1 and Q2 respectively. Most
inverters are designed such that
the power transistor turns on
when the optocoupler LED turns
on; this ensures that both power
transistors will be off in the event
of a power loss in the control
circuit. Inverters can also be
designed such that the power
the minimum dead time is zero
(i.e., there is zero time between
the turn-off of the very slowest
optocoupler and the turn-on of
the very fastest optocoupler).
(tPLH - tPHL max may allow shoot-
)
through currents, while longer
delays will increase the worst-case
dead time.
Calculating the maximum dead
time is slightly more complicated.
Assuming that the LED turn-on
delay is still exactly equal to
(tPLH - tPHL max
)
, it can be seen in
Figure 17 that the maximum dead
1-48
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