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AV02-0867EN - July 1, 2014
This expression can be rearranged to obtain
[(t
PLHmax
-t
PHLmin
)-(t
PHLmin
-t
PHLmax
)],
and further rearranged to obtain
[(t
PLH
-t
PHL
)
max
-(t
PLH
-t
PHL
)
min
],
which is the maximum minus the minimum data sheet
values of (t
PLH
-t
PHL
). The dierence between the maxi-
mum 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, opto coup lers with tight propagation delay
specications (and not just shorter delays or lower pulse-
width distortion) can achieve short dead times in power
inverters. The HCPL-4504/0454/J454 and HCNW4504
specify a minimum (t
PLH
- t
PHL
) of -0.7 µs over an operat-
ing temperature range of 0-70°C, resulting in a maximum
dead time of 2.0 µs when the LED turn-on delay is equal
to (t
PLH
-t
PHL
)
max
, or 1.3 µs.
It is important to maintain accurate LED turn-on delays
because delays shorter than (t
PLH
- t
PHL
)
max
may allow
shoot-through currents, while longer delays will increase
the worst-case dead time.
The LED signal to turn on Q2 should be delayed enough
so that an optocoupler with the very fastest turn-on
propagation delay (t
PHLmin
) will never turn on before an
optocoupler with the very slowest turn-o propagation
delay (t
PLHmax
) turns o. To ensure this, the turn-on of the
optocoupler should be delayed by an amount no less
than (t
PLHmax
- t
PHLmin
), which also happens to be the max-
imum data sheet value for the propagation delay dier-
ence specication, (t
PLH
- t
PHL
). The HCPL-4504/0454/J454
and HCNW4504 specify a maxi mum (t
PLH
- t
PHL
) of 1.3 µs
over an operating temperature range of 0-70°C.
Although (t
PLH
-t
PHL
)
max
tells the designer how much delay
is needed to prevent shoot-through current, it is insu-
cient to tell the designer how much dead time a design
will have. Assuming that the optocoupler turn-on delay
is exactly equal to (t
PLH
- t
PHL
)
max
, the minimum dead time
is zero (i.e., there is zero time between the turno of the
very slowest optocoupler and the turn-on of the very
fastest optocoupler).
Calculating the maximum dead time is slightly more
compli cated. Assuming that the LED turn-on delay is still
exactly equal to (t
PLH
- t
PHL
)
max
, it can be seen in Figure 17
that the maximum dead time is the sum of the maximum
dierence in turn-on delay plus the maxi mum dierence
in turno delay,
[(t
PLHmax
-t
PLHmin
)+(t
PHLmax
-t
PHLmin
)].
