18
Power Inverter Dead Time and Propagation Delay Specica-
tions
The HCPL-4504/0454/J454 and HCNW4504 include a
specica tion intended to help designers minimize “dead
time” in their power inverter designs. The new “propaga-
tion delay dierence” specication (t
PLH
- t
PHL
) is useful for
deter min ing not only how much optocoupler switch-
ing delay is needed to prevent “shoot-through” current,
but also for determin ing the best achievable worst-case
dead time for a given design.
When inverter power transis tors switch (Q1 and Q2 in
Figure 17), it is essential that they never conduct at the
same time. Extremely large currents will ow if there is
any overlap in their conduction during switching tran-
sitions, poten tially damaging the transistors and even
the sur rounding circuitry. This “shoot-through” current is
eliminated by delay ing the turn-on of one transistor (Q2)
long enough to ensure that the opposing transistor (Q1)
has completely turned o. This delay intro duces a small
amount of “dead time” at the output of the inverter dur-
ing which both transistors are o during switching tran-
sitions. Minimiz ing this dead time is an important design
goal for an inverter designer.
The amount of turn-on delay needed depends on the
propa ga tion delay characteristics of the optocoupler, as
well as the characteristics of the transistor base/gate drive
circuit. Consid er ing only the delay characteris tics 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 turn-on (t
PHL
) and
turno (t
PLH
) propagation delay specica tions, prefer-
ably over the desired operating temperature range. The
importance of these specications is illustrated in Figure
17. The waveforms labeled “LED1”, “LED2”, “OUT1”, and
“OUT2” are the input and output voltages of the opto-
coupler circuits driving Q1 and Q2 respectively. Most in-
verters are designed such that the power transistor turns
on when the optocoupler LED turns on; this ensures that
both power transistors will be o in the event of a power
loss in the control circuit. Inverters can also be designed
such that the power transistor turns o when the opto-
coupler LED turns on; this type of design, however, re-
quires additional fail-safe circuitry to turn o 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.
Figure 17. LED delay and dead time diagram.
