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HCPL-4504-500E Datasheet, PDF (18/19 Pages) AVAGO TECHNOLOGIES LIMITED – High CMR, High Speed Optocouplers | |||
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Figure 17. LED delay and dead time diagram.
Power Inverter Dead Time and Propagation Delay Speciï¬ca-
tions
The HCPL-4504/0454/J454 and HCNW4504 include a
speciï¬cation intended to help designers minimize âdead
timeâ in their power inverter designs. The new âpropaga-
tion delay diï¬erenceâ speciï¬cation (tPLH- tPHL) is useful for
determining not only how much optocoupler switch-
ing delay is needed to prevent âshoot-throughâ current,
but also for determining the best achievable worst-case
dead time for a given design.
When inverter power transistors switch (Q1 and Q2 in
Figure 17), it is essential that they never conduct at the
same time. Extremely large currents will ï¬ow if there is
any overlap in their conduction during switching tran-
sitions, potentially damaging the transistors and even
the surrounding circuitry. This âshoot-throughâ current is
eliminated by delaying the turn-on of one transistor (Q2)
long enough to ensure that the opposing transistor (Q1)
has completely turned oï¬. This delay introduces a small
amount of âdead timeâ at the output of the inverter dur-
ing which both transistors are oï¬ during switching tran-
sitions. Minimizing this dead time is an important design
goal for an inverter designer.
The amount of turn-on delay needed depends on the
propagation delay characteristics of the optocoupler, as
well as the characteristics of the transistor base/gate drive
circuit. Considering only the delay characteristics of the
optocoupler (the characteristics of the base/gate drive
circuit can be analyzed in the same way), it is important
to know the minimum and maximum turn-on (tPHL) and
turnoï¬ (tPLH) propagation delay speciï¬cations, prefer-
ably over the desired operating temperature range. The
importance of these speciï¬cations is illustrated in Figure
17. The waveforms labeled âLED1â, âLED2â, âOUT1â, and
âOUT2â 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 oï¬ in the event of a power
loss in the control circuit. Inverters can also be designed
such that the power transistor turns oï¬ when the opto-
coupler LED turns on; this type of design, however, re-
quires additional fail-safe circuitry to turn oï¬ 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.
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