HCPL-314J Datasheet, PDF (12/15 Pages) Agilent(Hewlett-Packard) – 0.4 Amp Output Current IGBT Gate Drive Optocoupler

Selecting the Gate Resistor (Rg)
Step 1: Calculate R_g minimum from the I_OL peak specification. The IGBT and Rg
in Figure 24 can be analyzed as a simple RC circuit with a voltage supplied by
the HCPL-314J.
Rg ≥ ^{VCC – VOL}
     I_OLPEAK
  = ^{24 V – 5 V}
      0.6A
  = 32 Ω
The V_OL value of 5 V in the previous equation is the V_OL at the peak current of
0.6A. (See Figure 6).
Step 2:Check the HCPL-314J power dissipation and increase Rg if necessary.
The HCPL-314J total power dissipation (P_T) is equal to the sum of the emitter
power (P_E) and the output power (P_O).
PT = PE + PO
PE = IF • VF • Duty Cycle
PO = PO(BIAS) + PO(SWITCHING) = ICC • VCC + ESW (Rg,Qg)• f
= (ICCBIAS + KICC • Qg • f) • VCC + ESW (Rg,Qg) • f
where KICC • Qg • f is the increase in I_CC due to switching and KICC is a constant
of 0.001 mA/(nC*kHz). For the circuit in Figure 19 with IF (worst case) = 10
mA, Rg = 32 Ω,Max Duty Cycle = 80%,Qg = 100 nC, f = 20 kHz and T_AMAX =
85°C:
PE = 10 mA • 1.8 V • 0.8 = 14 mW
PO = (3 mA + (0.001 mA/(nC • kHz)) • 20 kHz • 100 nC) • 24 V + 0.4 µJ • 20 kHz
= 128 mW
< 260 mW (PO(MAX) @ 85°C)
The value of 3 mA for I_CC in the previous equation is the max. I_CC over entire
operating temperature range.
Since PO for this case is less than PO(MAX), Rg = 32 Ω is alright for the power
dissipation.
4.0
Qg = 50 nC
3.5
Qg = 100 nC
3.0
Qg = 200 nC
Qg = 400 nC
2.5
2.0
1.5
1.0
0.5
0
0
20
40
60
80 100
Rg – GATE RESISTANCE – Ω
Figure 20. Energy Dissipated in the HCPL-314J and for
Each IGBT Switching Cycle.
LED Drive Circuit Considerations for Ultra
High CMR Performance
Without a detector shield, the
dominant cause of optocoupler CMR
failure is capacitive coupling from
the input side of the optocoupler,
through the package, to the detector
IC as shown in Figure 21. The HCPL-
314J improves CMR performance by
using a detector IC with an optically
transparent Faraday shield, which
diverts the capacitively coupled
current away from the sensitive IC
circuitry. However, this shield does
not eliminate the capacitive coupling
between the LED and optocoupler
pins 5-8 as shown in Figure 22.
This capacitive coupling causes
perturbations in the LED current
during common mode transients
and becomes the major source of
CMR failures for a shielded optocou-
pler. The main design objective of a
high CMR LED drive circuit becomes
keeping the LED in the proper state
(on or off ) during common mode
transients. For example, the recom-
mended application circuit (Figure
19), can achieve 10 kV/µs CMR while
minimizing component complexity.
Techniques to keep the LED in the
proper state are discussed in the next
two sections.
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