HCPL-314J Datasheet, PDF (12/16 Pages) Agilent(Hewlett-Packard) – 0.4 Amp Output Current IGBT Gate Drive Optocoupler

12
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).
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.
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).
P_T = P_E + P_O
P_E = I_{F •} V_{F •} Duty Cycle
P_O = P_O(BIAS) + P_O(SWITCHING) = I_{CC •} V_CC + E_SW (Rg,Qg)_• f
= (I_CCBIAS + K_{ICC •} Qg _• f) _• V_CC + E_SW (Rg,Qg) _• f
where K_{ICC •} Qg _• f is the increase in I_CC due to switching and K_ICC is a
constant of 0.001 mA/(nC*kHz). For the circuit in Figure 19 with I_F
(worst case) = 10 mA, Rg = 32 Ω, Max Duty Cycle = 80%,
Qg = 100 nC, f = 20 kHz and T_AMAX = 85°C:
P_E = 10 mA _• 1.8 V _• 0.8 = 14 mW
P_O = (3 mA + (0.001 mA/(nC _• kHz)) _• 20 kHz _• 100 nC) _• 24 V +
0.4 µJ _• 20 kHz = 128 mW
< 260 mW (P_O(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 P_O for this case is less than P_O(MAX), Rg = 32 Ω is alright for the
power dissipation.
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
optocoupler. 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 recommended
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.