HCPL-J314 Datasheet, PDF (13/17 Pages) Agilent(Hewlett-Packard) – 0.4 Amp Output Current IGBT Gate Drive Optocoupler

13
Selecting the Gate Resistor (Rg)
Step 1: Calculate R_g minimum from the I_OL peak specification. The
IGBT and Rg in Figure 19 can be analyzed as a simple RC circuit with a
voltage supplied by the HCPL-J314.
Rg ≥ ^{VCC – VOL}
I_OLPEAK
= ^{24 V – 5 V}
0.6A
= 32 Ω
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
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).
Figure 20. Energy Dissipated in the
HCPL-J314 and for Each IGBT
Switching Cycle.
Step 2: Check the HCPL-J314 power dissipation and increase Rg if
necessary. The HCPL-J314 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 = 80 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-J314 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 opto-
coupler 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.