HCPL3140 datasheet(12/16 Pages) HP

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Selecting the Gate Resistor (Rg)

Step 1: Calculate Rg minimum from the IOL peak specification. The IGBT

and Rg in Figure 19 can be analyzed as a simple RC circuit with a

voltage supplied by the HCPL-3140/HCPL-0314.

The VOL value of 5 V in the previous equation is the VOL at the peak

current of 0.6A. (See Figure 6).

Step 2: Check the HCPL-3140/HCPL-0314 power dissipation and

increase Rg if necessary. The HCPL-3140/HCPL-0314 total power

dissipation (PT) is equal to the sum of the emitter power (PE) and the

output power (PO).

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 ICC 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 TAMAX = 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 = 80 mW

< 260 mW (PO(MAX) @ 85°C)

The value of 3 mA for ICC in the previous equation is the max. ICC over

entire operating temperature range.

Since PO for this case is less than PO(MAX), Rg = 32 Ω is alright for the

power dissipation.

24 V – 5 V

0.6A

= 32

Ω

Figure 20. Energy Dissipated in the

HCPL-0314 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-3140/HCPL-0314 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.

≥

VCC – VOL

IOLPEAK

Rg – GATE RESISTANCE –

Ω

100

1.5

4.0

1.0

3.5

Qg = 50 nC

Qg = 100 nC

Qg = 200 nC

Qg = 400 nC

3.0

2.0

0.5

2.5

HCPL3140 Datasheet(PDF) 12 Page - Agilent(Hewlett-Packard)

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