HCPL-J314 datasheet(13/16 Pages) AVAGO

13 / 16 page

Selecting the Gate Resistor (Rg)

Step 1:CalculateRgminimumfromtheIOLpeakspecifi-

cation.TheIGBTandRginFigure19canbeanalyzedas

asimpleRCcircuitwithavoltagesuppliedbytheHCPL-

J314.

VCC – VOL

Rg≥ ————

IOLPEAK

24V–5V

= ————

0.6A

= 32Ω

TheVOLvalueof5VinthepreviousequationistheVOL

atthepeakcurrentof0.6A.(SeeFigure6).

Step 2:ChecktheHCPL-J314powerdissipationandin-

creaseRgifnecessary.TheHCPL-J314totalpowerdissi-

pation(PT)isequaltothesumoftheemitterpower(PE)

andtheoutputpower(PO).

PT=PE+PO

PE = IF 6 VF 6 Duty Cycle

PO = PO(BIAS) + PO(SWITCHING) = ICC 6 VCC + ESW (Rg,Qg) 6 f

=(ICCBIAS+KICC 6 Qg 6 f) 6 VCC+ESW(Rg,Qg) 6 f

whereKICC 6 Qg 6 fistheincreaseinICCduetoswitch-

ingandKICC isaconstantof0.001mA/(nC*kHz).Forthe

circuitinFigure19withIF(worstcase)=10mA,Rg=32

Ω,MaxDutyCycle=80%,Qg=100nC,f=20kHzand

TAMAX=85°C:

PE =10mA 6 1.8V6 0.8=14mW

PO=(3mA+(0.001mA/(nC 6 kHz))6 20kHz6 100nC)6

24V+0.4µJ6 20kHz=80mW

<260mW (PO(MAX)@85°C)

Thevalueof3mAforICCinthepreviousequationisthe

max.ICCoverentireoperatingtemperaturerange.

SincePOforthiscaseislessthanPO(MAX),Rg=32Ωisall

rightforthepowerdissipation.

Figure 20. Energy dissipated in the HCPL-J314 and for

each IGBT switching cycle.

LED Drive Circuit Considerations for Ultra High CMR Perfor-

mance

Without a detector shield, the dominant cause of op-

tocoupler CMR failure is capacitive coupling from the

inputsideoftheoptocoupler,throughthepackage,to

the detector IC as shown in Figure 21. The HCPL-J314

improvesCMRperformancebyusingadetectorICwith

anopticallytransparentFaradayshield,whichdivertsthe

capacitivelycoupledcurrentawayfromthesensitiveIC

circuitry.However,thisshielddoesnoteliminatetheca-

pacitivecouplingbetweentheLEDandoptocouplerpins

5-8asshowninFigure22.Thiscapacitivecouplingcauses

perturbationsintheLEDcurrentduringcommonmode

transientsandbecomesthemajorsourceofCMRfailures

forashieldedoptocoupler.Themaindesignobjectiveof

ahighCMRLEDdrivecircuitbecomeskeepingtheLEDin

theproperstate(onoroff)duringcommonmodetran-

sients.Forexample,therecommendedapplicationcircuit

(Figure19),canachieve10kV/µsCMRwhileminimizing

componentcomplexity.

Techniques to keep the LED in the proper state are

discussedinthenexttwosections.

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

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