Overview of High PerformanceAnalog Optocouplers

Application Note 1357

Designing Analog CircuitsUsing the HCNR201Internally, the HCNR201 analogoptocoupler consists of twophoto detectors symmetricallyplaced between the input LED.Thus, the radiant flux receivedby each of the two photodetec-tors is essentially the same, andforms the basis for the input-out-put linear transfer response.Unlike most other optocouplers,where the LED at the input isdirectly controlled, for theHCNR201 the input photodetec-tor is generally placed in a servofeedback loop to control the LEDcurrent through the use of an ex-ternal op-amp. This feedbackloop has the most advantageouseffect of compensating for anytemperature related light outputdrift characteristics or othernonlinearities or aging effects ofthe LED.

Figure 1 shows the basictopology using the HCNR201 inthe servo feedback loop. TheHCNR201 is connected in aphotovoltaic mode, as the voltageacross the photo-diodes isessentially zero volt. For aphotoconductive operation thephoto-diodes are reverse biasedas shown in Figure 2.

The two op-amps shown are twoseparate LM158 packages, andnot two channels in a single dualpackage, otherwise galvanic

insulation is not present as thegrounds and Vcc are sharedbetween the two op-amps of thedual package. The op-amp alwaystries to maintain the same inputsvoltages at its two inputs in alinear feedback close loopconnection. Thus, the input sideop-amp always tries to place zerovolts across the photodiode 1(PD1). As noted before, in the

photo-voltaic mode of operation,the photodiode has either aforward bias or no bias appliedacross it. Thus, when the Vin=0V,there is no photodiode 1 current(IPD1) and so also is the IPD2 zero.This is because IPD2 = K3 x IPD1 bythe transfer gain K3 indicated inthe data sheet (K3 = IPD2/IPD1 =1).Now, if some positive polarityvoltage is applied at the input,

Figure 1. Positive Polarity Input Voltage Analog Isolation Amplifier using the HCNR201 InPhoto-Voltaic Mode

Figure 2. Positive Polarity Input Voltage Analog Isolation Amplifier using the HCNR201 InPhoto-Conductive Mode

HCNR200PD 1

HCNR200LED

HCNR200OC1

LM158 (1)

Vcc

C1100 pF

C2100 pF

VINR1

80 kΩ

R3150 Ω

R280 kΩ

_

+

1

2

_+

+

Vcc5.5V

2N3906

Vcc1

LM158 (2)VOUT

Optical Isolation

PD 2

R41KΩ

HCNR200LED

HCNR200OC1

HCNR200PD 1

150 kΩ

_

+

_+

+

1

1

2

Optical Isolation

R3

80 kΩR2 2N3904

Vcc

Vcc5.5V

Vcc5.5V

C1100 pF

LM158 (1)

VIN

Vcc 1

LM158 (2)

R280 kΩ

PD 2

Vcc 1

VOUTR4

1KΩ

2

the op-amp output would tend toswing to the negative rail (in thiscase the ground voltage) causingthe LED current to flow. The IPD1is now externally set by VIN andR1 (IPD1 = VIN/R1). The op-ampwill limit the LED current IF toan appropriate value required toestablish the externally set IPD1.The maximum full scale LED cur-rent is designed to keep it underthe absolute max rating of25 mA. Since, the op-amp is con-nected in a stable negativefeedback servo loop it is alsomaintaining the same voltagesacross its two inputs, in this casezero volts. The output voltage isjust IPD2 x R2. Thus, to establishthe transfer function followingequations can be written:

IPD1 = VIN/R1(input photo-diode current)

K3 = IPD2/IPD1 = 1(transfer gain indicated in thedata sheet)

IPD2 = K3 x IPD1

VOUT = IPD2 x R2

Solving the above equationsreadily yields the linear transferfunction asVOUT /VIN = K3 x R2/R1

Typically, the transfer gain K3=1,and is ±5% for the HCNR201 and±15% for the HCNR200. The in-put photo gain is represented byK1 parameter in the data sheetand is defined as IPD1/IF. Thedata sheet for the HCNR201 liststhis input current transfer ratioas (0.25 to 0.75)% for HCNR200and (0.36 to 0.72)% for theHCNR201. As indicated in thedata sheet for best linearity thephoto-diode current is con-strained between 5 nA to 50 µA.This implies that the Vin and R1combination at the input shouldconstrain the externally setmaximum photodetector currentat 50 µA. However, higher photo-detector currents up to 100 µAcan be easily set at higher LEDcurrents close to 25 mA.

Figure 2 shows the HCNR201biased in a photo-conductivemode of operation, where thephoto-diodes are forced intoreverse bias. In reverse bias thephoto-diode capacitance is loweras the depletion regions arelarger. Thus, for higher band-width response it may beadvantageous to use the photo-conductive configuration. Theequations to derive the transferfunction are similar to the photo-voltaic mode discussed earlier.

Figure 3. Bipolar Input Voltage Analog Isolation Amplifier using the HCR201

With R1 at 80 kohm an inputvoltage maximum of 4 volts willkeep the maximum photo-diodecurrent at 50 µA to achieve thelinearity indicated in the datasheet of the HCNR201. As notedbefore photo-diode currents upto 100 µA or higher can be easilyset if so desired.

Bipolar Input Voltage Analog CircuitUsing similar concepts as devel-oped for the positive-polarityinput voltage analog amplifierdiscussed before, it is quitestraightforward to developbipolar input voltage analogamplifier. Figure 3 shows thebipolar input voltage analog cir-cuit using the HCNR201 in theservo feedback loop.

This bipolar input voltage circuituses two HCNR200 or HCNR201optocouplers. The top half of thecircuit consisting of PD1, R1, DA,C1, R4 and optocoupler 1 (OC1)LED is for the positive inputvoltages. The lower half of thecircuit consisting of optocoupler2 (OC2) PD1, R2, BB and R5 andoptocoupler 2 (OC2) LED is forthe negative input voltages.

The diodes D1 and D2 helpreduce crossover distortion bykeeping both amplifiers active

BALANCE

HCNR200OC1PD 1

HCNR200OC1LED

HCNR200OC2LED

HCNR200OC2PD2

HCNR200OC1PD2

HCNR200OC2PD 1

_

_

_

_

+

+

+

+

_

_

+

+

Optical Isolation

2

VIN

R1180 kΩ R4

680 Ω

C130 pF

R2180 kΩ

C230 pF

R5680 Ω

RA50 kΩ

DB

Vcc

Vcc

Vcc

LM158 (1)

LM158 (1)

1

Vcc

DA

Vcc 1

Vcc 1

LM158 (2)

C330 pF

R5180 kΩ

R650 kΩ

VOUT

3

during both positive and nega-tive portions of the input signal.Balance control R1 at the inputcan be used to adjust the relativegain for the positive and negativeinput voltages. The gain controlR7 can be used to adjust theoverall transfer gain of theamplifier. The capacitors C1, C2,and C3 are the compensationcapacitors for stability.

Current to Voltage ConverterFor measurement of very smallcurrents such as transducer sen-sor currents, a simple analogcurrent-to-voltage circuit can bedesigned as shown in Figure 4.This circuit uses two HCNR200optocouplers. The input currentcan be of either polarity. Theupper limit for the IIN should beconstrained to 50 µA maximumto achieve the non-linearityspecifications of 0.05% indicatedin the data sheet.

The lower limit of the currentmeasurement depends upon themaximum dark current associ-ated with the photodiodes, whichare approximately in the neigh-borhood of 100 pA maximumover temperature. The twoHCNR200 devices in this configu-ration are essentially connectedin anti-parallel configuration.One HCNR200 then translatesthe positive input current to apositive voltage. The secondHCNR200 translates the negativecurrent into a negative outputvoltage.

The resistor R2 is chosen to givethe full scale output voltage as:

Vout = ± IIN R2 = full scale outputvoltage. Thus R2 would be100 kohm at 50 µA max inputcurrent for a full-scale outputvoltage of 5V. Photo diode cur-rents up to 100 µA or higher canalso be easily selected.

Figure 4. Current-to-Voltage Converter using the HCNR200

11

2

+

_

-Vcc

+

_

Optical Isolation

IIN

HCNR200OC1PD 1

HCNR200OC2PD 2

C1100 pF

+Vcc

R1

HCNR200OC1LED

LM158 (1)

HCNR200OC2LED

HCNR200OC2PD 2

HCNR200OC1PD 2

C2100 pF

R2

LM158 (2)

VOUT

Vcc 1+15V

Vcc 1-15V

Isolated 4-to-20 mA AnalogTransmitter CircuitIndustrial manufacturingenvironments very often requiremeasuring temperatures, pres-sures, or fluid levels in a harshelectrically noisy environment.Transmitting signals throughcurrent instead of voltage couldbe advantageous in such an envi-ronment. Very often the distancebetween the sensor stage to acontroller, typically a PLC or amicrocontroller could also be asizeable distance. Additionalrequirement in such an applica-tion could be for high voltageinsulation or galvanic insulationfor safety protection either ofoperators or expensive digitallogic. Both of these criticalrequirements can be easilyaddressed through the use ofoptically isolated 4 to 20 mAtransmitter and receiver circuits.

Figure 5 shows a 4-to-20 mAanalog transmitter circuitdesigned around the HCNR201.

A unique feature of this circuit isthat there is no need for an iso-lated power supply on the loopside of the optical circuit. Theloop current generator suppliesthe power supply voltage. Thezener Z1 establishes the voltagerequired by loop-side op-amp. Toestablish the transfer function,following equations are estab-lished:

IPD1 = VIN/R1

K3 = IPD2/IPD1 = 1(by the transfer gain indicated inthe data sheet)

The current division at the inter-section of R5, R4, and R3establishes the photo-diodecurrent (IPD2) portion of the loopcurrent. The resistors R3 and R5are essentially in parallel andform the actual current divider.Thus, IPD2 can be written as

IPD2 = ILOOP • (R5 / (R5 + R3))

Figure 5. Isolated 4-to-20 mA Analog Transmitter circuit using the HCNR200

Optical Isolation

LM158

1

2N3906 2N3904

Vcc

0.001 uF

VIN

R180 kΩ

HCNR200PD 1

HCNR200LED

R2150 Ω

Vcc5.5V

R8100 kΩ

Z15.1 V

R73.2 kΩ

2N3904

2N3904

0.1 µF

R6140 Ω

0.001 µF

LM158

R410 kΩ

R310 kΩ

HCNR200PD 2

R525 Ω

-ILOOP

+ILOOP

+

_

+_

4

Solving these equations yieldsthe transfer function as

K3•VIN/R1 = ILOOP• (R5/(R5+R3))

ILOOP/VIN = K3•(R5 + R3)/(R5 R1)

The resistor values have been soselected in this example thatwhen input voltage is 0.8 V theloop current formed is 4 mA, andwhen the input voltage is 4 V, theloop current formed is 20 mA.This assumes that the transferfunction K3 equals 1, which isthe case typically as indicated inthe data sheet for the HCNR201.

Isolated 4-to-20 mA AnalogReceiver CircuitThe 4-to-20 mA receiver circuitis similar in construction to the4-to-20 mA transmitter circuitdiscussed earlier. In the receivercase, the loop current is receivedat the input of the receiver, andthe output is a linear voltage rep-resentation of the input loopcurrent. Figure 6 shows the re-ceiver circuit.

Once again, no isolated powersupply is needed on the loop sideof the receiver circuit, as thepower supply is established bythe source supplying the loopcurrent. The zener Z1 establishesthe 5 V level for the Op-amppower supply. The loop currentis split at the junction of R3 andR2 and PD1. The resistors R1 andR3 are essentially in parallel, asthere is zero volts across thephoto-detector diode (PD1). Theservo op-amps forces zero voltsacross the PD1, and thus R1 andR3 form the current divider forthe loop current.

The transfer function for thereceiver circuit can be estab-lished by observing the followingequations

IPD1 = ILOOP • (R3/ (R3 + R1))

K3 = IPD2/ IPD1

VOUT = IPD2 • R5

Solving these equations leads usto the transfer function as

VOUT/R5 = K3•ILOOP•(R3/(R3 +R1))

VOUT /ILOOP = K3•R5 •R3/(R3 + R1)

The resistor values shown in thereceiver circuit are scaled suchthat when loop current is 4 mAthe output voltage is 0.8V. Whenthe loop current is 20 mA theoutput voltage is 4V. This againassumes that K3 (transfer func-tion) equals 1 which is typicallythe case as indicated in the datasheet for the HCNR201.

Wide BandwidthVideo Analog AmplifierFor wide-bandwidth video ana-log applications an amplifierdesign is shown in Figure 7. Thisis an ac input coupled and acoutput coupled circuit. The LEDinput current IF is set at a rec-ommended 6 mA for theHCPL-4562 or 10 mA for theHCNW4562 by selecting anappropriate value for the R4. Ifthe VCC1 on the input side is 5Vthe voltage VB established by theresistor divider R1 and R2 at thebase of Q1 (neglecting base cur-rent drop across R3) is approx.1.16V. This establishes the volt-age VE at the emitter of Q1 as0.56V. Adjust R4 to set the rec-ommended LED current at 6 mA.

Figure 6. Isolated 4-to-20 mA Analog Receiver Circuit using the HCNR200.

Figure 7. Wide Bandwidth Analog Isolation Amplifier Using the HCPL-4562.

Optical Isolation

-ILOOP

+ILOOP

1

+

_

_

+

R110 kΩ

R326 Ω

R210 kΩ

HCNR200PD 1

R4180 Ω

LM158

HCNR200LED

2N3906

0.001 µF

Z15.1 V

0.1 µF

HCNR200PD 2

0.001 µF

VOUT

LM158

Vcc5.5V

R580kΩ

1

2

2

2

2

VIN

C147µF

D11N4150

51Ω

500Ω

R3100ΩVB

R21.0 kΩ

R4 POT

Q1

R16.8kΩ

VCC1+5V

IF

1

2

3

4

8

7

6

5

HCPL-4562

KPD

VE Q1 2N3904Q3 2N3904

GAIN ~ KPD • •

R715 kΩ

R81.0 kΩ

Q2

Q3

R69.1 kΩ

VCC2 + (9 to 12) V

R9760Ω

Q4 C22µF

Vout

R11470 Ω

R10100 Ω

Q1 to Q4=2N3904

KPD=0.0032 TYPICALLY

R7R4

R9R10~

5

With 0.56V at VE the resistor R4is selected to be approx. 93Ω for6 mA of IF.

With a VCC2 supply between (9 to12) V, the value of R11 is selectedto keep the output voltage at mid-point of the supply at approx.4.25V with the collector currentICQ4 of Q4 at approx. 9 mA.

Where R11' is the parallelcombination of R11 and loadimpedance and fT4 is the unitygain frequency Q4. From thisequation one can observe that tomaximize the bandwidth onewould want to increase the valueof R11' or reduce the value of R9at a constant ratio of R9/R10.

ICQ4 ≤ 4.25V/470Ω ≤ 9 mA

The small signal model of the bi-polar transistors can determinethe overall voltage gain of thecircuit and gain stages involvedand is found to be

Figure 8. Optically Coupled Regenerative Audio Receiver.

REGENERATION

10KΩ 10KΩ

+

_

BT19VBIAS

T1

TO ANTENNA

C1 ANTTRIM

3

2 Anode

Cathode

GND

VB

Q1

Vo

Vcc 8

6

5

7

330

330

HCPL-4562

C627

3.3k

0.01µF

270µF

0.01µF

0.01µF

+

_

BT29VBIAS

3.3KΩ

0.01µF 22µF

Q2

0.1µF

0.1µF

AUDIOOUT

DG

S

RS27kΩ

L1C2

TUNING

GV ≈ VOUT/ VIN

≈ ∂ IPB/∂ IF [R7 R9 /R4 R10]

Where ∂ IPB/∂IF is the base photocurrent gain (photo diode cur-rent gain) and is indicated as atypical of 0.0032 in the datasheet. Adjust resistor R4 toachieve the desired voltage gain.The voltage gain of the secondstage (Q3) is approximatelyequal to

R9 / R10 • / [1 + sR9 (CCQ3 +1/(2πR11' fT4) ]

Optically Coupled RegenerativeAudio ReceiverA simple optically coupled regen-erative (OCR) RF audio receivercan be constructed using theHCPL-4562 where the tuningcontrol and regenerative controlare optically isolated from therest of the receiver circuit.2

Figure 8 shows one such regen-erative detector design, wherethe RF from the antenna is opti-

cally coupled to the base of theoscillator transistor.

In this design the optocoupler’stransistor is configured as aColpitt’s oscillator. The base cur-rent that controls the oscillationof the optocoupler output tran-sistor (Q1) is supplied by theoptical photon coupling from theinput LED IF modulation. The RFenergy from the antenna iscoupled to the LED by the tunedcircuit formed by T1 and C1. The10kohm potentiometer providesthe regeneration control at theinput of the LED.

It is possible to connect an audiotransformer directly in the col-lector circuit of Q1 to drive thehigh sensitivity and high imped-ance headphones. However, inthe design shown in Figure 8 theaudio is recovered by a high im-pedance MOSFET transistor Q2.The tuned circuit (L1, C2) is con-nected to the gate of this infinite

6

Figure 9. HCPL-7800/7840/788J Block Diagram.

impedance MOSFET transistorQ2 which has a minimal loadingimpact on the tuned circuit. Theaudio voltage is developed acrossRS (27 kohm). The simple RCfilter formed by RS and 0.1 µFcapacitor filters out the RFcomponent and passes the audiocomponent for the headphones.If necessary, one can connect anadditional amplification stage,along with further filtering, andan audio amplifier at the outputto drive low impedance head-phones.

Agilent's Isolation AmplifiersOptical isolation boundary inIsolation Amplifiers provideshigh common mode rejectioncapability. Sigma-Delta modula-tion and unique encoding/decoding technologies providehigh precision and stabilityperformance. All above perfor-mances rely on an integratedhigh-speed digital optocoupler totransmit signal across isolationboundary. Figure 9 is the func-tional block diagram. HCPL-788Jintegrates short circuit and over-load detection contributed tointelligent motor driver.

A 2nd order Σ-∆ modulator con-verts analog input signal intosingle bit data stream, which isedge-trigged by encoder. Highspeed encoded data transmitthrough optical coupling chan-nel, and is recovered to single bitstream by decoder. The digital-to-analog converter simplyconverts single bit stream intovery precise analog voltagelevels. The final analog outputvoltage is recovered by filteringthe DAC output. The filter wasdesigned to maximize bandwidthwhile minimizing quantizationnoise generated by the sigma-delta conversion process. Theoverall gain of the isolation am-plifier is determined primarilyby matched internal tempera-ture-compensated bandgapvoltage references, resulting invery stable gain characteristicsover time and temperature.

The typical performance such asoffset, gain tolerance,nonlinearity and temperaturedrift can be guaranteed by differ-ential output manner. Oneexternal op-amp has three func-tions: to reference the output

signal to the desired level (usu-ally ground), to amplify thesignal to appropriate levels, andto help filter output noise.

Single-pole output from isolationamplifier, like VOUT+ to GND2, canbe used to save cost by less op-amp and a few other components.

Absolute output from smartamplifier HCPL-788J is usuallyused to monitor AC currentvalue, regardless of polar of thecurrent. Absolute output can di-rectly connect to microcontrollerand simplify the design of outputsignal circuit.

VOLTAGEREGULATOR

FAULTDETECTOR

VOLTAGEREGULATOR

CLOCKGENERATOR

ISOLATIONBOUNDARY

ISO-AMPINPUT

ISO-AMPOUTPUT

EncoderLED DRIVECIRCUIT FILTER

RECTIFIER

FAULTFAULT

DETECTORCIRCUIT

DECODERAND D/A

ABSVAL

HCPL-788J ONLY

Σ∆Modulator

7

Shown in Figure 10, isolatedmodulator HCPL-7860/786J hasdirect Sigma-Delta signal outputwith modulation clock, whichcan be directly connected tomicroprocessor and converted to12-bit effective resolution digitaldata.

Table 1 shows an overview ofisolation amplifiers.

Figure 10. HCPL-7860/786J Block Diagram.

CLOCKGENERATOR

Encoder

VOLTAGEREGULATOR

LED DRIVECIRCUIT

DECODER

DETECTORCIRCUIT

CLOCKRECOVERY

ISOLATION BOUNDARY

ANALOGINPUT DATA OUTPUT

CLOCK OUTPUT

Σ∆Modulator

Table 1. Specifications Overview of Isolation Amplifiers.

Isolated Amplifier, HCPL- 7800 7800A 7840 788J

Gain Tolerance, % ±3 ±1 ±5 ±5

Max. Input Offset Voltage, mV 3 3 3 3

Max. Input Offset Drift Vs Temperature, mV/°C 10 10 10 10

VOUT 100 mV Max. Nonlinearity, % 0.2 0.2 0.2 0.4

Typ. Gain Drift Vs Temperature, ppm/°C 250 250 250 50

Max. Prop Delay, ms 9.9 9.9 9.9 20

Min. CMR at VCM = 1 kV, kV/ms 10 10 10 10

Package Type DIP8 DIP8 DIP8 SO16

IEC/EN/DIN EN 60747-5-2 [VIORM], VPEAK 891[1] 891[1] 891[1] 891[1]

UL [VISO], VRMS 3750 3750 3750 3750

Isolated Modulator, HCPL- 7860 786J

Max. Offset Drift Vs. Temperature, mV/°C 10 10

Max. Internal Reference Voltage Matching Tolerance, % 1 2

Min. CMR at VCM = 1 kV, kV/ms 15 15

Package Type DIP8 SO16

IEC/EN/DIN EN 60747-5-2 [VIORM], VPEAK 891[1] 891[1]

UL [VISO], VRMS 3750 3750

Note:

1. Option 060 is needed

8

General Voltage SensingWith Agilent’s isolation amplifi-ers, a designer can simplyeliminate extra noise affectionwhen sensing AC or DC voltage.A high voltage source Vs (Figure11) is divided by resisters Rs andR1 to get a typical voltage signal±200 mV from formula:

Vin = Vs Rs/ (Rs+R1)

Rs value should be relativelysmall to match with isolationamplifier’s input impedance,and to keep a relative biascurrent which does not affect theaccuracy of measurement. Forexample, HCPL-7840 inputimpedance 500 kΩ and a lessthan 1 kΩ Rs will have 0.4 µApeak bias current.

A capacitor C1 is connected aslow-pass filter to preventisolation amplifier from voltagetransients of input signal. Toobtain higher bandwidth, thecapacitor C1 can be reduced, butit should not be reduced muchbelow 1000 pF to maintain gainaccuracy of the isolationamplifier.

Single-pole output betweenVOUT+ to GND2 is usually appliedfor general voltage sensing forsaving cost.

General Current SensingA large current source can besensed by a shunt resistor RS,which converted current to avoltage signal Vin = IS RS (Fig-ure 12).

For example, to monitor a singlephase 240 VAC/1.2 kW lamp cur-rent, its peak current is:IS = ±(5 • 1.414) A = ±7.07 ARS is calculated at 28 mΩ whilethe peak current input voltageare ±198 mV. This resistor re-sults a power dissipation lessthan 1/4 W.

The power supply VDD1 in inputside of optocoupler can be avail-able from rectified and regulatedAC line, but the output sidepower supply VDD1 must be iso-lated to AC line.

A 39Ω resistor R1 and bypasscapacitor C2 are connected tofilter voltage transients frominput signal.

Single-pole output betweenVOUT+ to GND2 is usually appliedfor general current sensing forsaving cost.

Motor Current SensingInverter or servo motor driversimplement vector control fastand accurately with two moderncontrol loops: position feedbackby optical encoder and currentfeedback by optical isolatedamplifier.

Optical isolated amplifiersdirectly measure phases or railcurrent, replacing conventionalindirect measurement throughtransformer or Hall Effectsensor. The users had recognizedsignificant advantages ofoptocouplers: standard ICpackage, high linearity, lowtemperature draft. Thesefeatures provide opportunities tomake a compact, precise andreliable motor driver.

A typical application circuit inFigure 13 mainly consists ofshunt resister, isolated amplifierand a low cost op-amp.

The maximum shunt resistanceRS can be calculated by takingthe maximum recommendedinput voltage and dividing by thepeak current that should seeduring normal operation. For ex-ample, if a motor will have amaximum RMS current of 30 Aand can experience up to 50%overloads during normal opera-tion, then the peak current is63.3 A (= 30 • 1.414 • 1.5).Assuming a maximum input volt-age of 200 mV, the maximumvalue of shunt resistance in thiscase would be about 30 mΩ.

Figure 11. General Voltage Sensing Circuit.

HCPL-7800/7840VDD1 VDD2

VOUT+

VOUT-

GND2

VIN+

VIN-

GND1

+5V

C1

C2

R1

Rs

Vs

Figure 12. General Current Sensing Circuit.

HCPL-7800/7840VDD1 VDD2

VOUT+

VOUT-

GND2

VIN+

VIN-

GND1

+5V

C1

C2R1Rs

Is

Load

9

The particular op-amp used inthe post-amp circuit is notcritical. However, it should havelow enough offset and highenough bandwidth and slew rateso that it does not adverselyaffect circuit performance. Thegain is determined by resistorsR4 through R7, assuming thatR4= R5 and R6 = R7, the gain ofthe post-amplifier is R6/R4.

Bootstrap power supply isusually used to reduce cost andsize in motor driver. It eliminatesthe need for an isolated powersupply or a dc-dc converter. Abootstrap power supply for highside of a half bridge is shown inFigure 5, When designing a boot-strap power supply, thebootstrap components R1, R2, C1and C2 must be chosen to suffi-ciently power its load—theisolated half side of gate driveand current sensingoptocouplers.

When low IGBT is on, rail volt-age goes through R1, R2 and C1to charge capacitor C2 up to 18 Vand meanwhile supply toHCPL-3120 and regulator, whichpowers current sensor. Whenlow IGBT is off, C2 dischargesand distributes its current togate driver and regulator 78L05.The threshold voltage of boot-strap power supply is 15 V,which is required by gate driverHCPL-3120.

When low IGBT is off, the storedenergy on C1 will discharge toC5, which are together with DZ2to generate a negative voltagesource.

A bootstrap power supply forlow side of half bridge is identi-cal to high side circuit.

ConclusionThis paper has outlined andhighlighted the wide scope andapplications that are now pos-sible using sophisticated andhighly linear optocouplers.Designers can now choose andselect an appropriate analogoptocoupler available fromAgilent Technologies, Inc. thatmeets their end analog designcriteria. This includes high com-mon mode rejection capablecurrent or voltage sensingoptocouplers such as theHCPL-7800A or the HCPL-788J.Or the high linearityoptocouplers such as theHCNR201. Or the high band-width optocouplers such as theHCPL-4562.

Figure 13. Motor Current Sensing Circuit.

IN OUT

78L05

8

7

65 4

3

2

1Vcc N/C

Anode

Cathode

N/C

Vo

VoVee

HCPL-3120

R1C1

D1

R2

R3

RS

D2 DZ2 C5

C4

C3

M2

RG

M1

C2DZ118 V

HV-

12V

0.01µF

0.1µF

1 8

7

6

5

2

3

4

HCPL-7800

Vdd1 Vdd2

Vout+

Vout-

GND2

Vin+

Vin-

GND1

+5V

0.1µFC6

R42.00 KΩ

R5 2.00 KΩ

C8

R7

150pF

10.0 KΩ

-15 V

+

_

C9

C10

R6

C7

0.1µF

0.1µF

10.0KΩ

MC34081Vout

+15

150pF

MOTOR

HV+

References1. Photodiode Amplifiers: Op

Amp Solutions, Jerald G.Graeme, McGraw-Hill, NewYork, 1996.

2. “The OCR Receiver,” QST,Daniel Wissell, N1BYT, June1998, pp 35-38.

3. “Designing with AgilentTechnologies IsolationAmplifiers” Agilent Technolo-gies, Application Note 1078,Publication No. 5965-5976E,1999.

4. “Optocouplers for VariableSpeed Motor Control Elec-tronics in Consumer HomeAppliances” Jamshed N.Khan, Agilent Technologies,White Paper, Publication No.5980-1297E, 2000.

5. “Motor Drive and InverterDesign Using Optocouplers”Joe Pernyeszi, Mike Waltersand Jason Hartlove, Proceed-ing of PCIM, pp. 397-406,1995.

6. Optocoupler Designer’sGuide, Agilent Technologies,Publication No. 5988-4082EN,2002.

7. “Isolation Amplifiers Com-pared to Hall EffectiveDevices for Providing Feed-back in Power-ConversionApplications” D. Plant, Pro-ceeding of the Second SmallMotor International Confer-ence (SMIC), pp. 353-358,1996.

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Data subject to change.Copyright © 2004 Agilent Technologies, Inc.Obsoletes 5988-9262ENMarch 1, 20045989-0804EN