ACPL-796J-000E Datasheet ACPL-796J-000E, ACPL-796J-560E, ACPL-796J-060E | P13-P15

Application Information

Digital Current Sensing Circuit

Figure 17 shows a typical application circuit for motor control phase current sensing. By choosing the appropriate shunt

resistance, any range of current can be monitored, from less than 1 A to more than 100 A.

NON-

ISOLATED

5 V/3.3 V

FLOATING POSITIVE

SUPPLY

SENSE

–

DD1

GND1

MCLKIN

MDAT

DD2

GND2

0.1 PF

ISOLATION

BARRIER

GATE

DRIVE

CIRCUIT

HV+

HV–

0.1 PF

5.1 V

10 nF

MOTOR

R2 39 :

+–

ACPL-796JGND1 GND2

10 PF

Figure 17. Typical application circuit for motor phase current sensing.

Power Supplies and Bypassing

As shown in Figure 17, a  oating power supply (which in

many applications could be the same supply that is used

to drive the high-side power transistor) is regulated to 5

V using a simple zener diode (D1); the value of resistor

R1 should be chosen to supply su cient current from

the existing  oating supply. The voltage from the current

sensing resistor or shunt (R

SENSE

) is applied to the input of

the ACPL-796J through an RC anti-aliasing  lter (R2 and

C2). And  nally, a clock is connected to the ACPL-796J and

data are connected to the digital  lter. Although the appli-

cation circuit is relatively simple, a few recommendations

should be followed to ensure optimal performance.

The power supply for the isolated modulator is most

often obtained from the same supply used to power the

power transistor gate drive circuit. If a dedicated supply

is required, in many cases it is possible to add an addi-

tional winding on an existing transformer. Otherwise,

some sort of simple isolated supply can be used, such as

a line powered transformer or a high-frequency DC-DC

converter.

An inexpensive 78L05 three terminal regulator can also be

used to reduce the  oating supply voltage to 5 V. To help

attenuate high-frequency power supply noise or ripple, a

resistor or inductor can be used in series with the input of

the regulator to form a low-pass  lter with the regulator’s

input bypass capacitor.

As shown in Figure 17, 0.1 F bypass capacitors (C1 and

C3) should be located as close as possible to the input and

output power-supply pins of the isolated modulator. The

bypass capacitors are required because of the high-speed

digital nature of the signals inside the isolated modulator.

For better  ltering, an additional 10-μF capacitor can be

used. A 10 nF bypass capacitor (C2) is also recommended

at the input due to the switched-capacitor nature of the

input circuit. The input bypass capacitor also forms part of

the anti-aliasing  lter, which is recommended to prevent

high frequency noise from aliasing down to lower fre-

quencies and interfering with the input signal.

PC Board Layout

The design of the printed circuit board (PCB) should follow

good layout practices, such as keeping bypass capacitors

close to the supply pins, keeping output signals away from

input signals, the use of ground and power planes, etc. In

addition, the layout of the PCB can also a ect the isolation

transient immunity (CMR) of the isolated modulator, due

primarily to stray capacitive coupling between the input

and the output circuits. To obtain optimal CMR perfor-

mance, the layout of the PC board should minimize any

stray coupling by maintaining the maximum possible

distance between the input and output sides of the circuit

and ensuring that any ground or power plane on the PC

board does not pass directly below or extend much wider

than the body of the isolated modulator.

Shunt Resistors

The current-sensing shunt resistor should have low re-

sistance (to minimize power dissipation), low inductance

(to minimize di/dt induced voltage spikes which could

adversely a ect operation), and reasonable tolerance (to

maintain overall circuit accuracy). Choosing a particu-

lar value for the shunt is usually a compromise between

minimizing power dissipation and maximizing accuracy.

Smaller shunt resistances decrease power dissipation,

while larger shunt resistances can improve circuit accuracy

by utilizing the full input range of the isolated modulator.

The  rst step in selecting a shunt is determining how

much current the shunt will be sensing. The graph in

Figure 18 shows the RMS current in each phase of a three-

phase induction motor as a function of average motor

output power (in horsepower, hp) and motor drive supply

voltage. The maximum value of the shunt is determined

by the current being measured and the maximum rec-

ommended input voltage of the isolated modulator. The

maximum shunt resistance can be calculated by taking the

maximum recommended input voltage and dividing by

the peak current that the shunt should see during normal

operation. For example, if a motor will have a maximum

RMS current of 10 A and can experience up to 50%

overloads during normal operation, then the peak current

is 21.1 A (= 10 × 1.414 × 1.5). Assuming a maximum input

voltage of 200 mV, the maximum value of shunt resistance

in this case would be about 10 m.

Figure 18. Motor Output Horsepower vs. Motor Phase Current and Supply.

15 20 25 30

MOTOR PHASE CURRENT - A

(

rms

)

MOTOR OUTPUT POWER - HORSEPOWER

5350

440

380

220

120

The maximum average power dissipation in the shunt

can also be easily calculated by multiplying the shunt

resistance times the square of the maximum RMS current,

which is about 1 W in the previous example.

If the power dissipation in the shunt is too high, the resis-

tance of the shunt can be decreased below the maximum

value to decrease power dissipation. The minimum value

of the shunt is limited by precision and accuracy require-

ments of the design. As the shunt value is reduced, the

output voltage across the shunt is also reduced, which

means that the o set and noise, which are  xed, become

a larger percentage of the signal amplitude. The selected

value of the shunt will fall somewhere between the

minimum and maximum values, depending on the par-

ticular requirements of a speci c design.

When sensing currents large enough to cause signi -

cant heating of the shunt, the temperature coe cient

(tempco) of the shunt can introduce nonlinearity due to

the signal dependent temperature rise of the shunt. The

e ect increases as the shunt-to-ambient thermal resis-

tance increases. This e ect can be minimized either by

reducing the thermal resistance of the shunt or by using

a shunt with a lower tempco. Lowering the thermal resis-

tance can be accomplished by repositioning the shunt

on the PC board, by using larger PC board traces to carry

away more heat, or by using a heat sink.

For a two-terminal shunt, as the value of shunt resistance

decreases, the resistance of the leads becomes a signi -

cant percentage of the total shunt resistance. This has two

primary e ects on shunt accuracy. First, the e ective resis-

tance of the shunt can become dependent on factors such

as how long the leads are, how they are bent, how far they

are inserted into the board, and how far solder wicks up

the lead during assembly (these issues will be discussed

in more detail shortly). Second, the leads are typically

made from a material such as copper, which has a much

higher tempco than the material from which the resistive

element itself is made, resulting in a higher tempco for the

shunt overall. Both of these e ects are eliminated when a

four-terminal shunt is used. A four-terminal shunt has two

additional terminals that are Kelvin-connected directly

across the resistive element itself; these two terminals are

used to monitor the voltage across the resistive element

while the other two terminals are used to carry the load

current. Because of the Kelvin connection, any voltage

drops across the leads carrying the load current should

have no impact on the measured voltage.

Several four-terminal shunts from Isotek (Isabellenhütte)

suitable for sensing currents in motor drives up to 71 Arms

(71 hp or 53 kW) are shown in Table 11; the maximum

current and motor power range for each of the PBV series

shunts are indicated. For shunt resistances from 50 m

down to 10 m, the maximum current is limited by the

input voltage range of the isolated modulator. For the

5 m and 2 m shunts, a heat sink may be required due

to the increased power dissipation at higher currents.

Table 11. Isotek (Isabellenhütte) four-terminal shunt summary.

Shunt Resistor

Part Number

Shunt

Resistance Tol.

Maximum

RMS Current

Motor Power Range

120 V

- 440 V

m % A hp kW

PBV-R050-0.5 50 0.5 3 0.8 - 3 0.6 - 2

PBV-R020-0.5 20 0.5 7 2 - 7 0.6 - 2

PBV-R010-0.5 10 0.5 14 4 - 14 3 - 10

PBV-R005-0.5 5 0.5 25 [28] 7 - 25 [8 - 28] 5 - 19 [6 - 21]

PBV-R002-0.5 2 0.5 39 [71] 11 - 39 [19 - 71] 8 - 29 [14 - 53]

Note: Values in brackets are with a heatsink for the shunt.

When laying out a PC board for the shunts, a couple of

points should be kept in mind. The Kelvin connections to

the shunt should be brought together under the body

of the shunt and then run very close to each other to the

input of the isolated modulator; this minimizes the loop

area of the connection and reduces the possibility of stray

magnetic  elds from interfering with the measured signal.

If the shunt is not located on the same PC board as the

isolated modulator circuit, a tightly twisted pair of wires

can accomplish the same thing.

Also, multiple layers of the PC board can be used to increase

current carrying capacity. Numerous plated-through vias

should surround each non-Kelvin terminal of the shunt to

help distribute the current between the layers of the PC

board. The PC board should use 2 or 4 oz. copper for the

layers, resulting in a current carrying capacity in excess of

20 A. Making the current carrying traces on the PC board

fairly large can also improve the shunt’s power dissipa-

tion capability by acting as a heat sink. Liberal use of vias

where the load current enters and exits the PC board is

also recommended.

Shunt Connections

The recommended method for connecting the isolated

modulator to the shunt resistor is shown in Figure 17. V

of the ACPL-796J is connected to the positive terminal of

the shunt resistor, while V

– is shorted to GND1, with the

power-supply return path functioning as the sense line

to the negative terminal of the current shunt. This allows

a single pair of wires or PC board traces to connect the

isolated modulator circuit to the shunt resistor. By refer-

encing the input circuit to the negative side of the sense

resistor, any load current induced noise transients on the

shunt are seen as a common-mode signal and will not

interfere with the current-sense signal. This is important

because the large load currents  owing through the

motor drive, along with the parasitic inductances inherent

in the wiring of the circuit, can generate both noise

spikes and o sets that are relatively large compared

to the small voltages that are being measured across the

current shunt.

If the same power supply is used both for the gate

drive circuit and for the current sensing circuit, it is very

important that the connection from GND1 of the isolated

modulator to the sense resistor be the only return path

for supply current to the gate drive power supply in order

to eliminate potential ground loop problems. The only

direct connection between the isolated modulator circuit

and the gate drive circuit should be the positive power

supply line.

In some applications, however, supply currents  owing

through the power-supply return path may cause o set

or noise problems. In this case, better performance may

be obtained by connecting V

+ and V

– directly across

the shunt resistor with two conductors, and connecting

GND1 to the shunt resistor with a third conductor for the

power-supply return path, as shown in Figure 19. When

connected this way, both input pins should be bypassed.

To minimize electromagnetic interference of the sense

signal, all of the conductors (whether two or three are

used) connecting the isolated modulator to the sense

resistor should be either twisted pair wire or closely

spaced traces on a PC board.

The 39  resistor in series with the input lead (R2) forms

a low pass anti-aliasing  lter with the 10 nF input bypass

capacitor (C2) with a 400 kHz bandwidth. The resistor

performs another important function as well; it dampens

any ringing which might be present in the circuit formed

by the shunt, the input bypass capacitor, and the induc-

tance of wires or traces connecting the two. Undamped

ringing of the input circuit near the input sampling

frequency can alias into the baseband producing what

might appear to be noise at the output of the device.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

ACPL-796J-000E

Mfr. #:

Buy ACPL-796J-000E

Manufacturer:

Broadcom / Avago

Description:

Optically Isolated Amplifiers 5MHz-20MHz,5000 Vrms Sigma-Delta Modulatr

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ACPL-796J-000E

ACPL-796J-000E

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