HCPL-7860-000E Datasheet HCPL-786J-000E, HCPL-7860-300E, HCPL-7860-000E | P13-P15

Figure 17. Recommended Application Circuit.

MOTOR

HV-

HV+

SENSE

FLOATING

POSITIVE

SUPPLY

GATE DRIVE

CIRCUIT

DD1

IN+

IN-

GND1

DD2

MCLK

MDAT

GND2

CDAT SCLK

CCLK V

CLAT CHAN

MCLK1 SDAT

MDAT1 CS

MCLK2 THR1

MDAT2 OVR1

GND RESET

+ 5 V

HCPL-7860/

HCPL-786J

CONTROL

CIRCUIT

0.1 µF

0.01 µF

R2 39 Ω

5.1 V

HCPL-0872

Power Supplies and Bypassing

The recommended application circuit is 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 sup-

ply. The voltage from the current sensing resistor or shunt

(Rsense) is applied to the input of the HCPL-7860/HCPL-

786J (U2) through an RC anti-aliasing lter (R2 and C2). And

nally, the output clock and data of the isolated modulator

are connected to the digital interface IC. Although the

application circuit is relatively simple, a few recommenda-

tions 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 additional

winding on an existing transformer. Otherwise, some sort

of simple isolated supply can be used, such as a line pow-

ered 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 µF bypass capacitors (C1 and

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

and output power-supply pins of the isolated modulator

(U2). The bypass capacitors are required because of the

high-speed digital nature of the signals inside the isolated

modulator. A 0.01 µF bypass capacitor (C2) is also recom-

mended 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

frequencies and interfering with the input signal.

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

Voltage.

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 modu-

lator, due primarily to stray capacitive coupling between

the input and the output circuits. To obtain optimal CMR

performance, 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 particular

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 volt-

age. The maximum value of the shunt is determined by the

current being measured and the maximum recommended

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 cur-

rent 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 x

1.414 x 1.5). Assuming a maximum input voltage of 200

mV, the maximum value of shunt resistance in this case

would be about 10 mW.

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 mini-

mum and maximum values, depending on the particular

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 sig-

nal dependent temperature rise of the shunt. The eect

increases as the shunt-to-ambient thermal resistance

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 resistance 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.

15 20 25 30

MOTOR PHASE CURRENT - A (rms)

MOTOR OUTPUT POWER - HORSEPOWER

5 350

440

380

220

120

Table 3. Isotek (Isabellenhütte) Four-Terminal Shunt Summary.

Shunt Resistor

Part Number

Shunt Resistance Tol.

Maximum RMS Cur-

rent

Motor Power Range

120 V

- 440 V

mW % 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.

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 ad-

ditional 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 3; the maximum

current and motor power range for each of the PBV series

shunts are indicated. For shunt resistances from 50 mW

down to 10 mW, the maximum current is limited by the

input voltage range of the isolated modulator. For the 5

mW and 2 mW shunts, a heat sink may be required due to

the increased power dissipation at higher currents.

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 in-

crease 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.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P17

HCPL-7860-000E

Mfr. #:

Buy HCPL-7860-000E

Manufacturer:

Broadcom / Avago

Description:

Data Acquisition ADCs/DACs - Specialized Isolated Modulator

Lifecycle:

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HCPL-7860-000E

HCPL-7860-000E

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