15

Power Supplies and Bypassing

As shown in Figure 19, a floating 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 ensure sufficient current can be

supplied from the existing floating supply. The voltage

applied to the input of the ACPL-7970 through an RC anti-

aliasing filter (R2 and C2). And finally, a clock is connected

to the ACPL-7970 and data are connected to the digital

filter. Although the application 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

transistorgatedrivecircuit.Ifadedicatedsupplyisrequired,

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 powered trans-

former or a high-frequency DC-DC converter.

An inexpensive 78L05 three terminal regulator can also be

used to reduce the floating 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 filter with the regulator’s

input bypass capacitor.

As shown in Figure 19, bypass capacitors (C1a, C1b, C3a

and C3b) 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. A 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 filter, which is recommended to

prevent high frequency noise from aliasing down to lower

frequencies 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 affect 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 affect 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 first step in selecting a shunt is determining how

much current the shunt will be sensing. The graph in

Figure 20 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 recom-

mended 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 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 m

Ω.

The maximum average power dissipation in the shunt

can also be easily calculated by multiplying the shunt re-

sistance times the square of the maximum RMS current,

which is about 1 W in the previous example.

Figure 20. Motor Output Horsepower vs. Motor Phase Current and Supply.

MOTOR PHASE CURRENT - A (rms)

15

40

0

35

25

10

30

20

5

5

35

30

0

20

25

15

10

440 V

380 V

220 V

120 V