Fairchild AN-7511 User Manual

Page 9

Application Note 7511 Rev. A1

FIGURE 13A. MOTOR CURRENT AND VOLTAGE ARE SHOWN

HERE, FOR LIGHT LOADS

FIGURE 13B. MOTOR CURRENT AND VOLTAGE ARE SHOWN

HERE, FOR HEAVY LOADS.

To complete the design of the 6-step motor drive, it’s necessary to consider protection circuitry for the output IGTs. The drive receives its
power from a switching supply already containing provisions for protection from line over-voltage and under-voltage and transient effects.
However, you still have to guard the power switches against unwanted effects on the output lines and the possibility of noise or other
extraneous signals causing gate-drive timing errors.

The best protection circuit must match the characteristics of the power switch and the circuit’s bias conditions. The IGT is very rugged
during turn-on and conduction, but it requires time to dissipate minority carriers when turning off high currents and voltages. An analysis
of the possible malfunction condition

FIGURE 14. THE LOWEST COST SENSOR IMAGINABLE, A PIECE OF COPPER WIRE SERVES AS THE CURRENT MONITOR IN THIS SYS-

TEM. THE CHOPPED AND AMPLIFIED VOLTAGE DROP ACROSS THE WIRE TRIGGERS A GATE-DRIVE SHUT-OFF CIRCUIT
UNDER FAULT CONDITIONS.

TRACE

VERTICAL

HORIZONTAL

1A/DIV

1mSEC/DIV

50V/DIV

1mSEC/DIV

TRACE

VERTICAL

HORIZONTAL

3A/DIV

2mSEC/DIV

100V/DIV

2mSEC/DIV

LINE

INPUT

DISABLE

GATE DRIVE

TURNOFF

ADJUST

50 TO 320V DC

5V DC

UPPER 3

LOWER 3

dI/dt LIMIT

24V DC

CHOPPER

I LIMIT

10A

20A

ISOLATION

IGT

SWITCHES

MOTOR

CONTROL

AND

TIMING

ISOLATION

RECTIFIER

AND

FILTER

SWITCHING

POWER

SUPPLY

RECYCLE

TIME

COMPARATOR

AND LATCH

AV = 100

ISOLATION

Application Note 7511