Fairchild AN-7511 manual 1A/DIV

Application Note 7511

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FIGURE 13A. MOTOR CURRENT AND VOLTAGE ARE SHOWN HERE, FOR LIGHT LOADS

TRACE	VERTICAL	HORIZONTAL
A	1A/DIV	1mSEC/DIV
B	50V/DIV	1mSEC/DIV

FIGURE 13B. MOTOR CURRENT AND VOLTAGE ARE SHOWN HERE, FOR HEAVY LOADS.

TRACE	VERTICAL	HORIZONTAL
A	3A/DIV	2mSEC/DIV
B	100V/DIV	2mSEC/DIV

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

AC

LINE INPUT

		50 TO 320V DC
RECTIFIER	SWITCHING
						dI/dt LIMIT
AND	POWER				24V DC
FILTER	SUPPLY						UPPER 3
		5V DC	CONTROL			ISOLATION
				AND			LOWER 3
		HV	TIMING
		ADJUST

ISOLATION ISOLATION ISOLATION

										CHOPPER
		HV		GATE DRIVE			TURNOFF
	DISABLE
RECYCLE			COMPARATOR		I LIMIT				AC
TIME			AND LATCH					AV = 100
						10A
						20A

IGT

SWITCHES

MOTOR

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.

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1