Fairchild AN-7511 manual Isolate With Galvanic Impunity, Signal Path Isolator

Application Note 7511

Apiezoelectric coupler operationally similar to a pulse-train drive transformer, but potentially less costly in high volume is a small, efficient device with isolation capability ranging to 4kV. What’s more, unlike optocouplers, they require no auxiliary power supply. The piezo element is a ceramic component in which electrical energy is converted to mechanical energy, transmitted as an acoustic wave, and then reconverted to electrical energy at the output terminals Figure 5A.

ON

CONTROL

INPUT

OFF PULSE TRANSFORMER

	+
1N914
1N914	IGT
2N5232	C1
	-
1k

The piezo element’s maximum coupling efficiency occurs at its resonant frequency, so the control oscillator must operate at that frequency. For example, the PZT61343 piezo coupler in Figure 5B’s driver circuit requires a 108kHz, ±1%-accurate astable multivibrator to maximize mechanical oscillations in the ceramic material. This piezo element has a 1W max

FIGURE 6A. PROVIDING HIGH ISOLATION AT LOW COST, PULSE TRANSFORMERS ARE IDEAL FOR DRIVING THE IGT. AT SUFFICIENTLY HIGH FREQUENCIES, C1 CAN BE THE IGT’S GATE-EMITTERCAPACITANCE ALONE.

power handling capability and a 30mA p-p max secondary current rating. The 555 timer shown provides compatible waveforms while the RC network sets the frequency.

Isolate With Galvanic Impunity

Do you require tried and true isolation? Then use transformers; the IGT’s low gate requirements simplify the design of independent, transformer-coupled gate-drive supplies. The supplies can directly drive the gate and its

ON OFF

CONTROL

INPUT

		+
1N914
		IGT
	C	R
		-
1N914	RC = 3∝SEC

discharge resistor Figure 6, or they can simply replace the level-shifting supplies of Figure 2. It’s good practice to use pulse transformers in drive circuitry, both for IGT’s and MOSFETs, because these components are economical, rugged and highly reliable.

FIGURE 6B. A HIGH-FREQUENCY OSCILLATOR IN THE TRANS- FORMER’S PRIMARY YIELDS UNLIMITED ON- TIME CAPABILITY.

In the pulse-on, pulse-off method Figure 6A, C1 stores a positive pulse, holding the IGT on. At moderate frequencies (several hundred Hertz and above), the gate-emitter capacitance alone can store enough energy to keep the IGT on; lower frequencies require an additional external capacitor. Use of the common-base n-p-n bipolar transistor to discharge the capacitance minimizes circuit loading on the capacitor. This action extends continuous on-time capability without capacitor refreshing; it also controls the gate-discharge time via the 1kΩ emitter resistor.

				VARIABLE
220V AC				DC VOLTAGE
	THREE-PHASE				THREE-PHASE
3φ 60Hz		SWITCHING						φ
	BRIDGE				IGT	3
		REGULATOR	VOLTAGE	TIMING			INDUCTION
	RECTIFIER				INVERTER
								MOTOR
			ENABLE	AND DRIVE

			ADJUST VOLTAGE			ENABLE		CURRENT
			I	I	I	LOWER		SENSE
							LEGS	SIGNAL
			5V
LOW VOLTAGE		POWER SUPPLY	VOLTAGE	MOTOR	I				TACHO-
TRANSFORMER							OVERLOAD
		FOR CONTROL	CONTROLLED	CONTROL	SHUT DOWN				METER
RECTIFIER							PROTECTION
	24V DC	CIRCUITS	OSCILATOR	LOGIC					FEEDBACK
FILTER					DRIVE
					OSCILLATOR
			24V

I	SIGNAL PATH ISOLATOR
	EG: OPTOCOUPLIER PIEZO COUPLER

FIGURE 8. THIS 6-STEP 3-PHASE-MOTOR DRIVE USES THE IGT-DRIVE TECHNIQUES DESCRIBED IN THE TEXT. THE REGULATOR AD- JUSTS THE OUTPUT DEVICES’ INPUT LEVELS; THE VOLTAGE-CONTROLLED OSCILLATOR VARIES THE SWITCHING FREQUENCY AND ALSO PROVIDES THE CLOCK FOR THE 3-PHASE TIMING LOGIC. THE V/F RATIO STAYS CONSTANT TO MAINTAIN CONSTANT TORQUE REGARDLESS OF SPEED.

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1