Fairchild AN-7511 manual Piezoelectric Couplers Provide 4-kV Isolation

Application Note 7511

Piezoelectric Couplers Provide 4-kV Isolation

Using a high-frequency oscillator for pulse-train drive Figure 6B yields unlimited on-time capability. However, the scheme requires an oscillator that can be turned on and off by the control logic. A diode or zener clamp across the trans- former’s primary will limit leakage-inductance flyback effects.

age-doubler circuitry improves the turn-on time and also pro- vides long on-time capability. Although this design uses only a 5V supply on the primary side of a standard trigger trans- former, it provides 15V gate-to-emitter voltage.

To optimize transformer efficiency, make the pulses’ voltage x time products equal for both the On and the Off pulses. In situations where the line voltage generates the drive power, a simple relaxation oscillator using a programmable unijunc- tion transistor can derive its power directly from the line to provide a pulse train to the IGT gate.

The circuit shown in Figure 7 accommodates applications involving lower frequencies (a few hundred Hertz and

OSCILLATOR

1N914
1:2	0.001	IGT
	∝F
		4.7k
1N914
	0.001∝F

below). The high oscillator frequency (greater than 20kHz) helps keep the pulse transformer reasonably small. The volt-

FIGURE 7. THIS DRIVING METHOD FOR LOW-FREQUENCY SWITCHING PROVIDES 15V TO THE IGT’S GATE

D7
L1
		D1		D3	D5
C1	Q1		Q3		Q5
R		R		R
325V								INDUCTION
10A
								MOTOR
		D2		D4	D6	NOTES:
						Q	- Q = D94FR4
						1	6	220V
R						D1 - D7 = 1N3913
	Q2	R	Q4	R	Q6	D8 - D13 = 1N914
						R = 4.7k, 1/ W
							2
						C1 = 100∝F, 400V
						L1 = 40∝H

FIGURE 9A. THE POWER INVERTER’S DRIVE CIRCUIT USES SIX IGTS TO DRIVE A 2-HP MOTOR.

φA	180o						VAB
	Q	1	ON	15o DELAY			0	t
	0					t
				Q2ON			VBC
φB				Q3ON			0	t
	0					t
					Q4ON
							VCA	t
φC							0

0		Q5ON	ILA
	Q6ON	t
			0	t

ILA

0 t

ILB

	0	t
ILB

0 t

ILC ILC

	0	t
0	t

FIGURE 9B. THE TIMING DIAGRAM SHOWS THAT EACH IGT CONDUCTS FOR 165o× OF EVERY 360o CYCLE; THE DELAY IS NECESSARY TO AVOID CROSS CONDUCTION.

FIGURE 9C. THE THREE WINDINGS’ VOLTAGES AND CUR- RENTS ARE SHOWN. NOTE THAT ALTHOUGH COSTLY SNUBBER NETWORKS ARE ELIMINAT- ED, FREEWHEELING DIODES ARE NEEDED; THE IGTS HAVE NO INTRINSIC OUTPUT DIODE.

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1