Fairchild AN-7511 manual Use Optoisolation To Avoid Ground Loops, Take Some Driving Lessons

Insulated-Gate Transistors Simplify AC-Motor

Speed Control

Application Note September 1993 AN-7511

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An IGT’s few input requirements and low On-state resistance simplify drive circuitry and increase power efficiency in motor- control applications. The voltage-controlled, MOSFET-like input and transfer characteristics of the insulated-gate transis- tor (IGT) (see EDN, September 29, 1983, pg 153 for IGT details) simplify power-control circuitry when compared with bipolar devices. Moreover, the IGT has an input capacitance mirroring that of a MOSFET that has only one-third the power- handling capability. These attributes allow you to design sim- ple, low-power gate-drive circuits using isolated or level-shift- ing techniques. What’s more, the drive circuit can control the IGT’s switching times to suppress EMI, reduce oscillation and

VCC

ON

OFF

R1

R3

R2

LOAD

15 ≤	VCCR2	≤ 25V
	R-------------------1+ R2

R3 CONTROLS tOFF

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noise, and eliminate the need for snubber networks.

Use Optoisolation To Avoid Ground Loops

The gate-drive techniques described in the following sections illustrate the economy and flexibility the IGT brings to power control: economy, because you can drive the device’s gate directly from a preceding collector, via a resistor network, for example; flexibility, because you can choose the drive circuit’s impedance to yield a desired turn-off time, or you can use a switchable impedance that causes the IGT to act as a charge- controlled device requiring less than 10 nanocoulombs of drive charge for full turn-on.

Take Some Driving Lessons

Note the IGT’s straightforward drive compatibility with CMOS, NMOS and open-collector TTL/HTL logic circuits in the common-emitter configuration Figure 1A. R3 controls the turn- off time, and the sum of R3 and the parallel combination of R1 and R2 sets the turn-on time. Drive-circuit requirements, however, are more complex in the common-collector configuration Figure 1B.

In this floating-gate-supply floating-control drive scheme, R1 controls the gate supply’s power loss, R2 governs the turn-off time, and the sum of R1 and R2 sets the turn-on time. Figure 1C shows another common-collector configuration employing a bootstrapped gate supply. In this configuration, R3 defines the turn-off time, while the sum of R2 and R3 controls the turn- on time. Note that the gate’s very low leakage allows the use of low-consumption bootstrap supplies using very low-value capacitors. Figure 1 shows two of an IGT’s strong points. In the common-emitter Figure 1A, TTL or MOS-logic circuits can drive the device directly. In the common-collector mode, you’ll need level shifting, using either a second power supply Figure 1B or a bootstrapping scheme Figure 1C.

FIGURE 1A. SIMPLE DRIVING AND TRANSITION-TIME

CONTROL

		VCC
		CONTROL
		INPUT		R1
ON					R2
	OFF		15V

R1	CONTROLS GATE	LOAD
SUPPLY POWER LOSS
R2 CONTROLS tOFF
R1	+ R2 CONTROLS tON

FIGURE 1B. A SECOND POWER SUPPLY

			15 ≤	VCCR2	≤ 25V
				R-------------------1+ R2
			R3 CONTROLS tOFF
			R2 + R3 CONTROLS tON
R1			τ « ------------------------------------------------	5C
	R2	R3	ICEO + IGES + 2IR

OFF

ON

LOAD

FIGURE 1C. BOOTSTRAPPING SCHEME

In the common-collector circuits, power-switch current flowing through the logic circuit’s ground can create problems. Optoisolation can solve this problem (Figure 2A.) Because of the high common-mode dV/dt possible in this configuration, you should use an optoisolator with very low isolation capaci- tance; the H11AV specs 0.5pF maximum.

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1