Fairchild AN-7511 manual Application Note, Pulse-Transformer Drive Is Cheap And Efficient

Application Note 7511

For optically isolated “relay-action” switching, it makes sense to replace the phototransistor optocoupler with an H11L1 Schmitt-trigger optocoupler (Figure 2B).) For applications requiring extremely high isolation, you can use an optical fiber to provide the signal to the gate-control photodetector. These circuit examples use a gate-discharge resistor to control the IGT’s turn-off time. To exploit fully the IGT’s safe operating area (SOA), this resistor allows time for the device’s minority carriers to recombine. Furthermore, the recombination occurs without any current crowding that could cause hot-spot forma- tion or latch-up pnpn action. For very fast turn-off, you can use a minimal snubber network, which allows the safe use of lower value gate resistors and higher collector currents.

VCC
	R1
	R2	R3
	C
	CONTROL
	INPUT
OFF	H11AV2
ON		LOAD

FIGURE 2A. AVOID GROUND-LOOP PROBLEMS BY USING AN OPTOISOLATOR. THE ISOLATOR IGNORES SYS- TEM GROUND CURRENTS AND ALSO PRO- VIDES HIGH COMMON-MODE RANGE.

VCC = 300V
	43k
	1N5061	5.6k	5.6k
	10∝F	5.6k
CONTROL
	35V
INPUT
OFF
ON
	H11L1		LOAD

FIGURE 2B. A SCHMITT-TRIGGER OPTOISOLATOR YIELDS “SNAP-ACTION” TRIGGERING SIMILAR TO THAT OF A RELAY.

Pulse-Transformer Drive Is Cheap And Efficient

Photovoltaic couplers provide yet another means of driving the IGT. Typically, these devices contain an array of small silicon photovoltaic cells, illuminated by an infrared diode through a transparent dielectric. The photovoltaic coupler provides an isolated, controlled, remote dc supply without the need for oscillators, rectifiers or filters. What’s more, you can drive it

directly from TTL levels, thanks to its 1.2V, 20mA input parameters.

Available photovoltaic couplers have an output-current capability of approximately 100∝A. Combined with approximately 100kΩ equivalent shunt impedance and the IGT’s input capacitance, this current level yields very long switching times. These transition times (typically ranging to 1 msec) vary with the photovoltaic coupler’s drive current and the IGT’s Miller-effect equivalent capacitance.

Figure 3 illustrates a typical photovoltaic-coupler drive along with its transient response. In some applications, the photovoltaic element can charge a storage capacitor that’s subsequently switched with a phototransistor isolator. This isolator technique - similar to that used in bootstrap circuits provides rapid turn-on and turn-off while maintaining small size, good isolation and low cost.

In common-collector applications involving high-voltage, reac- tive-load switching, capacitive currents in the low-level logic cir- cuits can flow through the isolation capacitance of the control element (eg, a pulse transformer, optoisolator, piezoelectric coupler or level-shift transistor). These currents can cause undesirable effects in the logic circuitry, especially in high- impedance, low-signal-level CMOS circuits.

		I	DIG22			+
	ON			IGT
		OFF			-
CONTROL
	INPUT

OUTPUT

CURRENT

INPUT
CURRENT	0	1	2ms

FIGURE 3. AS ANOTHER OPTICAL-DRIVE OPTION, A PHOTO- VOLTAIC COUPLER PROVIDES AN ISOLATED, REMOTE DC SUPPIY TO THE IGT’S INPUT. ITS LOW 100∝A OUTPUT, HOWEVER, YIELDS LONG

IGT TURN-ON AND TURN-OFF TIMES.

The solution? Use fiber-optic components Figure 4 to elimi- nate the problems completely. As an added feature, this low- cost technique provides physical separation between the power and logic circuitry, thereby eliminating the effects of radiated EMI and high-flux magnetic fields typically found near power-switching circuits. You could use this method with a bootstrap-supply circuit, although the fiber-optic sys- tem’s reduced transmission efficiency could require a gain/speed trade-off. The added bipolar signal transistor minimizes the potential for compromise.

©2002 Fairchild Semiconductor Corporation Application Note 7511 Rev. A1