July 17, 1997

Transformer driver controls bidirectional switch

Gerald Grady, Maxim Integrated Products, Sunnyvale, CA

You can use a transformer-driver IC to simply control an on/off switch that connects V_SUPPLY to a load (Figure 1a). V_SUPPLY can be positive, negative, or an ac voltage. Only the MOSFETs' maximum V_DS ratings, which are 50V for Q₁ and Q₂, limit the magnitude.

The transformer's primary winding and driver IC operate on 5V, generating an isolated secondary waveform that D₁ and D₂ rectify to produce a 10V V_GS for the n-channel MOSFETs. V_GS is isolated, constant, and unaffected by changes in V_DS with respect to ground. Because the combination of one MOSFET and negative V_GS would allow current flow in the off state (because of forward bias on the MOSFET's internal parasitic diode), the circuit in Figure 1 connects the two MOSFETs source to source. Their internal diodes then oppose each other, which blocks unwanted current flow to either polarity in the off state.

Shutting down IC₁ turns off the switch by removing V_GS from the MOSFETs. A 5V input to IC₁'s SD (shutdown) pin turns off the switch; an input of 0V turns it on (Figure 1b). The speed of this turn-off time depends on the value of R₁. Lower values reduce turn-off delay but cause higher supply current. For R₁=1 kilohm, the supply current is 24 mA. If speed isn't an issue, you can lower the supply current to 5 mA by substituting a larger R₁.

Alternative switching techniques have drawbacks. Relays, for instance, have switch bounce and high power consumption of about 0.5W. The maximum V_GS rating for most power MOSFETs, which is about 20V for standard devices and about 15V for logic-level devices, makes it difficult to stand off voltages greater than 15V. You can level shift the gate voltage, but this approach wastes power. In addition, the larger gate resistor required for a higher voltage slows the switching speed.

The operating frequency of the MAX845 ranges from 500 to 675 kHz. Thus, these high frequencies allow the transformer to be small. For this design, the transformer is about 0.25 in. square and less than 0.125 in. high. (DI #2059)

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