Machine Design

High-Voltage ICs

Several chip makers can now produce integrated circuits that operate directly from ac lines. These ICs are expected to drastically change the makeup of power supplies. For one thing, they may eliminate the need for transformers for some types of supplies. However, it appears that one of the first types to be affected will be low-current versions in nonisolated applications.

A high-voltage IC process is credited for the first major change in low-power supplies in 20 years. Single-chip supplies were, in the past, not feasible because breakdown-voltage barriers prevented conventional ICs from use in off-line applications. Transformers were needed to step the voltage down to a safe level for the electronics. Conventional linear ICs typically operate over a ±15-V range. Maximum voltage is limited by the concentration of the background dopant in the silicon or the level of electrical isolation between transistors, whichever is most restrictive.

The ICs are fabricated on low-resistivity silicon wafers to reduce power loss and improve device speed. Wafer resistivity is lowered by heavy doping. But pn junctions formed in heavily doped material have low breakdown voltages. High-voltage discrete transistors, on the other hand, are made with high-resistivity, lightly doped silicon substrates.

Isolation is usually achieved by surrounding each transistor with a pn junction. When the junction is reverse biased, the depletion region blocks leakage current flow into or out of the device. The drawback with junction isolation is that it takes up wafer space and requires activation power.

Breakdown voltage improvements in high-voltage ICs are possible for two reasons. One, the starting substrate material is high-resistivity silicon. Secondly, isolation junctions are replaced by a solid insulator. A dielectric layer of glass a few microns thick isolates each transistor. Dielectric isolation takes up less space and consumes no power.

One of the first applications of the high-voltage IC process is a single-chip power supply called the HV-1205. The 1205 survives inrush currents of up to 1.2 A and is immune to both spikes and severe brownout. Full power is delivered through sags down to 30 Vac. The IC packs into an eight-lead miniDIP the functions of a transformer, rectifier, and a three-terminal regulator. It hooks directly to any industrial ac line and can deliver clean low-voltage dc power.

An advanced switching design provides up to three outputs in the range of 5 to 25 Vdc at up to 50 mA. In industrial uses, for instance, the chip can simultaneously power a 24-V relay, provide 10-Vdc sensor excitation, and source a 5-V logic signal. A battery backup configuration allows uninterrupted operation during power loss. The IC automatically charges the battery during normal ac operation.

The 1205 is basically a nonisolated switching power supply that operates at line frequency. A high-efficiency switching preregulator rectifies and reduces the incoming line voltage, while a linear postregulator removes ripple and provides load regulation. Because the chip is functionally simple, it is easy to design into protected power systems.

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