DocID025494 Rev 1

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AN4390

Description

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Description

Uninterruptible power supplies (UPS) are widely used to avoid a sudden loss of data or

information caused by a power supply disconnection (blackout). Power and voltage spikes,

supply frequency changes, noise and radio frequency interference affect negatively the

load. In

Figure 1, the basic schematic of a "standby UPS" is shown; it is the most common

type of UPS present in the market [

1]. Surge suppressor and input filter protect the output

equipment respectively from current spike and noise coming from input line. During an input

supply failure, the transfer switch enables the battery-powered path.

Figure 1. Standby UPS schematic

The UPS DC-AC section is formed by a H-bridge supplied by the battery voltage, which is

converted into a usable AC voltage for the output equipment (

Figure 2).

Figure 2. DC-AC section: single-phase inverter

Q1-Q2 and Q3-Q4 are driven to allow current to flow through the load to both directions.

When Q1 and Q2 are in ON state, the voltage across the load is positive and the current

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