1. Field of the Invention
The present invention relates generally to uninterruptible power supplies. More specifically, the preferred embodiments of the present invention relate to an uninterruptible power supply with controllable leakage current.
2. Background Discussion
Uninterruptible power supplies (UPSs) are commonly used to provide power to critical equipment that must not experience even short duration brownouts or blackouts. For example, computer servers, computer networks, telecommunications electronics and medical devices are often powered by an uninterruptible power supply.
A UPS device typically has an AC-DC-AC converter and backup battery that is activated in case the alternating current (AC) line power is temporarily disconnected or falls below a voltage threshold.
FIG. 1 shows a conventional UPS circuit according to the background art. The conventional UPS circuit includes an input rectifier (having switches Q1, Q2) and an output inverter (having switches Q3, Q4) connected in series. The input rectifier converts AC input power to DC power, and the output inverter converts the DC power to AC output power. Typically, the switches Q1, Q2, Q3, Q4 will be MOSFET devices or insulated gate bipolar transistors (IGBTs), though switches Q1 Q2 can also be diodes. The switches Q1, Q2, Q3, Q4 are controlled by a gate drive circuit (not shown). Storage capacitors C1, C2 store DC power. Typically, an isolation transformer T is provided. The input rectifier switches Q1, Q2 receive AC line power and provide direct current (DC) power to the inverter switches Q3, Q4. The inverter switches Q3, Q4 are typically operated according to a pulse-width modulation (PWM) scheme. The PWM waveform is filtered by output inductor Lo and output capacitor Co to create a smooth AC output waveform. With this arrangement, the voltage and frequency of the AC output can be accurately controlled and will be independent of the AC input.
Leakage current is often a problem with the conventional circuit of FIG. 1 and similar UPS circuits. Leakage current results when AC signals (egg from the PWM waveform produced by the inverter switches Q3, Q4) couple to ground, the device chassis, or nearby electronic components. A typical UPS providing a kilowatt of power may experience several hundred microamperes of leakage current. In some applications, leakage current can produce ground loop currents, noise, and disturbances in sensitive electronic circuits. Leakage current is particularly a problem in medical device electronics because medical devices are often very sensitive to noise and ground loop current. Leakage current can be dangerous in medical applications as it may cause a malfunction of critical life support or monitoring equipment.
Accordingly, there is a need in the art for a UPS circuit that has reduced leakage current. It would be particularly beneficial to provide a UPS circuit that has a leakage current less than an adjustable maximum level. Also, it would be beneficial to provide a UPS circuit that allows the maximum leakage current to be controlled by a user.