Inverter offers design flexibility

Nihal Kukaratna, Arthur C Clarke Institute, Katubedda Moratuwa, Sri Lanka -- EDN, 6/5/2000

You may occasionally need a substitute for a commercial dc/ac inverter. A typical application is in an uninterruptible power supply (UPS). The circuit in Figure 1 is a flexible, low-component-count inverter with closed-loop voltage regulation. The advantages of the circuit are that it works from a 12V car battery (or from higher battery voltages with minor modifications), it offers closed-loop voltage regulation, and phase locking with a commercial power supply is possible.

The circuit is designed around a MOS-gate driver family, such as International Rectifier's (www.irf.com) IR215X family (IC₅). This IC drives the gates of power MOSFETs Q₃ and Q₄ through NAND gates (IC₆). A 555-timer-based 100-Hz oscillator (IC₂) feeds the MOS-gate driver's frequency-generation block through a divide-by-two circuit (IC₄). The MOS-gate drivers' low and high outputs drive the power MOSFETs' gates through IC₆. The combination of the MOS-gate driver and the IC₆ NAND gates maintains the necessary deadband to prevent simultaneous conduction of the power-MOSFET pair. A voltage-feedback sample, compared with the 1.2V reference source in a MAX951, IC₇, provides a closed-loop feedback to vary the value of the constant-current source comprising Q₇ and the optoisolator, IC₈. This variable constant-current source varies the monostable output of IC₃, which feeds the IC₆ NAND gates. The feedback system thus maintains the proper pulse width in the gate drivers.

You can easily modify the (square-wave) circuit for a sinusoidal output by adding a few components between IC₃ and IC₆ (Reference 1). For higher outputs, you need change only the battery voltage and the power MOSFETs. The circuit in Figure 1 is a 200-VA unit. Figure 2 shows the output-regulation curve for different battery-voltage inputs. Figure 3 shows the efficiency-versus-load curve. The inverter circuit has 81 to 93% efficiency for loads of 15 to 180W. Using a tape-wound, powdered-iron-core transformer as T₁, you can package the unit in a 100-in.³ volume for a 230V, 50-Hz emergency power source. (DI #2538).

REFERENCE

1. Kularatna, N, and P Silva, "New Approach to Sine-Wave Inverters for UPS and Emergency Power Supplies Using Low-Cost MOS Gate Drivers," Proceedings of Power Systems World Conference (PCIM 98), pg 332.

2. N Kularatna and M de Silva, "DSP-Based Sine-Wave Inverters for UPS and Emergency Power Supplies with MOS Gate Drivers," Proceedings of Power Systems World Conference (PCIM 99), pg 296.