With increased dependency on electric power for various domestic, commercial purposes and the seemingly declining capacity of power utilities in many countries, the need for additional backup power sources is on the rise. Various modules are already available to address these different needs. However, most modules are either too expensive, too bulky, or too rigid in their power capacity, capability, and flexibility.

The circuit described here is an off-line uninterruptible power supply (see the figure). It has an expandable power stage design that can be easily modified for use with power ranges from as low as 100 W to as high as 5000 W with forced cooling.

The design is based on the LM3524D, a popular industrial-grade, pulse-width-modulation (PWM) controller. This device is fully self-contained and has all of the necessary logic built-in to ensure a compact and cost-effective product. The controller offers:

• Complementary power drive output stage with 180° out-of-phase switching.
• Stable oscillator with shutdown capability and noise suppression.
• Current-sense and voltage-sense circuitry.
• Dead time between the two complementary switching stages to prevent cross conduction and subsequent core saturation.
• Low voltage cutoff.

The on-chip oscillator frequency is controlled by the RC pair R2 and C1, and the required frequency is calculated as f_OSC = 1/(R2 × C1). Depending on the country, f_OSC = 50 Hz or 60 Hz. The low-power drive stage, containing transistors Q1 and Q2, is driven by the outputs at pin 12 and 13 of the controller; out-of-phase switching is handled by its own internal logic.

The final switching stage is comprised of transistors Q5 and Q6 wired across the main step-up transformer. These transistors are switched on in tandem to drive current through the two halves of the primary winding of transformer T2, independently.

Power diodes D2 and D3 are included to protect the power-stage transistors against reverse currents. Switching pulses from the drive transistors are transferred to the power stage via transistors Q3 and Q4, which serve as current amplifiers.

Resistor R10 is used to feed back the current to the current-sense monitor via pin 5. The current-sense logic monitors this feedback and triggers shutoff in event of either overload or power-stage failure.

The value of R10 is calculated as I_MAX = 200 mV/R10, where I_MAX is proportional to the power rating of the load, and 200 mV is the minimum required potential drop across R10 to trigger a shutdown by the current-sense logic.

The transformer T1 is connected to the standard mains supply and the rectified dc is fed to the shutdown pin (pin 10). This pin serves to turn off the LM3524 during the presence of the standard mains supply. If the mains supply fails, the bias on the shutdown pin is removed, thus starting the oscillator and hence the drive stages.

The power stages and transformer T2 must be appropriately rated for the required output power. The power transistors (Q3, Q4, Q5, and Q6) can be paralleled with similar devices for increased power-handling capacity. In addition, forced cooling can be used to achieve an even higher output power rating. However, for low- or medium-output power ratings, these transistors must be mounted on a large heat sink. Transformer T2 is a standard 12 V:0:12 V to 230-240 V step-up winding, while T1 is a small 230-V to 12-V step-down transformer rated for 50 mA on the secondary windings.

Additional filter and shaping circuitry can be added on the secondary winding of T2 to obtain a near sine-wave output. The RC network comprising R3 and C2 is included for compensation.

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