**Digital control of an uninterruptible power supply (UPS)using an ASIC
**Uninterruptible power supply systems (UPS) are necessary for all applications where electronic systems have to work also in case of power failure (i.e. computer centers,
hospital equipment, communication equipment etc.). Many mainframe computers are fed by UPS systems.

UPS systems conventionally consist of a synchronous generator, a fly-wheel for short-time energy storage and a battery powered motor or diesel engine.

By progress in power electronics static power converters can be realized, especially for mid-range output power (i.e. 5 - 50 kVA). In recent times there is increasing demand for UPS systems with low-distortion output voltages and sinusoidal input currents. Even at non-linear loads producing output currents with high harmonic content, sinusoidal output voltage is required. Due to this, even at unsymmetrical loads a static inverter can be better than rotating converters.

On this background the Anton Piller GmbH & Co. KG asked the IAM GmbH to design a high performance digital control together with an application specific integrated circuit (ASIC) containing the new control structure.

The mains rectifier is a four-quadrant, three-phase inverter using fast switching power transistors. Modified main circuits can be chosen for one-quadrant or single-phase applications. In this configuration the dc-link voltage is higher than the normally rectified line voltage. The rectifier inverter operates as a three-phase step-up-chopper. For decreased DC voltages an additional auto transformer can be used.

The control scheme is shown in Fig. 2. Controlled quantities are the DC-link voltage and the phase currents. Pl-algorithms are used for the current loop. The reference is made up of an amplitude calculated by the DC-voltage regulator multiplied with the sine functions of the mains phase angle.

To generate the sine values a PLL-algorithm is implemented. Regulating the DC-link voltage has two conflicting goals: to achieve a stable DC-link voltage and to produce sinusoidal input currents. Especially in single-phase application and with unsymmetrical inverter loads the DC-link voltage has to vary to achieve power storage in the DC-link capacitor. Therefore the DC-control consists of two independent control circuits: a fast PI-controller normally working at its upper limit is responsible for avoiding over voltage; by a slower combination of battery current controller and battery voltage controller a well-filtered operation is achieved.

The control scheme of the output inverter is shown in Fig. 3. The Controlled quantity is the output voltage. An underlying current control loop is implemented for protection of the power devices. The output value is transformed to switching signals by pulse width modulation (PWM).

PI-controllers for AC quantities normally have phase and amplitude errors, which are not tolerable. On the other hand, PI-controllers provide predictable behavior at all operating modes. To reduce undesired effects feed-forward signals from the reference and output voltage are used. To eliminate the influence of the changing DC link voltage on the gain of the current controller the current control output is multiplied with the reciprocal value of the DC voltage.

The reference for the current controller is taken from the voltage controller. A modified PI-controller was chosen; special algorithms are implemented to cope with the saturation problem of the output transformer. The output of the voltage controller is limited to the current capability of the devices.

This ASIC containing both peripheral and signal processing hardware is a very cost-effective solution for this UPS system. The disadvantage of having hardware red algorithms was acceptable due to the fixed specification of the UPS system and counterbalanced by an universal control structure performed by the ASIC with many switches and load able parameters.

All inner current and voltage control loops are performed with a sampling rate equivalent to the switching frequency of the power devices (10 or 20 kHz).

Output inverter and controlled rectifier both contain equivalent circuitry and signal processing pulse width modulator, current control, sine-generation, DC-link measurement and adaptation. The whole UPS family consists of both one- and three-phase systems. Integrating the total functions for a three-phase rectifier and inverter into one ASIC would be overloaded for single-phase applications. Therefore it was decided to put the control of a three-phase inverter into one circuit. This can be used for the three-phase output inverter, the three-phase rectifier and also for both a one-phase rectifier and inverter.

Some additional circuitry was included. The measurement of the phase difference for the PLL-control of the line frequency is supported by the ASIC using phase angle captures functions. To support the microcontroller to determine the effective output voltage and current load the integrated squares of the measured values are calculated during a period of the sine wave frequency.

The repetition cycle of all calculations performed by the ASIC is the switching frequency of the inverter legs. This frequency normally amounts to 20 kHz. At the beginning of the cycle the A/D-conversion is started. The ASIC calculates the references and the UD-voltage controller. After at most 15 µs the A/D-conversion is ready and the calculation of the controllers starts being performed in 10 µs for all six PI-algorithms. New PWM-values are given to the modulator at the middle of the switching cycle. In the remaining second half of the cycle the ASIC performs the calculation of sum of squares of the measured values.

Using a controlled step-up chopper the input currents of the single-phase controlled rectifiers are pure sinusoidal. This transforms non-linear loads at the output side into linear resistive loads at the line-side (Fig. 6).

input current and a mixed resistive non-linear load

without transfer to bypass operation

operation after approx. 100 ms

A more detailed description of this work you can find in following publication done together with the Anton Piller GmbH & Co. KG: