System and method for off-line ups

An uninterruptible power supply system, comprising:

• a backup power source;
• an output configured to provide an AC output voltage to a load;
• a power conversion circuit coupled to the backup power source and the output for generating the AC output voltage;
• and
• means for controlling the power conversion circuit to generate, during a portion of an output cycle, a substantially sinusoidal output voltage using a plurality of pulse width modulation (PWM) control signals, detecting that the output voltage has reached a maximum output voltage level, clamping the output voltage by controlling an inverter to reduce the output voltage to a steady state output voltage reference level, and recording a phase angle at which the output voltage has reached the maximum output voltage level.

An uninterruptible power supply (UPS) system, comprising:

• a backup power source;
• an output configured to provide a substantially sinusoidal AC output voltage to a load;
• a power conversion circuit coupled to the backup power source and the output for generating the AC output voltage from the backup power source in a backup mode of operation;
• and
• a controller coupled to the power conversion circuit, the controller configured to:
• operate the power conversion circuit during a portion of an output cycle using a plurality of pulse width modulation (PWM) control signals that when applied to the power conversion circuit causes the AC output voltage to be generated;
• compare the AC output voltage to a reference voltage at each of a plurality of time intervals during the portion of the output cycle;
• calculate a PWM control signal scaling factor based on each of the comparisons to the reference voltage;
• maintain the AC output voltage at substantially the reference voltage by adjusting a width of at least one of the PWM control signals using the PWM control signal scaling factor;
• detect that the output voltage has reached a maximum output voltage level;
• clamp the output voltage by controlling an inverter to reduce the output voltage to a steady state output voltage reference level;
• and
• record a phase angle at which the output voltage has reached the maximum output voltage level.