EL SEGUNDO, CA. December 1998 - International Rectifier (IR^Ò ), the power expert, now offers a revolutionary new soft start solution for AC converters that promises to provide unprecedented levels of system reliability and safety to AC motor drive, welding, induction heating, UPS, power supply and other applications that run off the AC line. An industry first, the new IR1110 Soft Start Controller IC drives three SCRs in a half-controlled, 3-phase input bridge while providing extensive built-in charging, protection and supervisory functions.

"For some time, power conversion system advances have been focused on the high-frequency output or inverter stages of line driven systems," explained David Tam, vice president for control IC products at IR. "In fact, the input or AC converter stage is often the weak link in the entire power conversion system, with the DC bus capacitor the most bulky and problematic component in any high-power system. Our new Soft Start Controller IC is the first highly integrated solution available to control an AC converter and protect the DC bus capacitor from damage and catastrophic failure."

"The new IR1110 IC increases AC converter reliability and safety, adding protection features to prolong DC bus capacitor lifetime by preventing serious damage from DC bus short circuits. The device provides an array of status feedback signals for versatile system control and promotes up-time by eliminating idling during power dip ride through. The new IR1110 also takes up 75 percent less board space than traditional soft charge methods, and even improves motor drive power at low speed by up to 100 percent."

"Traditional soft charging circuits use a bulky combination of series power resistors and bypass relays with associated circuitry to protect the DC bus capacitor," Tam said. "This approach also has limited functionality. For example, the inverter stage cannot operate while charging the bus capacitor, momentary power dips cause system reset and long down time, and capacitor short circuits induce large inrush current that potentially burn out the input module, capacitor or PCB.

The IR soft start solution is based on a single IC that drives three SCRs in a half-control, 3-phase input bridge. With low power consumption the IC is able to run off the SCR snubber derived power supply. The charging, protection and supervisory functions include:

• SOFT DC BUS CAPACITOR CHARGING -- Through phase control of the SCRs at a programmable rate, resulting in prolonged reliability and lifetime for the DC bus capacitor.
• CURRENT LIMIT PROTECTS AGAINST DC BUS SHORT CIRCUIT -- The IR1110 solution limits the inrush current and allows safe shut down of the AC drive system, eliminating catastrophic damage to the input module, capacitor and/or burning PCB foil.
• FAST RAMP-BACK RECOVERY FROM A TRANSIENT LINE LOSS -- The IR1110 solution starts powering a motor even during bus voltage ramp-back, eliminating idle time. Traditional AC drives suffer down time when power dips, because the drive cannot engage the inverter stage while bus voltage is ramping back.
• DYNAMIC DC BUS VOLTAGE REGULATION -- Regulating the bus voltage as required by the inverter stage will deliver more power in low speed motor operation with the same size IGBT. Dynamic bus regulation also suppresses high voltage spikes and motor insulation failure, extending AC motor life.
• VERSATILE LINE DIAGNOSTICS -- Feedback signals for low- or high-line voltage and single- or three-phase loss are provided by the IR1110, allowing designers to choose either shutdown or continuous operation in the event of single-phase loss.

IR1110 Datasheet

A Reference Design Kit is available, and includes a compact, 5 x 6-inch, self-contained AC converter system that runs off 230V/460V/575V 3-phase AC lines and provides a power range of 10 to 20HP using IRKH91-16 ADD-A-Pak input modules.