subject: As the output of the bridge rectifier approaches its maximum voltage [print this page] As the output of the bridge rectifier approaches its maximum voltage
Conventional bridge rectifier circuits are not well equipped to handle high start-up currents and short-circuits which stress circuit components such as fuses, bridge diodes and smoothing capacitors. Typically, for example, the occurrence of an output short circuit destroys a protection fuse between the power supply and the bridge rectifier, requiring replacement of the fuse. To limit the in-rush current at turn-on, conventional circuits include a thermistor or a relay in series with the output of the rectifier bridge. However, thermistor protection, although inexpensive, is suitable only for low-power applications. Relay protection, while more reliable and efficient than a thermistor, presents a significant cost premium, typically 50% to more than 100% of the bridge rectifier cost, and occupies a significant amount of space, usually more than the bridge rectifier itself.
Therefore, a need exists for a self-contained bridge rectifier circuit which can be provided in a module or package similar to that of a conventional bridge rectifier, but which also provides in-rush current protection on start-up and short-circuit protection during operation without a significant cost premium.
The present invention overcomes the disadvantages of the prior art and achieves the objectives set forth above by providing an apparatus and method for rectifying an AC input voltage to produce a DC output voltage with control circuitry for automatically ramping the rectified output voltage up to a predetermined maximum output voltage. The control circuitry is preferably provided in the form of a power integrated circuit, such that the entire bridge rectifier circuit with the control circuitry can be packaged together in a standard-sized module.
The present invention includes a bridge rectifier circuit with at least one silicon controlled rectifier (SCR). The conduction interval of the SCR is governed by a ramped-up voltage provided by the control circuitry. The ramped-up voltage automatically widens the conduction interval of the SCR, generating a firing signal at the gate of the silicon controlled rectifier at increasingly earlier times in the input signal phase. During the initial portion of the voltage ramp-up, the control circuitry fires the SCR only during periods in which the AC input voltage has a negative slope.
As the output of the bridge rectifier approaches its maximum voltage, a continuous firing signal is provided to the gate of the silicon controlled rectifier to minimize current drain by the control circuitry.
Undesired firing of the SCR during spikes in the AC voltage is prevented by the control circuitry. Snubbers connected in parallel with the silicon controlled rectifier provide dV/dt immunity. Housekeeping power for the control circuitry is self-contained within the module.
The soft start bridge rectifier circuit using one or more SCRs to control the charging current of the dc bus capacitor is a prime candidate for motor control power trains. The circuit offers the following advantages when compared to an uncontrolled diode bridge with a precharge resistor and shorting relay: eliminating the precharge resistor and relay, saving space, improving reliability, providing automatic current limiting in the event of bus capacitor failure or output inverter leg failure, and providing regulation of the dc bus voltage thereby reducing the required voltage ratings of the bus capacitor and the inverter output switches.
