A dc-to-ac inverter employing a control low
power section and a power section. The low power section comprises a
square wave generator, cross-coupled NOR latches and low level
transistor switches for providing switching current to the power
section. The power section employs complementary FET drivers and power
switches connected to a battery or other source of dc and a
ferro-resonant transformer for changing dc to ac.
The power section is protected by a voltage
sensor. In a preferred embodiment, this voltage is proportional to the
current through the respective power switches and is used to unlatch the
applicable NOR latch, thereby causing shut off of the protected network
of the power section.
SUMMARY OF THE INVENTION
The invention embodiments disclosed herein pertain to dc-to-ac
inverters having a control or low voltage section and a high voltage or
power section. The low voltage section includes a square wave generator
for generating complementary square wave voltages at a nominal 60 Hz. Of
course, some systems operate with respect to other frequencies and,
therefore, 60 Hz is exemplary for a typical operating system. The square
wave voltages are respectively connected as one of the inputs to
latching networks comprised of cross-coupled NOR gates, which, in turn,
activate low voltage switching transistors. Another of the inputs to the
respective NOR-gate latches is from the respective networks of the
power section of the circuit. When a condition occurs in such section so
as to indicate a condition of concern, the NOR gate receives an input,
that causes the NOR latch to unlatch and therefore causes the related
transistor switch to remove the enabling current for the related power
driver. In one embodiment, the collector voltage of the power
transistors are monitored for out of saturation occurrence. In another
embodiment, emitter currents through the power transistors are monitored
for excessive values.
The power section of the circuit incorporates field effect transistors (FET's) for driving respectively connected power transistors into saturation in push-pull fashion. The outputs of these power transistors are connected to the primary winding of a ferro-resonant transformer.
The ferro-resonant transformer converts the applied square wave input to a relatively undistorted sine wave, which is further filtered before being applied to the load, typically an HID lighting system, a computer, or the like.
Some loads are quite phase sensitive, and, therefore, when normal power distribution fails and an emergency system is placed into operation, the phase of the two systems must be synchronized. Therefore, one embodiment of the present invention provides a phase lock system for doing this. The square wave portion of the inverter just described is derived from the power distribution line, with a back-up emergency square wave generator operating without load. A multiplexer steering network selects the power distribution-derived input except in case of failure, when it switches in the output of the emergency generator. A phase detector system together with a voltage controlled oscillator assures that the square wave applied to the latching part of the control circuit does not jump from one portion of a cycle to another, but only gradually drifts over a number of cycles to lock in on the frequency of the emergency system.
The power section of the circuit incorporates field effect transistors (FET's) for driving respectively connected power transistors into saturation in push-pull fashion. The outputs of these power transistors are connected to the primary winding of a ferro-resonant transformer.
The ferro-resonant transformer converts the applied square wave input to a relatively undistorted sine wave, which is further filtered before being applied to the load, typically an HID lighting system, a computer, or the like.
Some loads are quite phase sensitive, and, therefore, when normal power distribution fails and an emergency system is placed into operation, the phase of the two systems must be synchronized. Therefore, one embodiment of the present invention provides a phase lock system for doing this. The square wave portion of the inverter just described is derived from the power distribution line, with a back-up emergency square wave generator operating without load. A multiplexer steering network selects the power distribution-derived input except in case of failure, when it switches in the output of the emergency generator. A phase detector system together with a voltage controlled oscillator assures that the square wave applied to the latching part of the control circuit does not jump from one portion of a cycle to another, but only gradually drifts over a number of cycles to lock in on the frequency of the emergency system.
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