Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-06-02
2001-03-27
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S132000
Reexamination Certificate
active
06208529
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to isolated DC-DC converters and more particularly to buck derived converters which feature zero voltage switching of the controlled power switches and zero current switching of the non-controlled switches.
BACKGROUND OF THE INVENTION
DC to DC converters are frequently used to convert DC voltage, provide galvanic isolation of the output from the input, and to regulate the output. DC to DC converters are also frequently used as a portion of AC to DC power supplies. For example such power'supplies are employed in Telecommunication or Cellular Power Systems to provide isolated 24 Volt or 48 Volt power to the system batteries and paralleled load. DC to DC converters are also frequently used to convert and isolate one DC voltage from another. For example, +24 Volt cellular site power is converted to −48 Volt for co-located telecommunications equipment by using DC/DC converters.
Of the many topologies that can be used for DC-DC converters, those that are buck derived are often preferred for medium (+24 V, 48 V) and low (5 V, −3 V) voltage outputs. This is due to the non-pulsating output current and ease of control as the output voltage is directly proportional to the duty cycle of the switching devices. A common buck derived converter is the forward converter discussed in “The Forward Converter in Switched-Mode Power Supplies”, Philips Application Note #474, Jul. 4, 1975. This converter features good component load factors (see “Converter Component Load Factors, A Performance Limitation of Various Topologies” Bruce Carsten, PCI'88 Munich, Germany), relative simplicity, ease of control and non pulsing output current. Another buck derived converter is the half bridge exemplified in “Switched Mode Power Supplies—Highlighting a 5 V 40A Inverter Design”, Motorola Application Note #AN-737, 1974. This converter is more complex with 2 primary power switches, and has the disadvantage of 2 output windings, in the power transformer. Other commonly used buck derived converters are the full bridge PWM controlled and the phase shifted bridge converters. The component load factor of the forward converter was improved with the use of an active reset switch as disclosed in “High Power SMPS Require Intrinsic Reliability”, Bruce Carsten, PCI'81 Proceedings pp. 118 to 133, Munich, Germany, September 1981.
One objective in designing a DC-DC converter is to achieve low switching losses and low EMI. This can be achieved by adding capacitors across the primary power switches and by operating the circuit in such a way to bring the voltage across them to zero before turning them on. This approach was disclosed in Jitaru U.S. Pat. No. 5,126,931 in the active reset forward converter by adding a saturable reactor, or third controlled switch, in series with the output winding of the transformer and reducing the magnetizing inductance of the transformer to allow the voltage of the main switch to ring down to zero before it is turned on.
Another class of DC/DC converters use asymmetrically controlled half bridge or full bridge switches, where either switch is on except during the switching times and the output is controlled by the relative duty cycle of the switches. Examples of such a converter are described in “Soft-Switched DC/DC Converter with PWM Control”, Ramesh Origanti et al, Proceedings of Intelec 93 Paris, France September '93. The first is similar to a flyback (buck/boost) converter. The second is somewhat similar to a buck derived converter but has a non-linear parabolic transfer function with maximum output at 50% duty cycle and would be difficult to control. Another such converter topology is described in “DC/DC Converter for High Input Voltage, For Switching with Peak Voltage of Vin/2, Capactive turn off Snubbing and Zero Voltage Turn on”, I. Barbi et al, PESC '98 Fukuoka, Japan May '98, and is similar to a phase shifted full bridge converter. This converter topology suffers from substantial extra current in the transformer windings when both half bridge sides are switched to the same input voltage. This circuit will have a poor transformer load factor except at 50% (full) duty cycle.
It is accordingly an object of the invention to provide a new and improved DC-DC converter most suitable for medium output voltages such as 24 Volts or 48 Volts DC.
An additional object of the invention is to provide a DC-DC converter with an isolated output which is proportional to the control duty cycle, and has a non-pulsating output current. A further object of the invention is to provide zero voltage switching of the controlled power switches (e.g. MOSFETS) and zero current switching of the output non-controlled switch (e.g. a diode) to maximize conversion efficiency and minimize EMI, without using an additional switch in series with the output. Another object of the invention is to provide a DC-DC converter with high component load factor for the switching devices and especially the transformer to minimize size and cost.
SUMMARY OF THE INVENTION
In one of its aspects the invention consists of a switch mode DC to DC converter comprising a half bridge or full bridge arrangement of asymmetlically controlled switches at the input stage, at least one capacitor in series with the primary winding, and an output stage including a series combination of a secondary DC blocking capacitor and the secondary winding, one end of the series combination being connected to an output inductor, and an output diode connected across the series combination.
The primary half bridge stage may be realized by two controlled switches in series with one another, the series combination being in parallel with the input terminals. Antiparallel diode means are associated with each switch and are poled to allow current to flow in a direction opposite to the normal direction of current flow in each switch. An inductor is in series with the primary winding and at least one primary DC blocking capacitor, the series combination of the inductor, primary winding and primary DC blocking capacitor being connected between one of the input terminals and the common point of said two controlled switches.
The primary DC blocking capacitor may in fact be two capacitors such that each capacitor is connected to one of the input terminals.
The primary stage may also be realized as a full bridge arrangement of switches. Two asymmetrically controlled switches are connected in series with one another and in parallel with the input terminals and with a further two asymmetrically controlled switches which are also connected in series with one another. Antiparallel diode means are associated with each switch and are poled to allow current to flow in a direction opposite to the normal direction of current flow in each switch. An inductor is in series with the primary winding and a primary DC blocking capacitor, the series combination of the inductor, primary winding and primary DC blocking capacitor being connected between the common point of the first two controlled switches on the one hand and the common point of the further two controlled switches on the other hand.
In another aspect, the invention comprises a clamp diode means and a clamping capacitor connected in series across the output diode. The clamp diode is poled to conduct opposite to the normal direction of current flow when the switch is ON. A controlled energy recovery switch is connected across the clamp diode means and is poled to conduct current in the opposite direction to the normal direction of current in the clamp diode means when the controlled switch is turned ON.
In another aspect of the invention, a converter with a half bridge or full bridge primary stage includes an output stage including a secondary DC blocking capacitor in series with the secondary winding, one end of the series combination being connected to a tap on an output inductor, and an output diode connected across the combination of the secondary winding and the secondary DC blocking capacitor. A cl
Argus Technologies Ltd.
Natan Epstein Beehler & Pavitt
Riley Shawn
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