Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-11-30
2001-09-18
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S127000
Reexamination Certificate
active
06292380
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a self-regulated synchronous rectifier. The invention relates in particular to a self-regulated synchronous rectifier used in an AC/DC or DC/DC converter.
The invention relates to a synchronous rectifier regulated by coupled winding or self-regulated by symmetrical or asymmetrical direct energy transfer. In the remainder of the text, the expression “self-regulated synchronous rectifier” also means “synchronous rectifier regulated by coupled winding”.
2. Description of the Prior Art
Asymmetrical conversion systems including an initial voltage source discharging into a transformer primary connected in parallel with a main switch are known in the art. The secondary of the transformer is connected in cascade with a self-regulated synchronous rectifier and a filter. The output of the filter discharges a regulated DC voltage into an application. In a conversion system of the above type the role of the self-regulated synchronous rectifier is:
to deliver to the application, via the filter, the energy transferred by the transformer in the on period of the main switch, and
to block the transfer during the off period of the main switch, the application being powered by the coil of the filter during the off period of the main switch.
An asymmetrical self-regulated synchronous rectifier includes two MOSFETs adapted to perform the above two functions. For example, the asymmetrical self-regulated synchronous rectifier includes:
a first and a second output of the rectifier,
a first MOSFET connected between the first secondary transformer end and the first output of the rectifier and having its gate connected to the second end of the secondary of the transformer, and
a second MOSFET connected between the first output of the rectifier and the second output of the rectifier and having its gate connected to the first transformer end.
The secondary voltage of the transformer controls the two MOSFETs.
For economic reasons, manufacturers wish to develop converters accommodating a wide range of input voltage in one and the same product. This implies that the secondary voltage of the transformer also varies within a wide range. However, the secondary voltage of the transformer is also the signal at the gate of the MOSFETs of the rectifier. There are limits on the voltages that can be applied to the gates of MOSFETs. If too high a voltage is applied to the gate of a MOSFET, the MOSFET may be destroyed. Protecting MOSFETs from an overvoltage at the gate by connecting the gate in series with a passive voltage divider bridge is known in the art. However, the voltage divider causes high losses if the transformer secondary voltage is too low or too high. The voltage applied to the gate of the MOSFETs is too low or too high for optimum control of the MOSFETs, i.e. with an optimum control dynamic range which is most economical in terms of losses. As a result, for low output voltages, the feasible power transferred is very low and for higher output voltages the dynamic range of the input voltage is small.
One object of the present invention is to propose a self-regulated synchronous rectifier in which the gate is protected against gate overvoltages, allowing wide variation of the input voltage combined with optimum performance in terms of output current and voltage.
SUMMARY OF THE INVENTION
The invention provides a self-regulated synchronous rectifier connected between a secondary transformer winding and an LC filter.
The self-regulated synchronous rectifier includes two MOSFETs each having a gate connected in series with a gate protection circuit.
According to the invention, each gate protection circuit includes a divider bridge and a switch connected in parallel with the divider bridge and having an open position and a closed position and the rectifier includes a control device for controlling the switches adapted to receive an input signal proportional to the input voltage of the rectifier and producing output signals for controlling the switches.
In an asymmetrical first embodiment the rectifier is connected between the secondary transformer winding, which has first and second transformer ends, and the LC filter, which has first and second filter inputs, and has:
first and second rectifier inputs respectively connected to the first and second transformer ends,
first and second rectifier outputs, the second rectifier output being connected to the second rectifier input,
a direct MOSFET connected between the first rectifier input and the first rectifier output and having a gate connected to the second rectifier input and in series with the gate protection circuit, and
a freewheel MOSFET connected between the first rectifier output and the second rectifier output and having a gate connected to the first rectifier input and in series with the gate protection circuit.
In a symmetrical second embodiment the secondary winding of the transformer includes first and second sub-windings of opposite phase, each of the sub-windings having first and second transformer ends, and an LC filter having first and second filter inputs.
The self-regulated synchronous rectifier includes:
first and second rectifier inputs respectively connected to the transformer ends of the first sub-winding defining a first subsystem,
first and second rectifier inputs respectively connected to the transformer ends of the second sub-winding defining a second subsystem, and
first and second rectifier outputs, the second rectifier output being connected to the second rectifier input of the first subsystem.
One MOSFET is connected between the first rectifier input of the first subsystem and the first rectifier output and has a gate connected to the second rectifier input of the first subsystem and in series with the gate protection circuit.
The other MOSFET is connected between the first rectifier input of the second subsystem and the first rectifier output and has a gate connected to the second rectifier input of the second subsystem and in series with the gate protection circuit.
In one embodiment the control device generates two output signals independent of each other and each signal controls one of the switches.
In another embodiment the control device generates two interdependent output signals and each signal controls one of the switches.
The control device includes means for generating at least one threshold value and means for comparing the input signal proportional to the rectifier input voltage with the threshold value and the output signals are a function of the direction of the comparison between the input signal proportional to the rectifier input voltage and the threshold value.
The rectifier can include voltage measuring means including a diode connected in series with a measurement capacitor between the second rectifier input and the first rectifier output and the input signal is taken off between the diode and the measurement capacitor.
One advantage of the present invention results from the dynamic control of the gate voltage, enabling the voltage divider bridge to be short circuited or not, according to the transformer secondary voltage. Thus, whatever the amplitude of the input voltage of the self-regulated synchronous rectifier, the gate voltages of the MOSFETs are optimized to limit losses and retain an optimum switching dynamic range. Accordingly, for wide variations in the input voltage it is possible, at the same voltage, and with the same overall volume, to feed more power into a converter including a rectifier in accordance with the invention.
REFERENCES:
patent: 4893228 (1990-01-01), Orrick et al.
patent: 5144547 (1992-09-01), Masamoto
patent: 5179512 (1993-01-01), Fisher et al.
patent: 5663877 (1997-09-01), Dittili et al.
patent: 5708571 (1998-01-01), Shinada
patent: 0 665 634 A1 (1995-08-01), None
patent: 0 884 829 A1 (1998-12-01), None
Ceunebrock Patrick
Diallo Almadidi
Alcatel
Patel Rajnikant G.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wong Peter S.
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