Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-09-11
2004-07-06
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S049000
Reexamination Certificate
active
06760235
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally related to control and operation of power converter devices, and, more particularly, to circuits and techniques that improve the startup performance of power converters using synchronous rectifiers.
DC/DC power converter devices are widely used in numerous applications, such as telecommunications and networking applications. A dc/dc converter is an electronics device that converts a raw dc (direct current) voltage input, usually with a certain variation range, to a dc voltage output that meets a set of specifications. With fast-growing technologies used in telecommunications equipment, the demands on the power density and conversion efficiency of dc/dc converters continue to increase. The converter typically includes a transformer, having primary and secondary windings wound around a common magnetic core. By opening and closing the main power switches for appropriate intervals, control over the energy transfer between the input and output is accomplished. The transformer provides an alternating voltage whose amplitude can be adjusted by changing the number of turns of the primary and secondary windings. Moreover, the transformer provides DC isolation between the input and the output of the converter. However, a transformer is not required in a non-isolated converter.
One of the most common DC/DC converter topologies is the forward converter. When the primary winding of the forward converter is energized by closing the primary switch, energy is immediately transferred to the secondary winding. Synchronous rectifier circuits are used in forward converters, as well as in flyback converters, buck converters, push-pull converters, and half-bridge converters, among others. In switching power supply circuits employing synchronous rectifiers, the diodes are replaced by power transistors to obtain a lower on-state voltage drop. The synchronous rectifier generally uses n-channel MOSFETs rather than diodes to avoid the turn on voltage drop of diodes that can be significant for low output voltage power supplies. The transistors are biased to conduct from source-to-drain (for an n-channel power MOSFET) when a diode would have been conducting from anode to cathode, and conversely, are gated to block voltage from drain-to-source when a diode would have been blocking from cathode to anode. Although MOSFETs usually serve this purpose, bipolar transistors and other semiconductor switches as also suitable.
In these synchronous rectifier circuits, the gate signals can be self-driven, i.e., the gate signal can be tied directly to the power circuit, or controlled-driven, i.e., the gate signal is derived from some point in the circuit and goes through some processing circuit before being fed to the MOSFET gate driver. In a power converter, the synchronous rectifier which conducts during the non-conducting period of the main power switch(switches) is called a freewheeling synchronous rectifier. The gate drive signal to a freewheeling synchronous rectifier plays a very important role in the startup process of a converter.
FIG. 1
shows conventional synchronous rectifiers in a forward converter
10
. In this example, a DC voltage input Vin is connected to the primary winding of the power transformer by a MOSFET power switch Q
1
. A clamp circuit arrangement is also provided to limit the reset voltage. The MOSFET power switch Q
1
is shunted by a series connection of a clamp capacitor Creset and a MOSFET switch device Q
2
. The conducting intervals of Q
1
and Q
2
are mutually exclusive. The voltage inertia of the capacitor Creset limits the amplitude of the reset voltage appearing across the magnetizing inductance during the non-conducting interval of MOSFET power switch Q
1
.
The secondary winding is connected to an output lead through a synchronous rectifier including MOSFET rectifying devices SR
1
and SR
2
. Each rectifying device includes a body diode. With the power switch Q
1
conducting, the input voltage is applied across the primary winding. The secondary winding is oriented in polarity to respond to the primary voltage with a current flow through inductor Lo, the load connected to the output lead and back through the MOSFET rectifier device SR
1
to the secondary winding. Continuity of the current flow in the inductor Lo when the power switch Q
1
is non-conducting is maintained by the current path provided by the conduction of the MOSFET rectifier device SR
2
. An output filter capacitor Co shunts the output of the converter.
Conductivity of the two rectifier devices SR
1
and SR
2
is controlled by the gate drive signals provided by the primary PWM (pulse-width modulated) control of switch Q
1
. The control signal to SR
1
and SR
2
can be derived from various ways, such as signals coupled from the power transformer T or other mechanisms that carry the primary PWM timing information. PWM includes, for example, an oscillator, a comparator, and a flip-flop. The output of the PWM provides a PWM drive signal.
In order to prevent transformer saturation and excessive heating or failure of the switch Q
1
during startup, the PWM drive signal on the primary switch Q
1
usually goes through a soft-start process. During a soft-start, the pulse-width of the gate signal to Q
1
gradually increases from a very small duty-ratio to its steady-state duty-ratio. Since the drive signal of the freewheeling synchronous rectifier SR
2
is by and large complementary to that of primary switch Q
1
, its duty ratio starts high and gradually reduces over the soft-start process. Consequently, during a startup, especially if the output has a pre-existing voltage (pre-bias) (which could be from other power sources in the system) before the converter starts, the large duty-ratio of SR
2
will build a negative current in the output inductor Lo, which may cause the output voltage to drop, and further resulting in disturbance to other voltages that are coupled to this output. This output voltage drop and the disturbance to other voltages may be unacceptable to some of the loads connected to these voltages.
Therefore, it would be desirable to control the drive signal to the rectifier device SR
2
during this startup process to address the above problem.
BRIEF SUMMARY OF THE INVENTION
Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof a circuit for controlling switching circuitry in a power converter device during a soft-start process wherein the pulse-width of a gate drive signal to a main switch of the power converter device gradually increases from a minimum duty-ratio to a steady-state duty-ratio. The circuit includes synchronous rectifier control circuitry adapted to gradually apply a gate drive signal to a freewheeling synchronous rectifier of the switching circuitry during the soft-start process by either controlling the amplitude or pulse-width of the gate drive signal to the freewheeling synchronous rectifier.
The synchronous rectifier control circuitry includes gate clamping circuitry. In one aspect of the invention, the gate clamping circuitry includes a diode series coupled to a resistor paralleled to a capacitor to provide voltage clamping of the gate drive signal applied to the freewheeling synchronous rectifier. In another aspect of the invention, the gate clamping circuitry comprises a transistor series coupled to a resistor paralleled to a capacitor to provide voltage clamping of the gate drive signal applied to the freewheeling synchronous rectifier. A diode is optionally series coupled with an emitter or base of the transistor to block the voltage when the freewheeling rectifier is gated low. The common of the gate clamping circuitry may also be coupled to a negative voltage potential. Resistors and capacitors may be provided in the main current path of the gate drive circuit of the synchronous rectifier.
The present invention further fulfills the foregoing needs by providing in another aspect thereof, a method for controlling switching circuitry in a power converter device during a soft-start pr
Jiang Yimin
Lin Feng
Mao Hengchun
Sun Ning
Beusse Brownlee Wolter Mora & Maire
McLeod Christine Q.
Netpower Technologies, Inc.
Riley Shawn
LandOfFree
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