Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-09
2003-09-09
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021060, C363S021140, C363S026000, C363S065000, C363S089000
Reexamination Certificate
active
06618274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed to a power system and more particularly to a method and apparatus for controlling a synchronous rectifier power converter.
2. Description of Related Art
In order to meet the ever-increasing demand for high speed and miniaturization of digital devices, microelectronic circuit voltage levels have been dropping. No longer are 5 V and 12 V power supplies dominant, but rather 3.3 V, 2.5 V, 1.8 V, and 1.5 V and others are becoming increasingly common as the standard voltages in many electronic devices. Traditional 5V and 12V power supplies typically use diodes to rectify secondary AC voltage to a DC voltage. These supplies allow the output current on the secondary side to “freewheel” during the time that the power switches on the primary side are off. However, as output voltage decreases, the power loss incurred in the rectifier diodes becomes very large compared to the output power. For example, using 0.5 V Schottky diodes in a 1V output power supply results in a power loss of approximately 33% of the output power in the rectifier circuits alone.
Further, high-power density is crucial in applications where the space for the power supply relative to the power output is limited. Thus, there is an ongoing quest to develop power supplies with increased density. Limiting a power supply to a small area requires that the power supply be efficient because the heat transfer capability decreases as the overall size of the power supply decreases. To achieve higher efficiency, synchronous rectifiers are increasingly used in low output voltage DC to DC converters. One with skill in the art understands that the use of a synchronous rectifier field effect transistor (“FET”) greatly improves the DC to DC conversion efficiency. The advantage of the synchronous rectifier FET is the very low “on resistance” of current FETs which can be even further reduced by paralleling multiple devices.
Although synchronous rectifiers are much more efficient than diode rectifiers at today's lower voltage levels, they are not without their drawbacks. One of the major problems is caused by the bi-directional nature of current flow in a synchronous rectifier. During startup, light-load, or shutdown conditions, for example, a significant reverse power flow may occur when several power modules using synchronous rectifiers are connected in parallel for highly reliable microelectronic systems or when a pre-bias voltage is applied to the output of the power supply. This phenomenon, which is well known in the art, can bring down the output bus voltage, thus causing malfunction or shutdown to downstream voltage-sensitive electronic devices.
For DC to DC converters using synchronous rectifiers that are connected in a parallel configuration, the bi-directional power flow characteristics can result in a very undesirable operating condition in which one converter drives the output of the other converter. When one or more converters are operating in this reverse power processing mode, the overall power system can be circulating a significant amount of current while actually delivering very little current to the converter output load. This results in an undesirably high power dissipation within the converters even under light-load or no load conditions.
Because of this effect, turn-on and startup transients have become major concerns in systems where two or more DC to DC converters employing synchronous rectifier FETs are connected in parallel without any or-ing diodes. If proper control of the synchronous rectifiers is not used in such a system, one of the converters may behave as a load, sinking current from the other converter, even under a no load condition. Such a system is inefficient and may result in abnormal startup of the system. The system transient response could also be detrimentally affected during the converter's transition from the reverse power processing mode to a forward processing mode.
In response to this problem, several solutions have been proposed. Eng (U.S. Pat. No. 6,101,104) proposes a control method and apparatus that senses the voltage across the synchronous rectifier, and turns the synchronous rectifier off when the voltage across the rectifier is about to change polarity. Ideally the synchronous rectifier is turned off at the approximate instant that the voltage across the rectifier reaches approximately zero. Thus, current flow in the reverse direction is avoided in the synchronous rectifier FET. Brkovic (U.S. Pat. No. 5,940,287) proposes a transient response network including a synchronous rectifier controller that senses the state of the power switch and then disables the synchronous rectifier device when the power switch has remained in a non-conducting state for a specified period of time. Boylan and Rozman (U.S. Pat. Nos. 6,038,154 and 6,191,964 B1) have proposed a control circuit for operating a synchronous rectifier in both a bi-directional mode and a uni-directional mode of operation as a function of characteristics of the power system employing the synchronous rectifiers. Based on the characteristic sensed, the control circuitry switches between the bi-directional and uni-directional mode of operation by enabling or disabling the synchronous rectifier FET. In the bi-directional mode, the control circuitry switches the synchronous rectifier FETs to rectify the substantially alternating current as well as the free-wheeling current. Bi-directional current flow is possible in this mode of operation. In the uni-directional mode, the synchronous rectifier FETs are disabled to act as a conventional diode rectifier (due to the intrinsic body diode of the FET), allowing only uni-directional current and thereby preventing reverse power flow because the diode only conducts in one direction.
Referring now to
FIG. 1
, a schematic diagram of a prior art clamped-mode forward converter circuit
100
with a synchronous rectifier circuit
130
is illustrated. This circuit is described in more detail in U.S. Pat. No. 6,191,964 B1 to Boylan et al., issued Feb. 20, 2001, entitled “Circuit and Method for Controlling a Synchronous Rectifier Converter,” and incorporated herein by reference as if fully set forth at length. The clamped-mode forward converter circuit
100
and its advantages are discussed in U.S. Pat. No. 5,303,138 to Rozman, issued on Apr. 12, 1994, entitled “Low Loss Synchronous Rectifier for Application to Clamped-Mode Power Converters,” and incorporated herein by reference as if fully set forth at length.
The clamped-mode forward converter circuit
100
comprises a voltage input V
IN
connected to a primary winding
110
of a power transformer by a power switch (e.g., MOSFET) Q
1
. The power switch Q
1
is shunted by series connection of a clamp capacitor Cclamp and a power switch Q
2
. The conducting intervals of the power switch Q
1
and the power switch Q
2
are mutually exclusive. The duty cycle of the power switch Q
1
is D and the duty cycle of the power switch Q
2
is 1-D.
A secondary winding
135
of the power transformer is connected to an output filter capacitor C
out
through an output filter inductor Lout and the synchronous rectifier circuit
130
, providing a substantially alternating current input to the synchronous rectifier
130
. The synchronous rectifier circuit
130
comprises control circuitry
150
and switching circuitry. A rectifying synchronous rectifier device SR
1
and a freewheeling synchronous rectifier device SR
2
comprise the switching circuitry. The switching circuitry may be realized with any suitable rectifier devices, although a low R
DS(on)
n-channel MOSFET is suitable for such applications. A diode D
1
and a diode D
2
are discrete devices placed in parallel with the synchronous rectifier devices SR
1
, SR
2
, respectively. However, the diodes D
1
, D
2
may represent an intrinsic body diode of a n-channel MOSFET.
A current sensing device
165
encompasses either a current shunt connected in series with the output, or a Hall effect current sense device in series wit
Boylan Jeffrey John
Chen Qing
He Jin
Hilburn Del Ray
Carstens David W.
Carstens Yee & Cahoon LLP
Innoveta Technologies
Sterrett Jeffrey
LandOfFree
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