Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With rectifier
Reexamination Certificate
2000-11-28
2004-08-03
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With rectifier
C324S701000, C363S021060
Reexamination Certificate
active
06771059
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to AC-to-DC converter systems. More particularly, this invention relates to an improved controller circuit design and configuration for use with synchronous rectification to achieve high power conversion efficiency.
2. Description of the Prior Art
Conventional art of design and manufacture of AC to DC converter systems is not able to satisfy the more advancing requirements now imposed by the high performance server technology. These requirements include smaller size, lower fan noise, high reliability, low cost and low power consumption. The power supply systems are now employed in the computer industries to convert various AC voltages ranging from one hundred to two hundred and forty volts to regulated DC voltages of 3.3, 5, 12 and −12 volts. Specifically, the difficulties arise from the facts that these requirements appear to constrain the designs of the power supply systems in opposite directions. On the one-hand the power supply is expected to produce more output power and be more reliable. On the other hand, the power supply is constrained by seemingly contradictory requirements that the system be made smaller, quieter, and cheaper. One of the best ways to satisfy these requirements is to increase the efficiency of the power supply system. This is because efficiency improvement would lead to reduction of heat generation thus allow for smaller size of a power supply to operate at a lower temperature that would increase the reliability and meanwhile require less noise generated by fans for heat-dissipation. Although, there may be a concern that a system designed for higher efficiency tends to be more complex, and this increases the production cost, such concerns are likely offset by the follow-on savings in heat dissipation, package, shipping, and cost reductions resulted from lower power consumption.
There are a number of ways to increase the efficiency of a power supply. A method is to reduce the losses in the output rectifier of a converter since these losses are relatively large compared to other losses. In a high frequency power converter as that shown in
FIG. 1A
, the standard devices for rectifying an output voltage of three to five volts are schottky diodes D
1
and D
2
.
FIG. 1A
is a generic representation of a basic forward converter showing a conventional circuit configuration of forward switching converter for a power supply system operated with a pulse width modulator controlling a main switching transistor Q
1
at the primary side. The Pulse Width Modulator is any one of many commercial integrated circuits, which can modulate a pulse width duty cycle based upon a feedback signal. Its output is an approximately 0 to 12V pulse waveform at a fixed frequency, e.g., a frequency of 100 KHz. This waveform drives the main switching transistor Q
1
. Transistor Q
1
acts as a power switch under control of the Pulse Width Modulator output. With a rectified input voltage source 400 volts DC, the waveform appearing at the output (drain) of Q
1
has a peak value of about 400 Volts or a peak-to-peak value of 800 Volts. The primary side is coupled to the secondary side with a transformer T
1
with the secondary side provided with rectifying diodes D
1
functioning as a forward output diode and D
2
as a freewheel output diode. The output load is coupled in series with an output filter inductor and in parallel to an output filter capacitor. For a typical 5 Volt DC output, the transformer has a turns ratio defined by Np:Ns of about 15:1 where Np is the number of turns of the primary side and Ns is the number of turns of the secondary side. The peak voltage into the anode of D
1
is around 25 Volts. When Q
1
is on, D
2
is reverse biased and D
1
is forward biased (the anode is positive relative to the cathode). During this time, a positively sloped output current flows through D
1
and L
1
to the output load. L
1
(output filter inductor) stores most of the transformer output energy pulse to produce a ramping current which is usually continuous. When Q
1
is off, D
1
is reverse biased and L
1
maintains a negatively-ramping current by forward-biasing D
2
as it discharges some of its stored energy. Except for resistive losses, the average voltage across C
1
is identical to the average voltage across D
2
. C
1
serves to filter the periodic and random perturbations from the DC output voltage so as to reduce them to acceptable levels.
As that shown in
FIG. 1A
, schottky diodes often cause a forward voltage drop of about 0.6 volts. Even if the power supply has no other losses, the voltage drops caused by the shottky diodes represent about ten to fifteen percent efficiency loss for a three to five volt output. In order to compensate for these losses, higher power is required prior to a rectification action taken by the schottky diodes. A higher power processed by prior stages of the power supply system tends to increase losses further during during these prior-stage-processing functions. As a result, the losses are compounded and the total efficiency losses are significantly increased. Conversely, the total power savings achieved by improving the output rectifier efficiency tend to have a reverse effect of compounding the improvement of the efficiency for the entire power supply system. There are methods to incrementally minimize the losses of the schottky diodes by optimizing the transformer and choosing the best schottky diodes. However, improvements of power losses achievable by using better schottky diodes are quite limited. Under the circumstances when a small performance improvement is required, better schottky diodes would generally be sufficient to satisfy the requirement. But when compared to another technique of applying a synchronous rectification, even the best schottky diodes would come short of matching the performance when synchronous rectification is employed for AC to DC conversion.
An effective method to increase the output rectifier efficiency is by implementing a controlled switch to achieve synchronous rectification. In the recent past, synchronous rectification was considered too exotic for commercial applications. The device most commonly used for the controlled switch is a MOSFET. Advancement in semiconductor technology has improved the cost/performance of the MOSFETs, and the power supply industry now begun to use synchronized rectification for performance improvement as the improvements achievable by the schottky are not sufficient to meet the demand of higher performance. The most commonly available switch is an n-channel MOSFET transistor that has an operation characteristic of providing a blocking voltage when the drain is positive relative to the source and the gate is at a zero or negative potential relative to the source. Due to an inherent drain-to body diode, there will be always a current even under a negative gate biased condition when the drain is negative relative to the source. This is normally considered as an undesirable feature for typical applications of the n-channel MOSFET. However, by providing a positive voltage to the gate, e.g., 10 volts relative to the source to turn on the n-channel MOSFET, the n-channel MOSFET will conduct a current with a very low voltage drop. This occurs regardless of the polarity of the voltage applied to the drain relative to the source. The MOSFET transistor thus provides an operation characteristic that is useful to function as a very efficient rectifier. Specifically, the rectifying function is achieved by adjusting the gate-source voltage to negative or zero to prevent a reverse current. And, conversely to generate a low voltage-drop conducting condition by adjusting the gate-source voltage to positive to provide a rectified current. The method is however depends on proper synchronization of the gate voltage to the variations of the relative source-drain potential.
FIG. 1B
shows a conventional synchronous rectified converter where the synchronization control signal is generated from the primary side. Spe
Deb Anjan K.
Delta Electronics , Inc.
Lin Bo-In
LandOfFree
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