Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-10-24
2004-07-06
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S021110, C363S021180
Reexamination Certificate
active
06760238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to the field of electric circuits. More particularly, the invention pertains to apparatus and method for DC/DC converters having high speed and high accuracy. Yet more particularly, this invention pertains to the control of the output voltage of DC/DC converters.
2. Description of Related Art
DC/DC converters are known to be used in various industries. The usage includes power supplies for computers, personal digital assistants, cellular phones and other hand held mobile electronic devices and systems. Each usage may have specific demands. Further, DC/DC converters have various types of output voltage control which utilizes pulse width modulation (PWM) type. In general, PWM type DC/DC converters can be subdivided into analog pulse width modulation control and digital pulse width modulation control.
Analog Pulse Width Modulation Control
Analog pulse width modulation control is by far the most prevalent method for controlling DC/DC Converters. A block diagram of a DC/DC converter with this type of control is shown in FIG.
1
. The control circuit blocks, which comprise the conventional analog pulse width modulation control circuit, are shown within the dotted section of FIG.
1
. The remainder of the circuitry of
FIG. 1
provides the actual DC/DC conversion function.
FIG. 1
shows a transformer isolated forward converter topology for the DC/DC converter function. The same type of control circuit, however, is used for non-isolated forward converter topology, isolated and non-isolated flyback, push pull, half bridge, full bridge, Sepic, Cuk, Weinberg, Severns, and other topologies. Although we will limit our discussion to the Isolated Forward Converter topology of
FIG. 1
, it should be understood that any other DC/DC converter topologies are controlled in the same manner.
The conventional analog control method regulates the DC Output voltage of a DC/DC converter by varying the ratio of the “on” time and “off” time of the DC/DC Converters power switch Q
1
. Control of the DC output voltage starts by using an error amplifier (U
2
) for comparing a sample of the output voltage to a voltage reference. The error amplifier outputs a voltage, which is proportional to the difference between a sample of the DC output and the voltage reference. Since the error amplifier has a very high DC gain (usually more than 10,000), a very small DC output error (less than 1 millivolt) will produce a very large change in error amplifier output voltage. The output of the error amplifier connects to the pulse width modulator circuitry comprised of a comparator U
3
, ramp generator U
4
, latch U
5
, and clock generator U
6
.
Waveforms associated with the pulse width modulation function are shown in FIG.
2
. The pulse starts when the clock generator turns on the latch. This turns on Power switch Q
1
through switch driver U
1
, and starts the flow of power through transformer T
1
, rectifier CR
1
, and output filter L
1
and C
1
.
The clock generator also starts a ramp (shown in FIG.
2
), which connects to one side of the comparator U
3
. The other leg of the comparator connects to the output of the error amplifier U
2
. The comparator U
3
changes state and turns off the latch U
5
when the ramp generator U
4
output becomes the same as the error amplifier output, which in turn shuts off transistor Q
1
. The process repeats when the clock generator U
6
turns on the latch U
5
again.
The pulse width generated by the above control circuit depends on the output of the error amplifier U
2
. If the DC output is too low, the error amplifier output increases, thereby increasing the pulse width driving transistor switch Q
1
, which increases the DC output. Conversely, if the DC output is too high, the error amplifier voltage decreases, thereby decreasing the pulse width driving transistor switch Q
1
, and decreasing the DC output. The DC output is thus accurately regulated by the action of the error amplifier U
2
.
The DC output not only needs to be accurately regulated, it also needs to be stable. DC outputs are known to possess an AC ripple component. Stability means that the DC output has an AC ripple component at the same frequency as the clock generator U
6
. In order for this to occur, for a fixed DC input voltage and for a fixed DC output load, the pulse width driving power switch Q
1
must not change from pulse to pulse,. This is shown in
FIG. 2
as a constant error amplifier output, which generates the same pulse width among the three pulses that are depicted. It is pointed out that those skilled in the state of the art should know how to choose stabilization components R
1
, C
2
, C
4
, R
3
, and C
3
so as to stabilize the DC output for a given clock generator frequency and output filter components L
1
and C
1
.
Also of importance in DC output stabilization is the choice of ramp renerator U
4
. If the ramp is fixed and unchanging, it is called voltage mode control. On the other hand, if the ramp is derived from the inductor current L
1
, it is called current mode control. The choice of stabilization components is different for these two types of ramp generators.
Analog PWM control methods for DC/DC converter have their drawbacks. One of them is the byproducts of stabilizing the DC output in that the error amplifier is slowed down. The DC output, therefore, is limited by the speed with which it can respond to a change in DC input voltage or DC output load current.
FIG. 3A
shows a typical response of the DC output voltage to a change in DC output load current. As the DC output load steps from one value to a higher value, the DC output voltage at first drops. The error amplifier eventually responds and corrects for this drop. Likewise, as shown in
FIG. 3B
, when the DC output load current steps from a higher value to a lower value, the DC output voltage first goes up, before the error amplifier responds and corrects for this increase. The speed with which the DC Output voltage corrects is called the transient response. The transient response is a complex combination of clock generator frequency, choice of output filter components, and choice of stabilization components. As a general rule, however, a well-stabilized DC/DC converter output cannot respond any faster than 50 to 100 clock generator cycles. This then is the limiting factor in the speed in which an analog pulse width modulated DC/DC converter can respond to changes in DC Input voltage or changes in DC output load current.
Digital Pulse Width Modulation Control
By digital pulse width modulation control, it is generally referred to the utilization of a voltage comparator without an error amplifier for the DC output voltage control of a DC/DC converter. Digital regulation started in the early days of transistor DC/DC switching regulators, going back to the 1970's. These regulators were called ripple regulators. A single comparator turned the transistor switch on and off based on the DC output voltage ripple. There was no clock generator or latch. The delays through the various circuit components determined the frequency of the regulators' operation.
The technique became more sophisticated, and resulted in a patent being granted in 1993 to Harry E. Wert (see U.S. Pat. No. 5,260,861). An equivalent block diagram of the DC output voltage control portion of the Wert patent is shown in FIG.
4
. Ac can be seen, the control of the DC output voltage starts by comparing a sample of the DC output voltage to a voltage reference by a voltage comparator, U
2
.
Waveforms associated with the pulse width modulation function are shown in FIG.
5
. The pulse starts when the clock generator U
4
turns on the latch U
3
. This turns on Power switch Q
1
through switch driver U
1
, and starts the flow of power through transformer T
1
, rectifier CR
1
, and output filter L
1
and C
1
.
The comparator U
2
changes state and turns off the latch U
3
when the sampled DC output becomes the same as the Voltage reference U
5
, which in turn shuts off transistor Q
1
. The process re
BC Systems, Inc
Brown & Michaels PC
Laxton Gary L.
Sherry Michael
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