Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Rectangular or pulse waveform width control
Reexamination Certificate
2000-05-16
2001-05-08
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Rectangular or pulse waveform width control
C327S143000, C327S531000, C327S544000
Reexamination Certificate
active
06229366
ABSTRACT:
BACKGROUND
1. Field of the Invention
The field of the present invention pertains to the field of power supplies and among other things to the regulation of power supplies.
2. Background of the Invention
Power supplies that convert an AC mains voltage to a DC voltage for use by integrated electronic devices, amongst other devices, are known. The power supplies are required to maintain the output voltage, current or power within a regulated range for efficient and safe operation of the electronic device. Switches that operate according a pulse width modulated control to maintain the output voltage, current, or power of the power supply within a regulated range are also known. These switches utilize an oscillator and related circuitry to vary the switching frequency of operation of the switch, and therefore regulated the power, current or voltage that is supplied by the power supply.
A problem with utilizing pulse width modulated switches is that they operate at a relatively high frequency compared to the frequency of the AC mains voltage, which results in a high frequency signal being generated by the power supply. This high frequency signal is injected back into the AC mains input and becomes a component of the AC mains signal. The high frequency signals are also radiated by the power supply as electromagnetic waves. These high frequency signals add to the Electromagnetic Interference (EMI) of the power supply, and in fact are the largest contributors to the EMI of the power supply. The EMI generated by the power supply can cause problems for communications devices in the vicinity of the power supply and the high frequency signal which becomes a component of the AC mains signal will be provided to other devices in the power grid which also causes noise problems for those devices. Further, the radiated EMI by the power supply can interfere with radio and television transmissions that are transmitted over the air by various entities.
To combat the problem of EMI, several specifications have been developed by the Federal Communications Commission (FCC) in the United States and the European Community (EC) have established specification that specify the maximum amount of EMI that can be produced by classes of electronic devices. Since power supplies generate a major component of the EMI for electronic devices, an important step in designing a power supply is minimizing the EMI provided by the power supply to levels with the acceptable limits of the various standards. Since, a power supply can be utilized in many different countries of the world, the EMI produced should be within the most stringent limits worldwide to allow for maximum utilization of the power supply.
A known way of minimizing the EMI provided by the power supply is by adding an EMI filter to the input of the power supply. An EMI filter generally utilizes at least one inductor, capacitor and resistor in combination. However, the greater EMI produced by the power supply the larger the components that are utilized as part of the EMI filter. The cost of the EMI filter is in large part determined by the size of the inductor and capacitor utilized. The longer the components, the higher the cost of the power supply. Further, simply utilizing an EMI filter does not address the radiated EMI.
Another problem associated with pulse width modulated switches results from operation of the power supply at start up. At start up, the voltage, current and power at the output of the power supply will essentially be zero. The pulse width modulated switch will then conduct for the maximum possible amount of time in each cycle of operation. The result of this is a maximum inrush current into the power supply. The maximum inrush current is greater than the current that is utilized during normal operation of the power supply. The maximum inrush current stresses the components of power supply and switch. Stress is specifically a problem for the switch, or transistor, the transformer of the power supply, and the secondary side components of the power supply. The stress caused by the maximum inrush current decreases the overall life of the power supply and increases the cost of the power supply because the maximum rating of the components used in the power supply to not destruct from the inrush currents will be greater than the maximum rating required for normal operation.
Further, when the pulse width modulated switch conducts for the maximum possible amount of time in each cycle of operation the voltage, current and power at the output of the power supply rise rapidly. Since the feedback circuit of the power supply often does not respond as fast as the operating frequency of the switch, the rapid rise of the voltage, current and power will often result in an overshoot of the maximum voltage in the regulation range which will cause damage to the device being supplied power by the power supply.
Referring to
FIG. 1
a known power supply that attempts to minimize EMI and reduce startup stress is depicted. A rectifier
10
rectifies the filtered AC mains voltage
5
, from EMI filter
120
, input by the AC mains to generate a rectified voltage
15
. Power supply capacitor
20
then generates a substantially DC voltage with a ripple component. The rectified voltage
15
with ripple component is provided to the primary winding
35
of transformer
40
that is used to provide power to secondary winding
45
. The output of secondary winding
45
is provided to secondary rectifier
50
and secondary capacitor
55
that provide a secondary DC voltage
60
at the power supply output
65
to the device that is coupled to the power supply.
In order to maintain the secondary DC voltage within a regulate range a feedback loop including an optocoupler
70
, zener diode
75
and a feedback resistor
80
provides a signal indicative of the voltage at the power supply output
65
to feedback pin
85
of pulse width modulated switch
90
. The voltage magnitude at the feedback terminal is utilized to vary the duty cycle of a switch coupled between the drain terminal
95
and common terminal
100
of the pulse width modulated switch
90
. By varying the duty cycle of the switch the average current flowing through the primary winding and therefore the energy stored by the transformer
40
which in turn controls the power supplied to the power supply output
65
is kept within the regulated range. A compensation circuit
105
is coupled to the feedback pin
85
in order to lower the bandwidth of the frequency of operation of the pulse width modulator.
Inrush currents are minimized at start up by use of soft start capacitor
110
. Soft start functionality is termed to be a functionality that reduces the inrush currents at start up. At this instant a current begins to flow through feedback resistor
80
and thereby into soft start capacitor
110
. As the voltage of soft start capacitor
110
increases slowly, current will flow through light emitting diode
115
of optocoupler
70
thereby controlling the duty cycle of the switch. Once the voltage of the soft start capacitor
110
reaches the reverse breakdown voltage of zener diode
75
current will flow through zener diode
75
. The approach described above will reduce the inrush currents into the power supply, however, it will be several cycles before the light emitting diode
115
will begin conducting. During the several cycles the maximum inrush current will still flow through the primary winding and other secondary side components. During these cycles the transformer may saturate, and therefore the transformer may have to be designed utilizing a higher core size than would be required for normal operation even with the use of soft start capacitor as in FIG.
1
.
To reduce the EMI output by the power supply an EMI filter
120
is utilized. Additionally, pulse width modulated switch
90
is equipped with frequency oscillation terminals
125
and
130
. Frequency oscillation terminal
125
and
130
receive ajitter current
135
that varies according to the ripple component of substantially DC voltage
25
. The ji
Balakirshnan Balu
Djenguerian Alex
Lund Leif
Blakely , Sokoloff, Taylor & Zafman LLP
Power Integrations, Inc.
Zweizig Jeffrey
LandOfFree
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