Power Converter with Reduced Power Dissipation

Abstract

A power converter with reduced power dissipation at light loads and method of operating the same. In one embodiment, the power converter includes an opto-isolator circuit configured to produce an output signal dependent on an output characteristic of the power converter. The power converter also includes a controller configured to control the output characteristic to a first regulated value when the output signal is greater than or equal to a threshold level, and control the output characteristic to a second regulated value less than the first regulated value when the output signal is less than the threshold level.

Claims

1 . A power converter, comprising: an opto-isolator circuit configured to produce an output signal dependent on an output characteristic of said power converter; and a controller configured to: control said output characteristic to a first regulated value when said output signal is greater than or equal to a threshold level, and control said output characteristic to a second regulated value less than said first regulated value when said output signal is less than said threshold level. 2 . The power converter as recited in claim 1 wherein said second regulated value comprises a level sufficient for said output characteristic to power said opto-isolator circuit. 3 . The power converter as recited in claim 1 wherein said controller is configured to control said output characteristic to said second regulated value in accordance with a voltage of a winding of a transformer. 4 . The power converter as recited in claim 3 wherein said voltage of said winding of said transformer is sensed with a diode and resistor-capacitor network. 5 . The power converter as recited in claim 1 further comprising a startup circuit configured to power said controller to initiate a switching action for a switch during a startup interval, said controller configured to terminate an operation of said startup circuit after said switching action for said switch commences. 6 . The power converter as recited in claim 5 wherein said operation of said startup circuit is configured to be reinitiated after a delay period when said switching action for said switch terminates. 7 . The power converter as recited in claim 5 wherein said operation of said startup circuit is configured to be reinitiated in accordance with said output signal from said opto-isolator circuit. 8 . The power converter as recited in claim 1 wherein said output signal is an inverted sense of said output characteristic. 9 . The power converter as recited in claim 1 wherein said controller is configured to disable a switching frequency of said power converter dependent on said output characteristic or another output characteristic of said power converter. 10 . The power converter as recited in claim 9 wherein said controller is turned off when said switching frequency of said power converter is disabled. 11 . A method, comprising: producing an output signal dependent on an output characteristic of a power converter; controlling said output characteristic to a first regulated value when said output signal is greater than or equal to a threshold level, and controlling said output characteristic to a second regulated value less than said first regulated value when said output signal is less than said threshold level. 12 . The method as recited in claim 11 wherein said second regulated value comprises a level sufficient for said output characteristic to power a circuit for producing said output signal. 13 . The method as recited in claim 11 wherein controlling said output characteristic to said second regulated value is performed in accordance with a voltage of a winding of a transformer. 14 . The method as recited in claim 13 wherein said voltage of said winding of said transformer is sensed with a diode and resistor-capacitor network. 15 . The method as recited in claim 11 further comprising powering a start up circuit for an initiation of a switching action for a switch during a startup interval, and terminating an operation of said startup circuit after said switching action for said switch commences. 16 . The method as recited in claim 15 further comprising reinitiating said operation of said startup circuit after a delay period when said switching action for said switch terminates. 17 . The method as recited in claim 15 further comprising reinitiating said operation of said startup circuit in accordance with said output signal from an opto-isolator circuit. 18 . The method as recited in claim 11 wherein said output signal is an inverted sense of said output characteristic. 19 . The method as recited in claim 11 further comprising disabling a switching frequency of said power converter dependent on said output characteristic or another output characteristic of said power converter. 20 . The method as recited in claim 19 wherein said method is disabled when said switching frequency of said power converter is disabled.
TECHNICAL FIELD [0001] The present invention is directed, in general, to power electronics and, more specifically, to a power converter with reduced power dissipation at light loads. BACKGROUND [0002] A switched-mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”). [0003] Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter. The duty cycle “D” is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value. [0004] Power converters designed to operate at low power levels typically employ a flyback power train topology to achieve low manufacturing cost. A power converter with a low power rating designed to convert ac mains voltage to a regulated dc output voltage to power an electronic load such as a printer, modem, or personal computer is generally referred to as a “power adapter” or an “ac adapter.” [0005] Power conversion efficiency for power adapters has become a significant marketing criterion, particularly since the publication of recent U.S. Energy Star specifications that require a power conversion efficiency of power adapters for personal computers to be at least 50 percent at very low levels of output power. The “One Watt Initiative” of the International Energy Agency is another energy saving initiative to reduce appliance standby power to one watt or less. These efficiency requirements at very low output power levels were established in view of the typical load presented by a printer in an idle or sleep mode, which is an operational state for a large fraction of the time for such devices in a home or office environment. A challenge for a power adapter designer is to provide a high level of power conversion efficiency (i.e., a low level of power adapter dissipation) over a wide range of output power. [0006] Numerous strategies have been developed to reduce manufacturing costs and increase power conversion efficiency of power adapters over a wide range of output power levels, including the incorporation of a burst operating mode at very low output power levels. Other strategies include employing an energy-recovery snubber circuit or a custom integrated controller, and a carefully tailored specification. Each of these approaches, however, provides a cost or efficiency limitation that often fails to distinguish a particular vendor in the marketplace. Thus, despite continued size and cost reductions of components associated with power conversion, no satisfactory strategy has emerged to reduce power converter dissipation at low load currents. [0007] Accordingly, what is needed in the art is a circuit and related method for a power converter that enables a further reduction in manufacturing cost while reducing power converter power dissipation, particularly at low load currents, that does not compromise end-product performance, and that can be advantageously adapted to high-volume manufacturing techniques for power adapters and other power supplies employing the same. SUMMARY OF THE INVENTION [0008] These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by advantageous embodiments of the present invention, including a power converter with reduced power dissipation at light loads and method of operating the same. In one embodiment, the power converter includes an opto-isolator circuit configured to produce an output signal dependent on an output characteristic of the power converter. The power converter also includes a controller configured to control the output characteristic to a first regulated value when the output signal is greater than or equal to a threshold level, and control the output characteristic to a second regulated value less than the first regulated value when the output signal is less than the threshold level. [0009] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS [0010] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: [0011] FIGS. 1 and 2 illustrate diagrams of embodiments of a power adapter including a power converter constructed according to the principles of the present; [0012] FIG. 3 illustrates a graphical representation of an exemplary output signal Iopto from an opto-isolator circuit verses an output characteristic according to the principles of the present invention; and [0013] FIG. 4 illustrates a schematic diagram of an embodiment of a power adapter including a power converter constructed according to the principles of the present invention. [0014] Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated, and may not be redescribed in the interest of brevity after the first instance. The FIGUREs are drawn to illustrate the relevant aspects of exemplary embodiments. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS [0015] The making and using of the present exemplary embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. [0016] The present invention will be described with respect to exemplary embodiments in a specific context, namely, a power adapter including a power converter operable at low load currents with reduced power dissipation. While the principles of the present invention will be described in the environment of a power converter, any application that may benefit from operation at low load with reduced power dissipation including a power amplifier or a motor controller is well within the broad scope of the present invention. [0017] Turning now to FIG. 1 , illustrated is a schematic diagram of an embodiment of a power adapter including a power converter constructed according to the principles of the present invention. The power adapter is configured to convert ac mains voltage to a regulated dc output voltage V out . A power train (e.g., a flyback power train) of the power converter (also referred to as a “flyback power converter”) includes a power switch Q main coupled to a source of electrical power (e.g., an ac mains 110 ) via an electromagnetic interference (“EMI”) filter 120 , and an input filter capacitor C in to provide a substantially filtered dc input voltage V in to a magnetic device (e.g., an isolating transformer or transformer T 1 ). Although the EMI filter 120 illustrated in FIG. 1 is positioned between the ac mains 110 and a bridge rectifier 130 , the EMI filter 120 may contain filtering components positioned between the rectifier 130 and a transformer T 1 . The transformer T 1 has primary winding N p and secondary winding N s with a turns ratio that is selected to provide the output voltage V out with consideration of a resulting duty cycle and stress on power train components. [0018] The power switch Q main (e.g., an n-channel field-effect transistor) is controlled by a controller (e.g., a pulse-width modulator (“PWM”) controller 140 ) that controls the power switch Q main to be conducting for a duty cycle. The power switch Q main conducts in response to gate drive signal V G produced by the controller 140 with a switching frequency (often designated as “f s ”). The duty cycle is controlled (e.g., adjusted) by the controller 140 to regulate an output characteristic of the power converter such as an output voltage V out , an output current I out , or a combination thereof. A feedback path (a portion of which is identified as 150 ) enables the controller 140 to control the duty cycle to regulate the output characteristic of the power converter. A circuit isolation element, opto-isolator 180 , is employed in the feedback path 150 to maintain input-output isolation of the power converter. The ac voltage or alternating voltage appearing on the secondary winding N s of the transformer T 1 is rectified by an auxiliary power switch (e.g., diode D 1 or, alternatively, by a synchronous rectifier, not shown), and the dc component of the resulting waveform is coupled to the output through the low-pass output filter including an output filter capacitor C out to produce the output voltage V out . The transformer T 1 is also formed with a third winding (e.g., a bias winding) N bias that may be employed to produce an internal bias voltage for the controller 140 employing circuit design techniques well known in the art. The internal bias voltage produced by the third winding N bias is a more efficient process than an internal bias voltage produced by a bias startup circuit or startup circuit that typically employs a resistor with a high resistance coupled to the filtered dc input voltage V in to bleed a small, bias startup current therefrom. [0019] During a first portion of the duty cycle, a current I pri (e.g., an inductor current) flowing through the primary winding N p of the transformer T 1 increases as current flows from the input through the power switch Q main . During a complementary portion of the duty cycle (generally co-existent with a complementary duty cycle 1 -D of the power switch Q main ), the power switch Q main is transitioned to a non-conducting state. Residual magnetic energy stored in the transformer T 1 causes conduction of a current I sec through the diode D 1 when the power switch Q main is off. The diode D 1 , which is coupled to the output filter capacitor C out , provides a path to maintain continuity of a magnetizing current of the transformer T 1 . During the complementary portion of the duty cycle, the magnetizing current flowing through the secondary winding N s of the transformer T 1 decreases. In general, the duty cycle of the power switch Q main may be controlled (e.g., adjusted) to maintain a regulation of or regulate the output voltage V out of the power converter. [0020] In order to regulate the output voltage V out , a value or a scaled value of the output voltage V out is typically compared with a reference voltage using an error amplifier (e.g., in output voltage controller 160 ) to control the duty cycle. The error amplifier in the output voltage controller 160 controls a current in a light-emitting diode (“LED”) of the opto-isolator 180 (similar to output voltage controller 260 as described hereinbelow with reference to FIG. 2 ). The controller 140 converts a resulting current produced in a transistor of the opto-isolator 180 to control the duty cycle. This forms a negative feedback arrangement to regulate the output voltage V out to a (scaled) value of the reference voltage. A larger duty cycle implies that the power switch Q main is closed for a longer fraction of the switching period of the power converter. Thus, the power converter is operable with a switching cycle wherein an input voltage V in is coupled to the transformer T 1 for a fraction of a switching period by the power switch Q main controlled by controller 140 . [0021] Turning now to FIG. 2 , illustrated is a block diagram of an embodiment of a power adapter including a power converter constructed according to the principles of the present invention. The power converter is advantageously formed to draw very low power from an input voltage source at very low output current. For instance, the power converter formed with the architecture illustrated in FIG. 2 can potentially draw one milliwatt or less when its load is substantially disabled. [0022] The power converter is coupled to a rectifier and input filter 210 that are coupled to ac mains 110 to provide a substantially filtered dc input voltage V in . A controller 230 controls a switching action for a power switch coupled to transformer (such as power switch Q main and transformer T 1 illustrated in FIG. 1 but not shown in FIG. 2 ) of a power train 240 to control an output characteristic of the power converter such as its dc output voltage V out . [0023] Initially, the controller 230 is in a nonoperational state during a startup interval due to absence of an internal bias voltage. A bias voltage startup circuit or startup circuit 220 is coupled to the dc input voltage V in to produce a startup bias voltage on lead 282 during the startup interval, which is sufficient to initiate operation of the controller 230 . Once the controller 230 has initiated its operation, which includes starting a switching action of at least one power switch, the controller 230 is able to efficiently produce its own internal bias voltage from an ac voltage produced across a transformer winding, such as transformer winding N bias , illustrated in FIG. 1 , of the transformer. This action is signaled to the bias voltage startup circuit 220 to disable production of the startup bias voltage, which is an inefficient process to produce a bias voltage, and thereby terminate the startup interval. Alternatively, initiation of operation of the controller 230 can be signaled on lead 280 to the bias voltage startup circuit 220 to terminate operation of the bias voltage startup circuit 220 by a voltage produced in the power train 240 in accordance with a transformer winding. A rectifier and output filter circuit as part of the power trains 240 is coupled to the transformer to produce the dc output voltage V out . [0024] An opto-isolator circuit 290 coupled to the dc output voltage V out (an output characteristic of the power converter) produces an output signal 281 in accordance with an output voltage controller 260 and an opto-isolator 270 . Preferably, the output voltage controller 260 senses the dc output voltage V out with an error-amplifier to produce an output voltage error signal 261 that is coupled to opto-isolator 270 . In the error amplifier, the resulting output voltage error signal 261 is produced with an inverted sense of the output characteristic. For example, if the dc output voltage V out exceeds a desired regulated value (a first regulated value), the output signal 281 from the opto-isolator 270 will have a low value. Correspondingly, if the dc output voltage V out is less than the desired regulated value, the output signal 281 from the opto-isolator 270 will have a high value. [0025] The output signal 281 from the opto-isolator 270 is coupled to the bias voltage startup circuit 220 and to the controller 230 . The controller 230 uses the output signal 281 to control the dc output voltage V out to the first, desired regulated value, such as five volts, when the output signal 281 from the opto-isolator 270 is greater than or equal to a threshold level, such as one microampere and to control the dc output voltage V out to a second regulated value less than the first, desired regulated value, such as 2.5 volts, when the output signal 281 from the opto-isolator 270 is less than the threshold level. The second regulated value is a level sufficient for opto-isolator circuit 290 to be powered by the output characteristic such as the dc output voltage V out . In an embodiment, the second regulated value is one-half the first regulated value, but other values for the second regulated value are contemplated within the broad scope of the present invention. [0026] The controller 230 is configured to control the output characteristic such as the dc output voltage V out to the second regulated value by sensing a voltage of a winding of a transformer, such as the transformer T 1 illustrated in FIG. 1 , which avoids the need for the opto-isolator circuit 290 to be operational at low values of the output characteristic. The voltage of the winding of the transformer can be sensed with a diode and a resistor-capacitor network acting as a low-pass filter coupled to the winding. Recall that the controller 230 terminates operation of bias voltage startup circuit 220 when a switching action for the power switch coupled to the transformer commences. The operation of the bias voltage startup circuit 220 is reinitiated when the output signal 281 from the opto-isolator circuit 290 is larger than a threshold value, which occurs when the output voltage is less then the first regulated value. This happens when the supply voltage of the controller 230 has dropped below a predetermined supply voltage such as its minimum supply voltage, which can happen when the switching frequency is very low. Otherwise, the bias voltage startup circuit 220 stays in an off state. [0027] The operation of the bias voltage startup circuit 220 is reinitiated after an optional delay period, such as ten milliseconds, when the switching action for the switch terminates. The controller 230 may be operational without the delay period if the bias voltage startup circuit 220 is activated at power on and the supply voltage of the controller 230 does not drop below a predetermined supply voltage such as its minimum supply voltage when the switching frequency is lowest. Operation of the bias voltage startup circuit 220 spans the startup interval. [0028] The controller 230 can be configured to disable a switching frequency of the power converter dependent on the output characteristic such as the dc output voltage V out or another output characteristic of the power converter such as an output current. For example, the controller 230 can disable the switching frequency if a duty cycle therein is less than a duty cycle threshold and/or the output characteristic such as the dc output voltage V out exceeds the first, desired regulated value. The operation of the controller 230 can be turned off when the switching frequency of the power converter is disabled. [0029] The operation of the power converter illustrated in FIG. 2 can be explained further as follows. The bias voltage startup circuit 220 starts the controller 230 . After startup, such as when sufficient bias voltage has been produced by the bias voltage startup circuit 220 , the controller 230 terminates the bias voltage startup circuit 220 to reduce power loss. At no load, the off time of the controller 230 is essentially so long that it goes into lockout. It stays unpowered until the dc output voltage V out falls lower than the desired, first regulated value of the output voltage controller 260 to avoid power loss. If the output signal 281 from the opto-isolator circuit 290 rises above a threshold level, the bias voltage startup circuit 220 is reinitiated (i.e., the startup interval is reinitiated). Thus, a minimal current produced by the opto-isolator circuit 290 reactivates the bias voltage startup circuit 220 . The controller 230 is started again and a small amount of power is transferred to the secondary side of the power trains 240 . Due to this action, the dc output voltage V out rises slightly above the desired first regulated value, and the controller 230 at low output current again goes into lockout. [0030] Turning now to FIG. 3 , illustrated is a graphical representation of an exemplary output signal Iopto (in milliamperes) from an opto-isolator circuit verses an output characteristic (e.g., an output voltage Vout in volts) according to the principles of the present invention. [0031] FIG. 3 illustrates a first regulated value (e.g., five volts) 304 of the output voltage V out and a second regulated value (e.g., 2.5 volts) 303 of the output voltage V out that is one-half of the first regulated value 304 . As shown in FIG. 3 , when the output voltage V out is greater than a lower output voltage operational limit 301 , the opto-isolator circuit produces an output signal Iopto with an inverted sense around a desired, regulated output voltage of five volts, saturating at a maximum opto-isolator value Iopto_max at the lower output voltages. Below the lower output voltage operational limit 301 , the opto-isolator circuit is not operational because it derives its bias voltage from the output voltage V out , and accordingly the opto-isolator circuit produces essentially a zero output signal Iopto. [0032] When the output signal Iopto from the opto-isolator circuit is less than a low threshold level 302 , the controller is thereby signaled that there is essentially no output voltage V out , and proceeds to regulate the output voltage to the second regulated value 303 by sensing a transformer winding of a transformer to enable the opto-isolator circuit to be operational again. Thus, the first regulated value 304 of the output voltage V out is active in a region 320 , and the second regulated value 303 of the output voltage V out is active in regions 310 , 330 . [0033] Turning now to FIG. 4 , illustrated is a schematic diagram of an embodiment of a power adapter including a power converter constructed according to the principles of the present invention. The power adapter includes a rectifier and input filter 410 constructed with diode bridge D 1 , filter capacitors C 1 and C 2 , and filter inductor L 1 to produce a filtered dc input voltage V in for the power converter. A resistor R 1 is included to provide an in-rush current limiting function when the power converter is initially coupled to an input voltage source such as an ac mains. [0034] A bias voltage startup circuit or startup circuit 420 is constructed with the series-circuit arrangement of resistors R 6 , R 7 , R 8 that are coupled to the dc side of the diode bridge D 1 to provide a small trickle-charge current to charge bias voltage filter capacitor C 11 . The three resistors R 6 , R 7 , R 8 are included in the circuit to accommodate a high dc input voltage as well as power dissipation in this portion of the circuit. When the bias voltage startup circuit 420 is disabled, a switch T 2 is turned off to disable current flow through the series-circuit arrangement of the resistors R 6 , R 7 , R 8 . During normal switching action of the power converter, the bias voltage filter capacitor C 11 is charged through a diode D 2 and a resistor R 18 that are coupled to a winding W 1 of a transformer in a power train 440 . The bias voltage startup circuit 420 is coupled to the filtered dc input voltage V in to produce a startup bias voltage on lead 425 during a startup interval, which is sufficient to initiate operation of a controller 430 (similar to lead 282 in FIG. 2 ). [0035] A controller 430 includes an integrated circuit IC 1 that produces a pulse-width modulated signal at the integrated circuit IC 1 terminal “G” to control the base of a switch T 1 . The collector of the switch T 1 is coupled to a primary winding P 1 of a transformer in the power train 440 to produce a switching action for the power converter. The power train 440 includes a diode D 3 , a capacitor C 3 , a resistor R 10 , and a resistor R 9 to absorb spikes and ringing produced across the primary winding P 1 of the transformer when the switch T 1 is turned off. The secondary windings S 1 , S 2 of the transformer are coupled in series in a flyback circuit topology to rectifier and output filter of the power train 440 . A diode D 100 provides the primary rectification function for the output voltage V out . Capacitors C 101 , C 102 , C 103 , C 104 and resistor R 100 provide filtering for the output voltage V out . The initiation of operation of the controller 430 can be signaled on lead 435 to the bias voltage startup circuit 420 to terminate operation of the bias voltage startup circuit 420 by a voltage produced in a power train 440 in accordance with a transformer winding (similar to lead 280 in FIG. 2 ). [0036] An opto-isolator circuit includes an output voltage controller 460 including an integrated circuit IC 3 that senses the output voltage V out of the power converter at integrated circuit IC 3 pin 6 and produces an error voltage control signal at integrated circuit IC 3 pin 1 that is coupled to an opto-isolator 470 through resistor R 105 . A resistor R 106 and capacitor C 107 in conjunction with the resistor R 105 provide pole-zero compensation for an output signal (a feedback signal) 485 produced by an opto-isolator 470 of the opto-isolator circuit (similar to output signal 281 of FIG. 2 ). The determination of the location of poles and zeros in a feedback circuit is well known in the art and will not be repeated here in the interest of brevity. [0037] The opto-isolator 470 produces a current at its output signal 485 that becomes a feedback voltage as this current is conducted through a resistor R 4 . The current is coupled through a resistor R 13 to the controller 420 . The current in the opto-isolator 470 also goes through a voltage divider formed with resistors R 12 , R 19 , which causes an offset to the feedback voltage. The resulting feedback voltage enables the controller 430 (in accordance with the output voltage controller 460 ) to regulate the duty cycle for the switch T 1 to control the output voltage V out of the power converter. A capacitor C 13 typically has very small capacitance. The capacitor C 13 is included for EMI-management purposes, and it has almost no effect on the control loop. [0038] The controller 430 senses the voltage at its pin U to control the output voltage V out . When the voltage at pin U is lower than a reference voltage it switches with a high (e.g., maximum) duty cycle. When the voltage at pin U is above the reference voltage, the switch T 1 is switched off until the voltage at pin U drops below the reference voltage. The voltage divider formed with resistors R 12 , R 19 in the controller 430 is proportioned for the second regulated value. Thus, the voltage is controlled to the second regulated value when there is no current in the opto-isolator 470 . When there is a current in the opto-isolator 470 , this pulls down the voltage level at pin U so that the output voltage V out rises until the voltage at pin U is at reference level. [0039] Therefore, the first regulated value can be controlled by the output voltage controller 460 by increasing and decreasing the current in the opto-isolator 470 . If the output voltage V out gets above the first regulated value, then the opto-isolator 470 current becomes zero. Then the voltage at pin U is much higher than the reference voltage because the capacitor C 5 is charged to the voltage corresponding to the first regulated value, while the voltage divider (R 19 , R 12 ) is set to the second regulated value. That causes a very long off time because the capacitor C 5 is discharged to the voltage corresponding to the second regulated value before the switch T 1 is switched on again (unless there is again current in the opto-isolator 470 ). During this time, the capacitor C 11 is discharged so that the supply voltage of the integrated circuit IC 1 drops below a level such as the minimum required supply voltage and the integrated circuit IC 1 is switched off. The integrated circuit IC 1 stays off until the bias voltage startup circuit 420 is activated either by current in the opto-isolator 470 or due to very low voltage at the capacitor C 5 (i.e., lower than the threshold voltage of the switch T 3 ). The diode-and-resistor-capacitor network D 5 , R 12 , R 19 , R 21 , C 5 , C 6 , C 10 senses a voltage of a winding of the transformer to allow the controller 430 to control the output voltage V out to the second (lower) regulated value. [0040] In Table I below, representative values are listed for the circuit elements illustrated in FIG. 4 . [0000] TABLE I C2 10 μF R3 150 kΩ R24 10 MΩ C3 150 pF R4 2 MΩ R100 0.1 Ω C5 100 nF R5 3.48 MΩ R102 392 kΩ C6 47 pF R6 8.25 kΩ R103 1 MΩ C7 100 pF R7 8.25 kΩ R104 1 MΩ C8 4.7 pF R8 8.25 kΩ R105 20 kΩ C9 47 pF R11 3 MΩ R106 274 Ω C10 27 pF R12 1.18 MΩ IC1 MC4LTE C11 100 nF R13 619 kΩ IC2 HCNW136 C13 4.7 pF R14 698 kΩ IC3 LPV511 C100 100 pF R15 10 kΩ L1 680 μH C101 220 μF R16 4.7 Ω T1 TS13003BCT C102 220 μF R17 5.62 Ω T2 BSS126 C103 220 μF R18 10 Ω T3 BS170 C104 100 nF R19 4.99 MΩ T4 BC847G-B C106 100 nF R20 10 MΩ T5 BC847G-B C107 22 nF R21 47.5 Ω R2 2.49 Ω C108 0 pF R22 6.19 MΩ C4 0 pF (optional) (optional) [0041] A flyback power converter with a power output rating of 5 watts was constructed as described previously hereinabove with no-load power dissipations as listed below in Table II. [0000] TABLE II Measurement time after startup, minutes AC Input Voltage, volts Input Power, mW 30 115 0.6 30 230 1.6 90 230 1.2 240 230 1.1 The input power shown in the table above declines slowly over time due to the decreasing leakage current of the primary bulk capacitors. These are aluminum-electrolytic capacitors which cause a significant part of the total power loss at no load due to their leakage current, which significantly decreases when the capacitors are charged to a constant voltage for some time. [0042] Thus, a power converter with reduced power dissipation at light loads and method of operating the same has been introduced herein. In one embodiment, the power converter includes an opto-isolator circuit configured to produce an output signal dependent on an output characteristic of the power converter. The power converter also includes a controller configured to control the output characteristic to a first regulated value when the output signal is greater than or equal to a threshold level, and to control the output characteristic to a second regulated value less than the first regulated value when the output signal is less than the threshold level. [0043] Those skilled in the art should understand that the previously described embodiments of a switched-capacitor power converter and related methods of operating the same are submitted for illustrative purposes only. While the principles of the present invention have been described in the environment of a power converter, these principles may also be applied to other systems such as, without limitation, a power amplifier or a motor controller. For a better understanding of power converters, see “Modern DC-to-DC Power Switch-mode Power Converter Circuits,” by Rudolph P. Severns and Gordon Bloom, Van Nostrand Reinhold Company, New York, N.Y. (1985) and “Principles of Power Electronics,” by J. G. Kassakian, M. F. Schlecht and G. C. Verghese, Addison-Wesley (1991). [0044] Also, although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the processes discussed above can be implemented in different methodologies and replaced by other processes, or a combination thereof. [0045] Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

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