Feedback circuit for non-isolated power converter

A feedback circuit for a power converter (e.g., a non-isolated converter) is disclosed. The feedback circuit may include a sense circuit coupled to receive an output current of the converter. A sense voltage may be generated across the sense circuit and a voltage-to-current converter may be used to convert the sensed voltage into a feedback signal representative of the output current. The voltage-to-current converter may include a variable shunt regulator, resistor, and transistor. A voltage across the shunt regulator may change in response to a change in voltage across the sense circuit, and the feedback signal may change in response to a change in the voltage across the shunt regulator. A controller may be coupled to receive the feedback signal from the feedback circuit and may control switching of a power switch to regulate the output current based at least in part on the feedback signal.

BACKGROUND

The present disclosure relates generally to power converters and, more specifically, to feedback circuits for power converters.

2. Description of Related Art

Electronic devices are typically used with power conversion circuits. Switched mode power converters are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current (ac). In a switched mode power converter, a high voltage ac input is converted to provide a well-regulated direct current (dc) output. In operation, a switch, included in the switched mode power converter, is utilized to control the desired output by varying the duty ratio (typically the ratio of the on time of the switch to the total switching period) and/or varying the switching frequency (the number of switching events per unit time). More specifically, a switched mode power converter controller may determine the duty ratio and/or switching frequency of the switch in response to a measured input and a measured output.

Conventional power converters include a controller that may be configured to provide a regulated voltage and/or a regulated current at the output of the power converter. In general, a regulated power converter may also be referred to as a power supply. One type of conventional controller monitors a voltage at the output of the power converter in order to provide a regulated output voltage while another type of controller monitors a current at the output in order to provide a regulated output current. One way to measure the output current is to include a sense resistor at the output of the power converter such that the output current flows through the sense resistor and the resultant voltage dropped across the sense resistor is proportional to the output current. However, the voltage dropped across the sense resistor is typically large and often referenced to a voltage level different than that of the power converter controller. Thus, additional circuitry, such as an opto-coupler or a bias winding, is often needed to level shift the voltage across the sense resistor in order to interface with the controller. However, these components can be bulky and expensive.

Additionally, for some conventional applications, the input of the power converter may be galvanically isolated from the output of the power converter. In general, galvanic isolation prevents dc current from flowing between the input and the output of the power converter Implementing galvanic isolation, however, usually requires additional circuitry, such as a magnetic coupler or an opto-coupler, which adds cost to the power converter.

DETAILED DESCRIPTION

For embodiments of the present disclosure, a power converter controller controls switching of a switch to regulate an output current in response to the output current. In addition, a power converter, in accordance with embodiments disclosed herein, may be non-isolated and may also include a feedback circuit that directly measures the output current without the need for isolation between the output and the controller.

FIG. 1is a functional block diagram illustrating an example power converter100and a load124. The illustrated example of power converter100is shown as including input terminals101and103(collectively referred to herein as the “input” of the power converter), an input capacitor104, a positive input voltage rail138, an input voltage sense circuit108, a controller110, a feedback circuit122having a sense circuit126(shown in this example as including sense resistor RSENSE126), an output capacitor120, an input return106, a switch112, diodes114and116, an inductor118, an output return140, and output terminals142and144(collectively referred to herein as the “output” of the power converter). While in this example sense circuit126includes sense resistor126, it should be appreciated that other current sense circuits known to those of ordinary skill in the art may be used. Also shown inFIG. 1is an input voltage YIN102, an input voltage sense signal130, a feedback signal132, a drive signal128, an output current IO136, and an output voltage VO134.

Power converter100is a non-isolated power converter. For example, in the illustrated embodiment, the input of power converter100is electrically coupled to the output (e.g., dc current is able to flow between input terminals101/103and output terminals142/144). During operation, power converter100provides a regulated output voltage VO134and/or output current IO136to load124from an unregulated input voltage VIN102. In one embodiment, the input of power converter100receives input voltage VIN102from a rectifier circuit (discussed below), which in turn is coupled to receive an unregulated ac input voltage from a source (not shown), such as a conventional wall socket. In another embodiment, the input of power converter100receives a dc input voltage from a source (not shown). As shown inFIG. 1, input terminal101is coupled to positive input voltage rail138, while input terminal103is coupled to input return106.

FIG. 1further illustrates input capacitor104as having one terminal coupled to positive input voltage rail138and another terminal coupled to input return106. As shown inFIG. 1, input capacitor104is coupled to receive the input voltage VIN102. In one embodiment, input capacitor104provides a filtering function for noise, such as electro-magnetic interference (EMI) or other transients. For other applications, the input capacitor104may have a capacitance large enough such that a dc voltage is applied at the input of the power converter100. However, for power converters with power factor correction (PFC), a small input capacitor104may be utilized to allow the voltage at the input of the power converter100to substantially follow the rectified ac input voltage YIN102. As such, the value of the input capacitor104may be chosen such that the voltage on the input capacitor104reaches substantially zero when the rectified ac input voltage YIN102reaches substantially zero.

FIG. 1further illustrates switch112as having one terminal coupled to input return106and another terminal coupled to diode116. Diode116is then coupled to diode114and inductor118. Diode116is coupled to prevent reverse current flow in switch112. However, it should be appreciated that diode116may be optional. Inductor118is further coupled to one end of capacitor120and feedback circuit122. As shown inFIG. 1, diode114is coupled to the positive input voltage rail138and inductor118.

The terminals of capacitor120are shown inFIG. 1as being coupled between positive input voltage rail138and inductor118. Load124is shown as being coupled between output terminals142and144. In operation, output capacitor120produces a substantially constant output current IO136, output voltage VO134, or a combination of the two, which is received by load124.

During operation, load124may receive substantially constant power. Load124may also be a load where the output voltage varies as a function of the output current in a predetermined and known manner. For example, output voltage VO134may be substantially proportional to output current IO136. In one embodiment, load124may be an LED array, as will be discussed in further detail below.

Feedback circuit122is coupled to sense output current IO136from the output of power converter100to produce feedback signal132. Feedback circuit122is further coupled to controller110such that feedback signal132is received by controller110. Feedback signal132may be a voltage signal or a current signal that is representative of output current IO136. It is recognized that a voltage signal and current signal each may contain both a voltage component and a current component. However, the term “voltage signal” as used herein means that the voltage component of the signal is representative of the relevant information. Similarly, the term “current signal” as used herein means that the current component of the signal is representative of the relevant information. By way of example, feedback signal132may be a current signal having a voltage component and a current component, where it is the current component that is representative of output current IO136.

As shown inFIG. 1, input voltage sense circuit108is coupled to sense the input voltage VIN102. In one embodiment, input voltage sense circuit108detects the peak voltage of input voltage YIN102. Input voltage sense circuit108is also coupled to generate input voltage sense signal130, which may be representative of the peak voltage of input voltage YIN102. In another example, input voltage sense signal130may be representative of the average voltage of input voltage YIN102. Input voltage sense signal130may be a voltage signal or a current signal that is representative of input voltage YIN102.

Controller110is coupled to generate a drive signal128to control the switching of switch112. Controller110may be implemented as a monolithic integrated circuit or may be implemented with discrete electrical components or a combination of discrete and integrated components. In addition, switch112receives the drive signal128from the controller110.

Switch112is opened and closed in response to drive signal128. It is generally understood that a switch that is closed may conduct current and is considered on, while a switch that is open cannot substantially conduct current and is considered off. In one embodiment, switch112may be a transistor, such as a metal-oxide-semiconductor field-effect transistor (MOSFET). In one example, controller110and switch112form part of an integrated control circuit that is manufactured as either a hybrid or monolithic integrated circuit.

As shown inFIG. 1, controller110outputs drive signal128to control the switching of switch112in response to feedback signal132and in response to input voltage sense signal130. In one embodiment, the drive signal128is a pulse width modulated (PWM) signal of logic high and logic low sections, with the logic high value corresponding to a closed switch and a logic low corresponding to an open switch. In another embodiment, drive signal128is comprised of substantially fixed-length logic high (or ON) pulses and regulates the output (shown as output current IO136, output voltage VO134, or a combination of the two) by varying the number of ON pulses over a set time period.

In operation, drive signal128may have various drive signal operating conditions, such as the switch on-time tON(typically corresponding to a logic high value of the drive signal128), switch off-time tOFF(typically corresponding to a logic low value of the drive signal128), switching frequency fs, or duty ratio. As mentioned above, load124can be a constant load. Thus, during operation, controller110may utilize feedback signal132and input voltage sense signal130to regulate the output (e.g., output current IO136). For example, a reduction in the input voltage sense signal130may correspond to the input voltage sense circuit108sensing a lower value of the input voltage YIN102. Thus, controller110may extend the duty ratio of drive signal128to maintain a constant output current IO136in response to this reduction in the input voltage sense signal130.

In one example, controller110may perform PFC, where a switch current (not shown) through switch112is controlled to change proportionately with the input voltage YIN102. By way of example, controller110may perform PFC by controlling the switching of switch112to have a substantially constant duty ratio for a half line cycle of the ac input voltage (not shown). In general, the ac input voltage (not shown) is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. In another example, the controller110may perform PFC by sensing the switch current and comparing the integral of the switch current to a decreasing linear ramp signal.

As discussed above, load124may be a substantially constant load that does not vary during operation of the power converter.FIG. 2Aillustrates an LED array224, which is one possible implementation of load124ofFIG. 1. As shown, LED array224includes N number of LEDs (i.e., LED 1 though LED N). As further shown,FIG. 2Bis a diagram illustrating a circuit model of the LEDs included in the LED array224ofFIG. 2A. LEDs246,248,250, and252are circuit models of LEDs 1, 2, 3, and N, respectively, ofFIG. 2A. That is, LED 1 may be represented by the model LED246, which includes an ideal diode D1, a threshold voltage VD1and a series resistance RS1. Thus, LED246will generally conduct current when the voltage across LED246exceeds threshold voltage VD1and the current through LED246will be proportional to the voltage across it due in part to series resistance RS1.FIG. 2Cis a graph illustrating a relationship between output current and output voltage of the circuit model of LEDs ofFIG. 2B. As shown inFIG. 2C, the sum of the threshold voltages VD1through VDNrepresents a minimum voltage VMINnecessary to turn on the LEDs. That is, LED array224will generally not conduct current until the output voltage VOexceeds the minimum voltage VMIN. Also, shown inFIG. 2Cis that for output voltages VOgreater than the minimum voltage VMIN, the output current IOis generally proportional to the output voltage VO. In other words, as the output current IOis reduced through LED array224, a proportional reduction in voltage across the series resistance RS1, RS2, . . . RSNoccurs as well, thus, reducing the overall output voltage VO.

In the examples where load124includes an LED array similar or identical to array224, it can be desirable to have a well-regulated output current IO136to generate a uniform brightness. If the output current IO136(or output voltage) is not properly regulated, a flickering effect can be produced by the LED array224.

FIG. 3is a circuit diagram of an example input voltage sense circuit308, in accordance with an embodiment of the present disclosure. Input voltage sense circuit308is one possible implementation of input voltage sense circuit108ofFIG. 1. The illustrated example of input voltage sense circuit308includes a diode354, resistors355,357,358, and361, a capacitor359, and nodes356and360. Also shown inFIG. 3are positive input voltage rail338(e.g., positive input voltage rail138), input return306(e.g., input return106), and input voltage sense signal330(e.g., input voltage sense signal130).

In one embodiment, input voltage sense circuit308detects the peak voltage of input voltage VIN102. Input voltage sense circuit308is also coupled to generate input voltage sense signal330, which may be representative of the peak voltage of input voltage VIN102. Input voltage sense signal330may be a voltage signal or a current signal and is representative of input voltage VIN102.

During operation, the voltage between nodes356and360may be relatively high. Thus, the illustrated example of input voltage sense circuit308includes resistors357and358coupled in series between nodes356and360such that the voltage rating of each resistor is not exceeded during operation. Although,FIG. 3illustrates two resistors (i.e., resistors357and358) as coupled between nodes356and360, any number of resistors, including one or more, may be utilized such that the voltage rating of each resistor is not exceeded.

FIG. 4is a circuit diagram of an example feedback circuit422, in accordance with various embodiments. Feedback circuit422is one possible implementation of feedback circuit122ofFIG. 1. Feedback circuit422may generate feedback signal432(e.g., feedback signal132) that is representative of the output current IO136. Although feedback signal432that is generated by feedback circuit422is a current signal, it is recognized that feedback circuit422may include additional circuitry (not shown) to generate feedback signal432as a voltage signal and still be in accordance with the teachings disclosed herein.

Feedback circuit422includes diode462between positive input voltage rail438(e.g., positive input voltage rail138) and resistor464. More specifically, the anode of diode462may be coupled to positive input voltage rail438and the cathode of diode462may be coupled to one end of resistor464. Resistor464may be further coupled to node465. Further shown as included in feedback circuit422is a capacitor474coupled between node465and one end of sense circuit426. In the example illustrated, sense circuit426includes sense resistor RSENSE426. However, it should be appreciated that other known current sense circuits may be used.

Feedback circuit422is shown as further including capacitor472coupled to node465, shunt regulator468, and resistor476. Further, one end of capacitor472is coupled to the cathode of the shunt regulator468while the other end of capacitor472is coupled to the reference of the shunt regulator468. One end of resistor476is also coupled to the reference of the shunt regulator468while the other end of resistor476is coupled to capacitor478and resistor480. Resistor480is coupled to output return440and sense circuit426. Capacitor478is further coupled to the opposite terminal of sense circuit426.

As mentioned above, feedback circuit422may further include shunt regulator468. In the example illustrated, the cathode of shunt regulator468is coupled to node465, while the anode of shunt regulator468is coupled to transistor470.

Feedback circuit422may further include a voltage-to-current converter that includes resistor466, transistor470, and shunt regulator468. Resistor466may be coupled to node465and the emitter of transistor470. Transistor470may include a PNP bipolar junction transistor coupled to operate in the linear region of the transistor. Transistor470may have its base coupled to shunt regulator468and may be coupled to output feedback signal432. As discussed above, feedback signal432may be a current signal that is representative of output current IO136. In one embodiment, feedback signal432is at least substantially proportional to the output current IO136.

In operation, an output current IO136flows from load124to node481, causing a sense voltage to be generated across the sense circuit426(shown in this example as including sense resistor RSENSE426). The sense voltage is proportional to the output current IO136. This sense voltage is filtered by resistor480and capacitor478. The sense voltage also causes a voltage VSHto be formed across shunt regulator468. Voltage VSHmay be filtered by capacitor474and resistor464allows the voltage at node465to vary. The voltage across resistor466is proportional to the voltage VSHacross the cathode and anode of the shunt regulator468. For example, the voltage across resistor466is substantially equal to voltage VSHminus the emitter-base VEBvoltage of transistor470(e.g., approximately 0.7 V). The current entering the emitter of transistor470is substantially equal to the current across resistor466. In the example shown, the emitter current is substantially equal to the voltage across resistor466divided by the resistance of resistor466. For a transistor470with a large beta value, the collector current (i.e., feedback signal432) is substantially equal to the emitter current. In the example shown, the emitter current is substantially equal to (VSH-VEB)/(resistance of resistor466). Voltage VSHacross shunt regulator468decreases as the output current increases. As such, the feedback signal432also decreases with increasing output current. Similarly, voltage VSHacross shunt regulator468increases as the output current decreases. As such, the feedback signal432also increases with decreasing output current.

In the illustrated example, the value of the various components may be selected to set the value of feedback signal432such that feedback signal432is within an operating range of the controller (e.g., controller110).

Accordingly, embodiments of the present disclosure provide for a feedback circuit, such as feedback circuit422, that provides a feedback signal that is representative of the output current IO136of the power converter without the need for additional isolation circuitry, as discussed above with conventional systems. As shown inFIGS. 1 and 4, the output of power converter100may not be electrically isolated from controller110by way of feedback circuit122or422.

FIG. 5is a circuit diagram of an example power converter500having a feedback circuit similar or identical to that shown inFIG. 4and an input voltage sense circuit similar or identical to that shown inFIG. 3. Power converter500is one possible implementation of power converter100ofFIG. 1. In one embodiment, load124may include an LED array, such as LED array224ofFIG. 2A, and power converter500, a rectifier circuit (not shown), and the LED array may be packaged together into a single apparatus, such as an LED lamp (e.g., an LED light bulb). The LED lamp including power converter500, rectifier, and LED array224may be designed to be interchangeable with, and serve as a replacement for, conventional incandescent or compact fluorescent light bulbs.

AC input terminals101and103may be coupled to receive a rectified ac input voltage VIN102from a rectifier circuit (not shown). The rectifier circuit may include a full-wave bridge rectifier operable to receive an unregulated ac input voltage from a power source, such as a conventional wall socket, and output the rectified input voltage VIN102.

As shown inFIG. 5, integrated control circuit511is a low-side controller. That is, the switch112is coupled to the input return106. For the example shown, integrated control circuit511has a source terminal S that is coupled to input return106. Integrated control circuit511is shown inFIG. 5as including other terminals in addition to the source terminal S (i.e., bypass terminal BP, reference terminal R, input voltage terminal V, feedback terminal FB, and drain terminal D, etc.). As shown inFIG. 5, input voltage terminal V is coupled to receive input voltage sense signal130. As mentioned above, input voltage sense signal130may be a current signal. Thus, input voltage terminal V may be configured to sink the current received from input voltage sense circuit108. Further shown inFIG. 5is feedback terminal FB coupled to receive feedback signal132. As also mentioned above, feedback signal132may be a current signal and thus, feedback terminal FB may be configured to sink the current received from feedback circuit122. In one example, reference terminal R is coupled to source terminal S through resistor R1to provide controller510with a reference with which to compare the other signals received by the controller. In one embodiment, the feedback signal132and input voltage sense signal130may both be referenced with respect to the source terminal S.

AlthoughFIG. 5illustrates switch112as including a MOSFET, switch112may also be a power switching device including a bipolar transistor or an insulated gate bipolar transistor (IGBT).