Self-oscillating boost DC-DC converters with current feedback and digital control algorithm

In a method and system for controlling a direct current to direct current (DC-DC) converter includes an inductor coupled to receive a voltage input at an input terminal. A diode is coupled in series between the inductor and an output terminal of the DC-DC converter. A switch is coupled between the inductor and a ground reference. The switch receives a control signal from a controller for adjusting a duty cycle of the DC-DC converter. The duty cycle controls an output voltage at the output terminal. The controller generates the control signal in response to receiving a feedback signal, which is derived as a predefined function of a voltage feedback signal indicative of the output voltage and a current feedback signal indicative of a current flowing through the inductor.

BACKGROUND

The present disclosure relates generally to the field of power supplies, and more particularly to techniques for efficiently controlling a direct current to direct current (DC-DC) converter used in power supplies.

Switching DC-DC converters (also referred to as regulators) have been used to provide direct current (DC) power to electrical/electronic devices such as integrated circuits (ICs), digital signal processors, radio frequency (RF) circuit devices, printed circuit boards, and the like, due to their improved power conversion efficiency compared to non-switching regulators. Switching DC-DC converters regulate an average DC output voltage by selectively storing energy in an inductor during a charge cycle, e.g., during an on time of a switching element. The energy stored in the inductor is selectively transferred to charge an output capacitor in discrete packets during a discharge cycle, e.g., during an off time of the switching element. Thus, the charge and discharge cycles are controlled by the switching element such as a MOSFET by adjusting the on time and off time of a current flowing through the inductor. By comparing the voltage across the output capacitor to a reference voltage the inductor current is controlled to provide a desired output voltage.

Maintaining the desired output voltage while accommodating variations in the load and/or the input voltage may be difficult with many traditional DC-DC converters. For example, selecting a long on time with a short off time may favor a full load condition but may impair light load performance. While selecting a short on time with a long off time may improve light load performance but may impair performance at full load. In addition, a selection of the on time or the off time that may be too short may increase the converter's vulnerability to noise. Similarly, accommodating variations in input voltage with or without variations in the load may further degrade performance.

Therefore, a need exists to provide an improved method and system for efficiently controlling a DC-DC converter. Additionally, a need exists for a technique to determine the on and off time of self-oscillating boost converters for an improved performance against variations in the load and the input voltage, and for improved susceptibility to noise. Accordingly, it would be desirable to provide an improved DC-DC converter, absent the disadvantages found in the prior methods discussed above.

SUMMARY

The foregoing need is addressed by the teachings of the present disclosure, which relates to self-oscillating boost DC-DC converters having current feedback and a digital control algorithm. According to one embodiment, in a method and system for controlling a DC-DC converter includes an inductor coupled to receive a voltage input at an input terminal. A diode is coupled in series between the inductor and an output terminal of the DC-DC converter. A switch is coupled between the inductor and a ground reference. The switch receives a control signal from a controller for adjusting a duty cycle of the DC-DC converter. The duty cycle controls an output voltage at the output terminal. The controller generates the control signal in response to receiving a feedback signal, which is derived as a predefined function of a voltage feedback signal indicative of the output voltage and a current feedback signal indicative of a current flowing through the inductor.

In a particular embodiment, a method of controlling a DC-DC converter includes receiving a current feedback signal indicative of a current flowing through an inductor of the DC-DC converter and receiving a voltage feedback signal indicative of an output voltage of the DC-DC converter. The current feedback and the voltage feedback signals are compared to provide a feedback signal to a controller. A duty cycle of the DC-DC converter, and hence the output voltage, is controlled by the controller responsive to the feedback signal.

Several advantages are achieved by the method and system according to the illustrative embodiments presented herein. The embodiments advantageously provide for an improved self-oscillating DC-DC power conversion technique to adapt to changing load and input voltage conditions in a cost effective manner. The digital control algorithm advantageously deploys constraint-based control to make the converter more robust to accommodate the changing load and input voltage. In addition, digital control algorithm filters out coupling noise from the feedback signal to improve power conversion performance. Thus, the improved power conversion technique advantageously regulates the output voltage in both continuous current mode (CCM) and discontinuous control mode (DCM).

DETAILED DESCRIPTION

Novel features believed characteristic of the present disclosure are set forth in the appended claims. The disclosure itself, however, as well as a preferred mode of use, various objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings. The functionality of various circuits, devices or components described herein may be implemented as hardware (including discrete components, integrated circuits and systems-on-a-chip), firmware (including application specific integrated circuits and programmable chips) and/or software or a combination thereof, depending on the application requirements.

Many traditional switching DC-DC converters may have difficulty in maintaining a desired output voltage while accommodating variations in the load and/or the input voltage. In addition, a selection of the on time or the off time that may be too short may increase the converter's vulnerability to noise. This problem may be addressed by an improved self-oscillating DC-DC converter. In the improved system and method, a current feedback signal indicative of the inductor current and an output voltage feedback signal is provided to a controller for adjusting the on and off times. A digital control algorithm is added to the controller to make the DC-DC converter more robust to accommodate changes in load, accommodate variations in the input voltage, and improve immunity to noise.

According to one embodiment, in a method and system for controlling a DC-DC converter includes an inductor coupled to receive a voltage input at an input terminal. A diode is coupled in series between the inductor and an output terminal of the DC-DC converter. A switch is coupled between the inductor and a ground reference. The switch receives a control signal from a controller for adjusting a duty cycle of the DC-DC converter. The duty cycle controls an output voltage at the output terminal. The controller generates the control signal in response to receiving a feedback signal, which is derived as a predefined function of a voltage feedback signal indicative of the output voltage and a current feedback signal indicative of a current flowing through the inductor.

FIG. 1depicts an electronic system, generally designated100, that includes a direct current to direct current (DC-DC) converter110providing power to a load120, according to an embodiment. An inductor130is coupled to receive a voltage input140at an input terminal142. In a particular embodiment, the voltage input140is a DC voltage applied across the input terminal142and a reference196such as ground. A diode150is coupled in series between the inductor130and an output terminal152. A switch160, which is the switching element in the DC-DC converter110, is coupled between the inductor130and the reference196. In a particular embodiment, the switch160is a MOSFET device. A capacitor180is connected between the output terminal152and the reference196to store the energy received from the inductor130. In depicted embodiment, an output voltage Vout170is provided to the load120that is coupled between the output terminal152and the reference196. In a particular embodiment, the load may vary with the application. Examples of the load may include ICs, digital signal processors (DSPs), radio frequency (RF) circuit devices, printed circuit boards, and the like. The DC-DC converter110has a self-oscillating feature since the DC-DC converter110is independent of an internal and/or external generated periodic input such as a clock.

A control signal162is used to control, e.g., open or close, the switch160. During a charge cycle, the switch160is closed (or on state) thereby providing a charge path between the input terminal142and the reference196. That is, during on time tONof the switch160, an inductor current132flows through the inductor130and the switch160. During a discharge cycle, the switch160is opened (or off state) thereby disconnecting the charge path between the input terminal142and the reference196. Since the inductor current132may not change instantaneously, the inductor current132charges the capacitor180during the discharge cycle, e.g., during an off time tOFFof the switch160. Additional detail of the charge and discharge cycle is described inFIG. 3.

The diode150is used as a rectifier to allow the inductor current132to flow from an input energy storage element, e.g., the inductor130, to the output charge storage element, e.g., the capacitor180, during the discharge cycle when the switch160is open. However, the diode150is reverse-biased during the charge cycle when the switch160is closed.

The time duration of the on or off state of the switch160controls a duty cycle of the self-oscillating DC-DC converter110. The duty cycle is generally defined as tON/(tON+tOFF). Thus, adjusting the duty cycle controls an average power output provided to the load120by adjusting the output voltage Vout170in response to the changes in the load120.

The controller190controls the operation (e.g., on or off control) of the switch160by asserting (or high) or deasserting (or low) the control signal162in response to receiving a feedback signal188. In the depicted embodiment, the controller190includes a digital control algorithm194for generating the control signal162in response to the feedback signal188. Additional details of the digital control algorithm194are described inFIG. 2. In a particular embodiment, the feedback signal188is derived as a predefined function of two inputs. In the depicted embodiment, the predefined function is a comparator186. The comparator186receives at the positive input a current feedback signal184indicative of the inductor current132and a voltage feedback signal182is received at the negative input indicative of the output voltage Vout170. The comparator186asserts the feedback signal188when the current feedback signal184is greater than the voltage feedback signal182. The comparator186deasserts the feedback signal188when the current feedback signal184is not greater than the voltage feedback signal182. In a particular embodiment, the comparator186may be included in the controller190.

The current feedback signal184is provided by a current feedback circuit178connected in parallel with the load120, e.g., coupled between the output terminal152and the reference196. In a particular embodiment, the current feedback circuit178includes resistors R1168, R2166and R3164connected in series between the output terminal152and the reference196. Junction of R1168and R2166form a node N1158and junction of R2166and R3164form a node N2156. A portion Iinj154of the inductor current132is injected into the current feedback circuit178at node N2156via a sense switch134. The sense switch134is connected in parallel with the switch160and both the sense switch134and the switch160are controlled by the control signal162. In a particular embodiment, the sense switch134is a MOSFET device.

In a particular embodiment, the values of R1168, R2166and R3164selected may depend on application factors such as on-resistance of the switches160and134. The range of values for R1168and R2166may be hundreds of kilo ohms and the range of values for R3164may be 50 ohms to a few hundred ohms.

During the charge cycle, the inductor current132will be divided between the two parallel paths, e.g., first via the switch160to the reference196and second via sense switch134, N2156and R3164to the reference196. Ratio of the current flowing through each path is inversely proportional to the path resistance. Thus, injected current Iinj154is calculated as a percent of the inductor current132and is therefore indicative of the inductor current132. Voltage VN2at node N2156is indicative of the DC voltage offset introduced by the injected current Iinj154and is calculated by Equation 100.
VN2=Iinj*R3   Equation 100

Voltage VN1at node N1158is proportional to the output voltage Vout170and is biased by the DC voltage offset calculated by Equation 100. Thus, Equation 110 calculates voltage VN1as follows:
VN1=Vout*((R2+R3)/(R1+R2+R3)+(Iinj*R3)   Equation 110
Voltage VN1at node N1158, which is calculated by adding a first component and a second component of Equation 110, is provided as the current feedback signal184indicative of the inductor current132.

An integrator176provides the voltage feedback signal182. The integrator is connected in cascade with a voltage feedback circuit174. The voltage feedback circuit174is connected in parallel with the load120and is similar to the current feedback circuit178, e.g., coupled between the output terminal152and the reference196. In a particular embodiment, the voltage feedback circuit174includes the resistors R1168, R2166and R3164connected in series between the output terminal152and the reference196. Within the voltage feedback circuit174, junction of R1168and R2166form a node N3172. Voltage VN3at node N3172is proportional to the output voltage Vout170and is calculated by Equation 120 as follows:
VN3=Vout*((R2+R3)/(R1+R2+R3)   Equation 120
Thus, voltage VN3at node N3158is provided as a second voltage feedback signal148to the integrator176, the second voltage feedback signal148being indicative of the output voltage Vout170.

The integrator176compensates for the DC voltage offset VN2introduced by the injection of current Iinj154calculated in Equation 110 by integrating a difference between the second voltage feedback signal148and an output voltage reference Vref146over a predefined time period T to generate the voltage feedback signal182. In a particular embodiment, the voltage feedback signal VFB182182is calculated by Equation 130 as follows:

VFB⁢182=∫0T⁢k*(VN⁢3-Vref)*⁢ⅆt+IcEquation⁢⁢130
where k and Icare constants. In a particular embodiment, the comparator186and the integrator176may be included in the controller190.

While the DC-DC converter110is illustrated as a boost (step-up) converter inFIG. 1, it is contemplated that the improved control technique described herein is independent of the topology of the DC-DC converter. That is, the digital control algorithm194, voltage and current feedback based control techniques may be applied to other known DC-DC converter types such as buck (step-down), buck-boost and Cuk (step-up or step-down with inverse polarity), and charge-pump converters.

FIG. 2is a block diagram illustrating details of a digital control algorithm ofFIG. 1, according to an embodiment. In a particular embodiment, the digital control algorithm194includes logic components to control timing aspects of the charge and discharge cycles. The switch160is closed (on state) by asserting the control signal162when the feedback signal188is deasserted (or low). The switch160is opened (off state) by deasserting the control signal162when the feedback signal188is asserted (or high). During the charge cycle, the switch160is closed, the inductor current132builds up and the output voltage170droops. During the discharge cycle, the switch160is open, the inductor current132decays and the output voltage170rises.

In the depicted embodiment, the digital control algorithm194includes a minimum off time logic component210when the load120is heavy, e.g., when the load120draws a load current above a threshold. A higher load current draws a higher inductor current build-up for the current feedback signal184to overcome a droop in the voltage feedback signal182to cause a change of state of the feedback signal188. Therefore, the switch160is closed for longer time duration and thus has a longer tONon time. When the switch160is opened, the minimum off time logic component210will over ride the feedback signal188and impose a minimum off time to ensure adequate charge transfer takes place every cycle. In addition, the minimum off time digitally filters out any coupling noise. Exemplary waveforms associated with the minimum off time logic component210are described with reference toFIG. 4A.

In the depicted embodiment, the digital control algorithm194includes a minimum on time logic component220when the load120is light, e.g., when the load120draws a load current below a threshold. A lighter load current causes a lighter inductor current build-up resulting in tripping the feedback signal188from low to high during the charge cycle when the switch160is closed. However, the minimum on time logic component220over rides the feedback signal188and imposes a minimum on time to digitally filter out any coupling noise. Due to the light load condition, the feedback signal188remains high after the minimum off time imposed by the minimum on time logic component220expires. Hence, the switch160remains open as long as the feedback signal188remains high. Exemplary waveforms associated with the minimum on time logic component220are described with reference toFIG. 4B.

In the depicted embodiment, the digital control algorithm194includes a maximum on time logic component230when the load120is heavy, e.g., when the load120draws a load current above a threshold and when the voltage input140is low, e.g., less than a threshold. Due to the low the voltage input140, build-up of the inductor current132is slow during the charge cycle. To reduce the output voltage170from drooping further, the maximum on time logic component230imposes a maximum on time that switches the switch160to open and thereby charge the capacitor180independent of the state of the feedback signal188. Exemplary waveforms associated with the maximum on time logic component230are described with reference toFIG. 4C.

In a particular embodiment, the logic for the minimum on time, minimum off time and maximum on time is performed during every charge/discharge cycle. The particular values for the minimum on time, minimum off time and maximum on time may vary depending on application factors such as the load120, switching frequency, and variation in input voltage140. In a particular embodiment, a typical value for the minimum on time is 0.5 microseconds, for the minimum off time it is 1 microsecond, and for the maximum on time it is 6 microseconds. In another embodiment, a maximum duty cycle (DCMAX) may be calculated by Equation 200 as follows:
DCMAX=(tMAX-ON/(tMAX-ON+tMIN-OFF))   Equation 200
where tMAX-ONis the maximum on time and tMIN-OFFis the minimum off time.

In a particular embodiment, the time period, and hence the switching frequency, for the charge and discharge cycle may vary. That is, the combined time period for consecutive charge and discharge cycles may vary. The minimum on time and the minimum off time constraints imposed by the digital control algorithm194may limit a maximum achievable switching frequency of the self-oscillating DC-DC converter110.

In a particular embodiment, the digital control algorithm194may include logic to not limit the maximum off time for the discharge cycle thereby enabling operation of the DC-DC converter in a continuous current mode (CCM) and in a discontinuous control mode (DCM). In addition, a wider range of input voltage140and the load120may be accommodated by not limiting the maximum off time. In this embodiment, the minimum duty cycle may approach 0 as the maximum off time approaches a large number.

FIG. 3illustrates waveforms associated with a charge and discharge cycle of DC-DC converter110ofFIG. 1, according to one embodiment. In this illustration, at time t=t0310, the switch160is closed initiating the charge cycle. The inductor current132(IIND) increases from an initial value I1312at time t0310to an increased value of I2314at time t1320. The diode150is reversed biased, and the output voltage Vout170droops from an initial value V1316at time t0310to a decreased value V2318at time t1320. When the sense switch134is closed, the injected current Iinj154jumps from Izero326to an initial value I3324at time t0310and ramps up to an increased value of I4322at time t1320. The time duration of the charge cycle is the on time period tON316. At t=t1320, the switch160is opened to initiate the discharge cycle. Due to the forward bias on the diode150, the inductor current132drops from the previous value of I2314at time t1320to I1312at t2330to transfer the energy from the inductor130to the capacitor180. The output voltage Vout170increases from an initial value V2318at time t1320to an increased value V1316at time t2330. At t=t1320, the sense switch134is opened and the injected current Iinj154drops from the initial value I4322to Izero326. The time duration of the discharge cycle is the off time tOFF328. At t=t2330, the switch160is closed and the charge and discharge cycle repeats.

FIG. 4Aillustrates waveforms associated with a minimum off time logic component210ofFIG. 2, according to one embodiment. At time t=t3410, a spike or glitch occurs in the feedback signal188due to noise. The change of state in the feedback signal188causes the minimum off time logic to be activated, thereby deasserting the control signal162to open the switch160for a minimum off time tMIN-OFF420. At time t=t4430, the feedback signal188changes state, but this change has no effect on the control signal162which keeps the switch160open. A traditional DC-DC converter may have discharged momentarily causing poor regulation.

FIG. 4Billustrates waveforms associated with a minimum on time logic component220ofFIG. 2, according to one embodiment. At time t=t5440, a spike or glitch occurs in the feedback signal188due to noise. The change of state in the feedback signal188causes the minimum on time logic to be activated, thereby maintaining the control signal162at a high level to keep the switch160closed for a minimum on time tMIN-ON450. At time t=t6460, the glitch clears and feedback signal188changes state, but this change has no effect on the control signal162which keeps the switch160closed. A traditional DC-DC converter may have not allowed a sufficient inductor current build up resulting in poor regulation.

FIG. 4Cillustrates waveforms associated with a maximum on time logic component230ofFIG. 2, according to one embodiment. At time t=t7470, a change of state in the feedback signal188causes the maximum on time logic to be activated, thereby maintaining the control signal162at a high level to keep the switch160closed for a maximum on time tMAX-ON480. At time t=t8490, a change of state on the feedback signal188has no effect on the control signal162which keeps the switch160open. A traditional DC-DC converter may have not allowed a sufficient discharge resulting in poor regulation.

FIG. 5Ais a flow chart illustrating a method of controlling a DC-DC converter, e.g., the DC-DC converter110, according to an embodiment. At step510, a current feedback signal such as the current feedback signal184is received. At step520, a voltage feedback signal such as the voltage feedback signal182is received. At step530, a comparison of the current feedback signal and the voltage feedback signal is made to provide a feedback signal such as the feedback signal188. At step540, a duty cycle of the DC-DC converter is controlled by a controller such as the controller190responsive to the feedback signal.

Various steps described above may be added, omitted, combined, altered, or performed in different orders. For example, in a particular embodiment, receiving the current feedback signal at step510may include performing multiple steps described inFIG. 5B.

FIG. 5Bis a flow chart illustrating a method of receiving a current feedback signal, according to an embodiment. At step5102, a first component indicative of the output voltage is received. At step5104, a second component indicative of the current flowing through the inductor is received. At step5106, the first component is added to the second component, e.g., as calculated by Equation 110, to provide the current feedback signal.

Although illustrative embodiments have been shown and described, a wide range of modification, change and substitution is contemplated in the foregoing disclosure and in some instances, some features of the embodiments may be employed without a corresponding use of other features. Those of ordinary skill in the art will appreciate that the hardware and methods illustrated herein may vary depending on the implementation. For example, while certain aspects of the present disclosure have been described in the context of the system100having one or more devices, those of ordinary skill in the art will appreciate that the systems and processes disclosed are capable of being implemented using hardware, software, and firmware components including systems-on-a-chip (SoC) or a combination thereof.

The methods and systems described herein provide for an adaptable implementation. Although certain embodiments have been described using specific examples, it will be apparent to those skilled in the art that the invention is not limited to these few examples. For example, although a boost type DC-DC converter has been described, it is contemplated that additional DC-DC converter types such as buck, buck-boost, Cuk and others may be similarly controlled to improve performance against variations in the load and the input voltage, and for improved susceptibility to noise. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or an essential feature or element of the present disclosure.