Switch-mode power supply with frequency adjustment in discontinuous conduction mode

A switch-mode power supply and control circuitry that reduces variation in the switching frequency of the power supply with changes in loading. In one example, a switch-mode power supply includes an inductor, a transistor, and control circuitry. The transistor is coupled to the inductor, and configured to charge the inductor. The control circuitry is coupled to the transistor. The control circuitry is configured to turn off the transistor for a first time period. The first time period is a function of voltage across the inductor during the first time period. The control circuitry is also configured to determine whether the switch-mode power supply is operating in continuous conduction mode or discontinuous conduction mode. The control circuitry is further configured to add a predetermined fixed interval to the first time based on the switch-mode power supply operating in discontinuous conduction mode.

BACKGROUND

A switch-mode power supply is an electronic circuit that converts an input direct current (DC) supply voltage into one or more DC output voltages that are higher or lower in magnitude than the input DC supply voltage. A switch-mode power supply that generates an output voltage lower than the input voltage is termed a buck or step-down converter. A switch-mode power supply that generates an output voltage higher than the input voltage is termed a boost or step-up converter.

Some switch-mode power supply topologies include a drive/power transistor coupled at a switch node to an energy storage inductor/transformer. Electrical energy is transferred through the energy storage inductor/transformer to a load by alternately opening and closing the switch as a function of a switching signal. The amount of electrical energy transferred to the load is a function of the ON/OFF duty cycle of the switch and the frequency of the switching signal. Switch-mode power supplies are widely used to power electronic devices, particularly battery powered devices, such as portable cellular phones, laptop computers, and other electronic systems in which efficient use of power is desirable.

SUMMARY

A switch-mode power supply and control circuitry that reduces variation in the switching frequency of the power supply with changes in loading are disclosed herein. In one example, a switch-mode power supply includes an inductor, a transistor, and control circuitry. The transistor is coupled to the inductor, and configured to charge the inductor. The control circuitry is coupled to the transistor. The control circuitry is configured to turn off the transistor for a first time period. The first time period is a function of voltage across the inductor during the first time period. The control circuitry is also configured to determine whether the switch-mode power supply is operating in continuous conduction mode or discontinuous conduction mode. The control circuitry is further configured to add a predetermined fixed interval to the first time based on the switch-mode power supply operating in discontinuous conduction mode.

In another example, a switch-mode power supply control circuit includes a transistor, a flip-flop, a comparator, and a delay circuit. The transistor includes a source terminal that is coupled to ground. The flip-flop includes an output terminal that is coupled to a gate terminal of the transistor. The comparator includes a first input terminal and a second input terminal. The first input terminal is coupled to a drain terminal of the transistor. The second input terminal is coupled to ground. The delay circuit includes a first input terminal and a second input terminal. The first input terminal of the delay circuit is coupled to an output terminal of the comparator. The second input terminal of the delay circuit is coupled to a timer circuit. The output terminal of the delay circuit coupled an input of the flip-flop.

In a further example, switch-mode power supply control circuit includes an inductor terminal, an input voltage terminal, an output voltage terminal, a first transistor, a second transistor, and control circuitry. The inductor terminal is for connecting an inductor to the switch-mode power supply control circuit. The first transistor configured to pull the inductor terminal to ground. The second transistor is configured to connect the inductor terminal to the output voltage terminal. The control circuitry is coupled to the first transistor and the second transistor. The control circuitry is configured to turn off the first transistor for a first time period that is a function of voltage at the input voltage terminal and voltage at the output voltage terminal. The control circuitry is also configured to determine whether the switch-mode power supply is operating in continuous conduction mode or discontinuous conduction mode. The control circuitry is further configured to add a predetermined fixed interval to the first time based on the switch-mode power supply operating in discontinuous conduction mode.

DETAILED DESCRIPTION

In switch-mode power supplies (e.g., DC-DC converters) that operate in continuous conduction mode and in discontinuous conduction mode, the power supply switching frequency can differ substantially between the two modes. Such differences in operating frequency may be undesirable in applications that seek to minimize the frequency range of power supply noise.

FIG. 1shows a schematic for an example switch-mode power supply100that exhibits substantial variation in switching frequency when operating in discontinuous conduction mode versus continuous conduction mode. The switch-mode power supply100is a boost converter, and includes an inductor102, a transistor104, a transistor106, and control circuitry132. The transistor104and/or the transistor106may be a negative (N) channel metal oxide semiconductor field effect transistor (MOSFET) or a positive (P) channel MOSFET in various implementations of the100. The inductor102is coupled to the transistor104and the transistor106. The transistor104, when activated, connects the inductor102to ground to induce current flow in the inductor102(i.e., to charge the inductor102). The transistor106, when activated, connects the inductor102to the output terminal134. When the transistor104is active, the transistor106is inactive, and when the transistor106is active the transistor104is inactive. When the transistor106is active, the voltage at node130rises and current flows from the inductor102to the output terminal134to power a load circuit and/or charge a filter capacitor. The load circuit and filter capacitor are not shown inFIG. 1.

The control circuitry132controls the activation and deactivation of the transistor104and the transistor106. The control circuitry132includes a flip-flop108, a timer circuit138, and a feedback circuit136. The flip-flop108is coupled to the transistor104and the transistor106. A first output of the flip-flop108drives the transistor104, and a second (complementary) output of the flip-flop108drives the transistor106. The output switching of the flip-flop108ensures that the one of the transistor104and the transistor106that is current on is turned off before the one of the transistor104and the transistor106that is currently off is turned on. For example, high-to-low switching of the outputs of the flip-flop108is faster than low-to-high switching of the outputs of the flip-flop108, which provides a delay, or dead time, between when one of the transistors is turned off and the other is turned on. The flip-flop108turns the transistor104on and the transistor106off to charge the inductor102, and turns the transistor104off and the transistor106on to connect the node130to the output terminal134. The flip-flop108may include a gate driver circuit at each of the outputs connected to the transistor104or the transistor106to provide the current needed to charge the gate capacitances of the transistor104and the transistor106.

The feedback circuit136monitors the voltage at the output terminal134and resets the flip-flop108based on the voltage at the output terminal134. The feedback circuit136includes a voltage divider made up of resistor110and resistor112, an error amplifier114, and a comparator116. The error amplifier114sets the current threshold for turning off the transistor104based on the voltage at the output terminal134(as divided by the resistor110and the resistor112) and a reference voltage. When the current flowing through the transistor104(i.e., the current flowing in the inductor102) exceeds the threshold set by the error amplifier114, the output of the comparator116is activated to turn off the transistor104and turn on the transistor106.

The timer circuit138controls the duration of time during which the transistor104is turned off. When the flip-flop108turns off the transistor104, the voltage at the node130rises and initiates operation of the timer circuit138. The timer circuit138includes a comparator118, a current source124, a capacitor126, a switch128, and a voltage divider formed by resistor120and resistor122. When the flip-flop108turns off the transistor104, the switch128is open and the current source124generates a current that charges the capacitor126. The current source124may be resistor in some implementations. When the voltage across the capacitor126exceeds the voltage provided to the comparator118by the resistor120and the resistor122, then the output of the comparator118is activated. Activation of the output of the comparator118causes the flip-flop108to turn off the transistor106and turn on the transistor104. Activation of the output of the comparator118also causes the switch128to close and discharge the capacitor126in preparation for timing the transistor104off time in the next cycle.

In the switch-mode power supply100, when operating in the continuous conduction mode, the time that the transistor104is turned off includes the time period generated by the timer circuit138(TOFF) and the time between when the flip-flop108turns off the transistor106and turns on the transistor104(i.e., dead time). However, when the switch-mode power supply100is operating in the discontinuous conduction mode, the time the transistor104is turned off includes only TOFF. When current flow in the inductor102is negative (as when the switch-mode power supply100is operating in the discontinuous conduction mode) turning off the transistor106causes the voltage at the node130to drop and charging of the inductor102begins before the transistor104is turned on. Thus, in the switch-mode power supply100, the time that the inductor102is discharged is longer when operating in continuous conduction mode, than when operating in discontinuous conduction mode, and in-turn the operating frequency of the switch-mode power supply100is higher when operating in discontinuous conduction mode than when operating in continuous conduction mode.

FIG. 2shows an example timing diagram for switching of the switch-mode power supply100in continuous conduction mode. In the continuous conduction mode the current in the inductor102is always positive. While the transistor104is turned on, in interval206, the current204in the inductor102is positive and the voltage at node130(VSW) is approximately zero (e.g., the voltage dropped across the transistor104). At the end of the interval206, the flip-flop108turns off the transistor104at time208and, after expiration of time TDEAD1, turns on the transistor106at time210. At the expiration of TOFF, at time214, the flip-flop108turns off the transistor106, and after expiration of time TDEAD2, the flip-flop108turns on the transistor104at time216. After the transistor104is turned on at time216the inductor102begins to charge. Thus, the inductor102discharges from time208to time216when the switch-mode power supply100is operating in the continuous conduction mode. The switching frequency of the100when operating in continuous conduction mode is:

FIG. 3shows an example timing diagram for switching of the switch-mode power supply100in discontinuous conduction mode. In the discontinuous conduction mode the current in the inductor102is negative for a portion of the TOFFtime. While the transistor104is turned on, in interval306, the current304in the inductor102is positive and the voltage at node130(VSW) is approximately zero (e.g., the voltage dropped across the transistor104). At the end of the interval306, the flip-flop108turns off the transistor104at time308and, after expiration of time TDEAD1, turns on the transistor106at time310. In the TOFFinterval312the current in the inductor102falls below zero amperes. At the expiration of TOFF, at time314, the flip-flop108turns off the transistor106, and the voltage VSWfalls to zero or below, which in turn initiates charging of the inductor102. After expiration of time TDEAD2, the flip-flop108turns on the transistor104at time316. Thus, the inductor102discharges from time308to time314when the switch-mode power supply100is operating in the discontinuous conduction mode. The switching frequency of the100when operating in discontinuous conduction mode is:

FIG. 4shows an example graph of switching frequency versus output voltage for operation of an implementation of the switch-mode power supply100in continuous conduction mode and discontinuous conduction mode. The curve402shows operation frequency in continuous conduction mode, and the curve404shows operation frequency in the discontinuous conduction mode. In the continuous conduction mode, the switch-mode power supply100operates in a frequency range of about 1.32 megahertz (MHz) to about 1.5 MHz. In the discontinuous conduction mode, the switch-mode power supply100operates in a frequency range of about 1.45 MHz to about 1.65 MHz. For this implementation of the switch-mode power supply100, the frequency variation between operation in continuous conduction mode and discontinuous conduction mode is up to about 200 kilohertz (KHz)406.

FIG. 5shows a schematic for an example switch-mode power supply500with reduced variation in switching frequency when operating in discontinuous conduction mode versus continuous conduction mode. The switch-mode power supply500is a boost converter, and includes an inductor102, a transistor504, a transistor506, and control circuitry532. The transistor504and/or the transistor506may be an N channel MOSFET or a P channel MOSFET in various implementations of the500. The inductor102is connected to an inductor input terminal530(also referred to herein as node530), and coupled to the transistor504and the transistor506. The transistor504, when activated, connects the inductor102to ground to induce current flow in the inductor102(i.e., to charge the inductor102). A drain terminal504D of the transistor504is coupled to the inductor502and to a source terminal506S of the transistor506, and a source terminal504S of the transistor504is coupled to ground. The transistor506, when activated, connects the inductor102to the output terminal534. When the transistor504is active, the transistor506is inactive, and when the transistor506is active the transistor504is inactive. When the transistor506is active, the voltage at node530rises and current flows from the inductor102to the output terminal534to power a load circuit and/or charge a filter capacitor. The load circuit and filter capacitor are not shown inFIG. 5.

The control circuitry532controls the activation and deactivation of the transistor504and the transistor506. The control circuitry532includes a flip-flop508, a timer circuit538, a feedback circuit536, a comparator542, and a delay circuit544. The flip-flop508is coupled to the transistor504and the transistor506. A first output508A of the flip-flop508is coupled to and drives the gate terminal504G of the transistor504, and a second (complementary) output508C of the flip-flop508is coupled to and drives the gate terminal506G of the transistor506. The output switching of the flip-flop508ensures that the one of the transistor504or the transistor506that is currently on is turned off before the one of the transistor504or the transistor506that is currently off is turned on. For example, high-to-low switching of the outputs of the flip-flop508is faster than low-to-high switching of the outputs of the flip-flop508, which provides a delay, or dead time, between when one of the transistors is turned off and the other is turned on. The flip-flop508turns the transistor504on and the transistor506off to charge the inductor102, and turns the transistor504off and the transistor506on to connect the node530to the output terminal534. The flip-flop508may include a gate driver circuit at each of the outputs connected to the transistor504or the transistor506to provide the current needed to charge the gate capacitances of the transistor504and the transistor506.

The feedback circuit536monitors the voltage at the output terminal534and resets the flip-flop508based on the voltage at the output terminal534. The feedback circuit536includes a voltage divider made up of resistor510and resistor512, an error amplifier514, and a comparator516. The error amplifier514sets the current threshold for turning off the transistor504based on the voltage at the output terminal534(as divided by the resistor510and the resistor512) and a reference voltage. When the current flowing through the transistor504(i.e., the current flowing in the inductor102) exceeds the threshold set by the error amplifier514, the output of the comparator516is activated to reset the flip-flop508, which the turns off the transistor504and turns on the transistor506. The comparator516includes an input terminal516A that is coupled to the source terminal504S of the transistor504, and an output terminal516C that is coupled to an input terminal508D of the flip-flop508.

The timer circuit538controls the time duration during which the transistor504is turned off. When the flip-flop508turns off the transistor504, the voltage at the node530rises and initiates operation of the timer circuit538. The timer circuit538includes a comparator518, a current source524, a capacitor526, a switch528, and a voltage divider formed by resistor520and resistor522. When the flip-flop508turns off the transistor504, the switch528is open and the current source524generates a current that charges the capacitor526. The capacitor526includes a first terminal526A that is coupled to an input terminal518A of the comparator518, and a second terminal526B that is coupled to ground. The current source524may be a resistor in some implementations. When the voltage across the capacitor526exceeds the voltage provided to the comparator518at input terminal518B by the resistor520and the resistor522, then the output of the comparator518is activated. Activation of the output of the comparator118causes the flip-flop108to turn off the transistor106and turn on the transistor104. Activation of the output of the comparator518also causes the switch528to close and discharge the capacitor526in preparation for timing the transistor504off time in the next cycle. The switch528includes a control terminal528C that is coupled to an output terminal518C of the comparator518, a terminal528A that is coupled to the first terminal526A of the capacitor526, and a terminal528B that is coupled to the second terminal526B of the capacitor526.

The comparator542and the delay circuit544compensate for the increase in switching frequency when operating in the discontinuous conduction mode in the switch-mode power supply100. The comparator542includes an input terminal542A that is coupled to the node530(i.e., terminal502A of the inductor502) and a drain terminal504D of the transistor504, and an input terminal542B that is coupled to ground. The comparator542compares the current in the inductor102to zero to determine whether the current in the inductor102is positive or negative. If the current flowing in the inductor102is negative, then the switch-mode power supply500is operating in discontinuous conduction mode, and otherwise is operating in continuous conduction mode.

The output terminal542C of the comparator542is coupled a control input terminal544A of the delay circuit544. An output terminal518C of the comparator518is coupled to an input terminal544B of the delay circuit544. An output terminal544C of the delay circuit544is coupled to an input terminal508B of the flip-fop508. When the output of the comparator542indicates that the switch-mode power supply500is operating in discontinuous conduction mode, the delay circuit544routes the output of the comparator518to the flip-flop508through a delay element. When the output of the comparator542indicates that the switch-mode power supply500is operating in continuous conduction mode, the delay circuit544bypasses the delay element to route the output of the comparator518directly to the flip-flop508. The delay element may delay the output of the comparator518by about the time between when the flip-flop108turns off the transistor106and when the flip-flop108turns on the transistor104(i.e., TDEAD2).

FIG. 6shows a schematic diagram for an example delay circuit600suitable for use in switch-mode power supply500. The delay circuit600includes a delay element602and a switch604. The delay element602may include a number buffer devices connected in series to provide a selected delay. The switch604includes an input terminal604A that is coupled to the input terminal600B of the delay circuit600and the input terminal602A of the delay element602, a control terminal604B that is coupled to the control terminal600A of the delay circuit600, and an output terminal604C that is coupled to the output terminal602B of the delay element602and the output terminal600C of the delay circuit600. The switch604routes signal around the delay element602when closed. In the switch-mode power supply500, the switch604is closed when the switch-mode power supply500is operating in continuous conduction mode and open when operating in discontinuous conduction mode.

When the switch-mode power supply500is operating in the continuous conduction mode, the time that the transistor504is turned off includes the time period generated by the timer circuit538(TOFF) and the time between when the flip-flop508turns off the transistor506and turns on the transistor504(i.e., dead time) as shown inFIG. 2. When the switch-mode power supply100is operating in the discontinuous conduction mode, the comparator542and the delay circuit544increase the time that the transistor506is turned on by the delay time of the delay circuit544. Thus, in the switch-mode power supply500, the time that the inductor102is discharged is increased when operating in discontinuous conduction mode to better match the discharge time of the inductor102when operating in continuous conduction mode. Thus, the switch-mode power supply500reduces the difference in operating frequency when operating in discontinuous conduction mode and continuous conduction mode.

FIG. 7shows an example timing diagram for switching of the switch-mode power supply500in discontinuous conduction mode. In the discontinuous conduction mode the current in the inductor502is negative for a portion of the TOFFtime. While the transistor504is turned on, in interval706, the current704in the inductor502is increasing and the voltage at node130(VSW) is approximately zero (e.g., the voltage dropped across the transistor504). At the end of the interval306, the flip-flop108turns off the transistor504at time708and, after expiration of time TDEAD1, turns on the transistor506at time710. In the TOFFinterval712, defined by the timer circuit538, the current in the inductor502is decreasing and falls below zero amperes. At the expiration of TOFF, at time714, the signal generated by the comparator518is delayed by the delay circuit544for a time Delay that is approximately equal to TDEAD2. At time716, the signal generated by the comparator518has propagated through the delay circuit544and triggered the flip-flop508to turn off the transistor106. When the transistor106turns off, the voltage VSWfalls to zero or below, which in turn initiates charging of the inductor102. After expiration of time TDEAD2, the flip-flop508turns on the transistor504at time716. Thus, by delaying the output signal generated by the timer circuit538for about time TDEAD2when the switch-mode power supply500is operating in discontinuous conduction mode, variation the frequency of operation of the switch-mode power supply500is reduced. Thus, the time that the transistor504remains turned off is a function of the voltage across the inductor502while the transistor504is off with additional compensation when operating in discontinuous conduction mode.

When operating in discontinuous conduction mode, the time that the transistor504is turned off is:

TotalOffTime=KRCVINVOUT+TDEAD,
where:K is the resistance of resistor522divided by the sum of the resistance of resistor520and the resistance of the resistor522;R is the resistance of a resistor serving as the current source524; andC is the capacitance of the capacitor526;VINis the voltage at the voltage input terminal548;TDEADis the time delay from switching of the output508C to switching of the output508A of the flip-flop508.

When operating in continuous conduction mode, the time that the transistor504is turned off is:

FIG. 8shows an example graph of switching frequency versus output voltage for operation of an implementation of the switch-mode power supply500in continuous conduction mode and discontinuous conduction mode. The curve802shows operation frequency in continuous conduction mode, and the curve804shows operation frequency in the discontinuous conduction mode. In the continuous conduction mode, the switch-mode power supply500operates in a frequency range of about 1.32 megahertz (MHz) to about 1.5 MHz. In the discontinuous conduction mode the switch-mode power supply500operates in a frequency range of about 1.32 MHz to about 1.48 MHz. For this implementation of the switch-mode power supply500, the frequency variation between operation in continuous conduction mode and discontinuous conduction mode is up to about 20 KHz806.

Various components of the500may be provided on an integrated circuit as a switch-mode power supply control circuit. For example, the components546, or a subset thereof, may be provided as an integrated circuit switch-mode power supply controller.

FIG. 9shows a block diagram for a wireless device900that includes a switch-mode power supply with reduced variation in switching frequency when operating in discontinuous conduction mode versus continuous conduction mode in accordance with the present disclosure. The wireless device900may be a smartphone, a tablet computer, or other device that includes circuitry for wireless radio frequency communication. The wireless device900includes a switch-mode power supply902, a wireless transmitter904, a wireless receiver906, processing and control circuitry908, and user interface circuitry910. The switch-mode power supply902is an implementation of the switch-mode power supply500. The switch-mode power supply902provides power to the wireless transmitter904. In some implementations of the900, the switch-mode power supply902may provide power to other circuitry, such the wireless receiver906, the processing and control circuitry908, and/or the user interface circuitry910. Because the switch-mode power supply902operates over a narrower range of switching frequencies than implementations of a switch-mode power supply that lack the low load off time compensation disclosed herein, the noise generated by the switch-mode power supply902is confined to a narrower band than other switch-mode power supplies. As a result, the noise produced by switching of the switch-mode power supply902is more easily isolated from noise sensitive frequency bands. For example, the switch-mode power supply902may switch in a relatively narrow frequency band that is well outside the range analog signals in the wireless receiver906. Such isolation may be more difficult to achieve with the wider operational frequency range of switch-mode power supplies that lack the low load off time compensation disclosed herein.

The wireless transmitter904includes circuitry for wirelessly transmitting data provided by the processing and control circuitry908. For example, the wireless transmitter904may include an encoder, a modulator, an upconverter, a power amplifier, etc.

The wireless receiver906includes circuitry for extracting data from a detected wireless signal and providing the extracted data to the processing and control circuitry908. For example, the wireless receiver906may include a downconverter, decoder, a demodulator, and/or other circuitry.

The processing and control circuitry908includes circuitry for controlling the wireless transmitter904, wireless receiver906, and user interface circuitry910, and for processing data received from the wireless receiver906or user interface circuitry910, or data for use by the wireless transmitter904or the user interface circuitry910. For example, the processing and control circuitry908may include a processor, such as general-purpose microprocessor or a digital signal processor and memory that stores instructions that are executed by the processor.

The user interface circuitry910includes circuitry for providing information to or receiving information a user. For example, the user interface circuitry910may include a display device, such a liquid crystal or organic light emitting diode display device and associated control circuitry, touch screen circuitry, or other user input/output circuitry. The user interface circuitry910receives data from and provides data to the processing and control circuitry908.