Switching power supply control with phase shift

A control system for a switching power supply shifts the phase of a PWM signal in response to a change in operating conditions. The phase may be shifted by resetting an oscillator that controls the PWM signal. Phase shift logic may include a sample-hold circuit that holds the value of an error signal when the PWM signal switches state. The held error signal may be compared to the real-time error signal, preferably with a user configurable offset. The output of the phase shift logic may be used to reset the oscillator.

BACKGROUND

FIG. 1illustrates a conventional switching power supply control system that utilizes pulse width modulation (PWM). A power converter10controls the flow of power from a power source VCCto the load12in response to a pulse width modulation signal PWM. The power converter includes a single switching transistor Q1, inductor L1, and diode D1arranged in a classic buck configuration. An error amplifier16generates an error signal VERRby amplifying the difference between a feedback signal FB, which provides some measure of the output, and an input control signal VIN. A controller14generates a PWM signal that controls the switching transistor in the power converter in response to VERR, so as to provide a regulated output, typically a constant voltage, to the load.

FIG. 2illustrates an example waveform of the PWM signal generated by the controller14. At time t0, the PWM signal switches to the ON state which turns on the switching transistor in the power converter10. A time t1, the PWM signal switches to the OFF state, thereby turning the switching transistor off. The pulse width is the amount of time from t0to t1during which the PWM signal is in the ON state. The waveform ofFIG. 2has a period T which is the time between successive ON transitions of the PWM signal. To control the amount of power to the load, the controller varies the pulse width (duty cycle) of the PWM signal by varying the time t1at which the PWM signal switches off. For example, if the amount of power being supplied to the load is too low, the controller may leave the switch on longer until t1′ so that the transistor switch in the power converter is on for a longer portion of the switching cycle. Conversely, if the amount of power being supplied to the load is too low, the controller may leave the switch on only until t1″ so that the switch is on for a shorter portion of the switching cycle. The on time may be varied during each switching cycle to provide continuous output regulation.

The example described above may be referred to as trailing edge modulation because the second switching event (the turn off event in this case) during each switching cycle is varied. In leading edge modulation, the first switching even (e.g., turn on time) is varied, while the PWM signal turns off at a fixed time.

Various techniques have been suggested for improving the transient response of PWM switching power supplies. One technique involves the use of a voltage controlled oscillator (VCO) to control the switching cycle of the PWM signal. By applying an error signal to the VCO, the switching frequency of the PWM signal may be changed dynamically in an attempt to maintain a constant output voltage into a changing load. Another approach referred to as a glitch catcher circumvents the normal control loop to provide a temporary output current pulse directly from the supply VCCinto the load in an attempt to prevent the output voltage from falling in response to a sudden increase in the load.

DETAILED DESCRIPTION

This patent disclosure encompasses numerous inventions relating to switching power supplies. These inventive principles have independent utility and are independently patentable. In some cases, additional benefits are realized when some of the principles are utilized in various combinations with one another, thus giving rise to yet more patentable inventions. These principles can be realized in countless different embodiments. Only the preferred embodiments are described below. Although some specific details are shown for purposes of illustrating the preferred embodiments, other equally effective arrangements can be devised in accordance with the inventive principles of this patent disclosure. Thus, the inventive principles are not limited to the specific details disclosed herein.

FIG. 3illustrates an embodiment of a switching power supply having a control system according to the inventive principles of this patent disclosure. The embodiment ofFIG. 3includes power converter18that may embody any suitable switching power supply topology and therefore may include any appropriate number and combination of switches, transformers, inductors, capacitors, diodes, and the like. For ease of explanation, the power converter in this example may be assumed to have a single switching element requiring only a single PWM signal, but the inventive principles are not limited to any particular topology and may encompass embodiments that utilize multiple PWM signals.

The PWM signal shown inFIG. 3is generated by a controller22that has a provision for adjusting the phase of the PWM signal. The embodiment ofFIG. 3also includes phase shift logic24to shift the phase of the PWM signal in response to a change in operating conditions such as a change in the load20, the power source VCC, a control input signal, etc.

This phase shift may be better understood with reference toFIG. 4which illustrates an embodiment of a PWM signal according to the inventive principles of this patent disclosure. The switching cycle from t0to t2operates in a conventional manner with the PWM signal being switched off at time t1which may be varied to regulate the output to the load. The next cycle also begins with a conventional “on” pulse from t2to t3, the width of which is also modulated to control the output. However, at some point during the off time beginning at t3, the system detects a change in operating conditions, e.g., an increase in the load that increases the demand for power. If the PWM signal remains off until the normally scheduled turn on time at t4, the output may droop to unacceptable levels. In the embodiments ofFIG. 4, however, the turn on is accelerated with the PWM signal being quickly turned back on at t4′ to respond more quickly to the changed operating conditions. This imparts a phase shift to the PWM signal which reverts back to a normal period T during subsequent switching cycles.

The principles of phase shifting a PWM signal according to the inventive principles of this patent disclosure may be applied to all forms of PWM converter topologies and control techniques including leading edge modulation, trailing edge modulation, etc. For example, in an implementation with leading edge modulation, the PWM signal may be turned off immediately upon a change in operating conditions rather than waiting for the normal turn off time. The phase shift logic24illustrated inFIG. 3may be implemented in hardware, software, or any suitable combination thereof. Embodiments according to the inventive principles of this patent disclosure may also include additional apparatus such as error amplifiers, outputs sensors, etc. which may be separate from the apparatus shown inFIG. 3, or may be fabricated integral with the apparatus in a synergistic manner as will be described below.

FIG. 5illustrates another embodiment of a switching power supply having a control system according to the inventive principles of this patent disclosure. In the embodiment ofFIG. 5, the controller includes a comparator28which generates the PWM signal in response to a clock signal from an oscillator26and an error signal VERRfrom an error amplifier29. The phase shift logic32shifts the phase of the PWM signal by resetting the oscillator26. In this embodiment, the phase shift logic detects a change in operating conditions by monitoring the error signal VERR.

FIG. 6illustrates an embodiment of a phase shifting circuit for a switching power supply control system according to the inventive principles of this patent disclosure. A switch38, resistor R1and capacitor C1form a sample-hold circuit that tracks an error signal VERRwhen the switch is closed during the on time of the PWM signal. When the PWM signal turns off, a comparator40compares the sampled value of VERRthat is held on C1to the real-time value of VERRto monitor changes in operating conditions. The output of the comparator generates a reset signal that may be used to phase shift the PWM signal, for example, by resetting an oscillator that controls the PWM signal. An offset42may be added to an input of the comparator to provide noise immunity and/or set a threshold level for phase shifting the PWM signal. The offset may be user configurable as shown inFIG. 6. In an embodiment for a multi-phase power converter that utilizes more than one PWM signal, the PWM signals may be logically OR'ed so that the sample-hold circuit holds the sampled value whenever all phases are off.

FIG. 7illustrates another embodiment of a switching power supply having a control system according to the inventive principles of this patent disclosure. The embodiment ofFIG. 7includes a two-phase power converter with a PWM control system having multiple output representational signals summed for control purposes as described in U.S. Pat. No. 6,683,441 B2 which has a common inventor with the present patent disclosure. The output signal VCOMPfrom the voltage control feedback amplifier A2provides a convenient error signal for the phase shift logic39. The PWM signals PWM1and PWM2are combined in OR gate44to control the sample-hold switch38. The RESET output from comparator44resets the oscillator in controller210to phase shift the PWM signals in response to a change in operating conditions. Thus, a robust and synergistic control system with improved transient response may be realized by adding very few additional components and utilizing signals that already exist in a PWM control system.

The inventive principles of this patent disclosure have been described above with reference to some specific example embodiments, but these embodiments can be modified in arrangement and detail without departing from the inventive concepts. Thus, such changes and modifications are considered to fall within the scope of the following claims.