Adaptive controller for control of a DC-DC power supply

The present application provides a tunable compensator providing a control signal to control a switch in a power supply. A measurement is taken of the level of activity of the control signal. This measurement is used to introduce a bias into a tuner tuning the compensator when the amount of activity in the control signal drops.

FIELD

The present application relates to DC-DC power supplies and methods for controlling same.

BACKGROUND

DC-DC converters are devices which are employed to convert an input DC voltage to another DC voltage. DC-DC converters may be classified generally as linear or switching. A conventional arrangement for a switching DC-DC converter uses a power stage comprising one or more switching devices and one or more inductors or capacitors or both to convert an input voltage (Vin) to an output voltage (Vout). A controller is employed to try and maintain the output voltage at a desired set point. Conventionally, pulse width modulation is employed to control the operation of the switching devices within the power stage and accordingly the controller provides a control signal to a PWM module which operates the switching devices. A variety of different switching circuit topologies may be employed within the power stage which will be familiar to those skilled in the art, including for example the conventional buck and boost topologies. In a linear switching supply, the gain of a transistor or similar device is adjusted to achieve a desired output from an input voltage.

The output from the DC-DC converter is typically provided to a load which may be an electronic circuit comprising a plurality of components or just a single component, for example an LED light.

To improve the performance of the controller it is known to use adaptive control, an example of which is for example described in U.S. Ser. No. 12/439,802, which is assigned to the present assignee, the entire contents of which are hereby incorporated by reference. The use of adaptive control significantly improves the performance of controllers. In the referenced patent application, two controllers are provided which operate jointly in parallel and whose outputs are combined together in a balanced manner. The adaptive control function alters the balance point between the two controllers to bias the control towards one or other of the controllers. An advantage of this approach is that stability is ensured notwithstanding the control function being changed by the adaptive elements. In these arrangements, the adaptive controller is effectively tuned with a single degree of freedom, which provides stability in the tuning process by allowing the overall control function to vary between a cautious and an aggressive function.

Adaptive controllers require excitation in order provide the necessary information for adaptation to occur. Where persistent excitation does not exist, the adaptive controller may not tune optimally, or may fail to tune at all. It is possible to excite the adaptive controller by introducing an external stimulus such as pseudo-random noise, or a periodic signal, but the disturbance this causes to voltage regulation is often unacceptable. The term external used in this context refers to the excitation being external to the adaptive control circuit, rather than its physical characteristics. Similarly a non-linearity may be introduced into the control loop to introduce a disturbance. Thus for example, in U.S. Pat. No. 7,586,767 which measures the characteristics of limit cycle oscillations to perform the tuning function, the limit cycle oscillations are caused by introducing a non-linearity in the control loop by altering the resolution of the PWM circuit. However again a disturbance is being introduced that may affect voltage regulation.

The present application seeks to improve the operation of adaptive controllers as employed generally in power supplies.

SUMMARY

The present application measuring the level of activity of the control signal from an adaptive controller introduces a bias into the adaptive controller as the amount of activity drops. It will also be appreciated that an external excitation source is not required in this technique.

More particularly, the present application provides a controller and\or method in accordance with the claims which follow.

The techniques of the present patent application may be applied within single or multiple phase control ICs.

DETAILED DESCRIPTION OF DRAWINGS

The present application is directed to switch mode power supplies, for example of the type where Pulse Width Modulation (PWM) is used to control one or more switching elements. PWM may be implemented using a PWM circuit which provides switching signals to one or more switches in the power supply. A controller controls the PWM circuit in an effort to maintain an output from the power supply (typically voltage) at a desired setpoint as shown in the arrangement ofFIG. 1. More specifically, the arrangement comprises a compensator20which operates to provide a control signal21to a digital pulse width modulation circuit22which in turn drives a power stage28to provide an output voltage (Vout). The compensator seeks to maintain the output voltage at a value dictated by a set point109(ref). The control function (parameters) of the compensator is tunable by a tuner24. The combination of the compensator and the tuner together provide an adaptive controller. The compensator may be any suitable controller, including for example P, PI or PID type controllers. The control function of the controller may have a plurality of parameters which may be varied or set. In turn the tuner using a suitable tuning algorithm may adjust or set one or more of these parameters.

In an exemplary arrangement using an adaptive controller design of the Assignee of the present application, the control function of the compensator is configured to have a single degree of freedom so that changes in controller parameters may be based on a single tuning value (α) which, in turn, is obtained from the tuner. An example of such an adaptive controller (as referenced above) is described in U.S. Ser. No. 12/439,802, which is assigned to the present assignee, the entire contents of which are hereby incorporated by reference. In the referenced patent application, two controllers are provided which operate jointly in parallel and whose outputs are combined together in a balanced manner to provide an overall control output. The adaptive control function alters the balance point between the two controllers to bias the control towards one or other of the controllers. The control function may be stated generally as: Cout=C1α+C2(1−α), where C1and C2are the outputs from each of the individual controllers, Coutis the overall output and α is the previously described tuning value having a range between 0 and 1. In the context of DPWM the control function will produce a duty cycle signal d which fed to a DPWM module. It will be appreciated that the function for the overall control function clearly has just one degree of freedom. It will be understood that other control functions are also available to provide a single degree of freedom and that accordingly the application is not to be construed as being limited to just this example. Having a single degree of freedom is beneficial in power supplies in that the controller transfer function is deterministic with respect to the tuning value α, and may therefore be determined for all values of α. Suitably, one of the controllers is designed to have a conservative control function with the second controller designed with a more aggressive control function. In operation, the tuner uses measurements from the compensator to determine the optimum balance point between the conservative and the aggressive controller. During periods of load inactivity, in which persistent excitation is lacking, the nature of the tuning function which for example may seek to minimize error, causes the balance to tend towards the conservative controller. After such a period without persistent excitation, the adaptive tuning should react quickly once load activity occurs in order to re-tune the controller to the new optimum point, i.e. the tuner will cause the controller to shift away from the conservative controller and towards the aggressive controller. However, the rate at which the tuning may occur may mean the controller's performance when a sudden change occurs is less than ideal. Thus, during these inactive periods, the present application seeks to stimulate the adaptive control function by tuning more towards the aggressive controller, which will provide more control activity in the loop, thereby providing persistent excitation, which allows the adaptive controller to adapt quickly to changing load conditions. Overall regulation performance over a wide range of load conditions may be improved by this method. It will be appreciated that this is achieved without disturbing the normal operation of the compensator control loop, i.e. no disturbances are required to be introduced into the compensator control loop.

It may be convenient to view of the adaptive mechanism of this application as having dual goals (or modes), a) optimize regulation and b) stimulate loop excitation; yielding an adaptive controller that reacts appropriately to a wide range of load conditions, and obviates the need for external excitation. In general, existing dual-control methods introduce external disturbances (probing signals) or a hard limit on the variance of the control signal at a pre-determined limit which produces an unacceptable level of noise on the regulated voltage output, and results in undesirable behavior in the tuning process.

The embodiment ofFIG. 1includes a biasing mechanism26to effect the dual mode adaptive control scheme described above, with a method shown inFIG. 2. This biasing mechanism measures60the amount of change on the output control signal21of the compensator and generates62a bias signal23for the tuner when the excitation is low, i.e. in situations where the controller may become conservative. In the tuner, the bias signal is effectively added\ subtracted (as appropriate)64to the α value generated within the tuner to provide a modified α value. In periods of persistent excitation, the biasing mechanism provides little or no bias signal and the tuner functions as before.

In the biasing mechanism, an activity measurement module, for example variance element30, measures the variance of the control signal and a measure of activity Δd is obtained. It will be appreciated that an absolute measure such as variance is required rather than an average since an average value will be approximately the same regardless of the amount of change, and the object is to measure the amount of activity. For example the average of a sine wave would be zero regardless of the values of the peaks, using an absolute value a measure is obtained. It will also be appreciated that various methods exist for determining a signal's level of activity such as its variance or a suitable mathematical norm such as the 2-norm or 1-norm. A low pass filter (32) is employed for providing a time average estimate 105 of the measured control activity to limit the effect of instantaneous values and provide an estimate of the long term signal variance. Suitably a gain is incorporated within the variance element or the low pass filter to normalize the maximum value of the variance value leaving the LPF to 1, i.e. the measure of activity is limited to a range between 0 and 1. This activity value is then subtracted from a value of 1. The resulting value will be close to 0 during periods of significant change in the control signal from the compensator and close to 1 in periods of no change.

This signal is then fed through a gain element34having a gain value107for limiting the effect of the bias signal. The resulting signal23is fed to the tuner where it is added to the internal tuning value α being provided to the compensator to provide a modified α′ which is used as a single tuning value107by the compensator. This signal α′ effectively weights the adaptive bias such that during periods of inactivity, where persistent excitation of the adaptive loop is lacking, the adaptive tuning is biased. It will be appreciated that in the context of the present exemplary embodiment, during periods of low activity on the control signal21, the bias mechanism26will generate a bias signal to tend the compensator towards the more aggressive of the two control functions. Conversely, the adaptive tuning remains unbiased during periods where excitation is sufficient to affect loop adaptation. The value ofKbias, and frequency response of the LPF, may be suitably chosen to determine the amount of loop excitation which is considered to be sufficient. It will be appreciated that the dual mode adaptive control mechanism forms a control loop whose goal is to ensure excitation of the adaptive tuner in which the level of excitation is controlled, minimizing the output voltage noise introduced by the excitation. It will be further appreciated that the value ofKbiasand the characteristics of the LPF may vary from system to system depending on the characteristics of the control functions being employed. Appropriate values for the characteristics of the LPF andKbiasmay be determined for example, by analysis of the loop involvingKbiasand the LPF. Whilst, the described arrangement of the biasing mechanism is effective. It will be appreciated that modifications and alternatives may be provided. For example, a threshold may be applied to the measured variance, i.e. whereby only when the variance drops below the threshold does the biasing mechanism become operable. It will be appreciated that such a threshold may be implemented at different points within the biasing mechanism. In any event, it will be appreciated that purpose of the biasing mechanism is not to determine\set the tuning value as this function is performed by the tuning algorithm of the tuner. Instead it is to compensate for the tuning algorithm becoming conservative during periods of low activity in the control loop. It will be appreciated that whilst several different embodiments have been described herein, that the features of each may be advantageously combined together in a variety of forms to achieve advantage. Thus for example, whilst the above description has been made with reference to an exemplary controller as employed by the present assignee, it will be appreciated that the method may also be applied and\or incorporated within other adaptive control schemes. Similarly, whilst the above system and method has been described generally with respect to a switch mode power supply, it will be appreciated that the technique may also be applied to non-switching (linear) power supplies. It will be appreciated that in such an arrangement, the DPWM would be replaced with a linear driving stage and the power stage would be a linear mode power stage.

The conductors as discussed herein may be illustrated or described in reference to being a single conductor, a plurality of conductors, unidirectional conductors, or bidirectional conductors. However, different embodiments may vary the implementation of the conductors. For example, separate unidirectional conductors may be used rather than bidirectional conductors and vice versa. Also, plurality of conductors may be replaced with a single conductor that transfers multiple signals serially or in a time multiplexed manner. Likewise, single conductors carrying multiple signals may be separated out into various different conductors carrying subsets of these signals. Therefore, many options exist for transferring signals.

Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code. Furthermore, the devices may be physically distributed over a number of apparatuses, while functionally operating as a single device.