ADAPTIVE CONTROLLER BASED ON TRANSIENT NORMALIZATION

Abstract

A controller is provided for controlling a power stage of a power converter according to a control law, the control law implementing a specific type of compensator and being pre-designed to generate an objective response of a default power converter for a default parameter value of a component of the power stage. The controller is further configured to determine an actual response for an actual parameter value of the component of the power stage and to alter the control law for the actual parameter value of the component of the power stage such that the actual response matches the objective response. The controller determines a degree of matching between the actual response and the objective response by filtering the actual response to generate a filtered actual response and integrating a product of the filtered actual response and a delayed actual response.

Description

FIELD OF THE INVENTION

The present invention relates to an adaptive controller based on transient normalization, a related power converter and a related method. The present invention specifically relates a to DC/DC conversion controller in a system that automatically characterizes the power stage of a DC-DC converter and tunes a compensator in response to the characterization.

BACKGROUND OF THE INVENTION

Prior art systems have addressed the issue of DC-DC converter tuning by manually adjusting the controller response through a means convenient and accessible to the user, e.g., [1]. In [2] it was recognized that a DC-DC conversion power stage had properties that led to a convenient method to scale a pre-designed compensator so that its open-loop crossover frequency and phase margin remained approximately constant as power stage parameters varied, lending itself to manual tuning by the end user without having to re-design the compensator.

Adaptive control methods have been applied to the problem of DC-DC converter tuning. In [3] and [4] non-parametric methods were applied, involving the addition of a sinewave disturbance [3] or induced loop oscillation [4] into the system to measure the loop characteristics such as phase margin. However, these methods may be affected by outside disturbances which may be common in Point of Load regulation applications for example, and furthermore, may introduce noticeable disturbances on the output voltage affecting regulation performance.

In [5] a model reference impulse response method is introduced in which two methods are proposed to characterize the impulse response of the system involving a one-time fast characterization of the system and a long-term statistical characterization. Whilst the statistical method proposed in [5] and [6] can be used online, the convergence time is too long for many applications due to the length of the required noise sequence and the introduced noise disturbance may be undesirable. The impulsive perturbation method suggested in [5] requires an experimental impulse to be introduced repeatedly whilst a 2-parameter search is carried out to determine the regulator parameters. This introduces disturbances during the tuning, suffers from similar drawbacks to [3, 4] regarding sensitivity to outside disturbances and non-optimum convergence in the presence of noise.

The controller of [7] shows how an LMS filter can be used in a feedforward controller to tune a single gain but does not address the issue of adaptive control of the general transfer function of the regulator. That issue is addressed in [8] and [9] where a prediction error filter (PEF) is used to tune the loop based on minimisation of the power of the prediction error. However, the pseudo-open loop requirement for controller adaptation may lead to initial output voltage regulation being far less than required, and the two-parameter control system may be prone to divergence and therefore an unstable controller may result in certain circumstances. These issues are addressed in [10] where the PEF is utilised to adjust the balance between two controllers. But there are several drawbacks, for example:

• • i) the requirement to pre-design and to implement two controllers is over complicated for many users;
  • ii) the iterative minimisation of the prediction error takes some time which means that regulation is compromised during the convergence time because the controller is initially too conservative;
  • iii) the requirement for common state variables in the two fixed controllers means they cannot be integral controllers and this lack of a capability to vary the integral gain of the controller is a limiting factor in preserving the pulse response of the system.

Also [8], [9] and [10] are all limiting the type of control structure to ARMA (zero/pole/non-integral) type structures with a feedforward element for steady-state regulation. The vast majority of controllers are PID and there is a distinct advantage in being able to tune or adjust PID compensators automatically without limitation.

PCT/EP2014/063987 relates to a method to adjust a compensator across the end users design space, e.g., output capacitance, and a means for the end user to configure the compensator such that the most suitable adjustment value is selected. Considering that it is advantageous to retain the response of the original system whilst a power stage parameter is changed, e.g., the capacitance C, it is desired to determine the manner in which the compensator must change in order to achieve a similar response. That is, this objective may be realized by designing a base compensator in the usual way, and devising a means to alter the compensation according the new value of C for example so as to maintain system performance. It has been shown that this relationship can be maintained, for example, if the proportional gain K_p, the integral gain K_i and and the differential gain K_d of a PID controller are altered from the original values in the following manner:

F=C _new /C,

K _inew =K _i*sqrt(F),

K _dnew =K _d *F,

K _pnew =K _p *F,

where K_i, K_p and K_d represent the gains of the original PID controller (i.e. original set of compensator coefficients), K_inew, K_pnew, K_dnew represent the altered values respectively to maintain the system response and C_new and C represent the new and original value of the bulk capacitance. It will be clear that the capacitance C is being used as an example and the system is not limited in this regard but can also adjust for variations in other parameters (e.g. L), in a similar way.

The following references have been cited:

• [1] Circuit and method for changing transient response characteristics of a DC/DC converter module, U.S. Pat. No. 7,432,692, 2008;
• [2] Kelly, A., A system and Method for Design and Selecting Compensators for a DC-DC Converter, WO 2015/000916 A1;
• [3] Morroni, J., R. Zane, and D. Maksimovic, Design and Implementation of an Adaptive Tuning System Based on Desired Phase Margin for Digitally Controlled DC-DC Converters, Power Electronics, IEEE Transactions on, 2009, 24(2): p. 559-564;
• [4] Stefanutti, W., et al., Autotuning of Digitally Controlled Buck Converters based on Relay Feedback, in Power Electronics Specialists Conference, 2005, Recife, Brazil;
• [5] Costabeber, A., et al., Digital Autotuning of DC-DC Converters Based on a Model Reference Impulse Response, Power Electronics, IEEE Transactions on, 2011, 26(10): p. 2915-2924;
• [6] Miao, B., R. Zane, and D. Maksimovic, Practical on-line identification of power converter dynamic responses, in Applied Power Electronics Conference, 2005, Austin, Tex., USA: IEEE;
• [7] Kelly, A. and K. Rinne, Control of DC-DC Converters by Direct Pole Placement and Adaptive Feedforward Gain Adjustment, in Applied Power Electronics Conference, 2005, Austin, Tex.: IEEE;
• [8] Kelly, A. and K. Rinne, A self-compensating adaptive digital regulator for switching converters based on linear prediction, in Applied Power Electronics Conference and Exposition, 2006, APEC '06, Twenty-First Annual IEEE, 2006;
• [9] Kelly, A., A self compensating closed loop adaptive control system U.S. Pat. No. 7,630,779, USPTO; and
• [10] Kelly, A. Adaptive control system for a DC-DC power converter, United States Patent Application 2010/005723.

Utilizing the tuning method as disclosed in PCT/EP2014/063987 provides a suitable controller to preserve the system response characteristics of a DC-DC converter as the power stage parameters vary and is compatible with standard PID control structures. But a means to characterize the power stage and to vary the controller characteristics automatically in response to the characterized power stage is lacking. Nothing in the prior art is suitable for performing this characterization: quickly (ideally in response to a single load-step disturbance on the output of the regulator for example); not introducing additional disturbances; is insensitive to outside disturbances.

Therefore what is required is a system that will automatically characterize the power stage of a DC-DC converter and tune a compensator in response to the characterization.

DISCLOSURE OF THE INVENTION

It is an objective of the present disclosure to provide a controller for a power converter that automatically characterizes the power stage of a DC-DC converter and tunes a compensator in response to the characterization.

The present invention relates to a controller for controlling a power stage of a power converter according to a control law, the control law implementing a specific type of compensator and being pre-designed to generate an objective response of a default power converter for a default parameter value of a component of the power stage.

The response may be response to any loop disturbance, for example a load step response. It may be any similar loop disturbance with an observed effect on the output voltage to the load. For example, disturbances in the duty cycle or voltage set-point would also produce a load response on the output voltage which can be designed to be the objective response.

The controller is further configured to determine an actual response for an actual parameter value of the component of the power stage and to alter the control law for the actual parameter value of the component of the power stage such that the actual response matches the load step response. The controller determines a degree of matching between the actual response and the objective response by filtering the actual response to generate a filtered actual response and integrating a product of the filtered actual and a delayed actual response.

A filter used for filtering the actual response may comprise an inverse filter of the objective response such that an actual response that exactly matches the objective response results in a zero output from the filter apart from a first sample of the filtered actual response.

Preferably, the delayed actual response is delayed by one sample to compensate for the first sample of the filtered actual response that is non-zero even if the filter is an inverse of the objective response. Thus, by delaying the actual response by one sample yields a zero value for the degree of matching between the actual and the objective response.

The controller may be further configured to scale the degree of matching between the actual response and the objective response to alter the control law, either by a linear time invariant gain or by a gain that is dependent on the magnitude of the actual response, for example a constant divided by a 2-norm of the actual response.

The scaled degree of matching is provided to a compensator and used for tuning the compensator accordingly. The compensator may be a PID compensator.

Thus, the controller operates reliably in the presence of disturbances. No additional disturbances are required. In order to retain system features that are desirable in existing DC-DC converters, including nonlinear control features, the controller is applicable to a standard, single pre-designed default compensator including an integral compensator such as a PID.

The controller can be operated as either

• • i) a one-time online characterization of the system, requiring fast system identification;
  • ii) operate online continuously adapting to changes over time.

The controller may be further configured to alter the control law by selecting another type of compensator of a plurality of types of compensators and by altering the control law implementing the other type of compensator for an actual parameter value of the component of the power converter such that an objective response of the other type of compensator corresponding to the default power converter matches the actual response.

The controller may be further configured to optimize for a type of compensator of the plurality of types of compensators and the corresponding control law.

The present invention further relates a power converter comprising a controller as described above and a non-volatile memory for storing the plurality of types of compensators.

The present invention further relates to a method for controlling a power stage of a power converter according to a control law, the control law implementing a specific type of compensator. The method comprises: providing the control law being pre-designed to generate an objective response of a default power converter for a default parameter value of a component of the power stage; determining an actual response for an actual parameter value of the component of the stage; and altering the control law for the actual parameter value of the component of the power stage such that the actual response matches the objective response by determining a degree of matching between the actual response and the objective response by filtering the actual response to generate a filtered actual response and integrating a product of the filtered actual response and a delayed actual response.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the accompanying drawings, wherein

FIG. 1 shows the load step responses; and

FIG. 2 shows the load step response characterization system; and

FIG. 3 shows vector u when a=0.5 (objective response); and

FIG. 4 shows vector y when a=0.5; and

FIG. 5 shows vector u when a=0.2; and

FIG. 6 shows vector y when a=0.2; and

FIG. 7 shows vector u when a=0.8; and

FIG. 8 shows vector y when a=0.8; and

FIG. 9 shows vector u when a=−0.5; and

FIG. 10 shows vector y when a=−0.5; and

FIG. 11 shows vector u resulting from a 2^nd order impulse response; and

FIG. 12 shows vector y resulting from a 2^nd order impulse response; and

FIG. 13 shows an automatically tunable compensator; and

FIG. 14 shows the output voltage and inductor current of a buck converter with characterization turned on at 4.0 ms resulting in improved Load-Step response thereafter.

DETAILED DESCRIPTION OF THE INVENTION

The load-step response is a very important dynamic characteristic of DC-DC converters, but the response is dependent on both the loop gain/phase and the open-loop output impedance of the converter. Although the loop gain/phase alters the closed-loop output impedance, converters with similar loop characteristics may have different load-step responses. Therefore, an approach based on characterizing the shape of the load-step response is advantageous compared to methods that characterize the loop bandwidth/phase-margin.

In order to characterize the load-step response it is necessary to have an objective load-step response that represents the desired response. The characterization method identifies the salient features of the load-step response in comparison to the objective load-step response. Bearing in mind, that the magnitude of the response varies with load-step magnitude and edge-rate for example, a method involving some function of the difference, i.e. subtraction, between the response and the objective response would be problematic. Referring to FIG. 1 the objective load step (a) represents the characteristics of the desired response; the under-damped (b) and over-damped (c) responses are shown for comparison.

In order to characterize the load-step response and quantify how well it matches the objective response the load step response (u), is applied to a filter 21 as shown in FIG. 2.

The filtered actual load step response is multiplied, see stage 23, by the actual load-step response and integrated by integrator 24 in order to ascertain the degree of matching between the actual load step response and the objective load step response.

A delay 22 is required to remove the first sample from the filter. The filter may be designed as an inverse filter of the objective load-step response such that an actual load-step response that exactly matches the objective response results in a zero output from the filter, neglecting the first sample, and therefore the integral of the product of the filtered and original actual load step response is zero.

For example, considering an objective load step response represented by the vector u (FIG. 3), where u=[1, a, a², a³, . . . , aⁿ], applied to a filter whose impulse response is vector h where h=[1, −a]. The resulting signal from the filter is vector y (FIG. 4), where y=u. h and therefore y=[1, a-a, a²-a², . . . , aⁿ-aⁿ] which simplifies to y=[1, 0, 0, 0, . . . , 0]. Assuming zero valued signals apriori, delaying u by one sample yields u′ where u′=[0, 1, a, a², a³, . . . , aⁿ] and the result of the integral of the product is therefore v, where v=u′. y=0.

Now considering u=[1, b, b², b³, . . . , bⁿ] applied to a filter whose impulse response is vector h where h=[1, −a]. The resulting signal from the filter is y=[1, b-a, b²-ab, b³-ab², . . . , bⁿ-ab^n-1]. When b>a, the vector y simplifies to a vector of positive values (neglecting the first value), and the result of the integral of the product is therefore positive (FIG. 7, FIG. 8). When b<a, the vector y simplifies to a vector of negative values (neglecting the first value), and the result of the integral of the product is therefore negative (FIG. 5, FIG. 6).

Negative values of the parameter ‘a’ model an oscillatory response (FIG. 9), which results in a vector y (FIG. 10), whose integral of the product (neglecting the first value), is negative.

Therefore, it is clear that the proposed characterization system yields a value whose magnitude and sign is a measure of matching between the actual response and the objective response with a zero result value for an exact match to the objective response, a positive result value when ‘a’ is greater than the desired value and a negative result when ‘a’ is less than the desired value or negative.

A simple two-tap (first order) FIR filter has been considered for clarity of explanation but it is clear that higher order FIR filters or IIR filters may be employed to characterize higher order objective responses. For example the objective response vector equal to the impulse response of a filter whose transfer function is (1−0.1z⁻¹)/(1−1.3z⁻¹+0.36z⁻²) is illustrated in FIG. 11. FIG. 12 shows this is correctly characterized by the 2nd order IIR filter whose transfer function is: (1−1.3z⁻¹+0.36z⁻²)/(1−0.1z⁻¹).

The output of the characterization system of FIG. 2 may be used to adjust a PID compensator as shown in FIG. 13, where the compensator block 133 is a component of a DC-DC converter and the scaling block interfaces 132 the characterization block 131 to the compensator. The compensator 133 is adjusted by the adjustment value w.

The scaling block 132 may be suitably

• • i) a linear time invariant gain;

ii) a gain that is responsive to the magnitude of the signal being characterized (u) e.g. K|u| where |u| represents the 2-norm of u or another suitable function. The advantage of (ii) is that the resulting signal from the characterization block is amplified more if it is resulting from a small input signal u. Therefore it represents a greater requirement for adjustment in the compensator than if the same signal resulted from a large input signal u.

FIG. 14 shows the output voltage and inductor current of a buck converter with characterization turned on at 4.0 ms resulting in improved Load-Step response thereafter. The adjustment value w is also shown to characterize the pulse immediately resulting in improved compensator tuning after only one load-step pulse, as required.

Because of the characterization is carried out on the load-step pulse response as described it is clear that this method may operate with non-linear compensators, for example where different compensators are activated according to the system state at a specific instance in time, and furthermore is compatible with non-linear DPWM restart techniques.

Following characterization the adjustment value w may be stored in NVM to be applied when the converter is next powered up following power down. Also, the adjustment value (or the like) may be communicated over a communication bus (serial or parallel) to provide information regarding the characterization of the response which would be useful in the design and quality control of the end power system. For example, if it was observed that the value had changed since the previous characterization or was very different from expected then the user may be alerted to act accordingly (on an impending component failure for example).

It is clear that such combinations and others would be very beneficial.

Claims

1. A controller for controlling a power stage of a power converter according to a control law, the control law implementing a specific type of compensator and being pre-designed to generate an objective response of a default power converter for a default parameter value of a component of the power stage; the controller being further configured to determine an actual response for an actual parameter value of the component of the power stage;the controller being further configured to alter the control law for the actual parameter value of the component of the power stage such that the actual response matches the objective response by determining a degree of matching between the actual response and the objective response by filtering the actual response by an inverse filter of the objective response to generate a filtered actual response and integrating a product of the filtered actual response and a delayed actual response.

2. The controller according to claim 1, wherein the delayed actual response is delayed by one sample.

3. The controller according to claim 1, wherein a filter used for filtering the actual response comprises an inverse filter of the objective response such that an actual response that exactly matches the objective response results in a zero output from the filter apart from a first sample of the filtered actual response.

4. The controller according to claim 1, wherein the controller is configured to scale a degree of matching between the actual response and the objective response to alter the control law.

5. The controller according to claim 4, wherein the controller is configured to scale the degree of matching by a linear time invariant gain.

6. The controller according to claim 4, wherein the controller is configured to scale the degree of matching by a gain that is dependent on the magnitude of the actual response.

7. The controller according to claim 6, wherein the controller is configured to scale the degree of matching by a constant divided by a 2-norm of the actual response.

8. The controller according to claim 1, further configured to alter the control law by selecting another type of compensator of a plurality of types of compensators and by altering the control law implementing the other type of compensator for an actual parameter value of the component of the power converter such that an objective response of the other type of compensator corresponding to the default power converter matches the actual response.

9. The controller according to claim 8, further configured to optimize for a type of compensator of the plurality of types of compensators and the corresponding control law.

10. A power converter comprising: a controller according to claim 9, and a non-volatile memory for storing the plurality of types of compensators.

11. A method for controlling a power stage of a power converter according to a control law, the control law implementing a specific type of compensator, the method comprising: providing the control law being pre-designed to generate an objective response of a default power converter for a default parameter value of a component of the power stage;determining an actual response for an actual parameter value of the component of the stage; andaltering the control law for the actual parameter value of the component of the power stage such that the actual response matches the objective response by determining a degree of matching between the actual response and the objective response by filtering the actual response by an inverse filter of the objective response to generate a filtered actual response and integrating a product of the filtered actual response and a delayed actual response.

12. The method according to claim 11, further comprising: delaying the actual response by one sample.

13. The method according to claim 11, wherein filtering the actual response comprises using an inverse filter of the objective response such that an actual response that exactly matches the objective response results in a zero output from the filter apart from a first sample of the filtered actual response.

14. The method according to claim 11, wherein altering the control law comprises: selecting another type of compensator of a plurality of types of compensators and altering the control law implementing the other type of compensator for an actual parameter value of the component of the power stage such that the actual response matches the objective response of the other type of compensator.

15. The method according to claim 13, further comprising: optimizing for a type of compensator of the plurality of types of compensators and the corresponding control law.