DE102018120821A1 - Determination of the small signal transfer function - Google Patents

Abstract

A controller (100) for a switching power supply (200), which has a primary side (210) and a secondary side (220) and is designed to transform an input power (U1) received via the primary side (210) into an output power (U2) and To provide via the secondary side (220), comprises: a first controller subunit designed to execute an open control loop for controlling the output power (U2) as a function of a measured signal (CS, ZCD), the measured signal (CS, ZCD) from depends on a variable to be controlled on the basis of the control loop; a second controller subunit configured to measure the small signal transfer function of the open loop by: generating a test signal of variable amplitude and frequency; Feeding the test signal into the open control loop by adding the test signal to the measured signal (CS, ZCD) to obtain a disturbed signal which is fed to the control loop; and determining a difference between the disturbed signal and the measured signal (CS, ZCD).

Description

TECHNICAL AREA

The present document relates to exemplary embodiments of a controller and exemplary embodiments of a method for measuring the small signal transfer function of an open control loop, which is carried out for regulating an output power of a switching power supply. The switching power supply can be a switching power supply with electrical isolation, e.g. a flyback converter.

BACKGROUND

Many functions of modern devices in automotive, consumer, and industrial applications, such as converting electrical energy and driving an electric motor or electrical machine, rely on power semiconductor devices.

For example, so-called insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs) and diodes, to name just a few, are used for various applications, including switches in power sources and power converters.

The group of power converters includes so-called switched mode power supplies. A switching power supply can have a primary side and a secondary side, which are galvanically separated from one another, for example. For example, the switched-mode power supply is designed to transform an input power (e.g. an input voltage or an input current) received via the primary side into an output power (e.g. an output voltage or an output current) and to provide it via the secondary side.

A controller is used to regulate the switching power supply. Such a controller can have an integrated control loop, which sometimes makes it difficult to check and verify the stability of the control loop.

SUMMARY

According to one embodiment, a controller for a switching power supply, which has a primary side and a secondary side and is designed to transform an input power received via the primary side into an output power and to provide it via the secondary side, a first controller subunit and a second controller subunit. The first controller subunit is designed to carry out an open control loop for controlling the output power as a function of a measured signal, the measured signal depending on a variable to be controlled on the basis of the control loop. The second controller subunit is designed to measure the small signal transfer function of the open control loop by: generating a test signal of variable amplitude and frequency; Feeding the test signal into the open control loop by adding the test signal to the measured signal to obtain a disturbed signal which is fed to the control loop; and determining a difference between the disturbed signal and the measured signal.

According to a further embodiment, a method for operating a switching power supply, which has a primary side and a secondary side and is designed to transform an input power received via the primary side into an output power and to make it available via the secondary side, comprises the following steps: Output power as a function of a measured signal, the measured signal depending on a variable to be controlled on the basis of the control loop; and measuring the open loop small signal transfer function by generating a variable amplitude and frequency test signal; Feeding the test signal into the open control loop by adding the test signal to the measured signal to obtain a disturbed signal which is fed to the control loop; and determining a difference between the disturbed signal and the measured signal.

Additional features and advantages will become apparent from the detailed description that follows, and from the drawings.

list of figures

• 1 schematisch und exemplarisch einen Schaltungsaufbau eines Schaltnetzteils mit einem Controller gemäß einer oder mehreren Ausführungsformen;
• 2 schematisch und exemplarisch die Verstärkung (G) der Kleinsignaltransferfunktion in Abhängigkeit der Frequenz eines Testsignals; und
• 3 schematisch und exemplarisch die Verstärkung (G) und die Phase (P) der Kleinsignaltransferfunktion in Abhängigkeit der Frequenz des Testsignals.

Elements in the drawings are not necessarily drawn to scale. Instead, priority is given to presenting principles of the invention. Reference symbols in the drawings can also designate corresponding elements. Show it:

• 1 schematically and exemplarily a circuit structure of a switching power supply with a controller according to one or more embodiments;
• 2 schematically and exemplarily the reinforcement ( G ) the small signal transfer function as a function of the frequency of a test signal; and
• 3 schematically and exemplarily the reinforcement ( G ) and the phase ( P ) the small signal transfer function depending on the frequency of the test signal.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the drawings, which form a part of the present disclosure, and in which certain exemplary embodiments in accordance with which the invention can be implemented are shown by way of illustration.

In the following, reference is made in detail to various exemplary embodiments, one or more examples of which are illustrated in the drawings. Each example is presented for illustration and should not be construed to limit the invention. For example, features that are shown or described as part of one embodiment may be used in - or in conjunction with - other embodiments to yield another embodiment. The present invention is also intended to include such modifications and alterations. The examples are described using specific language, which should not be construed as limiting the scope of the claims. The drawings are not to scale and are for illustration purposes only. Unless otherwise stated, identical elements or manufacturing steps are indicated by identical reference numerals in the different drawings for better clarity.

In the context of the present description, the terms “in electrical contact” and “electrically connected” are intended to describe that there is a low-resistance electrical connection or a low-resistance current path between two connections of the device described here.

Furthermore, in the context of the present description, the term “electrical insulation”, unless otherwise stated, is used in the context of its general understanding and is therefore intended to describe that two or more components are positioned separately from one another and that there is no ohmic connection which connects these components , Nevertheless, components that are electrically isolated from one another can be coupled to one another, for example mechanically coupled and / or capacitively coupled and / or inductively coupled. As an example, two electrodes of a capacitor may be electrically isolated from one another and simultaneously mechanically and capacitively coupled, e.g. by means of insulation, e.g. by means of a dielectric.

Special exemplary embodiments, which are described in the present description, also relate, without being restricted thereto, to a power converter with at least one power semiconductor component. In one exemplary embodiment, such a component can be designed to carry a load current which is to be supplied to a load and / or which is accordingly provided by means of a power source. For example, the power semiconductor device may include one or more active power semiconductor cells, such as a monolithically integrated diode cell and / or a monolithically integrated transistor cell and / or a monolithically integrated IGBT cell and / or a monolithically integrated RC-IGBT cell and / or a monolithic integrated MOS gated diode (MGD) cell and / or a monolithically integrated MOSFET cell and / or modifications thereof. A plurality of such diode cells and / or such transistor cells can be integrated in the component.

The term “power semiconductor component”, as used in the present description, is intended to denote a single component with high voltage blocking and / or current carrying suitability. In other words: such a power semiconductor component is intended for high currents, typically in the ampere range, e.g. up to 5 or 100 A, and / or voltages typically above 15 V, particularly typically up to 40 V and more, e.g. up to at least 500 V or more than 500 V, e.g. at least 600 V.

For example, the power semiconductor component described below can be a component that is designed to be used as a power component in a low, medium and / or high voltage application. For example, the term “power semiconductor device” as used in the present specification is not intended to refer to logic semiconductor devices used, for example, for storing data, processing data and / or for other types of semiconductor-based data processing.

1 shows schematically and exemplarily a circuit structure of a switching power supply 200 with a controller 100 according to one or more embodiments.

The switching power supply 200 is designed, for example, as a flyback converter. Therefore, the flyback converter is also used in the present case 200 spoken.

The switching power supply 200 has a primary side 210 and a secondary side 220 on. The primary side 210 of the switching power supply 200 can galvanically from the secondary side 220 be separated, for example by a transformer 230 ,

Via the primary side 210 receives the switching power supply 200 an input power, for example an input voltage U ₁ , The input voltage U ₁ can be an AC voltage, for example, between two input connections 2101 and 2102 drops. The input voltage U ₁ is a capacitor 211 fed, and then through a filter transformer 212 a rectifier 213 , The rectifier 213 For example, can be implemented as a diode rectifier (as illustrated) and a capacitor 2135 as well as the diodes 2131 to 2134 exhibit. The output voltage of the rectifier 213 is a storage capacitor (or smoothing capacitor) 214 fed. The output voltage of this capacitor 214 sets the input voltage for the active part of the switching power supply 200 represents.

1 shows a typical configuration of a flyback converter which is the component group 218 with the capacitor 2181 , the resistor connected in parallel 2182 as well as the diode 2183 includes. Parallel to this component group 218 is the primary side 231 of the transformer 230 switched, and the switching element in series with this parallel connection 215 , which is designed, for example, in the form of a MOSFET.

To control the operation of the switching element 215 is a controller 100 provided the switching element 215 at the control signal connection 2153 (eg a gate connection) supplies a control signal GD (eg a gate signal). For this there is a corresponding signal path between the control signal connection 2153 of the switching element 215 and a control signal output 102 of the controller 100 intended. Depending on the switching state of the switching element 215 there is a corresponding current flow in the parallel connection from the component group 218 and the primary side 231 , A secondary page 232 of the transformer 230 outputs an output current due to the diode 221 and the capacitor 222 in an output power U ₂ , for example a DC voltage, the capacitor 222 to the secondary mass 223 is switched.

The controller 100 that has a connector 101 ( GND ) on the primary mass 219 is switched, influenced by control of the switching element 215 the transformation of power from the input voltage U ₁ in the power of the output voltage U ₂ , For example, the controller 100 trained, a switching frequency and / or a duty cycle (relative duty cycle) of the control signal DG to vary to affect the transfer of power. For example, the controller includes 100 a first controller subunit (implemented in software, for example, not shown in the drawings), which is designed to regulate the output power as a function of a measured signal ( CS . ZCD ) execute an open control loop, the measured signal depending on a variable to be controlled on the basis of the control loop. This variable to be controlled can be, for example, the control signal DG his.

The measured signal can be a current signal CS which, for example, is indicative of the amplitude of the between the two load connections (e.g. source and drain) 2151 and 2152 of the switching element 215 flowing current. To detect this current signal CS can, for example, a measuring resistor 217 be provided between ground 219 and load connection 2152 (for example the source connection) is switched. The current signal CS can the controller 100 via a first measurement signal input 103 be fed.

The measured signal can additionally or alternatively be a time signal ZCD include, which is a suitable time for a switching operation of the switching element 215 indicates. For example, it is the time signal ZCD around the zero current detection signal known in connection with flyback converters. To generate this time signal ZCD can the switching power supply 200 another component group 216 include one on the transformer 212 coupled coil 2161 has, as well as the two resistors 2162 and 2163 , as schematically in the 1 is illustrated. The time signal ZCD can the controller 100 via a second measurement signal input 104 be fed.

The basic mode of operation and the basic structure of a flyback converter are known to the person skilled in the art, and there are no deviations from these basic properties of a flyback converter in the present case, so that further explanations of the switched-mode power supply are given 200 the exemplary implementation of a flyback converter is not discussed further here.

In one embodiment, the controller 100 to the primary side 210 of the switching power supply 200 coupled. The switching power supply 200 is in this embodiment on its primary side 210 regulated, and not its secondary side 220 , This manifests itself, for example, in the fact that the measured signal or the measured signals, for example the time signal ZCD as well as the current signal CS , the primary side 210 are taken, and that the variable to be controlled, such as the control signal DG , the primary side 210 is fed.

As I said, the controller can 100 comprise a first controller subunit which is designed to execute the open control loop for controlling the output power (for example the output voltage U ₂ ) depending on a measured signal (e.g. at least one of the signals CS and ZCD ), the measured signal from one to Base of the control loop depends on the size to be controlled. The variable to be controlled is, for example, the control signal DG for the switching element 215 , The open control loop can be in the controller 100 be integrated, for example by a software code that is in a memory (not shown) of the controller 100 is deposited.

Aspects of this document relate to the metrological determination of the small signal transfer function of the open control loop, that of the controller 100 by means of its first controller subunit in order to regulate the output power as a function of the measured signal, the measured signal depending on the variable to be regulated on the basis of the control loop.

The determination of the small signal transfer function can be expedient, for example, in order to check or verify the stability of the open control loop. The determination of the small signal transfer function can also be expedient in order to optimize the open control loop with regard to at least one control parameter.

In the case of switched-mode power supplies which are controlled by a controller on the secondary side, the determination of the small signal transfer function and thus the verification of the open control loop are usually unproblematic. In the case of switching power supplies regulated on the secondary side, e.g. the value to be controlled is compared on the secondary side with a reference. The comparison result is processed, for example, by a PI controller on the secondary side. The output signal of the controller is then e.g. transferred to the primary side via an opto-coupler in order to influence the GD signal there. Due to the necessary components of the signal processing chain, it is comparatively easy to introduce an interference signal into this chain and to measure the reaction of the system. Since this functionality can be fully integrated in a controller controlled on the primary side, there is no suitable access for influencing and measuring.

Compared to switched-mode power supplies regulated on the secondary side, the determination of the small signal transfer function in switched-mode power supplies that are controlled by a controller on the primary side is therefore rather problematic. For example, verifying the stability of the control loop requires complex measurements and optimization of transfer functions. At the same time, this type of verification of the stability of the control loop is complex and imprecise.

In the present case, a possibility to measure and / or independently optimize the small signal transfer function of the open control loop is proposed in terms of process engineering and / or controller technology.

To do this, the controller includes 100 for example a second controller subunit (not shown). The second controller subunit can also be in the controller 100 be integrated, for example in an integrated circuit (IC) of the controller 100 , the controller 100 especially on the primary side 210 of the switching power supply 200 can be used, i.e. for primary control of the switching power supply 200 , The two controller subunits can each be implemented as software code, both of which are, for example, from the controller 100 be held in a common or in separate integrated circuits (IC).

The second controller subunit is designed to measure the small signal transfer function of the open control loop by performing the following measures: generating a test signal of variable amplitude and frequency; Feeding the test signal into the open control loop by adding the test signal to the measured signal to obtain a disturbed signal which is fed to the control loop; and determining a difference between the disturbed signal and the measured signal.

The test signal is based on the current signal, for example CS added up and / or on the time signal ZCD , For example, when regulating the output voltage U ₂ the ZCD signal by means of the second controller subunit, information about the output voltage U ₂ taken. The test signal is then added to this information, which corresponds to the measured signal. In another exemplary embodiment, when the output current is regulated, information is taken from both the CS signal and the ZCD signal, which allow calculation of the output current. For example, the test signal is then added to the result of this calculation (which represents the measured signal).

At this point it should be made clear that the test signal is not completely with the current signal CS or the time signal ZCD must be overlaid. For example, the include CS respectively. ZCD Signals not only an indication of the size to be controlled, but also other information. For example, the test signal does not become the current signal received by the controller CS or time signal ZCD added directly, but to information that has been taken from the received signal.

The second controller subunit can be designed, for example, the current signal CS and / or the time signal ZCD to be received and evaluated before the addition is carried out, for example in that the variable to be controlled is calculated on the basis of the received signal. A signal processing unit can be provided for this calculation, for example. The test signal is then added to the calculation result, ie the measured signal.

For example, in one embodiment, the ZCD signal contains information about the output voltage only at a certain point in time U ₂ , The test signal is added only to the value measured at this point in time. The evaluation of other information of the ZCD signal should not be changed by the test signal (eg zero crossings).

Similarly, in another embodiment, e.g. the CS signal only at one point in the switching cycle is information that can be used to calculate the output current. Only this value is superimposed on the test signal, not the complete CS signal.

According to one embodiment, the addition between the test signal and the measured signal (for example the part derived from the current signal CS or the time signal ZCD) takes place within the controller 100 ,

The second controller subunit is designed, for example, to measure the small signal transfer function of the open control loop by determining the phase P and reinforcement G the small signal transfer function, which is described below with reference to the 2 and 3 will be explained in more detail.

The determination of the difference between the disturbed signal and the measured signal includes, for example, the determination of a phase difference and an amplitude difference, which is described using the example below:

$$Y_i={\displaystyle \sum _{N}y\left(n\right)}\cdot imag\left(s\left(n\right)\right)$$

$$U_r={\displaystyle \sum _{N}u\left(n\right)}\cdot real\left(s\left(n\right)\right)$$

$$U_i={\displaystyle \sum _{N}u\left(n\right)}\cdot imag\left(s\left(n\right)\right)$$

For example, in a first step, a first complex pointer is determined ( Y _r . Y _i ) by correlating the test signal and the test signal shifted by 90 ° with the measured signal, and determining a second complex pointer ( U _r . U _i ) by correlating the test signal and the test signal shifted by 90 ° with the disturbed signal. These four steps can be expressed mathematically as follows: $$Y_r={\displaystyle \underset{N}{\Sigma }y\left(n\right)}\cdot real\left(s\left(n\right)\right)$$

$$Y_i={\displaystyle \underset{N}{\Sigma }y\left(n\right)}\cdot imaG\left(s\left(n\right)\right)$$

$$U_r={\displaystyle \underset{N}{\Sigma }u\left(n\right)}\cdot real\left(s\left(n\right)\right)$$

$$U_i={\displaystyle \underset{N}{\Sigma }u\left(n\right)}\cdot imaG\left(s\left(n\right)\right)$$

Here y (n) specifies the measured signal (eg one of the signals mentioned above CS or ZCD ), and s (n) the test signal and U.N) the disturbed signal.

The parameter real ( s (n) ) stands for the real part of the test signal s (n) , and correspondingly the parameter imag ( s (n) ) for the imaginary part of the test signal s (n) , With a corresponding summation, this results in N The two complex pointers thus result in samples Y and U that have the corresponding real parts Y _r and U _r as well as the corresponding imaginary parts Y _i and U _i exhibit. The correlation can start at any time n. N According to one embodiment, covers several periods of the test signal and is, for example, in the order of several thousand samples.

The determination of a correlation result with the aid of a complex signal is known to the person skilled in the art e.g. known as Fourier analysis. However, a complete Fourier analysis is not absolutely necessary here, but only an evaluation of the amplitudes and phase position at individual frequencies, in accordance with one or more embodiments.

The first complex pointer Y represents, for example, the relative gain and phase of the measured signal y (n) related to the test signal s (n) ,

The second complex pointer U represents, for example, the relative gain and phase of the disturbed signal U.N) related to the test signal s (n) ,

The ratio of the two complex pointers Y and U is therefore representative of the phase and amplitude difference between the measured signal y (n) and the disturbed signal U.N) ,

The second controller subunit of the controller 100 measures the small signal transfer function e.g. by determining the amplitude and phase difference of the two complex pointers ( Y _r . Y _i ; U _r . U _i ). At this point, it should be pointed out that at least according to some embodiments for measuring the small signal transfer function, a complete back and forth transformation into / out of the complete frequency space is therefore not absolutely necessary.

According to a further embodiment, the second controller subunit is designed to measure the small signal transfer function for a multiplicity of different frequencies of the test signal. Referring to the 2 For example, the second controller subunit can be designed to determine at which frequency f _D of the test signal s (n) the reinforcement G the small signal transfer function is 0 dB.

For example, the frequency of the test signal s (n ) within a minimum frequency value f _L and a maximum frequency value f _H defined frequency band variable.

A first measurement of the small signal transfer function takes place, for example, by feeding in the test signal s (n) at a first intermediate frequency f _M that between the minimum frequency value f _L and the maximum frequency value f _H lies.

With switching power supplies, the control bandwidth is typically between a few Hz and a few kHz and depends on the application: If a converter stage provides PFC (Power Factor Correction) functionality on the input side, the bandwidth is typically a few Hz up to 100 Hz. If a converter is to react quickly to load changes at the output, the bandwidth is typically in the range of 200 Hz to 10 kHz.

According to one embodiment, the update rate is, for example, in the order of the switching frequency (e.g. up to 20 kHz). This enables test signals of low frequencies to be generated with high resolution. With higher frequencies (> 1 kHz) the frequency resolution tends to decrease.

A subsequent measurement of the small signal transfer function is carried out, for example, by feeding the test signal with a second intermediate frequency that is lower than the first intermediate frequency f _M is when for the first intermediate frequency f _M certain gain is less than 0 dB and greater than the first intermediate frequency f _M is when for the first intermediate frequency f _M certain gain is greater than 0 dB. For example, the gain depends G the small signal transfer function in a manner of frequency F as in 2 is exemplarily illustrated. For example, it is assumed that the gain G the small signal transfer function with increasing frequency F decreases, although this dependency need not be linear, as in 2 illustrated.

The reinforcement G the small signal transfer function is 0 dB (i.e. 1) if the following criterion is met: $${\text{Y}}_{\text{r}}{}^{\text{2}}+{\text{Y}}_{\text{i}}{}^{\text{2}}={\text{U}}_{\text{r}}{}^{\text{2}}+{\text{U}}_{\text{i}}{}^{\text{2}}$$

Determining at what frequency f _D the gain of the test signal G the small signal transfer function is 0 dB, it can be done iteratively. Depending on whether the expression Y _r ² + Y _i ² or less than the expression U _r ² + U _i ² (i.e. depending on whether the gain G If the small signal transfer function is greater or less than 1), the relevant frequency at which the measurement was carried out is the new minimum frequency value f _L or as a new maximum frequency value f _H Are defined.

The first intermediate frequency f _M and the other intermediate frequencies can be calculated in various ways, for example as an arithmetic, geometric or harmonic mean between the minimum frequency value f _L and maximum frequency value f _H , The respective further intermediate frequency can, for example, also depend on at least one of the gains in other embodiments G _L and G _H be determined, the small signal transfer function at the corresponding frequencies f _L and f _H having.

As in 3 (B) not only typically takes gain G the small signal transfer function with increasing frequency F Section. 3 (A) and 2 ), but also the phase P the small signal transfer function.

It may be desirable that the phase P the small signal transfer function at that frequency f _D where the gain G the small signal transfer function is approximately 1 (0 dB) (Y _r ² + Y _i ² = U _r ² + U _i ² ), is within a certain range, for example in the range from 30 ° to 60 °. For example, if such conditions are met, it can be said that the open control loop has sufficient stability, according to which undesired oscillations are avoided overshoots when regulating the output power.

For example, the measurements indicate that the phase P the small signal transfer function at frequency f _D is not in the desired range, the controller can 100 For example, be configured to independently modify one or more parameters of the open control loop. After modification, the measurement of the small signal transfer function described above can be carried out again until the phase P the small signal transfer function at frequency f _D is in the desired range.

According to one embodiment of the controller 100 the first controller subunit is designed to automatically modify the open control loop with respect to at least one parameter as a function of the measurement of the small signal transfer function by the second controller subunit. The second controller subunit can be designed to automatically initiate the renewed measurement of the small signal transfer function after modification of the open control loop. This is shown schematically in the 3 indicated where with regard to a first setting of the at least one parameter of the open control loop (measuring point 1 ) at the frequency f ₁ leading to reinforcement G 1, there is a phase P ₁ results, which is outside the desired range, for example. The open control loop is modified with regard to the at least one parameter (measuring point 2 ). Then the frequency leads f2 to a reinforcement G the small signal transfer function of 1 , and the phase is P2 that may already be within the desired range.

After the controller 100 then has been optimized with regard to the phase range of the open control loop, a further optimization with regard to the bandwidth can optionally take place, it being thereby ensured that the phase P at the frequency that results in a gain of 1 remains in the desired range.

In summary, in one embodiment, the controller 100 thus be configured to parameterize a controller function specified by the open control loop as a function of the phase of the small signal transfer function determined. The second controller subunit can be designed to determine again at which frequency after the parameterization of the controller function f _D of the test signal the gain of the small signal transfer function is 0 dB.

The controller 100 can also be designed to operate in either a normal mode or in a test mode, the second controller subunit feeding the test signal only during the test mode. After the optimization of the open control loop or the control function specified by it, the operation of the second controller subunit may no longer be necessary if the conditions remain the same if the stability of the open control loop has been verified.

Further modifications are conceivable. For example, in more complex embodiments it can be provided that gradients of the phases are determined as a function of frequency in order to optimize parameters of the open control loop, which e.g. Set so-called fringe frequencies.

The measurement of the gain and the phase described by way of example can of course include the execution of mathematical algorithms, such as the discrete Fourier transform (DFT) or the fast Fourier transform (FFT).

Not only the controller is suggested here 100 for the switching power supply 200 , but also a switching power supply that a controller as described above 100 having. The present document also relates to the operation of a switching power supply, the method corresponding to the operation of the controller described above 100 corresponds so that reference is made to the above.

The terms “with”, “containing”, “including”, “comprehensive”, “exhibiting” and similar are open terms which indicate the presence of the elements or features mentioned, but do not exclude additional elements or features.

With the foregoing breadth of variations and applications in mind, it should be understood that the present invention is not defined by the foregoing description or the accompanying drawings. Instead, the present invention is only defined by the following claims and their legal equivalents.

Claims (15)

Controller (100) for a switching power supply (200), which has a primary side (210) and a secondary side (220) and is designed to transform an input power (U ₁ ) received via the primary side (210) into an output power (U ₂ ) and to be made available via the secondary side (220), the controller (100) comprising: a first controller subunit designed to execute an open control loop for regulating the output power (U ₂ ) as a function of a measured signal (CS, ZCD), wherein the measured signal (CS, ZCD) depends on a variable (GD) to be controlled on the basis of the control loop; a second controller subunit designed to measure the small signal transfer function of the open control loop by: generating a test signal of variable amplitude and frequency; Feeding the test signal into the open control loop by adding the test signal to the measured signal (CS, ZCD) to obtain a disturbed signal which is fed to the control loop; - Determining a difference between the disturbed signal and the measured signal (CS, ZCD). Controller (100) Claim 1 , wherein determining the difference between the disturbed signal and the measured signal (CS, ZCD) the Determining a phase difference and an amplitude difference. Controller (100) Claim 1 or 2 , wherein the second controller subunit is designed to measure the small signal transfer function of the open control loop by determining the phase and amplification of the small signal transfer function. Controller (100) according to one of the preceding claims, wherein determining the difference between the disturbed signal and the measured signal (CS, ZCD) comprises: - determining a first complex pointer (Y _r , Y _i ) by correlating the test signal and the test signal shifted by 90 ° with the measured signal (CS, ZCD); - Determining a second complex pointer (U _r , U _i ) by correlating the test signal and the test signal shifted by 90 ° with the disturbed signal; the second controller subunit measuring the small signal transfer function by determining the amplitude and phase difference of the two complex pointers (Y _r , Y _i ; U _r , U _i ). Controller (100) according to one of the preceding claims, wherein the second controller subunit is designed to measure the small signal transfer function for a plurality of different frequencies of the test signal. Controller (100) Claims 3 and 5 , wherein the second controller subunit is designed to determine at which frequency (f _D ) of the test signal the gain of the small signal transfer function is 0 dB. Controller (100) Claim 6 , the frequency of the test signal being variable within a frequency band defined by a minimum frequency value (f _L ) and a maximum frequency value (f _H ), and a first measurement of the small signal transfer function being carried out by feeding in the test signal at a first intermediate frequency (f _M ), which lies between the minimum frequency value (f _L ) and the maximum frequency value (f _H ), and where a subsequent measurement of the small signal transfer function is carried out by feeding the test signal with a second intermediate frequency which is smaller than the first intermediate frequency (f _M ) if the the gain intended for the first intermediate frequency (f _M ) is less than 0 dB, and which is greater than the first intermediate frequency (f _M ) if the gain intended for the first intermediate frequency (f _M ) is greater than 0 dB. Controller (100) Claim 6 or 7 , wherein the second controller subunit is designed to determine the phase of the small signal transfer function when the interference signal is fed at the frequency (f _D ) at which the gain of the small signal transfer function is 0 dB. Controller (100) Claim 8 The controller (100) is designed to parameterize a controller function specified by the open control loop as a function of the determined phase of the small signal transfer function. Controller (100) Claim 9 , wherein the second controller subunit is designed, after parameterizing the controller function, to determine again at which frequency (f _D ) of the test signal the gain of the small signal transfer function is 0 dB. Controller (100) according to one of the preceding claims, further developed for operating in a normal mode and in a test mode, the second controller subunit feeding the test signal only during the test mode. Controller (100) according to one of the preceding claims, wherein the primary side (210) of the switching power supply (200) is galvanically isolated from the secondary side (220). Switched-mode power supply (200), wherein the switched-mode power supply (200) comprises a controller (100) according to one of the preceding claims. Switching power supply (200) after Claim 13 , wherein the switching power supply (200) is a flyback converter. Method for operating a switching power supply (200), which has a primary side (210) and a secondary side (220) and is designed to transform an input power (U ₁ ) received via the primary side (210) into an output power (U ₂ ) and via to provide the secondary side (220), the method comprising: - executing an open control loop for regulating the output power (U ₂ ) as a function of a measured signal (CS, ZCD), the measured signal (CS, ZCD) being based on one of the control loop variable to be controlled (GD); and - measuring the small signal transfer function of the open control loop by: - generating a test signal of variable amplitude and frequency; Feeding the test signal into the open control loop by adding the test signal to the measured signal (CS, ZCD) to obtain a disturbed signal which is fed to the control loop; - determining a difference between the disturbed signal and the measured signal (CS, ZCD)

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