# Overview, Comparison and Extension of Concepts for Current-Controlled, Adjustable Inductances

Guido Schierle, Jonas Pfeiffer, Klaus F Hoffmann

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Guido Schierle, Jonas Pfeiffer, Klaus F Hoffmann. Overview, Comparison and Extension of Concepts for Current-Controlled, Adjustable Inductances. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0162. hal-05108320

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# Overview, Comparison and Extension of Concepts for Current-Controlled, Adjustable Inductances

Guido Schierle¹ ID, Jonas Pfeiffer² ID and Klaus F. Hoffmann¹ ID

¹HELMUT SCHMIDT UNIVERSITY

²SUMIDA COMPONENTS & MODULES GMBH

Holstenhofweg 85

22043 Hamburg, Germany

Tel.: +49 / (0)40 - 6541 2768

E-Mail: guido.schierle@hsu-hh.de

URL: https://www.hsu-hh.de/lek/en/

Dr. Hans-Vogt-Platz 1

94130 Obernzell, Germany

Tel.: +49 / (0)8591 - 937 - 0

E-Mail: jpfeiffer@eu.sumida.com

URL: https://www.sumida.com

**Index Terms**—Device modelling, DC/DC converter, Magnetic device, Optimization, Passive component.

**Abstract**—Magnetic components are an important part of many DC/DC converters. The use of active premagnetization makes them adjustable, which means an additional degree of freedom in the design of the respective converter topology. However, when selecting the most suitable premagnetization method for the specific application, various impact factors should be taken into account to ensure an optimal design.

This paper presents, analyses and discusses impact factors such as the adjustability range of the inductance value, the magnetic feedback as well as the behaviour of the auxiliary circuit for different premagnetization methods and different inductor design approaches. The measurement verification was carried out using a buck converter.

## I. INTRODUCTION

Controllable inductors and transformers have been used in various applications over the past decades and are still relevant today in some areas, such as LED lightning drivers, as reported in [1], [2]. In current research, the technology is also being applied to other areas, such as renewable energy systems (see [3]) or energy storage systems (see [4]).

The use of controllable magnetic components in DC/DC converters provides an additional degree of freedom in the topology design. In particular, topologies such as LLC converters are of great interest for variable inductors, as [5] shows. In addition, Dual Active Bridge (DAB) converters (see [6], [7]) as well as transformerless converter topologies are the focus of recent investigations.

Regardless of the topology used, there are some challenges in terms of design and identifying the optimal concept for each application. Some of these parameters are investigated in this paper. Furthermore, an additional auxiliary current has to be provided for the active pre-magnetization of the current-controlled adjustable inductors. To ensure reliable operation, this auxiliary current should be as free of ripple as possible. However, due to the magnetic coupling of the main and auxiliary windings, a ripple can be transferred from the main winding to the auxiliary winding. This might lead to an impact on the auxiliary circuit and can even damage it in the worst case. This paper therefore presents various designs of premagnetizable inductors and investigate these in terms of their back coupling behavior.

The paper is divided into four main sections. In the first section, the basic operating principle of active premagnetization and its methods are explained and the prototype inductors used for the investigation are presented.

The measurement setup, including the parameters to be analyzed, is then elaborated in the following section.

Section IV presents the measurement results. Each prototype is addressed and the respective measurement data are analyzed in detail with regard to the parameters examined.

Finally, a comparison of the parameters is given for a better overview and the main findings are discussed.

## II. ACTIVE PREMAGNETIZATION AND ITS IMPLEMENTATION

To use the principle of active premagnetization, a second winding, usually called auxiliary winding or control winding, is required. This can be applied to the
<figure><img src="image_2.png" /><figcaption>Fig. 1. The three different methods of active premagnetization (acc. to [8]).
a): parallel biasing; b): mixed biasing; c): orthogonal biasing</figcaption></figure>

magnetic core in various forms in addition to the main
winding or load winding. The auxiliary current that flows
through the auxiliary winding generates an auxiliary
magnetic flux that impacts the permeability of the core
material. The impact can be applied locally to previously
defined core sections or affect the entire magnetic core.
By changing the permeability, the inductance value of
the magnetic component can be reduced. The magnetic
component becomes controllable. This effect is usually
infinitely variable.

According to [8], a distinction is made between the
following three methods based on the way in which the
magnetic main and auxiliary fluxes are aligned:

* Parallel biasing

* Mixed Biasing

* Orthogonal biasing

The three methods are shown in Fig. 1. A detailed
overview of the various premagnetization methods, their
mode of action, previous applications and possible uses
is given in [9] and [10].

Orthogonal and mixed premagnetization were used
in the investigations for this paper. The different imple-
mentations of active premagnetization in the magnetic
components used are briefly explained in the following
subsections.

A. *Orthogonal biasing (Paraformer approach)*

The first implementation is based on the principle of
the so called Parametric Transformer or Paraformer. It is
depicted in Fig. 2.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Design of a core using orthogonally biasing (Paraformer approach) [11]</figcaption></figure>

The basic functionality of the Paraformer is pre-
sented, analyzed and discussed in [12]. Further investiga-
tions, mathematical and graphical analyses and serveral
different design approaches are published in [13]–[16].

The used design visualized in Fig. 2 is similar to an
approach presented in [17]. However, the cores used for
this paper are toroidal cores instead of UU cores. The
two cores are connected in such a way that the magnetic
flux in the control core is always orthogonal to the main
flux in the load core. A manufactured and already further
developed prototype with the best achieved results so far
is displayed in Fig. 3.

The shown adjustable inductor is analyzed in detail
in [21], [22]. Further investigations on this concept
regarding non-linear effects depending on operation tem-
<figure><img src="image_4.png" /><figcaption>Fig. 3. Prototype of the orthogonally biased core (Paraformer approach) [18]</figcaption></figure>

perature, material mixtures and design calculations are published in [19]–[22].

## B. Orthogonal biasing (Drill hole approach)

Another approach to active premagnetization is the use of drilled holes in the core material. The holes can be manufactured subsequently by using an Ultrasonic/Sonic Driller/Corer (USDC). Further information about the manufacturing process is given in [10]. The auxiliary winding is guided through these drill holes so that the core material around the drilled hole is impacted by the auxiliary magnetic flux (see Fig. 1c). For the investigations of orthogonal biasing in [10] and [23] as well as the investigations published in this paper, a P 30/19 core was used. This pot core is already manufactured with a hole in the center and is therefore ideally suited. The manufactured prototype can be seen in Fig. 4.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Prototype of the orthogonally biased core (drill hole approach) [23]</figcaption></figure>

## C. Mixed biasing (Drill hole approach)

Drill holes can also be used when using mixed biasing. The technique is called Virtual Air Gap (VAG). Basic studies have been published in [24] and [25]. A detailed analysis and discussion of this premagnetization

method as well as a concrete application for power grid filtering is given in [26]. A scaling of the mixed premagnetization including the associated analysis is given in [27] and [28].

A construction drawing of the magnetic core used for the investigations presented in this publication shows Fig. 5.

E 34/14/9 – 3C90

<figure><img src="image_6.png" /><figcaption>Fig. 5. Construction drawing of the mixed biased core (drill hole approach) [10]</figcaption></figure>

It is an E 34/14/9 core with drilled holes in the yoke. The core was also used for investigations in [10] and [23]. The manufactured prototype is presented in Fig. 6.

<figure><img src="image_7.png" /><figcaption>Fig. 6. Prototype of the mixed biased core (drill hole approach) [23]</figcaption></figure>

# III. MEASUREMENT SETUP

In order to compare the mentioned concepts parameters to be examined were defined, which will be described fully later. This includes for example the measurement of the control range of inductance or a possible magnetic back coupling from the load to the control circuit. For the measurements of the magnetic feedback a buck converter has been used as in [22], operated with all presented adjustable inductors. The signal propagation from load to control winding was
investigated. Different to [22], the focus is only on a
possible transmission of energy from load to control
winding. To check this, the control windings were loaded
with both a 50 Ω and a 1 kΩ resistor respectively. At first,
the used buck converter is explained very briefly. A more
detailed description can be found in [22]. After that the
parameters of comparison are stated.

A. Buck converter

For the measurement of the magnetic feedback from
the load circuit to the control ciruit a buck converter has
been used, as mentioned. The main parameters of the
topology are given in the following enumeration:

• $V_{in} = 48$ V

• $V_{out} = 12$ V

• $\bar{i}_{L} = 12$ A

• $P_{out} = 144$ W

• $L_{var}$ differs due to different prototypes

• $f = 100$ kHz

The realized setup including the buck converter and
the investigated prototypes are shown in Fig. 7. The
measurement setup corresponds to the setup from [22],
so that comparability can also be achieved here. Further
information can be found there as well and are not the
focus of this publication.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Section of the real setup including the prototypes used</figcaption></figure>

The different inductors used for the comparison in
this paper are enumerated with the letters A-D. From
here on, this labeling is used for assignment in the rest
of the paper.

B. Parameters to be examined

This subsection presents the parameters that are com-
pared in this paper. These include geometric properties
like weight and volume, which are crucial for real power
electronic applications, because magnetic devices can
dominate regarding this. The simplicity of manufacturing
is also assessed.

One major parameter is the range of inductance control, which analyses how much the inductance value can be influenced with a defined upper limit of control current. In addition to that, the shape behavior of the inductances with an applied control current will be discussed. Finally the magnetic coupling will be measured and analyzed, because it is probable that there will be magnetic feedback from the load side to the control side (see [22]. The voltage across the load inductance of the buck converter causes the magnetic flux in the load core to vary over time. According to Faraday’s Law of Induction, these variations can induce voltages in the control windings. Such induced voltages could potentially damage sensitive components, so they need to be thoroughly examined. The inductance of the control side is the last parameter to be examined as this significantly influences the transient response of the adjustable inductances.

Efficiency and losses are not considered in detail
but clearly has to be taken into account in the future.

For this paper, the latest and therefore newest pro-
totype of the orthogonal biased current controlled ad-
justable inductor in toroidal form will be used and
therefore presented for the first time. It is depicted in
Fig. 8. In contrast to previous approaches, cut control
cores according to the dimension of the load core are
used in addition to the uncut control core and wound with
the same control winding. This is intended to achieve
a saturation amplifier without increasing the parasitic
air gap of the load core, which is created by cutting
the material. This saturation amplifier is intended to
increase the influence on the inductance and at the same
time reduce the required control current with regard to
subsequent energy efficiency.

<figure><img src="image_9.png" /><figcaption>Fig. 8. Latest version of orthogonal biased current controlled adjustable inductance in toroidal form, prototype D</figcaption></figure>
The calculations for this design regarding number of turns for load and control winding can be found in [21]. The load winding consists of $N_{load} = 6$ and the control winding has $N_{control} = 381$. The calculations and design rules for the above mentioned inductors using the drill hole approach are given in [10].

## IV. MEASUREMENT RESULTS

In this section the measurement results of the prototypes regarding range of inductance control and magnetic feedback are visualized, explained and discussed.

### A. Control Range of Inductance

To provide an overview of the possibilities of influencing the inductance of the different prototypes has been examined with the same measurement setup. It is already well described in [21] and therefore will not be explained any further in this publication. During the investigation a control current is applied which follows the E6 series with an upper limit of 31.62 A. The measurement curves with no control current applied and the maximum control current are highlighted in color. The results at a control current of 10 A are also highlighted and represent a compromise between the controllability of the inductance and the electrical losses within the control winding, as the current in these is taken into account quadratically.

1) *Prototype A:* Prototype A is an orthogonal biased construction following the drillhole approach, which is visualized in Fig. 4. The measurement results with different control currents applied can be seen in Fig. 9.

<figure><img src="image_10.png" /><figcaption>Fig. 9. Dependency of inductance on load current as a function of control current, prototype A</figcaption></figure>

It is recognizable that the control current has a clear impact on the inductance with an adjustment range of

around 20 %. It is also noticeable that the inductance is strongly dependent on the load current due to saturation. However, the applied control current influences the saturation effect. The higher the applied control current the lesser load current is required to saturate the core. Nevertheless, the inductance does not drop suddenly after a certain load current was reached as typical for inductors made from magnetic material with an airgap but almost linear right from the start. Around $I_{load} = 20$ A there is an intersection area with almost no controllability of the inductance. After that, the applied control current lead to a higher inductance which decreases more slowly with increasing load current. This prototype could be used when a wide range of inductance is required or to mitigate saturation effects. Similar results were achieved in [18] using the paraformer approach.

2) *Prototype B:* Prototype B is also an orthogonal biased construction following the drillhole approach but designed for a smaller inductance value but at the same time more turns on the control side. In Fig. 10 the inductance in dependence on load current and with applied control currents is presented.

<figure><img src="image_11.png" /><figcaption>Fig. 10. Dependency of inductance on load current as a function of control current, prototype B</figcaption></figure>

The measurement displays that almost the same behavior occurs as with prototype A. Following the way smaller inductance there is no saturation with no control current applied. Different to prototype A the control range increases at higher load currents up to over 50 %. This could be used for applications with differing levels of load current in combination with a well designed controller for the control current. Following from the higher number of turns in the control circuit less control current has a bigger effect, making it more efficient compared to prototype A.
3) *Prototype C*: Different to the previous constructions, prototype C is designed with mixed bias influence, coming from a drillhole approach as well. Fig. 11 displays the results.

<figure><img src="image_12.png" /><figcaption>Fig. 11. Dependency of inductance on load current as a function of control current, prototype C</figcaption></figure>

The inductance is completely independent on the load current with no control current applied, which means no saturation occurs. Applying a control current the inductance becomes dependent on the load current. First, the inductance drops when a control current is applied but then increases with rising load currents until the initial value is reached. This effect requires higher load currents the higher the applied control current is. Even higher load currents lead to saturation. This can be "delayed" due to the control current, making the inductance more stable over a larger range of the load current. Nevertheless, this construction requires high control currents to obtain wider range of influence. At the upper limit of the control current at a load current of around 5 A the inductance is around 50 % smaller than the original value. A lower control current of 10 A has a significant smaller impact. The influence could be increased by adapting the design, dependent on the requirements for a specific application. However, in combination with a fast controller this behavior can be used to ensure that the inductance never falls below a defined value in order to protect parts of the application.

4) *Prototype D*: The newest approach of the orthogonal paraformer approach is found in prototype D. Defined requirements in earlier investigations were independent on load current with the aim to achieve a wider range of influence on the inductance with simultaneously lower control current. Measurement results are visualized in Fig. 12. The initial inductance has been calculated to be around 20 µH as this is between the values of the other prototypes.

<figure><img src="image_13.png" /><figcaption>Fig. 12. Dependency of inductance on load current as a function of control current, prototype D</figcaption></figure>

The initial inductance was calculated correctly as the figure proves. At no and lower control currents slight saturation can be noticed. For higher control currents the inductance is completely independent on the load current in the presented range. The inductance control increased from around 20 % in [22] at the upper limit of control current to around 70 %. A control current of only 10 A still reduces the inductance over 50 %. To increase the independence on load current the parasitic air gap could be increased. This concept for example could be used to change the inductance within a resonant tank.

## B. Magnetic Feedback

As described earlier the magnetic back coupling from the load into the control circuit will be analyzed in this subsection, because induced voltages could potentially damage sensitive components. The main investigation concerns the transfer of energy, which is examined with a connected resistor. In all the following figures the green colored graph represents the ripple current of the buck converter (its amplitude is dependent on the inductance value of the load inductor). The blue colored graphs are presenting the measured voltage over the control winding respectively the current through the 1 kΩ resistor. The red colored graph describes the same only as for the 50 Ω resistor. No load condition is not shown. The current is measured to validate whether the measured voltages are real and transmit energy.

1) *Prototype A*: This prototype has the highest inductance value which means the smallest ripple current is expected, neglecting the effect of saturation of the inductor. In Fig. 13 the measured voltage over the resistors is depicted.
<figure><img src="image_14.png" /><figcaption>Fig. 13. Feedback voltage over 1 kΩ and 50 Ω resistor connected in series to control winding, prototype A</figcaption></figure>

Almost no induced voltage is measured, only spikes are detectable at the switching moments of the buck converter for both resistance values. The associated current measurement to check the energy transmission is given by Fig. 14.

<figure><img src="image_15.png" /><figcaption>Fig. 14. Resulting current of feedback voltage, prototype A</figcaption></figure>

It is noticeable that the current also has spikes at the switching moments of the buck but also within bewteen with a lower amplitude and higher tact. However, the resistance value does not seem to have such a big effect, since the one resistor is 20 times bigger then the other, but the spikes are almost the same in amplitude of around 30 mA. This has to be investigated more in detail in the future. A maximum current of 2.5 mA occurs between the switching events. The control circuit therefore has a current source behavior. Nevertheless, the risk potential of a magnetic feedback can be rated as low.

2) *Prototype B*: As prototype B is almost the same as prototype A similar results are expected. The

main difference is the way smaller initial inductance, leading to a way higher ripple current, combined with a higher number of turns within the control circuit. The measurements are displayed in Fig. 15.

<figure><img src="image_16.png" /><figcaption>Fig. 15. Feedback voltage over 1 kΩ and 50 Ω resistor connected in series to control winding, prototype B</figcaption></figure>

The feedback voltage looks almost similar, even in amplitude and also only occurs at the switching moments. The amplitude of ripple current does not seem to have any influence. The current through the resistors is visualized in Fig. 16.

<figure><img src="image_17.png" /><figcaption>Fig. 16. Resulting current of feedback voltage, prototype B</figcaption></figure>

Cleary detectable again are the spikes during the switching with little less amplitude. Even the current between the switching moments is nearly the same. Between both prototypes almost no difference can be observed.

3) *Prototype C*: Prototype C is based on a concept with mixed biasing, so that the magnetic back coupling is more likely to occur here as the magnetic fields are not at all points orthogonal to each other. The results for the mixed bias prototype can be seen in Fig. 17.
<figure><img src="image_18.png" /><figcaption>Fig. 17. Feedback voltage over 1 kΩ and 50 Ω resistor connected in series to control winding, prototype C</figcaption></figure>

The feedback voltage for 1 kΩ seem to represent the voltage over the load inductor. For 50 Ω the feedback voltages seem to show a transient response, typical for charging a capacitor which is parallel connected to a resistor. This could indicate a capacitive coupling on the load side of the prototype. Special interest therefore is on the current through the resistors, given in Fig. 18.

<figure><img src="image_19.png" /><figcaption>Fig. 18. Resulting current of feedback voltage, prototype C</figcaption></figure>

Once again peaks during the switching moments occur but with a smaller amplitude. Several additional peaks are recognizable, which follow a high-frequency trend. The current through the resistors does not follow the shape of the voltage. For this prototype, the control circuit seems to act as a voltage source as there is almost no current but the voltages are different. This has to be investigated more detailed, since an EMC interference seem likely. However, with peaks around 10 mA the risk potential is rated as low as well.

4) *Prototype D*: Finally, prototype D is investigated. No large magnetic coupling is to be expected, based on the results of [22] even with the turns ratio of 6:381.

However, a possible effect coming from the implemented saturation amplifier needs to be investigated and is presented in Fig. 19.

<figure><img src="image_20.png" /><figcaption>Fig. 19. Feedback voltage over 1 kΩ and 50 Ω resistor connected in series to control winding, prototype D</figcaption></figure>

Surprisingly, the strongest feedback is observable for 1 kΩ with a voltage following the form of the ripple current and an amplitude from -4 V to 4 V. The higher resistance does not seem to damp the control circuit as the feedback voltage for 50 Ω is much lesser (but also reproducing the form of the ripple current). It seems to be more of an energetic feedback, a higher resistance leads to a higher voltage, which keeps the power constant. The measured currents for the feedback voltages displayed can be seen in Fig. 20.

<figure><img src="image_21.png" /><figcaption>Fig. 20. Resulting current of feedback voltage, prototype D</figcaption></figure>

As for prototype C the current through the resistors do not have the same shape as the voltages which is different to the results in [22]. For this reason EMC interference seems very likely. The maximum current peaks are around 10 mA. Therefore the danger for a supply unit is rated as low as well.

In general, all prototypes have a low hazard potential due to magnetic feedback. However, this must be
checked with an improved measurement setup in the future to reduce or avoid EMC interference.

### C. Transient Response

To affect the load side, the control cores must generate a magnetic field that interacts with the main field of the load core. This is accomplished by applying a direct current to the control side. The control circuit also includes an inductance, which has to be taken into account, as high inductance can lead to sluggish system behavior. Therefore, the inductances of the control side were measured using a pulsed measurement method until the upper limit of the control current was reached with no load current applied.

Fig. 21 depicts the inductance of the control circuit of prototype A.

<figure><img src="image_22.png" /><figcaption>Fig. 21. Dependency of control inductance of prototype A on control current</figcaption></figure>

The initial inductance of the control circuit is around 10 µH, which is rated as low and drops strongly with increasing control currents. After 10 A is reached, the decline in inductance is significantly reduced.

Fig. 22 illustrates the inductance of the control circuit for prototype B.

<figure><img src="image_23.png" /><figcaption>Fig. 22. Dependency of control inductance of prototype B on control current</figcaption></figure>

The inductance is around 100 µH with no control current applied. It decreases rapidly with higher control currents. For currents greater than 10 A, the inductance remains constant.

The measured inductance of the control circuit of prototype C is visualized in Fig. 23.

<figure><img src="image_24.png" /><figcaption>Fig. 23. Dependency of control inductance of prototype C on control current</figcaption></figure>

With around 25 µH the inductance of the control circuit can be rated as low as well. Again after 10 A of control current are reached no more change in inductance value is observable.

For the prototypes A-C the control inductance decreased significantly with higher control currents applied until almost no further reduction was recognizable. However, Fig. 9 - Fig. 11 display, that an increase in control current still have an effect on the load side. This should be addressed in future investigations.

Finally, Fig. 24 presents the inductance of the control circuit for prototype D.

<figure><img src="image_25.png" /><figcaption>Fig. 24. Dependency of control inductance of prototype D on control current</figcaption></figure>

The measured control inductance is approximately 13 mH due to the 381 turns. This suggests that the transient response of the control circuit may be very slow. As the control current increases, there is a significant decrease in inductance due to the saturation of the core material. However, it is evident that the control inductance is not fully saturated at the desired 10 A control current. This observation aligns with the measurement
<table><caption>TABLE I
OVERVIEW OF COMPARED CRITERIA FOR THE DIFFERENT PROTOTYPES OF CURRENT-CONTROLLED INDUCTORS</caption><thead><tr><td>Prototype</td><td>Weight</td><td>Volume</td><td>Simplicity of manufacturing</td><td>Max. Range inductance control</td><td>Magnetic Feedback</td><td>Inductance of control circuit</td><td>Dependency on load current</td></tr></thead><tbody><tr><td>A</td><td>low (112g)</td><td>low</td><td>easy</td><td>20%</td><td>low</td><td>very low</td><td>high</td></tr><tr><td>B</td><td>low (119g)</td><td>low</td><td>easy</td><td>50%</td><td>low</td><td>low</td><td>medium</td></tr><tr><td>C</td><td>low (92g)</td><td>low</td><td>medium</td><td>50%</td><td>low</td><td>very low</td><td>high</td></tr><tr><td>D</td><td>high (781g)</td><td>high</td><td>complex</td><td>70%</td><td>low</td><td>high</td><td>very low</td></tr></tbody></table>

results of the load inductance, where further increases in control current above 10 A continue to have a noticeable effect.

## V. CONCLUDING COMPARISON

In this section chosen individual criteria of the different prototypes will be discussed. The comparative parameters can be found in the table I. Prototypes A - C have only a low weight compared to prototype D, based on the different construction setups. The same applies for the volume.

For the simplicity of manufacturing the number of work steps and their complexity have to be taken into account. Since the used core for prototypes A and B is a pot core with a hole in the center it is very simple to realize this design. For prototype C an Ultrasonic/Sonic Driller/Corer (USDC) must be used for the drill holes, making it more difficult to manufacture. The prototype D, on the other hand, has to be cut to size using a water jet cutter. The cores are then glued together and the cores must be wound. This process is more time-consuming and complex, making this prototype the most challenging.

The ranges of inductance control differ from 20% for prototype A, over 50% for both, B and C and up to 70% for prototype D. However, it has to be taken into account that the current load capacity for the prototypes A-C is below 10 A and for D below 31.62 A. Therefore, the stated range of inductance control can not be reached during steady state operation over a long period of time with its peak values.

The magnetic feedback from the load circuit to the control circuit was low in all the prototypes tested, so there is hardly any hazardous potential.

The inductance in the control circuit was rated as very low for the prototypes A and C with around 10 µH to 25 µH, low for prototype C with 100 µH and high for prototype D with almost 13 mH. This comes from the huge number of turns, necessary to create the required orthogonal magnetic field to influence the load inductance.

This needs to be considered regarding the transient response of the control circuit in a real application.

The dependence on the load current differs greatly in the various setups. Prototype A saturates strongly with increasing load currents. Prototype B is independent on the load current until a control current is applied. Increasing control currents lead to a higher dependency on the load current.

In summary, it can be stated that the prototypes presented are very different in terms of their characteristics. This leads to lots of potential areas of application. Every system offers advantages, but coming with disadvantages or challenges at the same time.

## VI. CONCLUSION

Current-controlled, adjustable inductors have already been used successfully in several applications in the past. They often contribute to an improvement in the overall circuit because they offer an additional degree of freedom in the design of the topology.

During the design process, a choice has to be made between different premagnetization methods and their individual advantages and disadvantages. A low ripple in the auxiliary current is usually an important factor here. This can be impacted by feedback from the main winding. This impact in turn depends on the selected premagnetization method.

In this paper several prototypes of adjustable inductors following different premagnetization methods and concepts are described and investigated. The focus was primarily on the range of inductance control as well as the magnetic feedback from the load circuit to the control circuit.

The results state, that all the prototypes investigated have low feedback. However, they differ significantly from one another, especially in terms of their enveloping volume, their manufacturability or the adjustability range of their inductance value. Depending on the focus of the application design, a selection can be made based on these parameters.
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