OBJECT OF THE INVENTION
[0001] The present invention relates to a magnetically integrated planar inductor to be
employed in energy conversion processes in which use is made of input and/or output
filtering means in a switched power supply converter. It is of special, but not exclusive
application, in switched power converters for low power and low voltage applications,
and, also, facilitates a high level of integration.
STATE OF THE ART
[0002] A compound inductor comprising two or more coupled windings for use in energy conversion
processes in which switching takes place, is known from the United States patent US-A-5.481.238,
being incorporated in the present patent application by reference.
[0003] An output filter that includes the compound inductor implemented according to the
above mentioned United States patent, can be used in some converters, like a buck
converter or a boost converter.
[0004] The compound inductor proposed in that patent comprises a first inductor having a
first winding on a first magnetic core, a second inductor having a second magnetic
core outside the winding coil of the first inductor, and a second winding around the
first winding and the second core. One end of the first winding and the corresponding
end of the second winding are electrically connected.
[0005] The main drawback of the inductor described in the United State patent mentioned
is its high cost, due to the fact that it is complicated to manufacture because specific
magnetic cores of different sizes and magnetic materials are used for obtaining appropriate
electrical characteristics for the output filter of the switched power converter.
Therefore the manufacturing process is not fully automated.
[0006] Consequently, it is necessary to develop filtering means comprising inductive means
having a manufacturing process which is fully automated and in which standardised
elements are employed.
[0007] As a consequence, the overall cost of the filtering means is reduced and, thereby,
that of the switched power converter in which it is equipped.
CHARACTERISATION OF THE INVENTION
[0008] In order to overcome the problems described above, the magnetic integration of a
planar inductor is proposed for use in some types of switched power converters having
filtering means to smooth the current that is flowing through the power converter,
and in which the planar inductor is the object of the present invention.
[0009] The design of the planar inductor of the invention alters the constructional process
of said inductor in order that the current fed to the load satisfies the zero-ripple
condition and, in addition, the manufacturing process employs standardised elements,
for example planar magnetic cores having an E type section, and which do not require
any special machining during said process.
[0010] As a result, the repeatability of the electrical characteristics of the planar inductor
is ensured during the manufacturing process, and consequently they are not affected
by the slight differences that could arise during said process. Consequently, the
cost of the planar inductor of the output filter is considerably reduced and, also,
the overall cost of the converter is diminished.
[0011] The switched power converter attains a high overall efficiency in the process of
converting a voltage received from a voltage source to an output voltage and, in addition,
it responds rapidly to variations in the load, finding particular use in those applications
requiring low power and low voltage with a low profile and reduced size.
[0012] The planar inductor comprises a magnetic core having an E type standardised core
section, on the outside legs of which are wound, respectively, a first coil and a
second coil that are magnetically coupled to each other for producing a significant
leakage inductance, such that one end of the first coil and the corresponding end
of the second coil are electrically connected in a second node. The result being that
the common magnetic flux observed respectively by both coils, is conducted through
the ferromagnetic circuit formed by the outside legs of the magnetic core, and the
magnetic flux associated with the leakage inductance is conducted by one outside leg
and the centre leg.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] A more detailed explanation of the invention is provided in the following description,
based on the figures attached, in which:
- figure 1 shows an electrical schematic of a buck converter according to the invention,
- figure 2 shows, in an elevation view of the inductor core used in the construction
of the buck converter, the distribution of the magnetic flux according to the invention,
- figure 3 shows a preferred embodiment of the inductor core used in the construction
of the buck converter according to the invention, and
- figure 4 shows another electrical schematic of a boost converter according to the
invention.
DESCRIPTION OF THE INVENTION
[0014] Figure 1 shows a single output buck converter that incorporates filtering means 14
connected directly to a load, said means being designed to smooth an output current
so that its ripple is zero, the zero ripple condition.
[0015] The buck converter is used merely as an example in order to achieve a better description
of the present invention, it being possible to use other types of switched converters,
all having an output or an input filter, achieving similar results.
[0016] The buck converter is connected via input terminals 11 and 12 to a voltage source,
representing one of the input terminals 12 a common voltage reference of said converter.
[0017] One of the input terminals 11 is connected to a first terminal 13-1 of a first switching
means 13, for example a field effect transistor, type MOSFET. A second terminal 13-2
is connected to an input terminal 14-1 of the filtering means 14. The first switching
means includes a third terminal (not shown) over which a signal is received to adapt
the ON and OFF periods of the first switching means 13 in order to produce a regulated
output voltage across output terminals 14-3 and 14-4.
[0018] A first node 16, situated on the conducting line that connects the second terminal
13-2 with the input terminal 14-1, is connected to a first terminal 15-1 of a second
switching means 15, and a second terminal 15-2 of the second switching means 15 is
connected to the common voltage reference of the converter, forming a rectifier branch.
For example, the second switching means 15 can be a unidirectional conducting device
such as a diode.
[0019] The terminal 14-2 of the filtering means 14 is also connected to the common voltage
reference. The filtering means 14 receives energy from the voltage source during the
ON periods of the first switching means 13, and produces across its output terminals
14-3 and 14-4 the smoothed output voltage and an output current with ripple close
to zero, which are respectively applied to the load, this being possibly telecommunications
equipment.
[0020] The filtering means 14 includes a planar inductor 20 comprising at least a first
coil 21 located on the conducting line that connects terminal 14-1 with terminal 14-3,
a series combination of a second coil 22 and a second storage capacitor 22-1 being
connected to a second node 24 situated between the terminal 14-1 and an end of the
first coil 21. Consequently the series combination is connected in parallel with the
second switching means 15.
[0021] A first storage capacitor 21-1 is connected in parallel with the load in order to
smooth the current which flows through the first coil 21, so that the means value
of this current is zero.
[0022] The two coils 21 and 22 of the inductor 20 are wound around a magnetic core 23, being
magnetically coupled to each other. The coupling coefficient between said coils 21
and 22 is less than unity due to the fact that they have a different number of turns.
[0023] The inductor 20 is constructed in such a manner that it introduces a significant
leakage inductance between said coils 21 and 22. The leakage inductance is represented
on figure 1 by means of a third coil 21-2, which is connected in series with an end
of the first coil 21 and with an end of the first capacitor 21-1.
[0024] In order to comply with the zero ripple condition in the output current, the magnitude
of the inductance of the third coil 21-2 has to reach a high value, due to the fact
that the current ripple is a function of the ratio between the magnitude of the voltage
drop across the third coil 21-2 and the value of its inductance; consequently the
latter has to be as high as possible.
[0025] The voltage seen by the inductance 21-2 is the difference in voltage between the
voltage on the second capacitor 22-1 and that on the first capacitor 21-1.
[0026] Figure 3 shows the preferred embodiment of the magnetic core 23 used in the construction
of the inductor 20 in order to reach the results desired with the invention. The core
23 comprises at least two standardised core sections, and at least one of the sections
is of an E type, i.e. a three-leg core.
[0027] Figure 2 shows each of the coils 21 and 22 respectively wound around an outside leg
of the type E section, and where one end of the first coil 21 and the corresponding
end of the second coil 22 are electrically connected at the second node 24.
[0028] In order to simplify the manufacturing process of the inductor 20, the turns of each
of the coils 21 and 22 are laid on printed circuit board, being possibly of the multi-layer
type. As a result, the manufacturing process of the planar inductor 20 of the invention
is relatively insensitive to tolerances and guarantees a high degree of repeatability
of said process.
[0029] The operation of the buck converter is explained as follows: during a period when
the first switching means 13 is conducting, the same power as is applied across the
input terminals 11 and 12 is applied to the input terminals 14-1 and 14-2 of the filtering
means 14, which is greater than the voltage present across the output terminals 14-3
and 14-4.
[0030] As a result, there is a growing current flowing from the terminals 11 and 12 charging
the capacitors 21-1 and 22-1. The inductor 20 starts to charge, storing a certain
amount of energy as well as feeding current to the load. During this time interval,
there is a direct transfer of power from the input to the output.
[0031] At the moment when the first switching means 13 is turned OFF, the incoming current
to the filtering means 14 cannot be cut off abruptly, for which reason the second
switching means 15 starts to conduct.
[0032] At this moment, the voltage applied to the input of the filtering means 14 is less
than that applied to the terminals 14-3 and 14-4. Consequently, the current through
the filtering means 14 has the same direction as before, but is decreasing. At the
moment when the first switching means 13 resumes conduction, the second switching
means 15 stops conducting and the cycle is re-initiated.
[0033] The main objective of the magnetic integration of the planar inductor 20 is to shunt
the alternating current (AC) received by the filtering means 14 into the second coil
22; consequently, the first coil 21 only receives the direct current (DC), whereby
the output voltage is smoothed more easily. As a result, the series combination of
the second coil 22 and the second capacitor 22-1 receives only AC current with a mean
value equal to zero. Consequently, the output current satisfies the zero ripple condition.
[0034] The electrical characteristics of the filtering means 14 are unchanged by the incorporation
of the series combination mentioned. Thus, the inductor 20 is designed to confine
the common magnetic flux observed respectively by the two coils 21 and 22, through
the ferromagnetic circuit formed by the outside legs of the core 23. And the flux
observed by the third coil 21-2 is conducted through the ferromagnetic circuit formed
by the centre leg and the outside leg on which the first coil 21 is wound. The centre
leg has a predetermined air gap 23-1, in order to limit the magnitude of this last
flux.
[0035] Likewise, the outside legs have a second air gap, which is much less than the first
air gap 23-1 of the centre leg, for limiting the magnitude of the common magnetic
flux and prevent the core 23 from saturating.
[0036] Figure 4 shows the filtering means 14 of the invention in a switched power converter
of the boost type. Its operation is not described here, as it is similar to that of
the buck converter.
[0037] From the foregoing description, it can be deduced that the switched power converter
comprising the filtering means 14 of the invention is preferentially implemented with
planar integrated magnetic devices, the coils of which are laid on printed circuit
boards, demonstrating little sensitivity to variations arising during the manufacturing
process that can be automated through the use of standardised elements. The converter
in question is compact in size and low in profile, being particularly useful in those
applications which require both low power and low voltage, i.e. for supplying power
to end loads.
1. Magnetic integration of a planar inductor (20) for a switched power converter including filtering means (14) connected directly
to a load to supply it with a current having zero ripple; said planar inductor (20)
comprising a magnetic core (23) formed by at least one section of an E type standardised
core, a first coil (21) and a second coil (22) that are magnetically coupled to each
other in order to obtain a leakage inductance (21-2); one end of said first coil (21)
and the corresponding end of said second coil (22) being electrically connected at
a common second node (24); characterised in that each of said first coil (21) and said second coil (22) is wound around a
respective first and second outside leg of said standardised core section for conducting
the common magnetic flux observed respectively by both the first coil (21) and the
second coil (22) through the ferromagnetic circuit formed by said outside legs of
said magnetic core (23).
2. Magnetic integration of a planar inductor (20) according to claim 1, characterised in that said first coil (21) has a number of turns that is different from the number
of turns of said second coil (22).
3. Magnetic integration of a planar inductor (20) according to either one of claims 1 or 2, characterised in that the flux observed by said leakage inductance (21-2) is conducted through
the ferromagnetic circuit formed by the centre leg and the outside leg on which said
first coil (21) is wound.
4. Magnetic integration of a planar inductor (20) according to claim 3, characterised in that the magnitude of said leakage inductance (21-2) is limited by means of a
predetermined first air gap (23-1) located in the centre leg of said magnetic core
(23).
5. Magnetic integration of a planar inductor (20) according to claim 1, characterised in that the magnitude of the flux observed respectively by said first coil (21) and
said second coil (22) is limited by means of a second air gap located, respectively,
in said outside legs of said magnetic core (23), and being of a value less than that
of said first air gap (23-1).