XIX International conference for students and young scientists «MODERN TECHNIQUE AND TECHNOLOGIES»
Section 2: Instrument making
MAGNETIZING CURRENT RESTRICTION
Beykov М. V.
Scientific adviser: Prof. Dr. Kazantsev Y.M.
Language advisor: Prof., Dr. Troitsky O.Y.
Institute of Non-destructive testing at National Research Tomsk Polytechnic University,
30, Lenin av., Tomsk, 634050, Russia
E-mail: Beykovmv@gmail.com
1. Introduction
It’s well-known that when full-bridge switching
mode power supply (SMPS) is designed the attention
should be paid to transformer reversal. Ideally the
magnetic core of transformer is switched by symmetric volt-seconds applied to the transformer within a
pulse period. However, in practice the power semiconductor on-state voltages, ohmic losses, finite transistor turn-off times, delay times of the transistor gate
drive circuits and the ripple of the input filter capacitor voltage cause a deviation of the actual voltage
applied to the transformer primary side from the ideal
voltage shape which is assumed for the calculation of
the relative on-time of the power transistor in order to
guarantee a transformer volt-seconds balance. It may
result in the unbalance of positive and negative voltseconds applied to transformer. It causes displacement of magnetic hysteresis loop from point O up to
point O1 (fig.1).
Abstract
For a full-bridge switching-mode power supplies
different turn-on and turn-off delay times of power
semiconductors and different on-state voltages of the
valves would cause an unbalance of the positive and
negative volt-seconds applied to the high frequency
transformer within a pulse period and would result in
transformer core saturation.
This paper propose a concept for actively ensuring
a symmetric magnetization with switching frequency
of the transformer magnetic core of power supply
based on measurement and restriction of the magnetizing current. The magnetizing current is determined
by subtraction of the transformer primary and the secondary currents being weighted according to the
transformer turns ratio. Currents are measured by two
DC-current sensors. Subtraction is realized by special
circuit based on digital signal processor.
Fig.1. Hysteresis loop displacement
A simmetrization of the magnetic core switching
by passive means, i.e. by the insertion of ohmic resistor or by an artificial increase of parasitic wire resistances is not possible or sufficient for high efficient
systems.
Inserting an air-gap in the transformer core would
result in an increase of the maximum tolerable magnetizing current [1], which would reduce the resistance values required for a passive symmetrization.
However, a magnetizing current being too high in
magnitude results in a significant distortion of the
mains current and in an increase of the conduction
losses of the power semiconductors.
Alternatively, as proposed in [1], [2] a blocking
capacitor could be connected in series with the transformer primary for ensuring the symmetric magnetic
reversal. The voltage which then actually is applied to
the transformer primary winding does only contain
components with switching frequency. However, as it
has become clear by a closer experimental analysis of
Furthermore, it may bring about one-way magnetic core saturation and huge current surges (fig.2).
Fig.2. Magnetizing current
It reduces system reliability and efficiency due to
additional losses, and increases electromagnetic influence.
229
XIX International conference for students and young scientists «MODERN TECHNIQUE AND TECHNOLOGIES»
Section 2: Instrument making
this concept, a resonance between the main inductance of the transformer and the blocking capacitor
might occur.
Therefore, the appropriate way to prevent the
SMPS against one-way magnetic core saturation is an
actual problem.
and secondary transformer current. Then these signals
are subtracted according to the transformer ratio
Finally, there is a signal prorated to magnetizing
current that is used for restriction of magnetic saturation. It provides the signal with high speed response
and sensitivity.
2. Measurement of the core reversal
There is a lot of approaches to measure the magnetizing current. Some of them are based on magnetic
short-circuit of flux via additional cores. It assumes to
use split-type magnetic core or to split or drill the core
that decreases durability of magnetic core. So that it
makes it non-manufacturing. Besides, these methods
are low speed response and low sensitive. Therefore,
they are uncommonly used.
Another way is to use special reversal sensors that
provide signal prorated magnetizing current.
The easiest way to realize such kind of sensor is to
use two DC-current sensors measuring the primary
3. Magnetizing current restriction
Protection SMPS against one-way magnetic saturation is obtained by magnetizing current restriction.
The point is to limit the magnetizing current under
acceptable level depended on other factors and can be
determined experimentally.
This method has a huge disadvantage: the level of
magnetizing current restriction is fixed and it results
in undesirable current surges while low primary voltage is occurring (fig.3). It reduces efficiency of SMPS
and increase electromagnetic influence.
To avoid this operation condition it’s considered to
use additional sensor to measure the primary voltage.
Fig.3. Magnetizing current at low primary voltage
Sensor output signal is used to calculate the acceptable level of magnetizing current restriction according to the primary voltage. It allows to reduce the
undesirable current surges (fig.4) under any primary
voltage and obtained appropriate result.
Fig.4. Magnetizing current at low primary voltage
4. Conclusion
Without active control of the magnetizing current
there is the possibility of an unreliably high magnetizing current possibly resulting in a saturation of the
transformer of SMPS. If methods are employed which
allow higher magnetizing current and passive
symmetrization, the increased magnetizing current
distorts the ideal input current shape.
The presented approach to provide the one-way
saturation protection allows to design SMPS having
high efficiency, reliability and low electromagnetic
influence.
References
1. Patel R. Detecting impending core saturation
in switching-mode power converters. Proceeding of
the 7th National solid-state power conversion conference, pp. B-3/1–B-3/11 (1980).
2. Pankau J., Leggate D., Schlegel D., Kerkman
R,. and Skibinski G. High frequency modeling of current sensors. Proceeding of 14th IEEE applied power
electronics conference, Dallas, Vol. 2, March 14-18,
pp. 788-794 (1998).
230