HYBRID ZERO MAGNETIZATION SENSING IN FLYBACK
SWITCHING CONVERTER
FIELD OF INVENTION:
5 [001] The present subject matter described herein, in general, relates to field of
power converters, and in particularly relates to hybrid zero magnetization sensing
in Flyback switching converter.
BACKGROUND AND PRIOR ART:
[002] Generally, in the electronic circuits, high switching losses in power
10 devices are present. Main reason behind the high switching losses is hard
switching. In the hard switching, voltage and current overlaps during metaloxide-
semiconductor field-effect transistor (MOSEFT) switching ON and OFF.
To overcome the high switching losses, zero current switching is introduced to
reduce the switching losses.
15 [003] Conventionally, the existing method use reflected voltage type sensing to
over the high switching loss problem. In the voltage reflected method, reflected
voltage is sensed to detect zero current crossing through sense winding which is
the reflected voltage from the secondary winding to the auxiliary winding of the
Flyback converter. The method uses the reflected voltage to sense zero crossing in
20 the circuit. Further, these methods of sensing zero crossing is unable to sense the
zero crossing during startup, overload, and short current situations, as voltage in
secondary in all these conditions is not enough to sense the z&o crossing.
[004] US patent No. US4754385 relates to a dual transistor flyback converter
having a pair of synchronously driven switching transistors for switching voltage
25 to a primary of a flyback transformer. The current sensing circuit comprises a
current transformer placed in series with the flyback transformer secondary
circuit. In accordance with another feature of the invention, the dual transistor
flyback converter includes a load current sensing circuit for sensing load current
delivered by the transformer secondary. The current sensing circuit includes a
current transformer having a primary winding connected between the low voltage
output terminal of the converter and the transformer secondary. The invention has
the output current sensing using transformer, which then uses the signal to
regulate the current.
5 [005] US patent No. US6958920 relates to a switching power converter and
method of controlling an output voltage thereof using predictive sensing of
magnetic flux provides a low-cost switching power converter via primary-side
control using a primary-side winding. An integrator generates a voltage that
represents flux within a magnetic element by integrating a primary-side winding
10 voltage. A detection circuit detects the end of a half-cycle of post-conduction
resonance that occurs in the power magnetic element subsequent to zero energy
level in the power magnetic element. In the invention, magnetic flux is sensed by
voltage in primary side.
[006] US patent No. US7561446 relates to a transformer-coupled buck-boost
15 DC-DC power converter. An active clamp circuit is provided to form a resonant
circuit with the transformer to control the slew rate of the secondary current and
allow the secondary switch to be turned ON at conditions of zero voltage and
relatively low current. The architecture of the double-clamped ZVS buck-boost
topology operating in the first operating mode supports ZVS operation of the
20 primary switches and ZCS and ZVS operation of the secondary switch. In the
invention, ZVS is achieved in Buck Boost aid to achieve ZVS, an active clamp
circuit is used which uses transistor and capacitor.
[007] US Patent No US8213 192 relates to switching voltage regulator samples
signals corresponding to a flyback voltage on an auxiliary winding on a primary
25 side of the switching voltage regulator. The flyback voltage functions as feedback
from the output voltage on the secoridary side. On detection of presence of the
flyback voltage, samples corresponding to the flyback voltage are stored until the
flyback voltage falls below a threshold voltage. In the invention, only auxillary
side voltage is sensed to control the voltage and to achieve ZVS.
[OOS] US patent No. US9083254 relates to primary-side sampled feedback
system includes a sample acquisition phase during which the voltage across the
clamp capacitor is sensed as a measure of the primary-reflect output voltage. In
the invention, only auxiliary side voltage is sensed to control the voltage and to
5 achieve ZVS.
[009] US patent No. US7002815 relates to a switching type power converter
circuit includes a step-down converter circuit, a DC/AC converter circuit coupled
to the step-down converter circuit, and a rectifier circuit coupled to the DC/AC
converter circuit. In one embodiment, the DC/AC converter operates with near
10 50% duty cycle and with substantially zero-voltage, sub.s ta.n tially minimum
current switching in a resonant mode. An auxiliary step down converter may be
added. An ACIDC converter front end with a full-wave bridge, an RF filter, and a
power factor correction circuit may also be added. Losses in a switch can be
reduced by establishing a zero-voltage condition across a switch while the switch
15 transitions from the OFF state to the ON state. Consequently, some embodiments
may include features to establish a zero (or near zero) voltage condition across the
switch at turn on of the switch. Losses in a switch can be reduced by establishing
a zero-current condition while the switch transitions from the on state to the off
state. As such, some embodiments may include features to establish a zero-current
20 condition at turn off times of the switch. The phrase "turn off times of the switch"
refers to the instant at which the switch starts to transition from the on state to the
off state. The invention does not talk about sensing method. It only talks about the
ZVS which is achieved.by using one type of sensing which voltage.
[0010] US patent No. US8669744 relates to a zero voltage switching (ZVS)
25 technique for use in isolated and non-isolated switching power converters and
regulators, e.g., based upon buck, boost, bl~ck-boost, and double-clamped
topologies is disclosed. During a reverse energy phase of the converter operating
cycle, energy is transferred in reverse from the load or the clamp capacitor to the
inductor, allowing the current in the inductor to increase in magnitude with a
30 reverse polarity, building up reverse energy. The reverse energy may be used for
I charging and discharging parasitic and other circuit capacitances for ZVS. The
I reverse energy phase is adjusted based upon circuit operating conditions, so that
I the amount of energy stored in the inductor L at the end of the reverse energy
phase is approximately equal to, but .preferably no greater than, that required to
5 turn the switches ON at substantially zero voltage. In the invention, reverse
energy from load or clamp capacitor is used for achieving ZVS. It only reutilizes
I the reverse energy.
1 [0011] ITS patent No. US8791591 relates to the power converters having two or
more outputs. Zero-current switching multiple-output-regulator ("ZCS MOR")
10 converts power from an input source via a transformer to a main output and one or
more auxiliary outputs. Each output is coupled to a respective winding of a
transformer preferably by switches controlled as synchronous rectifiers. The
synchronous rectifier for each auxiliary ciutput preferably turns on at the start of
current flow in its respective winding and off 'as its respective current returns to
15 zero, independently of the other outputs. However the invention talks about cross
regulation of two outputs. In this invention, ZCS is achieved by using reflected
voltage. '
[0012] Thus, in view of the above prior arts, it is clear that the available methods
sense zero magnetization using reflected voltage only. Further, these methods are
20 not operated in the several conditions like noflight load, startup, circuit overload,
and short circuit. In other language, it can be said that the conventional methods
are based on the sensing of reflected voltage and based on the sensed reflected
voltage sense zero magnetization in the circuit.
[0013] Therefore it is desirable in the Flyback Switching Converter or power
25 converters to sense the zero crossing during noflight load, startup, circuit
overload, and short circuit current situations. Further there is need of a circuit
which works on both voltage signal and current signal in auxiliary and secondary
winding respectively. In other words, there is need of hybrid sensing in the
flyback switching converter or any power converter to sense voltage and current
signals to sense zero magnetization in low voltage - high current as well as high
voltage and low current conditions.
OBJECTS OF THE INVENTION:
[0014] The principal objective of the present invention is to provide zero
5 magnetization sensing in most of the operating conditions like startup, over load,
short circuit and no/light load condition in powcr convcrtcrs.
[0015] Another object of the present invention is to provide reliable and stable
operation of converter in boundary conduction mode under wide input operating
voltage range under normal as well as no/light load, over load and short circuit
10 conditions and capacitive startup conditions.
[0016] Yet another object of the present invention is to provide zero
magnetization sensing signal under critical conditions of very low output current
with highlnormal voltage and low voltage with higher current.
[0017] Still another object of the present invention is to avoid magnetic saturation
15 and fludcurrent walking issues in power converter by providing zero
magnetization.
SUMMARY OF THE INVENTION:
[0018] The present invention relates to a zero magnetization sensing in normal,
startup, over load, and short circuit condition to avoid magnetic saturation and
20 fludcurrent walking issues in Flyback switching converter. Further, zero
magnetization is sensed by both reflected voltage signal of auxiliary winding and
current signal in secondary winding of the converter. Both signals are
mergedlcombined to provide proper signal to sense zero magnetization in wide
input voltage operating range in most of the operating conditions like startup,
25 no/light load, over load, short circuit conditions. Combined current and reflected
voltage for zero magnetization sensing overcomes the issue by providing sensing
signal under very low current - high voltage and low voltage - high current
operation of converter. The present subject matter provides hybrid sensing by
adding voltage and current signals to sense zero magnetization in low voltage -
high current as well as high voltage and low current conditions.
[0019] In order to further understand the characteristics and technical contents of
the present subject matter, a description relating thereto will be made with
5 reference to the accompanying drawings. However, the drawings are illustrative
only but not used to limit scope of the present subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] It is to be noted, however, that the appended drawings illustrate only
typical embodiments of the present subject matter and are therefore not to be
10 considered for limiting of its scope, for the invention may admit to other equally
effective embodiments. The detailed description is described with reference to the
accompanying figures. In the figures, the left-most digit(s) of a reference number
identifies the figure in which the reference number first appears. The same
numbers are used throughout. the figures to reference like features and
15 components. Some embodiments of system or methods in accordance with
embodiments of the present subject matter are now described, by way of example,
and with reference to the accompanying figures, in which:
[0021] Figure 1 illustrates circuit diagram of the flyback switching converter to
sense zero magnetization, in accordance with an embodiment of the present
20 subject matter;
[0022] Figure 2 illustrates CT based current sensing, in accordance with an
embodiment of the present subject matter;
[0023] Figure 3 illustrates resistor based current sensing, in accordance with an
embodiment of the present subject matter;
25 [0024] Figure 4 illustrates Hall Effect based current sensing, in accordance with
an embodiment of the present subject matter;
[0025] Figure. 5(a), (b), and (c) illustrate the detection of zero magnetization in
normal operating condition, in accordance with the present subject matter;
[0026] Figure. 6(a), (b), and (c) illustrate the detection of zero magnetization in
start up and short circuit condition, in accordance with the present subject matter;
and
[0027] Figure. 7(a), (b), and (c) illustrate the detection of zero magnetization in
5 No and light load condition, in accordance with the present subject mattcr.
[0028] The figure depicts embodiments of the present subject mattcr for the
purposes of illustration only. A person skilled in the art will easily recognize from
the following description that alternative embodiments of the structures and
methods illustrated herein may be empleed without departing from the principles
10 of the disclosure described herein /
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0029] The subject matter disclosed herein relates to zero magnetization sensing
in normal, startup, over load, and short circuit condition to avoid magnetic
saturation and fludcurrent walking issues in Flyback switching converter. The
15 present subject matter illustrates a method and circuits for sensing the zero
magnetization in the power converters.
[0030] Further, zero magnetization is sensed by both reflected voltage signal of
auxiliary winding and current signal in secondary winding of the converter. Both
signals are mergedlcombined to provide proper signal to sense zero magnetization
20 in wide input voltage operating range in most of the opcrating conditions like
startup, nollight load, over load, short circuit conditions. Combined current and
reflected voltage for zero magnetization sensing overcomes the issue by providing
sensing signal under very low current - high voltage and low voltage - high
current operation of converter. The present subject matter provides hybrid sensing
25 by adding voltage and current signals to sense zero magnetization in low voltage -
high current as well as high voltage and low current conditions.
[0031] As explained above in the background section, converters are not able to
sense zero magnetization in the several conditions like nollight load, startup,
circuit overload, and short circuit. Further, it can be said that the conventional
converters are based on the sensing of reflected voltage and based on the sensed
reflected voltage sense zero magnetization in the circuit.
[0032] In the present subject matter, the power converter circuit senses the zero
magnetization in conditions having very low amplitude signals during nollight .
5 load, whereas voltage based sensing provides low amplitude signal during
overload, short circuit and startup conditions. In low voltage and high current
condition and vice versa condition, hybrid sensing is able to maintain amplitude
of the signal to detect the zero magnetization. To sense the zero magnetization in
these conditions hybrid sensing is used in which both voltage and current signals
10 being used to detect the zero crossing. Hence, the present invention aims to
combine two kind of signal to generate zero magnetization sensing in most of the
operating conditions, to avoid magnetic saturation and flux walking issues in wide
operating range of flyback converter.
[0033] These and other advantages of the present subject matter would be
15 described in greater detail with reference to the following figures. It should be
noted that the description merely illustrates the principles of the present subject
matter. It will thus be appreciated that those skilled in the art will be able to devise
various arrangements that, although not explicitly described herein, embody the
principles of the present subject matter and are included within its scope.
20 [0034] Figure 1 illustrates the circuit diagram of the flyback switching converter
to sense zero magnetizati0n:The present circuit includes two transformers T1 and
T2. The present converter includes a current sensing circuit (I SENSE) 1, a
voltage sense circuit (V SENSE) 2, a combiner circuit 3, and a pulse width
modulating (PWM) control circuit unit 4. The current sensing circuit (I SENSE) 1
25 is connected with the transformer T2. The current sensing circuit (I SENSE) 1 can
be CT based current sensing, (as shown in figure 2), resistor based current sensing
(as shown in figure 3), and Hall Effect based current sensing (as shown in figure
4). All these current sensing techniques are well known in the art. Further, the
. current sensing circuit (I SENSE) 1 senses the current in secondary winding of the
30 transformer TI. The current sensing circuit (I SENSE) .1 generates a current signal
based on the sensed current in the transformer T2. The current sensing circuit (I
SENSE) 1 senses the current flowing in secondary winding of transformer T1
through CT. An Electric magnetic field (EMF) is generated in the current sensing
circuit (I SENSE) 1 which is used for zero magnetization sensing. Current flowing
5 in the secondary winding of transformer T1 is sensed by T2, which generates the
emf in T2. This emf is in proportion to the amplitude of current flowing. The emf
is used to sense zero magnetization.
[0035] The voltage sensing circuit (V SENSE) 2 is connected with the
transformer TI. The voltage sensing circuit (V SENSE) 2 senses the voltage in
10 auxiliary winding of the transformer TI. The voltage sensing circuit (V SENSE) 2
generates a voltage signal based on the sensed voltage in the transformer TI. The
voltage sensing circuit (V SENSE) 2 is connected to auxiliary winding of
transformer T1. Due to mutual inductance effect of transformer the emf is
generated in auxiliary winding. This emf is sensed by the V sense circuit 2 to
15 detect zero magnetization.
[0036] The combiner circuit 3 combines the generated current signal and voltage
signal, and sends the combined signal to the PWM control' circuit unit 4. The
combiner 3 gets the signals' from both V sense and I sense circuit. The combiner
circuit 3 maintains the signal amplitude in a range which is supported by PWM
20 control unit. At the time of startup, light load, and short circuit conditions voltage
signal is low and current is high. The I sense circuit 1 senses the current and
provides signal to the combiner circuit 3. Where as in case of over load condition
current is low and voltage is high. The V sense circuit 2 provides signal to
combiner 3 to sense zero magnetization. In normal operating mode both I sense
25 and V sense circuit signals are used to sense zero magnetization hence named as
Hybrid sensing.
[0037] Figure. 5(a), (b), and (c) illustrate the detection of zero magnetization in
normal operating condition, in accordance with the present subject matter. Figure.
5(a) illustrates the voltage signal sense by the voltage sensing circuit (V SENSE)
30 2 in normal operating condition. In the normal operating condition, voltage signal
has the amplitude which is above the threshold level to detect the zero
magnetization. Further, zero magnetization is detected by the PWM control unit
on the threshold level. Figure. 5(b) illustrates the current signal sense by the
current sensing circuit (I SENSE) 1 in normal operating condition. In normal
5 operating conditions, the current signal has amplitude above the threshold level
which is required to detect the zero magnetization. Figure. 5(c) illustrates a
combiner signal generate by the combiner circuit 3 in normal operating condition.
The combiner circuit 3 combines both the current signal and the voltage signal,
and generates a combined signal having combined amplitude which is above the
10 threshold level. Therefore, the combiner circuit 3 combines both the signals and
detects zero magnetization when the combined amplitude is above the threshold
level.
[0038] Figure. 6(a), (b), and (c) illustrate the detection of zero magnetization in
start up and short circuit condition, in accordance with the present subject matter.
15 Figure. 6(a) illustrates the voltage signal sense by the voltage sensing circuit (V
SENSE) 2 in start up and short circuit condition. In the start up and short circuit
condition, voltage signal has very low amplitude which is below the threshold
level to detect the zero magnetization. Further, the zero magnetization cannot be
detected by the PWM control unit on voltage signal. Figure. 6(b) illustrates the
20 current signal sense by the current sensing circuit (I SENSE) 1 in start up and
short circuit condition. In start up and short circuit condition, the current signal
has good/high amplitude above the threshold level which is required to detect the
zero magnetization. Further, the zero magnetization can be detected by the PWM
control unit on current signal in this condition. Figure. 6(c) illustrates a combiner
25 signal generate by the combiner circuit 3 in start up and short circuit condition.
The combiner circuit 3 combines both the current signal having amplitude above
the threshold level and the voltage signal having amplitude below the threshold
level to maintain amplitude. Further, the combiner circuit 3 generates a combined
signal having amplitude above the threshold level to detect the zero
30 magnetization. The combiner circuit 3 uses the amplitude of current signal to
maintain the amplitude above the threshold level as shown in the figure 6(c).
Accordingly, the PWM control unit 4 detects zero magnetization based on the
generated combined signal which has amplitude above the threshold level.
[0039] Figure. 7(a), (b), and (c) illustrate the detection of zero magnetization in
No and light load condition, in accordance with the present subject matter. Figure.
5 7(a) illustrates the voltage signal sense by the voltagc scnsing circuit (V SENSE)
2 in No and light load condition. In the No and light load condition, voltage signal
has amplitude above the threshold level to detect the zero magnetization. Further,
the zero magnetization can be detected by the PWM control unit on voltage signal
only. Figure. 7(b) illustrates the current signal sense by the current sensing circuit
10 (I SENSE) 1 in No and light load condition. In No and light load condition, the
current signal has low amplitude which is below the threshold level. The present
current signal cannot be used to detect the zero magnetization. Therefore, the zero
magnetization cannot be detected by the PWM control unit on current signal in
present condition. Figure. 7(c) illustrates a combiner signal generate by the
15 combiner circuit 3 in No and light load condition. The combiner circuit 3
combines both the current signal having amplitude below the threshold level and
the voltage signal having amplitude above the threshold level to maintain
amplitude. Further, the combiner circuit 3 generates a combined signal having
amplitude above the threshold level to detect the zero magnetization. The
20 combiner circuit 3 uses the amplitude of voltage signal to maintain the amplitude
of combined signal above the threshold level as shown in the figure 7(c).
Accordingly, the PWM control unit 4 detects zero magnetization based on the
generated combined signal which has amplitude above the threshold level.
[0040] The PWM co'ntrol circuit unit 4 detects the exact zero magnetization point
25 in all operating conditions like normal, nollight load, over load, short circuit and
start up conditions. Every Boundary conduction mode or quasi resonant mode of
the PWM controller has input to sense zero magnetization. The combined signal
from the combiner circuit 3 is used to sense the zero magnetization. The present
hybrid sensing of zero magnetization crossing in power converters reduces
30 switching losses in wide input voltage range. Output current and reflected voltage
in windings of the transformers TI and T2 is being used to give the feedback to
the control circuit for the sensing of zero magnetization. Accordingly, zero
crossing can be detected even in startup, short circuit and over load condition.
Further, present circuit ensures every new switching cycle starts at zero 1
5 magnetization, ensuring flux is reset before starting the next cycle. If above .e.
r&*
condition is not fulfilled then next cycle will start before resetting of flux which
will .lead to successive accumulation of flux (Flux walking) in magnetic core, I
I
I
ultimately leading to magnetic saturation.
I
[0041] Both the current sensing circuit (I SENSE) 1 and the voltage sensing
10 circuit (V SENSE) 2 are placed in series of the secondary winding and the
auxiliary winding of the transformer T2 and T1 respectively. The signals
generated by the current sensing circuit (I SENSE) 1 and the voltage sensing
circuit (V SENSE) 2 are combined together to sense proper zero magnetization in
transformer. The combination of two signals helps to maintain proper amplitude
15 of the signal to sense zero magnetization in all operating conditions like nollight
load, over load, short circuit and start up conditions.
[0042] Although embodiments for the present subject matter have been described
in language specific to structural features, it is to be understood that the invention
is not necessarily limited to the specific features described. Rather, the specific
20 features' and methods are disclosed as embodiments for the present subject matter.
Numerous modifications and adaptations of the systeddevice of the present
invention will be apparent to those skilled in the art, and thus it is intended by the
appended claims to cover all such modifications and adaptations which fall within
the scope of the present subject matter.

We claim:
1. A power converter (100) to detect Zero magnetization to avoid magnetic evL5&
saturation and fluxlcurrent waling issues, the power converter comprises: % c.
a current sensing circuit (I SENSE) (1) to sense current in
transformer T2 and generate a signal based on the sensed current;
a voltage sensing circuit (V SENSE) (2) to sense reflected voltage
in transformer T1 and generate a. signal based on the reflected
sensed voltage;
a combiner circuit (3) combines the generated signals by the
current sensing circuit (I SENSE) (1) and the voltage sensing
circuit (V SENSE) (2) to generate a combined signal;
a pulse width modulating (PWM) control circuit unit (4) receives
the combined signal to detect zero magnetization in the power
converter.
15 2. The power converter (100) as claimed in claim 1, wherein the current
sensing circuit (I SENSE) (1) is coupled with secondary winding of the
transformer T2.
3. The power converter (100) as claimed in claim 1, wherein the voltage
20 sensing circuit (V SENSE) (2) is coupled with auxiliary winding of the
transformer T 1.
4. The power converter (100) as claimed in claim 1, wherein the power
converter is flyback switching converter.
5. The power converter (100) as claimed in claim 1, wherein the combiner
circuit (3) combines the generated current signal and the generated voltage
signal to maintain amplitude of the combined signal above zero
magnetization detection point.
6. A method to detect Zero magnetization in power converters to avoid
magnetic saturation and fluxlcurrent walking issues, the method comprises
sensing, by current sensing circuit (I SENSE) (I), current in
transformer T2;
generating, by current sensing circuit (I SENSE) (I), current signal
based on the sensed current in the transformer 2;
sensing, by voltage sensing circuit (V SENSE) (2), reflected
voltage in transformer TI;
generating, by voltage sensing circuit (V SENSE) (2), voltage
signal based on the sensed reflected voltage in the transformer T1;
combing, by combiner circuit (3), the generated current signal and
the voltage signal and generating a combined signal;
detecting, by pulse width modulating (PWM) control circuit unit
(4), zero magnetization in the power converter based on the combined
signal.
7. The method as claimed in claim 6, wherein the current sensing circuit (I
SENSE) (1) is coupled with secondary winding of the transformer T2.
8. The method as claimed in claim 6, wherein the voltage sensing circuit (V
SENSE) (2) is coupled with auxiliary winding of the transformer TI.
9. The method as claimed in claim 6, wherein the power converter is flyback
25 switching converter.
10. The method as claimed in claim 6, wherein the combiner circuit (3)
combines the generated current signal and the generated voltage signal to
maintain amplitude of the combined signal above zero magnetization
detection point