Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reissue Patent
1999-12-16
2002-11-05
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000, C363S131000
Reissue Patent
active
RE037898
ABSTRACT:
TECHNICAL FIELD
A flyback-type self-oscillating DC—DC converter uses the primary winding side circuit of a transformer to regulate the output voltage of a power supply.
BACKGROUND OF THE INVENTION
Switching power supplies offer remarkable advantages in terms of volume, weight and electrical efficiency if compared with traditional transformer-type power supplies functioning at the mains frequency. However, due to the complexity of the electronic circuitry employed, these switching power supplies are rather costly. One of the architectures most frequently used is based on the use of a flyback-type, DC—DC converter.
In a flyback system, energy is stored within the primary winding inductance of the transformer during a conduction phase of a power transistor (switch), functionally connected in series with the primary winding and is transferred to the secondary winding of the transformer during a subsequent phase of non-conduction of the switch, which is driven at a relatively high frequency, for example, by a local oscillator having a frequency in the order of tens of kHz.
In switching power supplies, the voltage at the input of the DC—DC converter is not regulated. Commonly, in a power supply connectable to the mains, the input voltage of the converter is a nonregulated voltage as obtained by rectifying the mains voltage by a Wien bridge and leveling it by a filtering capacitor. Therefore this voltage is a nonregulated DC voltage whose value depends on the mains voltage that can vary from 180 VAC to 264 VAC in Europe and from 90 VAC to 130 VAC in America.
A diagram of a flyback-type, self-oscillating primary side circuit of a power supply connectable to the mains is shown in FIG.
1
.
At the turning on instant, the voltage V
INDC
produces a current i in the resistance R
1
that has normally a high ohmic value. This current charges the gate-source capacitance of the power switch T
1
, which, in the example shown, is an isolated-gate, field effect transistor. The gate-source voltage increases in time according to the following approximate equation:
V
GS
=
i
·
t
C
GS
where V
GS
indicates the voltage between the gate and the source of transistor T
1
. C
GS
is the gate-source capacitance, i is the current that flows through R
1
and t is time.
When the voltage V
GS
reaches the threshold value V
THR
, the transistor begins to drive a current I
P
while the drain voltage V
DS
decreases because of the voltage drop provoked by the current I
P
on the inductance L of the primary winding N
P
of the transformer.
Therefore, a voltage equal to V
INDC
−V
DS
is generated at the terminals of the primary winding N
P
. This voltage, reduced according to the turn ratio N
1
/N
P
between the primary winding N
P
and the auxiliary winding N
1
belonging to the self-oscillating circuit, is also applied between the gate node G and the common ground node of the circuit through a capacitor C
2
. This voltage, which is in phase with the voltage present on the primary winding N
P
, provokes a further increase of the voltage between the gate node G and the source node S of the transistor T
1
, which therefore is driven to a state of full conduction. Therefore the voltage on the inductance L of the primary winding N
P
is approximately equal to the rectified input voltage V
INDC
and the current that flows through the primary winding of the transformer has a value given by the following equation:
I
P
=
V
INDC
·
t
L
On the other hand, the current I
P
also flows in the resistance R
2
provoking a voltage drop thereon given by I
P
·R
2
. Even this voltage drop grows linearly in time until it reaches conduction threshold value V
BE
of the second (transistor) switch T
2
.
By entering into a state of conduction, the transistor T
2
shortcircuits toward the ground node and
reduces the voltage at
the gate node G of the transistor T
1
, which therefore turns off. Initially the current I
P
continues to flow thus increasing the voltage V
DS
well above the input voltage V
INDC
. Therefore, a flyback voltage develops on the primary winding inductance L that has an opposite polarity to that of the voltage present during a conduction phase of the switch T
1
. This flyback voltage, reduced in terms of the turn ratio N
1
/N
P
, is also applied between the gate node G of the transistor T
1
and the common ground node of the circuit and further contributes to keep the transistor T
1
in an off condition, having a negative polarity as referred to the ground potential.
During a
high
conduction phase of the transistor
T
1
T
2
, the energy accumulated in the inductance L of the primary winding of the transformer transfers completely into the secondary circuit that is only partially depicted in FIG.
1
. This occurs during such an OFF or FLYBACK phase of operation of the transistor T
1
. When this phase of energy transfer is over, the
voltage
voltages
on the primary winding N
P
and on the winding N
1
of the self-oscillation circuit are null and therefore a new cycle can start again.
The above-mentioned system typifies a common flyback architecture where the primary current I
P
rises linearly from zero up to a peak value given by the following equation:
I
P
=
V
BE
R
2
(
1
)
during a conduction phase of transistor T
1
.
The relevant waveforms of the circuit are shown in FIG.
3
.
Upon a variation of the input voltage V
INDC
, the conduction time T
ON
of transistor T
1
varies according to the following expression:
T
ON
=
L
·
V
BE
V
INDC
·
R
2
=
L
·
I
P
V
INDC
(
2
)
Therefore, the frequency of oscillation is inversely proportional to the rectified mains voltage.
In the majority of applications, the output voltage must be regulated to make it independent from input voltage variation, in other words from the value of the rectified mains voltage.
Commonly in the majority of applications, control of the output voltage is implemented in the secondary circuit. These regulating use feedback control loops that normally sense the secondary voltage level provide this information to the primary circuit via an electrical isolation device, for example, a photo-coupler. These solutions are very efficient but they are also relatively expensive. Even alternative known solutions implementing an output voltage regulation by regulating the current flowing through the primary winding of the transformer during conduction phase of the switch, imply the realization of one or more auxiliary windings and a remarkable complication of the primary circuit.
SUMMARY OF THE INVENTION
It has now been found a surprisingly simple and effective system for regulating the output voltage through the primary circuit of the transformer of a DC—DC converter. The system of this invention does not require any additional winding because it exploits the auxiliary winding N
1
of the self-oscillation circuit for implementing the desired regulation of the voltage output by the secondary circuit of the transformer-type converter.
In practice, the method of this invention consists in realizing a discharge current circulation loop of the energy transferred in the auxiliary winding of the self-oscillation circuit during a phase of conduction of the transistor that switches the primary winding and in summing a signal representative of the level of energy on the control node of a driving stage of the switch to regulate its conduction interval.
Practically, the circulation loop of the discharge current relative to the energy stored in the auxiliary winding of the self-oscillation circuit, reproduces electrically the discharge current circulation loop of the energy that is stored in the secondary winding of the transformer. Through a process of self-redistribution of the energy that is stored in the primary winding inductance during the conduction phase of the switch, the system regulates the energy that is transferred from the primary to the secondary winding of the transformer so as to keep substantially uniform the output voltage that develops on the secondary winding of the transformer. This regulat
Jorgenson Lisa K.
Santarelli Bryan A.
Sterrett Jeffrey
STMicroelectronics S.r.l.
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