Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-10-31
2003-07-08
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021160
Reexamination Certificate
active
06590789
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of power converters of switched-mode power supply type. The present invention more specifically relates to isolated power supplies, that is, power supplies having no common point between the input voltage (for example, the A.C. supply system) and the regulated D.C. output voltage. The isolation is obtained by means of a transformer having a primary winding associated with a pulse-width modulation controlled switch, and having a secondary winding associated with a diode and with a capacitor providing the output voltage.
2. Discussion of the Related Art
FIG. 1
shows a conventional example of a switched-mode power supply of the type to which the present invention applies. Two input terminals P and N receive an A.C. voltage Vac, for example the mains voltage. Voltage Vac is rectified, for example in a fullwave manner, by means of a diode bridge
1
. The A.C. input terminals of bridge
1
are connected to terminals P and N and its rectified output terminals
2
and
3
provide a voltage Vr. Voltage Vr is generally smoothed by means of a capacitor C
1
connected between terminals
2
and
3
which form the input terminals of the actual switched-mode power supply.
The converter of
FIG. 1
is a so-called flyback converter in which a transformer
4
with inverted phase points has its primary winding
5
connected in series with a switch
6
between terminals
2
and
3
. The phase point of winding
5
is connected to a terminal of switch
6
, the other terminal of which is connected to terminal
3
. Switch
6
is connected in switched mode and at a non-audible high frequency (generally greater than 20 kHz). A secondary winding
7
of transformer
4
is associated with a capacitor C
2
across the terminals Sp and Sn of which is provided D.C. output voltage Vout. The phase point of winding
7
is connected to terminal Sp by a diode D
1
, the cathode of diode D
1
being connected to terminal Sp. The other terminal of winding
7
is connected to terminal Sn.
When switch
6
is on, the phase point of winding
7
is at a negative potential. Diode D
1
thus is off and a current is stored in primary winding
5
. Upon turning off of switch
6
, the phase points of windings
5
and
7
both become positive. Diode D
1
is forward biased. Capacitor C
2
is then charged with the power transferred to secondary winding
7
.
Switch
6
(for example, a MOS transistor) is, in the example of
FIG. 1
, integrated in a circuit
10
with its electronic control circuit. An example of such an integrated circuit, sold by STMicroelectronics Company, is known under trade name VIPER. Circuit VIPER includes an input terminal Vdd intended for receiving a positive power supply, a voltage reference terminal Vss connected to ground, and a terminal FB receiving an error signal. Finally, a terminal
12
is connected to the drain of the integrated N-channel transistor, the source of which is connected to terminal Vss. The gate of transistor
6
is connected at the output of a control circuit
11
(CTRL). Circuit
11
includes a comparator (not shown), a first input of which receives an internal voltage reference and a second input of which is connected, internally, to the positive supply terminal. A VIPER circuit is controlled by a current. The control, that is, the modification of the width of control pulses of switch
6
, is performed by, for example, using compensation loop integrated circuit
10
, which itself attempts to maintain its supply voltage (Vdd-Vss).
Thus, in an application to a switched-mode converter, terminal Vdd is connected, by a diode D
2
, to the phase point of an auxiliary winding
8
of transformer
4
. The anode of diode D
2
is connected to the phase point of the winding. The other terminal of auxiliary winding
8
is connected to reference terminal
3
of the rectified voltage. Auxiliary winding
8
has the function of providing the supply voltage of circuit
10
. Terminal FB is connected to the midpoint
13
of a series connection of a zener diode DZ and of a capacitor C
3
. A capacitor C
4
for filtering the local supply voltage is connected between terminal Vdd and terminal
3
, the latter being connected to terminal Vss of circuit
10
.
In the assembly of
FIG. 1
, the output voltage is set by the value of the zener diode and the transformation ratio between primary and secondary windings
5
and
7
. Auxiliary winding
8
, which gives an image of the output voltage, is used, the auxiliary winding being directly in phase with secondary winding
7
. The voltage in this winding
8
is thus proportional to the voltage in secondary winding
7
.
A disadvantage of the converter of
FIG. 1
is that the regulation of output voltage Vout is not very accurate. This disadvantage is illustrated by
FIG. 2
, which shows the characteristic of output voltage Vout according to the current lout taken by the load connected across terminals Sp and Sn of the converter. It can be considered that, for a nominal voltage Vnom for which the converter is sized, a regulation to more or less 10% of this nominal voltage is obtained for currents ranging between two respectively minimal and nominal values Imin and Inom. Currents Imin and Inom correspond, in practice, to respectively 10% and 100% of the maximum current for which the converter is sized.
When the current surge of the load supplied by the converter is smaller than value Imin, voltage Vout significantly increases as the current decreases. This phenomenon is, among others, due to the fact that noise (voltage peaks) present at the beginning of each demagnetization cycle of auxiliary winding
8
is no longer negligible as compared to the demagnetization period, which is very short. These peaks then strongly influence the value of the voltage across auxiliary winding
8
. Capacitor C
4
then charges to the maximum value of these peaks.
Between values Imin and Inom, voltage Vout slightly decreases (between +10 and −10% of nominal value Vnom) as the demagnetization period increases. The noise peaks at the beginning of each demagnetization period become more and more negligible.
When the current drawn by the load becomes greater than value Inom, the decrease slope of voltage Vout strongly increases. This is due to the fact that the duty cycle used by the converter is maximum. The output voltage level then cannot be maintained.
More and more often, the low current range (under Imin) is used for power saving reasons (for example, during stand-by periods of the circuits powered by the converter).
To obtain an accurate regulation of output voltage level Vout even for a low current, it is conventionally necessary to provide a regulation of the voltage at the transformer secondary.
FIG. 3
shows an example of a converter implementing such a conventional solution. It shows a transformer
4
having primary and secondary windings
5
and
7
with inverted phase points and having an auxiliary winding
8
providing a supply voltage to a VIPER-type circuit
10
. Rectifying bridge
1
and capacitor C
1
have not been shown in
FIG. 3
but are of course present. As compared to the assembly of
FIG. 1
, zener diode DZ is replaced with a phototransistor T of an optocoupler
14
, the diode D of which conveys a measurement signal coming from the secondary of transformer
4
. The anode of diode D is connected, by a resistor R, to D.C. output terminal Sp. The cathode of diode D is connected, by a resistor R
1
in series with a zener diode DZ
1
, to terminal Sn, the anode of diode DZ
1
being connected to terminal Sn. When the output voltage reaches the threshold voltage of diode DZ
1
in series with the D.C. voltage across diode D of the optocoupler, a current flows through these elements, as well as through the optotransistor. This current flow causes a decrease in the power sent to the secondary by reducing the peak current in switch
6
. The gain between the current on terminal FB and this peak current is indeed negative. The more the current is increased on terminal FB, the
Berhane Adolf Deneke
Morris James H.
STMicroelectronics S.A.
Wolf Greenfield & Sacks P.C.
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