Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-02-14
2001-09-25
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056070
Reexamination Certificate
active
06295213
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel switching power converter that clamps the peak turnoff voltage of the power switching transistor in the converter, and recycles the energy stored in the leakage inductance in the converter's transformer back to the input supply by means of a second converter that also can provide auxiliary power to the power converter's control circuit and/or other circuits. As a result, power conversion efficiency is improved, EMI is minimized, and a source of auxiliary power provided. The present invention preferably uses a forward converter as the main converter for converting a first DC voltage to a second DC voltage and a two switch buck converter to transfer power from a clamp capacitor back to the input supply and to generate a source of auxiliary power to power the control circuit of the forward converter.
BACKGROUND OF THE INVENTION
The forward converter is a common circuit topology used to transform electric energy from a source at a given potential to a destination load at a different potential. A typical forward converter includes a transformer having a primary winding and at least one secondary winding. The primary winding of the transformer is coupled to a source of power, usually DC power, via a primary switch or transistor. The secondary winding is coupled to a load via an output rectifier circuit comprising two commutating diodes and an output filter. The primary switch generally comprises a semiconductor switching device such as a FET or bipolar-junction transistor (BJT). When the primary winding is energized by the closing of the primary switch, i.e., the ON-period of the switch, energy is immediately transferred to the secondary winding, hence the name forward converter. In a typical forward converter, energy is stored in the transformer magnetizing and leakage inductances during the ON-period of the switch. During the OFF-period, the voltage across the transformer primary winding reverses, stored energy is dissipated, and magnetic flux in the core is reset. It is necessary to limit the voltage generated during the OFF-period to avoid damage to the switch or transistor.
More particularly, during the conduction or ON period of the switching transistor, current is transferred from the primary DC power source through the transformer to the output circuit. During the OFF period of the switching transistor, the magnetizing current in the transformer is returned to the primary DC source, resetting the flux in the transformer core, prior to the next cycle of operation.
FIGS. 1 and 2
illustrate two versions of a conventional forward converter wherein a third winding of the converter's transformer is used to limit the maximum reset voltage and to reset the core of the transformer when the primary switch is opened. Forward converter
1000
is shown in the circuit of FIG.
1
and comprises a transformer
1010
having a primary winding
1020
and a secondary winding
1022
, a resistor
1026
in parallel with a clamp capacitor
1025
connected in series with a diode
1030
, all of which is connected in parallel across the primary winding
1020
, an auxiliary circuit
1040
connected to a third winding
1045
, and a primary FET switch
1050
. On the secondary side of transformer
1010
are the two commutating diodes
1055
,
1060
which are coupled to an inductor
1070
to provide output power (Vout) to a load
1072
and output capacitor
1074
.
Resistor
1026
acts to dissipate the energy in clamp capacitor
1025
. Clamp capacitor
1025
along with diode
1030
limit the maximum reset voltage across switch
1050
. Auxiliary circuit
1040
provides a source of power Vcc to the converter's control circuit via a diode
1075
and a capacitor
1080
. Power is coupled to the converter from an input power source (Vin) which is connected across the series combination of the primary winding
1020
and the primary switch
1050
. The power dissipated by the clamp in forward converter
1000
is often 5% to 10% of the output power (Vout), which is too high to be wasted as heat in the clamp resistor
1026
.
Another simpler method for limiting the voltage generated during the OFF-period of a power supply system is shown in the forward converter
2000
shown in FIG.
2
. The forward converter
2000
includes a third winding
2022
but it is connected across the input supply Vin just by a diode
2030
. In this example, the third winding usually has the same number of turns as the primary, which means that the peak voltage developed across the switching transistor
2050
during the OFF period is twice the primary DC supply voltage. For a nominal rectified line input of 300V DC, the peak switch voltage would therefore be 600V, requiring a switching transistor voltage rating of at least 700V in such an example. Note that the maximum permissible conduction period or duty cycle of switching transistor
2050
is usually 50% of the total cycle time, to allow time for the transformer flux to be reset during the OFF period and avoid core saturation.
The above mentioned clamp winding methods improve the efficiency of a power converter, but they have disadvantages such as: a) the clamp voltage is proportional to, and increases with, the input voltage; b) the clamp winding can not be perfectly coupled to the primary winding, and so is unable to clamp all of the energy; and c) the high frequency oscillations between the coupling inductance and stray capacitances is a source of EMI.
It is also known in the art to employ other clamping methods to improve the efficiency of a power converter, such as the active clamp, but these methods do not completely resolve the foregoing disadvantages. See, for example, U.S. Pat. No. 4,441,146 wherein the third winding is eliminated and replaced by a series combination of a storage capacitor and an auxiliary switch coupled across either the primary or secondary winding. The auxiliary switch is operated counter to the primary switch, i.e., it is open when the primary switch is closed and closed when the primary switch is open.
Utilizing auxiliary converters to power a control circuit is also known. For instance, power for a control circuit is normally taken from a winding on the power transformer or from the input supply via an auxiliary converter. This auxiliary converter may be a linear regulator, which has been proven to be very inefficient, or a free running switching converter such as a flyback or blocking oscillator. A typical auxiliary converter, such as a free running converter, can generate a wide range of low frequency oscillations, resulting in EMI, due to the varying beat frequency between the main and auxiliary converters. Such a free-running converter is often unacceptable for certain telecommunication applications, such as telephone exchanges. The disadvantage of using these types of auxiliary converters is that it necessitates the costly and time-consuming task of synchronizing the auxiliary converter to the main converter.
Representative devices that utilize various clamping arrangements combined with converters to protect switching devices from high voltage transients and to recover the energy stored in the clamp during the OFF-period by using a switching means are described as follows: U.S. Pat. No. 4,607,322 to Henderson discloses an energy recovery snubber that includes a push-pull converter and clamp capacitor that returns energy to a supply by a second switch and by the windings on a transformer; U.S. Pat. No. 4,286,314 to Molyneux-Berry relates to an inverter circuit for minimizing switching power losses comprising a switch, clamp capacitor, and a second switch connected to an inductor coupled to a clamp capacitor, to resonantly discharge energy for each cycle, while returning that energy to the supply input; U.S. Pat. No. 4,438,486 to Ferraro for a low loss snubber for power converters involves a clamp capacitor, energy retrieved via separate (synchronized) flyback converter, where the FET in the flyback detects a higher voltage than the clamp, and experiences switching loss
Astec International Limited
Coudert Brothers
Patel Rajnikant B.
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