Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-06-06
2001-12-25
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056070
Reexamination Certificate
active
06333861
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to forward converters and, more particularly, to a low loss snubber and transformer core reset circuit used in forward converters.
BACKGROUND OF THE INVENTION
The forward converter is the preferred power supply topology for many applications, including providing power to telecommunications equipment. Forward converters provide a regulated output voltage to a load which is smaller than the input voltage supplied by an associated input power supply. A conventional forward converter
10
is illustrated in FIG.
1
. As shown in
FIG. 1
, a power switch S
1
is coupled in series with the primary winding
14
of a transformer. Coupled in parallel across the primary winding
14
is an RCD network comprising a resistor
16
, a capacitor
17
and a diode
18
. The RCD network is used to reset the transformer core of the converter by recycling the magnetizing energy from the primary winding
14
of the transformer back to the input supply of the converter during the off period of S
1
. It also limits the peak voltage across S
1
. The secondary side of the converter
10
includes a forward rectifier
20
coupled to the secondary winding
15
of the transformer, a free-wheeling rectifier
22
and an output filter consisting of choke inductance
24
and capacitor
26
. The output filter provides a substantially dc output voltage to a load R
L
.
The conductivity of S
1
is controlled by applying a suitable waveform to the gate
12
of S
1
. The waveform applied to the gate
12
of S
1
is typically provided by a feedback control circuit (not shown) which supplies a pulsed control signal using pulse width modulation (PWM), for example, to regulate the output voltage level. When S
1
is turned on, i.e. conducting, the input voltage, V
IN
, is applied across the primary winding
14
of the transformer. A secondary voltage V
S
is developed across a secondary winding
15
of the transformer and is applied across the forward output rectifier
20
. Current and power flows to the choke inductor
24
and output capacitor
26
(which forms an LC output filter) and the load, R
L
. Assuming the output capacitor
26
is sufficiently large and ignoring diode drops and losses, the voltage across the choke inductor
24
will be equal to V
S
−V
OUT
. In this fashion, the current flowing in the choke inductor
24
will increase linearly with time and can be described by di
L
/dt=(V
S−
V
OUT
)/L
O
, where L
O
is the size of the choke inductor
24
.
When S
1
is turned off. i.e., rendered non-conducting, the secondary voltage V
S
will reverse. However, the current in the choke inductor
24
will continue to flow in the forward direction rendering the free-wheeling diode
22
conductive. This permits the current to continue to circulate in the circuit loop bounded by the free-wheeling diode
22
, choke inductor
24
, output capacitor
26
and load R
L
. The current in the choke inductor
24
then decreases with time and may be represented by di
L
/dt=−V
OUT
/L
O
.
A drawback associated with conventional forward converters, as described above, is that the RCD network connected across S
1
can be a significant source of power loss. Power is lost in resistor
16
when switch S
1
is on during the time that capacitor
17
is discharging, and when switch S
1
is off during the time that a magnetizing energy is being returned to the input supply of the converter. The highest power loss is typically due to the discharging of capacitor
17
and this power loss increases with larger values of capacitor
17
capacitance. The value of capacitor
17
is selected to minimize the peak voltage as switch S
1
turns off and to provide a half cycle reset via the transformer primary winding
14
. Generally, the value of capacitor
17
is selected for the transformer reset function since it is greater than the value of capacitance required to minimize the peak voltage due to the effects of the leakage inductance of the transformer.
SUMMARY OF THE INVENTION
The aforementioned and related drawbacks associated with switching losses in conventional forward converters are substantially reduced or eliminated by the present invention. The present invention is directed to a forward converter employing a novel low loss snubber and transformer reset circuit which: (1) resets the transformer core of the forward converter by exchanging energy with the secondary winding inductance of the transformer when the power switch is turned off; and (2) eliminates the power loss associated with forward converters caused by primary side RCD networks. The low loss snubber and transformer reset circuit also reduces the voltage stresses across the power switch and the forward rectifier by making the resonant frequency of the secondary winding inductance and the snubber capacitor of the network less than the switching frequency of the power switch.
In an exemplary embodiment of the present invention, the forward converter comprises a transformer having a primary winding and a secondary winding; a power switch connected in series with the transformer and coupled to an input power source, the power switch capable of being alternately switched between an on period and an off period such that an ac voltage is generated across the secondary winding of the transformer in response thereto; an output filter operative to provide a substantially constant dc voltage to an output load; a forward rectifier operative to provide a forward conduction path between the secondary winding and the output filter; a freewheeling rectifier operative to provide a secondary side current path in connection with the output filter; and a snubber network circuit coupled between a secondary winding and the freewheeling rectifier and operative to reset the transformer core during the off period of the power converter.
An advantage of the present invention is that it provides a resonant reset circuit which provides a half cycle reset of the transformer of the forward converter.
Another advantage of the present invention is that it improves the operating efficiency of forward converters by minimizing the power loss associated with transformer reset and snubber circuits used in forward converters.
Still yet another advantage of the present invention is that it is easy to manufacture and implement.
A feature of the present invention is that it is inexpensive to manufacture.
REFERENCES:
patent: 5278748 (1994-01-01), Kitajima
patent: 5828559 (1998-10-01), Chen
patent: 6008630 (1999-12-01), Prasad
Astec International Limited
Berhane Adolf Deneke
Coudert Brothers
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