Converter secondary circuit having an auxiliary output

Details Converter secondary circuit having an auxiliary output Converter secondary circuit having an auxiliary output

2003-02-06

2004-08-03

Berhane, Adolf (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

Reexamination Certificate

active

06771523

ABSTRACT:

The present invention relates to a converter secondary circuit having an auxiliary output.
BACKGROUND OF THE INVENTION
A converter usually receives electrical power from a power supply having an input voltage and delivers electrical power to a load at an output voltage. The voltage at the main output is regulated by the primary of the converter to guarantee that a constant voltage is applied to the load even if the input voltage and/or the output current vary. It is often desirable to add one or more auxiliary outputs, which must be regulated independently by a secondary circuit.
The patent document EP 0 954 088 describes a secondary circuit for a converter having a main output and an auxiliary output. That converter comprises:
a first synchronous rectifier which is self-controlled, that generates a first rectified voltage for the main output and receives at its input a voltage supplied by a first transformer, and
a second synchronous rectifier that generates a second rectified voltage for said auxiliary output and that receives at its input a voltage supplied by a second transformer whose primary is the choke for filtering the first rectified voltage.
The second synchronous rectifier is controlled by an auxiliary regulation control circuit having two terminals connected to respective terminals of the secondary of the first transformer to synchronize the second rectifier.
FIG. 1
shows a converter secondary circuit with a main output, described in patent document EP 0 618 666.
An AC signal from the primary is present at the terminals
1
A and
1
B of the transformer
1
. This signal is rectified by a rectifier and filtered by a filter to obtain the required DC voltage at the terminals
2
A and
2
B of the main output
2
.
The self-controlled synchronous rectifier comprises a freewheel MOSFET
3
and a direct MOSFET
4
and the filter is an LC circuit
5
,
6
. Synchronous rectification using MOSFETs considerably reduces losses. The gate of the freewheel transistor
3
is connected to the terminal
1
B of the transformer
1
and the gate of the direct transistor
4
is connected to the terminal
1
A of the transformer
1
. Because of these connections, switching of the transistors
3
and
4
is automatically synchronized with the primary circuit.
The above kind of converter secondary circuit can be provided with an auxiliary output
7
.
FIG. 1
shows a prior art auxiliary output circuit.
The auxiliary output
7
also has a controlled synchronous rectifier consisting of a direct MOSFET
9
and a freewheel MOSFET
10
and a filter consisting of an LC circuit
11
,
12
and is regulated by an auxiliary regulation control circuit
8
.
The auxiliary regulation control circuit
8
has a control input
8
A connected to the terminal
7
A of the auxiliary output
7
, a synchronization input
8
B connected to the terminal
1
A of the transformer
1
, and two outputs
8
C and
8
D connected to the gates of the transistors
9
and
10
, respectively.
The auxiliary regulation control circuit creates a time-delay for controlling the energy transmitted on each switching cycle. This time-delay is a function of the control voltage V
Cmd
.
To avoid the need for a bidirectional switch, it is the unidirectional voltage V
Rect
after rectification in the secondary circuit that feeds the auxiliary output
7
via the MOSFETs
9
and
10
and the LC filter
11
,
12
.
FIG. 3
shows the theoretical waveforms of the signals in the above kind of circuit.
The voltage at the drain of the direct transistor
9
is the voltage V
Rect
rectified by the secondary circuit. An integrator converts the output voltage into a DC voltage V
Cmd
and, by comparison with a voltage ramp V
Rmp
, a binary signal V
Ctrl
is obtained that controls the turning on and off of the transistors
9
and
10
, as shown by the graphs Q
D
and Q
RL
. The resulting voltage V
Dly
is therefore shifted by a particular time-delay before it is filtered to obtain the voltage at the auxiliary output
7
.
The means used to achieve this are known in the art and are explained in more detail in the description given hereinafter.
The voltage ramp V
Rmp
is synchronized to the voltage at the terminal
1
A of the transformer
1
and therefore to voltage V
Rect
.
This represents the ideal operation. In reality, on each switching of the direct transistor
9
of the auxiliary output synchronous rectifier, an unwanted voltage drop occurs in the rectified voltage V
Rect
, as shown in FIG.
4
. This unwanted voltage drop leads to parasitic synchronization of the voltage ramp V
Rmp
and to poor operation of the circuit, the output voltage falling and the converter ceasing to function.
One solution would be to add a filter to cancel this parasitic effect. However, the overall efficiency of the converter would then decrease because of the time-delay introduced by the filter. Moreover, if the auxiliary output current is too high, a filter of that kind is no longer effective.
OBJECTS AND SUMMARY OF THE INVENTION
The invention solves this problem and, to do so, proposes a converter secondary circuit having a main output and an auxiliary output, the secondary circuit comprising a synchronous rectifier which is self-controlled and generates a-rectified voltage for the main output and said auxiliary output including a second synchronous rectifier which receives at its input said rectified voltage and is controlled by an auxiliary regulation control circuit having a synchronization input, which circuit is characterized in that said synchronization input receives a synchronization signal which is at zero when said rectified voltage is not at zero.
If the first synchronous rectifier consists of a main MOSFET and a freewheel MOSFET, the gate of the freewheel transistor is connected to a terminal of a transformer at a synchronization point, said synchronization input of said auxiliary regulation control circuit is connected to the secondary circuit, and said synchronization input is connected to said synchronization point.
Said synchronization input is advantageously connected in the auxiliary regulation control circuit to a rising edge detector, a falling edge detector or a level detector.
Said second synchronous rectifier preferably consists of a direct MOSFET and a freewheel MOSFET.


REFERENCES:
patent: 6128203 (2000-10-01), Canizales Teran
patent: 6297970 (2001-10-01), Hemena et al.
patent: 6445597 (2002-09-01), Boylan et al.
patent: 6459600 (2002-10-01), Farrington et al.
patent: 6487095 (2002-11-01), Malik et al.
patent: 6490183 (2002-12-01), Zhang
patent: 0 954 088 (1999-11-01), None
patent: 0 989 660 (2000-03-01), None

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