Dissipative clamping of an electrical circuit with a clamp...

Details Dissipative clamping of an electrical circuit with a clamp... Dissipative clamping of an electrical circuit with a clamp...

2003-11-20

2004-11-02

Sterrett, Jeffrey (Department: 2838)

Electric power conversion systems

Current conversion

Including automatic or integral protection means

C363S056120

Reexamination Certificate

active

06813171

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrical circuits and, more specifically, the present invention relates to electrical circuit clamping.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronic devices. In a known switched mode power supply, a low frequency (e.g. 50 or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated direct current (DC) output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop.
To illustrate,
FIG. 1
is a schematic of a known forward power converter
101
. A switch Q
1
103
turns on and off in response to a control
105
to provide a regulated DC output voltage V
OUT
129
from an unregulated DC input voltage V
IN
127
. In one embodiment, control
105
and switch Q
1
103
are included in a switching regulator, which may be used to regulate the output voltage V
OUT
129
. This topology is well known and its operation is well documented.
Every forward converter must have a way to set the voltage on the primary winding
107
of the transformer
109
during the time when the switch Q
1
103
is off. A popular way to set the voltage is with a clamp network
111
connected across the primary winding
107
. The known clamp network
111
illustrated in
FIG. 1
includes a resistor
113
, a capacitor
115
and a diode
117
and absorbs and dissipates parasitic energy from the transformer
109
that is not delivered to the load
119
nor returned to the input
121
. The balance of energy into the clamp network
111
through diodes
117
and energy dissipated in
113
determines a clamp voltage V
CLAMP
123
that is necessary prevent saturation of the transformer
109
.
FIG. 2
shows with idealized waveforms how the voltage V
SWITCH
125
on switch Q
1
103
is related to the input voltage V
IN
127
and the clamp voltage V
CLAMP
123
. The clamp voltage V
CLAMP
123
must be high enough to prevent saturation of the transformer
109
, but low enough to keep the voltage V
SWITCH
125
below the breakdown voltage of switch Q
1
103
.
FIG. 3
shows the relationship between V
CLAMP
123
and V
IN
127
in a known power supply. As the input voltage V
IN
127
changes, the clamp voltage V
CLAMP
123
must be confined between the two boundaries shown in FIG.
3
. The maximum voltage boundary is a straight line determined by the breakdown voltage of switch Q
1
103
. The minimum voltage boundary is a curved line determined by the voltage necessary to keep the transformer
109
from saturation.
FIG. 3
shows how the clamp voltage V
CLAMP
123
behaves with an RCD network, such as that illustrated in clamp network
111
of FIG.
1
. When the power converter
101
operates in continuous conduction mode, the clamp voltage V
CLAMP
123
stays substantially constant in response to changes in V
IN
127
at given load. The presence of leakage inductance in the transformer
109
causes the clamp voltage V
CLAMP
123
to change with load
119
. It is higher for greater current and lower for less current. The result is a restricted range of permissible input voltage V
IN
127
that is shown in the shaded region of FIG.
3
.
SUMMARY OF THE INVENTION
Dissipative clamping methods and apparatuses are disclosed. In one aspect of the invention, a method includes switching a power supply input on an energy transfer element, regulating a power supply output by switching the power supply input on the energy transfer element, clamping a voltage on the energy transfer element to a clamp voltage and varying the clamp voltage in response to the power supply input. Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below.


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patent: 02-311171 (1990-12-01), None
patent: WO 02/060066 (2002-08-01), None

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