Electricity: electrical systems and devices – Safety and protection of systems and devices – Voltage regulator protective circuits
Reexamination Certificate
2000-03-03
2003-03-04
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Voltage regulator protective circuits
C361S058000, C361S090000
Reexamination Certificate
active
06529354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to DC:DC power converters, and more specifically to mechanisms for ensuring a non-destructive start at power-up, under no-load or light load conditions, and under overload conditions.
BACKGROUND OF THE INVENTION
Circuitry to implement DC:DC converters is known in the art. Such circuits receive an input-side DC voltage that is coupled to a input voltage (Vin) via a switching circuit that has a low on impedance and a high off impedance. The result is that Vin is sampled or chopped and transformer-coupled to an output side. On the output side, the transformer-coupled waveform is rectified and filtered to provide a regulated output voltage Vo that may be greater than or less than the input voltage Vin. Feedback from output to input can be used to regulate the sampling duty cycle or frequency to provide an acceptably efficient DC:DC converter in a small form factor.
The present invention may be used with many circuits that electronically sample an input voltage with a switch such that the magnitude of an output voltage can be varied by the parameters of the switch. Such circuits can encompass DC:DC converter topologies including push-pull, and feed forward among others. By way of example,
FIG. 1A
depicts a so-called voltage-fed push-pull DC:DC converter
10
, according to the prior art, as having an input or primary side
20
and an output or secondary side
30
. The input and output sides are essentially demarcated by transformer T
1
, which has input side or primary windings W
1
, W
2
, and output side or secondary windings W
3
-
1
,W
3
-
2
and W
4
-
1
,W
4
-
2
. In some applications, windings W
1
and W
2
are identical, and center tapped windings W
3
-
1
,W
3
-
2
,W
4
-
1
,W
4
-
2
are identical. However, in general the various sets of windings may differ from each other. Because transformer T
1
isolate the input side and the side, transformer-coupled topologies such as shown in
FIG. 1A
are sometimes referred to as isolated DC:DC converters.
The input side
20
of the converter is coupled to a source of DC potential Vin that in some applications may be pre-regulated with a pre-regulator
40
whose output potential is controlled within a known tolerance. In other applications, pre-regulation is omitted and feedback
50
is used to modulate pulse width of drive signals output from a control circuit
60
, to regulate the output voltage(s), shown here as V
01
, V
02
.
In
FIG. 1A
, input voltage, which may be the output potential from pre-regulator
30
, is sampled or chopped using push-pull switching transistors Q
1
, Q
2
and respective transformer T
1
primary windings W
1
, W
2
. Control circuit
50
provides complementary drive signals to the input leads of Q
1
, Q
2
such that when Q
1
is on, Q
2
is off, and vice versa. Although Q
1
and Q
2
are shown as switching an end of primary windings W
1
, W
2
to ground potential, it is understood that ground potential implies a stable potential. Stated differently, if desired a potential other than 0 V DC might instead be switchably coupled to an end of primary windings W
1
and W
2
. This understanding that ground is simply a convenient reference potential shall apply throughout this disclosure.
On the converter output side
30
, center-tapped secondaries W
3
-
1
, W
3
-
2
, and W
4
-
1
, W
4
-
2
of transformer T
1
step-up or step-down the chopped waveforms, which are rectified by diodes D
1
, D
2
and inductor L
1
-capacitor C
1
, and by diodes D
3
, D
4
and inductor L
2
-capacitor C
2
. As described below, in an attempt to reduce voltage stress on the output side rectifier components and to reduce EMI it is customary to insert snubbers, typically a series-coupled resistor-capacitor, across each output winding of T
1
.
Feedback loop
50
can sample the DC output voltages, here shown as Vo
1
, Vo
2
, to control the pulse width (or duty cycle) and/or frequency of the Q
1
, Q
2
drive signals generated by control circuit
60
. The secondary windings may output different magnitudes Vo
1
, Vo
2
and the number of windings may be greater or less than two.
It can be difficult to ensure that system
10
(and DC:DC converter topologies other than voltage fed push-pull) operates in a safe mode initially upon start-up or power-on. For example, until output capacitors C
1
or C
2
become charged, the voltage output Vo
1
, Vo
2
sensed by feedback loop
50
can remain close to zero. Control circuit
60
may falsely interpret this feedback information as commanding more output voltage, e.g., there should be an increase in duty cycle, frequency, and/or amplitude of the drive signals to switches Q
1
and Q
2
.
But until C
1
and C
2
begin to charge-up, it is normal that the reported output voltage immediately upon start-up will be close to zero. Yet unless feedback loop
50
and/or control circuit
60
can distinguish the start-up under-voltage for Vo
1
, Vo
2
from a steady-state decrease in magnitude of Vo
1
, Vo
2
excessive inrush currents may be caused to flow through Q
1
and Q
2
, perhaps with destructive results.
Various techniques seeking to ensure a safe or soft start-up have been attempted in the art. For example, for a time immediately following power-up, control circuit
60
can impose a pulse-width modulation upon the drive signals to Q
1
, Q
2
to limit the maximum initial current that is allowed to flow through these switches. Control circuit
60
can then increase duty cycle from a guaranteed safe minimum initial start-up duty cycle to a normal operating duty cycle. In some applications, fairly complex circuitry may be required to ensure a safe soft start-up for a DC:DC converter.
Another problem associated with the circuitry of
FIG. 1A
occurs under no-load or light load condition, e.g., when neither Vo
1
nor Vo
2
is coupled to a sufficiently low load (not shown). Under such no-load or light-load conditions, an unregulated DC:DC converter can attempt to develop excessive output voltages Vo
1
, Vo
2
. Such over-voltage condition is not desirable and can unduly stress various components comprising system
10
. Further, when a suitable load is ultimately seen at Vo
1
, Vo
2
, the previous high over-voltage condition may contribute to excessive overshoot on the output waveforms, with possible damage to the load(s) and/or system
10
.
FIGS. 1B and 1C
depict two non-isolating DC:DC converter topologies. In
FIG. 1B
, switch Q
1
is switched on and off digitally by an output signal from control circuit
60
. The result produces a chopped or sampled version of Vin at the junction of inductor L and switch Q
1
. This chopped signal is rectified, e.g., by diode D
1
and capacitor C
1
, to produce a DC output signal Vo
1
. The Vo
1
signal may be fed-back to control circuit
60
, which will then alter at least one parameter of the drive signal to Q
1
to try to maintain a desired level of Vo
1
.
Another non-isolated DC:DC converter topology is shown in FIG.
1
C. Again, Q
1
is switched on and off digitally by an output signal from control circuit
60
, and as a result, a sampled fraction of Vin is coupled to inductor L. The resultant sampled or chopped signal is rectified, here by inductor L and capacitor C
1
. The rectified output voltage Vo
1
may be fedback to control circuit
60
, which will then attempt to regulate Vo
1
by controlling a parameter of the drive signal to switch Q
1
.
In the configurations shown in
FIGS. 1A-1C
, switch Q
1
(and if present, Q
2
) may be called upon to conduct excessive current during start-up or power-up to the DC:DC converter circuit. Thus, there is a need for a soft-start mechanism for use with input voltage-sampled circuits, including DC:DC converters of various topologies, both isolating and non-isolating. It should be possible to implement such mechanism reliably without adding undue complexity to the system design. Further, a simple mechanism should be provided to safely limit output voltage developed by such circuits including DC:DC converters under no-load or light-load conditions. Preferably such mechanisms should
Goder Dimitry
Shteynberg Anatoly
Dorsey & Whitney LLP
Jackson Stephen W.
Switch Power, Inc.
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