Generating bias voltage in a voltage converter

Details Generating bias voltage in a voltage converter Generating bias voltage in a voltage converter

2000-04-04

2001-03-27

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S049000

Reexamination Certificate

active

06208534

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to transformer-coupled power supplies and more particularly to DC:DC power supplies that must generate a bias voltage in addition to generating an output voltage.
BACKGROUND OF THE INVENTION
Voltage converters receive an input voltage (Vin) that is AC in an AC:DC power supply, or DC in a DC:DC power supply, and generate an output DC voltage (Vout) therefrom. The Vout output voltage may be greater than Vin or less than Vin. In many applications, the input-to-output voltage conversion requires the presence of a bias voltage (Vbias) that may be different in magnitude than Vin or Vout. Vbias may be required to establish a reference voltage against which Vout is compared, or may be required to operate a feedback circuit that compares Vout against some other reference potential and changes pulse width, duty cycle, frequency, etc. of a drive signal used to generate Vout. In some circuits Vout may be +48 VDC, as is commonly required in telecommunications circuitry, but generating Vout requires a bias potential of perhaps +12V, regulated to within ±10% or so.
FIG. 1A
depicts a prior art DC:DC power supply
10
that converts an input voltage (Vin) to an output voltage rectified DC voltage (Vout), and generates and uses a lower bias voltage (Vbias) in the input:output conversion. It is understood that system
10
could instead be an AC:DC power supply, in which case Vin would represent an input AC voltage after it has been rectified.
In
FIG. 1A
, bias voltage generator circuit
20
creates Vbias from Vin, the conversion shown generically with a Zener diode Vz, a filter capacitor C, and a resistor R. Collectively, generator circuit
20
depicts a so-called linear bias voltage generator configuration. The breakdown voltage of Vz will determine Vbias, and may commonly be about 12 VDC.
FIG. 1A
, Vbias provides a DC Vbias potential to a control and driver circuit
30
(hereafter, control circuit) that modulates pulse width and/or repetition frequency of a drive signal provided to a switch Q
1
. As shown, switch Q
1
is coupled to the low potential end of a primary winding W
1
on a transformer T
1
, the other end of winding W
1
being coupled to Vin. Primary winding W
1
is commonly fabricated with a center tap (denoted as X) because a split primary winding tends to decrease transformer leakage inductance, although not all circuits make use of the center tap node.
In a fashion well known to those skilled in the relevant art, switch Q
1
opens and closes in response to a drive signal from circuit
30
. When Q
1
is closed Vin is impressed across the input or primary transformer winding W
1
, and essentially Vin is sampled or chopped. The resultant chopped signal is inductively coupled to the output or secondary transformer winding W
2
, where the signal is rectified and filtered to yield a DC voltage, Vout.
FIG. 1A
depicts a typical output configuration comprising a secondary winding W
2
, across which is placed series-coupled R-C snubbers to reduce transient peaks. AC voltage presented to the secondary winding is rectified by diodes D
1
, D
2
and the output low-pass filter, here comprising inductor L
1
and output capacitor C
1
. A load (not shown) is coupled to the Vout node.
Magnitude of Vout can be altered by changing duty cycle of the drive signal provided by circuit
30
to switch Q
1
. (In certain topologies, Vout magnitude can also be altered by changing the repetition rate or frequency of the drive signal to switch Q
1
.) Such drive signal changes are typically responsive to a signal fedback from Vout via a feedback circuit, shown generically as path
50
. As a result, circuit
30
can make compensating changes in the drive signal delivered to the input of switch Q
1
. For example, if the load or other factors cause Vout to decrease, feedback via path
50
can cause circuit
30
to increase duty cycle of the drive signal to switch Q
1
to increase magnitude of Vout.
Although bias generator circuit
20
functions well enough to generate Vbias, such linear regulators can be very inefficient in terms of wasting electrical power and dissipating heat. Further, if Vin should increase in magnitude, the magnitude of Vbias may remain constant, but substantial additional voltage may now be dissipated across resistor R, with resultant greater inefficiency. In some applications, Vin may remain constant, but may so large in magnitude, +300 VDC for example, that excessive dissipation across R (or equivalently functioning components) may result. On the other hand, if Vin decreases too much, the magnitude of Vbias may vary unacceptably. In short, prior art bias circuits that use a linear regulator are simply too inefficient, and do not provide efficient compensation against changes in magnitude of Vin.
FIG. 1B
depicts a second method used in the prior art to generate a bias potential. In this configuration, power transformer T
1
has been modified to add an auxiliary winding (Waux). The turns ratio (Naux:Npri) between Waux and primary winding W
1
determines magnitude of the potential to be rectified by diodes Da
1
, Da
2
, inductor La, and capacitor Ca
1
. In many common applications, the turns ratio is such that a rectified Vbias of about +12 VDC is generated. Note that the converter system shown also includes a startup circuit
40
to ensure proper operation of circuit
30
during and following application of input potential Vin.
While bias generating circuit
20
′ in
FIG. 1B
can be more energy efficient than circuit
20
in
FIG. 1A
, the luxury of adding auxiliary winding Waux may not always be available. For example, T
1
may lack the necessary additional connection pins on its winding bobbin with which to bring out the two leads associated with Waux. Although one might add the Waux winding and simply let the wire leads dangle if no additional pins were available, this approach is impractical in a serious design for a production circuit. But even if additional pins were available, having to include an additional Waux winding adds expense and weight, requires more copper wire, and undesirably adds to the overall form factor of T
1
. Further, some circuits implement T
1
as a planar transformer that is fabricated as part of a printed circuit board containing much of system
10
. Such planar transformers are difficult to modify, especially where the geometry of conductive traces on the printed circuit board is a consideration in the design of transformer T
1
. Thus, although circuit
20
′ in
FIG. 1B
can be used to generate Vbias, in some applications form factors associated with T
1
and/or circuit economy preclude generating Vbias with an auxiliary winding.
To summarize, there is a need for a bias voltage generator for use with AC:DC or DC:DC voltage converters that is more efficient than a linear bias generator. Such a bias voltage generator should not require an auxiliary converter transformer winding (with attendant cost, weight, and bulk) or require additional pin-out connections for the converter transformer. In addition, such bias generator should provide a measure of self-regulation such that as Vin varies, Vbias remains substantially constant. Finally, such bias voltage generator should function without requiring additional drive signals beyond what is already present in the voltage converter.
The present invention provides such a bias generator.
SUMMARY OF THE INVENTION
The present invention is used with a conventional forward power converter in which Vin is coupled to one end of a converter transformer primary winding, and the low potential end of the primary winding is coupled to a converter switch that is driven with a duty cycle Kdc by a control circuit. In a first embodiment, the present invention samples a fraction Kf of the Vin voltage using a bias switch to obtain a sampled version of the desired bias voltage Vbias. The sampled voltage is then filtered to provide the desired Vbias, whose magnitude is proportional to the product (Vin)·(Kf)·(Kdc).
Preferably the fraction

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