Abstract:
For use with a power converter, a bias voltage generator samples a fraction of the input voltage Vin using a bias switch coupled to a tap on the power converter transformer input winding. The bias switch is driven from the same control circuit that drives the converter switch, and the fraction of Vin sampled by the bias switch is coupled to a low pass filter to generate the bias voltage Vbias. Vbias≈(Vin)·(Kf)·(Kdc), where Kf is the fractional location of the transformer tap, and Kdc is duty cycle of the power converter switch. Vbias enjoys automatic compensation against variation in Vin because the power converter will automatically compensate Kdc to correct for Vin variation. The resultant bias voltage generator requires no additional transformer winding.

Description:
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 Kf of Vin is obtained by providing a tap on the transformer primary winding at a fraction Kf of the total number of turns on the winding, e.g., Kf=0.5 represents a center-tap. A preferably solid state bias switch is coupled in series between the Kf tap and the input to a low pass filter, whose output is Vbias. The bias switch is turned on and off, synchronously with the converter switch, preferably in response to the drive signal generated by the control circuit. 
     Feedback that is present in the voltage converter normally causes the control circuit to pulse width or otherwise modulate the converter switch drive signal (e.g., Kdc) to maintain a constant Vout even if Vin changes. As noted, Vbias≈(Vin)·(Kf)·(Kdc). Thus magnitude of Vbias will benefit from automatic compensation against changes in Vin. Further, this compensation is obtained without requiring having to generate additional control drive signals, without having to add auxiliary windings to the converter transformer, and without excessive heat dissipation. A desired magnitude of Vbias may be generated by designing the underlying voltage converter system such that the product (Vin)·(Kf)·(Kdc) will produce the desired magnitude of Vbias. It is not necessary that Kf=0.5, and in practice typical values for Kf are 0.3≦Kf≦1.0. 
     A second preferred embodiment avoids the difficulty associated with turning-on a so-called high-side switch, in which a bias switch is series-coupled with Vin at the high potential end of the converter transformer primary winding. In this embodiment a preferably solid state bias switch is driven from the low voltage side of the primary transformer winding, in essence using a control signal referenced to ground rather than to Vin. The bias switch is again coupled in series between the Kf tap on the converter transformer primary winding and the input to the low pass filter. However in this embodiment, the drive signal for the bias switch is taken from the junction of the converter switch and the low potential end of the primary winding. 
     Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A depicts voltage converter system with a linear bias voltage generator, according to the prior art; 
     FIG. 1B depicts a voltage converter system with a dedicated auxiliary transformer winding used to generate a bias voltage, according to the prior art; 
     FIG. 2 depicts a voltage converter system with a bias voltage generator, according to a first embodiment of the present invention; and 
     FIG. 3 depicts a voltage converter system with a bias voltage generator, according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 depicts a generic voltage converter system  100  that receives an input voltage Vin and converts that voltage to an output voltage Vout, whose magnitude may be greater than or less than Vin. A converter transformer T 1  essentially separates the input side from the output side of the converter system. Similar to what was described with respect to prior art systems  10 , a converter switch Q 1  is turned-on and turned-off with a duty cycle (denoted Kdc) by a drive signal output from control circuit  30 . As noted, circuit  30  typically modulates pulse width and/or frequency in response to a signal fedback via path  50  from Vout. A start-up circuit  40  is normally provided to ensure safe and reliable start-up operation of system  100 . 
     As was the case in the prior art systems shown in FIGS. 1A and 1B, control circuit  30  or perhaps other circuitry (not shown) typically requires a bias voltage Vbias that must be generated. In the configuration of FIG. 2, a bias voltage generator circuit or system  110  is provided that overcomes the various shortcomings in prior art bias voltage generators in generating the desired Vbias. 
     Bias generator  110  includes a bias switch Sw that is coupled to receive a fraction Kf of Vin. In FIG. 2, Sw is coupled to the resultant potential Kf·Vin via a tap X′ on the primary winding W 1  of the converter transformer T 1 . The high potential end of W 1  is coupled to Vin, and the low potential end of W 1  is coupled to a converter switch Q 1  that is switched-on and switched-off with a duty cycle Kdc responsive to control signals from circuit  30 . If winding W 1  has a number of windings of wire equal to N 1 , Kf will represent the portion of the windings between the Q 1 -end of W 1  and the tap position X′. By way of example, if N 1 =500, and the tap X′ is at 350 turns up from the Q 1 -end of W 1 , then Kf={fraction (350/500)}=0.7. If, for example, W 1  is center-tapped, with half of the N 1  windings above and half of the N 1  windings below then tap, then Kf would be 0.5. Although many off-the-shelf converter transformers T 1  have center-a tapped primary winding (e.g., Kf=0.5), the present invention is not restricted to use with center-tapped primary winding transformers. Further, a converter may be designed in such a manner that Kf=1.0, or such that there is no need to split the primary transformer winding. Indeed, practical values for Kf are in a range of about 0,3≦Kf≦1.0. 
     As driver circuit  30  switches converter switch Q 1  on and off with a duty cycle Kdc, the voltage present at tap X′ will be proportional to (Vin)·(Kf)·(Kdc). Preferably bias switch Sw is a solid state switch having an input lead coupled to tap X′, which thus couples at least a fraction Kf of Vin through the primary transformer winding to the bias switch. Sw has a drive lead coupled to a point in system  100  to be responsive to the drive signal generated by circuit  30 . Sw also has an output lead at which a sampled voltage is present proportional to (Vin)·(Kf)·(Kdc), which voltage represents an unfiltered Vbias signal. 
     The drive signal to bias switch Sw advantageously is generated by the same control circuit  30  that drives converter switch Q 1 , although in some instances an driver A may be used to provide any necessary phase inversion and/or buffering. The drive lead of bias switch Sw need not be coupled directly to the output of control circuit  30 , as it suffices if Sw is driven by a signal present in system  100  that is responsive to the output generated by control circuit  30 . 
     As shown in FIG. 2, the bias switch sampled portion of Vin is coupled to the input of a filter circuit  120 , shown here as inductor Lf, capacitor Cf. A free-wheeling diode Df is also shown in FIG. 2, to provide a path for current in the primary transformer winding W 1  when converter switch Q 1  is open. Collectively Lf and Cf form an LC lowpass filter that rectifies the portion of Vin that is sampled by bias switch Sw. The output from filter  120  is the desired bias voltage Vbias, shown in FIG. 2 as powering control circuit  30 . As noted, magnitude of Vbias is given by: 
     
       
           V bias≈( V in)·( Kf )·( Kdc ) 
       
     
     By way of example, assume that nominal Vin is +48 VDC and that control and driver circuit  30  (or indeed any other circuit associated with system  100 ) requires Vbias≈+12 VDC. The design of system  100  will of course take into account the desired magnitude of Vout, given Vin=+48 VDC. For a given turns ratio (Nprimary:Nsecondary) between primary and secondary transformer T 1  windings, Vout is given by: 
     
       
           V out≈ V in· Kdc ·( N secondary/ N primary) 
       
     
     where Kdc is the duty cycle of converter switch Q 1 , e.g., the percentage of the time Q 1  is on compared to the period of the Q 1  drive signal. 
     Assume that tap X′ on the converter transformer winding is indeed a center tap, in which case Kf=0.5. As such, the potential at X′ will be (0.5)(Vin), where Vin is assumed to be 48 VDC in the present example. Thus, the voltage sampled by bias switch Sw will be 0.5×48 VDC=24 VDC. 
     It is seen that if system  100  can be designed to have a nominal duty cycle Kdc of 50%, that Vbias will be about equal to 50%×24 VDC≈+12 VDC. A corresponding change can be made when specifying the turns ratio for transformer T 1  such that when Vin≈48 VDC, Vout will have the desired magnitude when the duty cycle Kdc is 50%. 
     Assume that for whatever reason, Vin increases in magnitude. The increased magnitude will tend to increase Vout, which undesired in voltage increase is fedback via path  50  to control circuit  30 . Since control circuit  30  will now want to return Vout to its nominal design value, control circuit  30  will decrease duty cycle Kdc accordingly. At the same time that Vin increased, the fraction of Vin sampled by bias switch Sw will of course increase, which would tend to undesirably increase Vbias. However the automatic reduction in duty cycle Kdc imposed by control and driver circuit  30  will tend to maintain Vbias at the desired level, e.g., +12 VDC. 
     In some applications, other circuit design constraints may preclude designing for a nominal duty cycle that is the value of Kdc needed to generate Vbias, e.g., Kdc=50% in the above example. In such case, where nominal duty cycle can not be designed around Vbias considerations, the location of the tap point X′ on the converter transformer winding W 1  would be selected based upon Vbias. In the above example, if system  100  design constraints required say a 33% duty cycle (Kdc=0.33), whereas designing around Vbias would require a 50% duty cycle, the nominal value of Vbias can still be generated simply by locating X′ at a winding location 75% up from the low potential end of the primary winding, e.g., Kf=0.75. As a result, the fraction of Vin sampled by bias switch Sw would be (Kf)(Vin)=(0.75)·(Vin)=(0.75)·(48 VDC)=36 VDC. With a duty cycle Kdc of 33%, the resultant Vbias would be (Kf)·(Vin)·(Kdc), or (0.75)·(48 VDC)·(0.33), or 12 VDC. 
     It is seen from the above description that Vbias is generated without the power dissipation associated with linear bias generators, and without having to provide additional transformer windings, or additional pin-out connections. Further, the voltage bias generator may be driven from the same control circuit that is already present to drive the converter switch. In contrast to prior art configurations, the present invention provides Vbias with automatic compensation for changes in Vin. 
     In FIG. 2, bias switch Sw may be described as a high-side switch in that it is essentially in series with the high voltage portion of primary winding W 1 , as contrasted with the converter switch Q 1 , which is operated as a low-side switch. In some instances it can be difficult to drive high-side bias switches. 
     Turning now to FIG. 3, a so-called low-side bias switch Sw is shown, e.g., a bias switch that essentially is referenced to the low voltage side of the converter transformer primary winding W 1 . A PNP bipolar transistor Q 2  is shown although other switch devices may instead be used, including MOS transistors. 
     Drive to bias switch Sw or Q 2  is obtained from the lower end of primary winding W 1  through a base current-limiting resistor R 1 . When control and driver circuit  30  outputs a drive signal that turns Q 1  on, the lower end of R 1  is essentially grounded, which forward biases Q 2 , turning Q 2  on. When circuit  30  turns Q 1  off, base current through R 1  ceases and Q 2  turns off. In this fashion a fraction of the Vin potential is sampled by bias switch Sw (Q 2 ) and is presented to lower pass filter  120 , whose output is Vbias. The same considerations described with respect to FIG. 2 apply to FIG.  3 . Note that although the drive or base lead of Sw is not connected directly to the output of control circuit  30 , Sw nonetheless turns on and off responsive to the output of circuit  30 , since Sw is driven by a signal present at Q 1  that is itself responsive to the output of circuit  30 . 
     To recapitulate, a desired magnitude of Vbias is generated by sampling a desired fraction of Vin with a bias switch, and then preferably rectifying the sampled voltage to yield Vbias. The magnitude of Vbias generated is proportional to (Vin)·(Kf)·(Kdc), where Kf is the fractional location of the primary winding tap X′, and Kdc is duty cycle with which the bias switch coupled to Vin at the winding tap X′ is driven. 
     If desired one could of course sample the output of Sw before presenting the voltage to low pass filter  120 , for example to decrease Vbias. In such configuration, an additional switch could be disposed in series between Sw and the low pass filter, and operated perhaps at a sub-multiple of the repetition rate of the output of circuit  30 . This configuration could be useful where for design considerations X′ must be center-tapped and system  100  must be operated at a duty cycle too high to generate a desired low voltage for Vbias. Essentially the additional switch would sample the bias switch sampled fraction of Vin to provide a decreased magnitude for Vbias. 
     Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims.