Abstract:
An amplifier is driven by DC voltage from a switchmode power supply, whereby the switchmode power supply includes on the primary side a primary winding and bias supply winding. The bias supply winding supplies a reflected voltage from a secondary winding to a bias supply capacitor. The bias supply capacitor drives the control circuit and provides a sensing to the control circuit. The power supply further includes an active clamp circuit for controlling the voltage stress on a main switch. In another embodiment, boost inductors and a balancing transformer are added on the primary side of the transformer to prevent overvoltage conditions at light loads.

Description:
RELATED APPLICATION 
       [0001]    The present invention is related to and takes priority from U.S. Provisional Patent Application Ser. No. 60/906,291, filed on Mar. 12, 2007, the teachings of which are incorporated herein to the extent they do conflict. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is related generally to DC power supplies, and more particularly to audio amplifiers using switchmode power supplies that use current fed topologies and employ active clamp circuitry to reduce switching losses and voltage stress. 
       DEFINITION OF TERMS 
     Power Factor Correction: 
       [0003]    Controlling the input current to the power supply such that it is proportional to the input voltage. Therefore the AC line is presented with a load resistive in character. 
       Single Stage: 
       [0004]    The power supply uses only one converter to isolate the AC line from the output and supply a DC output. 
       Current Fed: 
       [0005]    The converter transformer is fed by an inductive source during the on time of the output diodes. This occurs when the primary switch turns off. Therefore no inductors are needed on the secondary side to limit the primary current. 
       Active Clamp: 
       [0006]    An active clamp is implemented with a clamping switch connected with its source or emitter terminal in common with the drain or collector of the primary or main switch. This is shown in  FIG. 1 , as described below in detail. The other end of the clamping switch is connected to a clamp capacitor, which is in turn tied to an input capacitor. The active clamping technique has two benefits. One is that the energy in the leakage inductance of an associated transformer is transferred to the clamp capacitor through an antiparallel diode inherent or added to the clamping switch, thereby limiting the voltage stress seen by the main switch. Second is that by turning off the active clamp switch shortly before turning on the main switch, the current flowing from the clamp capacitor into the leakage inductance is diverted into the main switch, draining the charge stored across it. This lowers the voltage across the switch, so that turn-on losses are reduced. 
       Regulated Bias Supply: 
       [0007]    A method of supplying a bias supply to the control circuit is employed with the present invention through use of a bias winding on the transformer. Feeding this voltage through a resistive divider to a voltage error amplifier of a control circuit allows the supply to regulate its own bias supply. Regulation of the output voltage is achieved by the cross coupling of the bias supply winding and the output windings of the transformer. Because this coupling is not perfect, the output voltage will not be as highly regulated as if it was regulated directly. However, in many applications including audio amplifiers, this is actually desirable. A short circuit will cause the bias supply to drop below the under voltage cutoff point of a control IC, causing it to turn off. This provides protection against overloads and shorts without having to sense the secondary current. 
       BACKGROUND OF THE INVENTION 
       [0008]    Amplifiers are used to increase the voltage and current levels of a signal in order to drive a load. All amplifiers use a power supply that supplies the energy required to perform this task. The power supply converts the AC line voltage into DC voltages suitable for the amplifier. The power supply also isolates the line voltage from the DC output voltages for safety purposes. This is accomplished by the transformer. A conventional power supply in an amplifier includes a transformer that works directly off the 60 Hz line voltage, and tends to be relatively heavy and bulky. The output of many prior art AC to DC power supplies completely unregulated, with no current limiting. In many prior such power supplies, the input current typically has a power factor of less than 0.7, not meeting most international regulations. 
         [0009]    To resolve these problems, the use of switching power supplies was implemented. Now there was both line and load regulation, and current limiting. Power factor was still less that 0.7, and generally they could not work over the universal input voltage range of 90-265 VAC without some sort of adjustment. The supplies were implemented with voltage fed topologies such as the half bridge topology. 
         [0010]    To remedy the switching supply problems, a boost PFC (power factor correction) stage was added to the input of the power supply. The boost stage consists of a boost inductor coupled to a boost switch. The boost stage can be controlled to yield a high power factor. The resulting supply is fully regulated, overload protected, and power factor can exceed 0.95, with a fully universal input voltage range. Unfortunately, the addition of the PFC stage reduces the efficiency of the supply. Additional circuitry is necessary to limit inrush current. It also became apparent that in many cases with audio amplifiers that the fully regulated output voltage did not sound as good as the unregulated output of the conventional supply. This may be partly due to the use of large energy storage capacitors being located in the transformer primary side of the power supply, with only small filtering capacitors on the output or secondary side. This limits the instantaneous current available to the amplifier. The added stage also increases the parts count and complexity of the design. The present invention resolves these detriments associated with known switchmode supplies while maintaining their advantages. 
       SUMMARY OF THE INVENTION 
       [0011]    The foregoing objects are attained by the use of a power factor corrected, single stage, current fed, active clamped power supply. Implementations of the present invention provides for both the power factor correction function and output isolation function being performed by a DC power supply for an amplifier. This is partly accomplished by providing a means for regulating the voltage generated by a winding on the primary side of a transformer of the present amplifier power supply 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an embodiment of the invention utilizing the SEPIC topology in the power supply for an amplifier. 
           [0013]      FIG. 2  is a preferred embodiment of the invention utilizing an active clamped flyback topology. It shows the bias supply consisting of a bias winding on transformer  34 , bias supply diode  56 , and bias supply capacitor  54  deliver power and information about the output voltage to a PWM controller  30 . 
           [0014]      FIG. 3  is a preferred embodiment of the invention utilizing a double ended isolated boost topology with four outputs suitable for a multirail amplifier  62 . 
           [0015]      FIG. 4  is a block diagram of an implementation of the gate drivers for the main switch  48  (also  48   a  and  48   b ) and active clamp switch  50  (also  50   a  and  50   b ). 
           [0016]      FIG. 5  shows typical waveforms encountered in the operation of a current fed converter with an active clamp snubber for the embodiments of  FIGS. 1 and 2 . 
           [0017]      FIG. 6  shows a waveform chart relative to the embodiments of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Note that when the main switch  48  ( FIGS. 1 and 2 ) is turned off, or switches  48   a  and  48   b  ( FIG. 3 ) are turned off, the boost inductor  40  of  FIG. 1  (also magnetizing inductance of primary winding of transformer  34  of  FIG. 2 , and boost inductors  40   a  and  40   b  of  FIG. 3 ), divert current into the transformer  34 . The sudden rise in current through the transformer&#39;s leakage inductance will cause a large voltage spike across the main switch  48 , or main switches  48   a  and  48   b,  possibly causing failure of these switches. This voltage spike can be controlled by the use of a snubber. The active clamp performs the snubber function by allowing the boost inductor  40  current to flow into the clamp capacitor  52  while the current builds up through the leakage inductance of the transformer  34  ( FIG. 1 ). Similarly, in the embodiment of  FIG. 2 , the boost current flows into capacitor  52  from the primary winding of transformer  34 . Similarly, in the embodiment of  FIG. 3 , the boost current flows into capacitor  52  from the series circuit of boost inductor  40   b,  balancing transformer  66 , and switch  50   b.  The clamp capacitor  52  is sized so that the voltage increase from the boost inductor  40  current is low enough to remain within the limits of the main switch  48 , of the embodiment of  FIG. 1 , and similarly for the embodiments of  FIGS. 2 and 3 . Also the resonant frequency of the clamp capacitor  52  and the leakage inductance of main switch  48  should be less than the switch frequency, or the zero voltage switching feature may be lost. The problem of voltage spikes on the main switch  48  are common to all current fed topologies. 
         [0019]    For applications such as audio amplification, perfect regulation is not necessary. The expense and space consumed by an optocoupler and its associated circuitry can be eliminated. In  FIG. 1  the PWM control circuit  30  is shown sensing the voltage of its own bias supply consisting of bias supply diode  56  and bias supply capacitor  54 , so that the bias supply voltage is regulated by the voltage error amplifier  74  of control circuit  30 . Main switch  48  current is sensed by current sense resistor  44 . This allows the PWM control circuit  30  to both provide power factor correction and protect against overload. The output voltage is indirectly regulated by means of the coupling of the secondary side windings to the primary side bias supply winding of transformer  34 . Imperfections in the coupling of transformer  34  result in a sloped V-I curve for the power supply output, similar to an unregulated conventional supply.  FIG. 1  shows DC blocking capacitor  72 , used in SEPIC and Cuk converters to keep DC currents from flowing through the transformer  34 . 
         [0020]    In another embodiment of the invention as shown in  FIG. 3 , on the primary side of the transformer the double ended topology of a half bridge boost converter is used, rather than a single ended topology as in other embodiments depicted. In this manner, an isolated boost converter is provided. 
         [0021]    Current fed topologies suited for this application are the flyback, SEPIC (Single Ended Primary Inductance Converter), Cuk, and isolated boost. Due to high peak and RMS currents the flyback converter as shown in  FIG. 2  is only practical at low powers of less that 300 W using currently available devices. The SEPIC converter as shown in  FIG. 1  is a single ended topology suitable for power levels up to 600 W. For higher powers the double ended or bridge type isolated boost topologies are useful. An embodiment of this topology is shown in  FIG. 3 . Interleaved versions of the flyback and SEPIC converters will allow increased power without corresponding increases in component current stress. The  FIG. 3  embodiment is inherently interleaved. 
         [0022]    The amplifier may be either linear or switching in operation, and may have any number of supply voltages. This is shown in  FIGS. 1 ,  2  and  3 . As the power supply topologies discussed previously are all of the current fed variety, with the inductive element on the primary side, it is easy to add voltage outputs by just adding windings to the transformer  34 , as shown progressively in  FIGS. 1 ,  2 , and  3 . This avoids the use as in the prior art of coupled inductors with multiple windings, necessary when using a voltage fed design. The current fed design of the present invention provides bulk energy storage capacitors  24  (see FIGS. I and  2 ) that are located at the amplifier voltage supply rails, so the amplifier  60  of  FIG. 1 , and amplifier  26  of  FIG. 2 , have full use of the stored energy. 
       Active Clamp Implementation: 
       [0023]    Power factor control is easily implemented with a variety of integrated circuits available from numerous vendors. However, many Power Factor Control IC&#39;s with desirable characteristics, including single cycle control, have only one gate drive output. The active clamp technique requires complementary gate drive signals for both the main switch and clamp switch. A number of ways of generating this second clamp switch drive are possible. An embodiment shown in  FIG. 4  employs a second IC (Complementary Gate Driver) specifically designed to generate the complementary gate drive signals. An example of such an IC is the Texas Instruments UC3715. The UC3715 does not have a floating gate drive to drive the clamp switch. A third IC such as the International Rectifier IR 2113 can be used to drive the gates of both switches. It also has a higher gate drive current capability that the UC3715. In the future, an IC that combines single cycle control with active clamp gate driving circuitry will make it easier to implement this control function. 
         [0024]    The invention comprises an amplifier such as amplifiers  26 ,  60 , or  62  of  FIGS. 1 ,  2 ,  3 , respectively, with DC power supplied by a power converter utilizing a current fed topology and an active clamp snubber. In addition the supply can use a regulated bias supply to eliminate the use of an optocoupler and its associated circuitry.  FIG. 2  illustrates a preferred embodiment of the present invention. The associated amplifier power supply uses a current fed topology. This allows the secondary winding of transformer  34  of the power supply to deliver current directly into capacitors  24  without an intervening buck inductor. In addition, the associated power supply will incorporate an active clamp snubber including switch active clamp switch  50  and clamp capacitor  52 . This controls the turn off voltage stress on main switch  48 , and can be configured to reduce the turn on loss of main switch  48  as well. The reduction in current and voltage changes over time also reduces electromagnetic emissions and output diode  58  turn off losses as well. 
         [0025]    The power supply embodiments of the present invention as used to power amplifiers, in particular audio amplifiers, are novel and have numerous benefits as previously explained. Any of the topologies shown in  FIGS. 1 ,  2 , and  3  all contain these novel elements. The scope of the claims is not limited to these examples. 
         [0026]    With reference to the waveform chart of  FIG. 5 , for the embodiments of  FIGS. 1 and 2  at t 0 , main switch  48  is on and active clamp switch  50  is off. Current is increasing through boost inductor  40  ( FIG. 1 ), input bridge rectifier  28 , and main switch  48 . Magnetizing current from transformer  34  is going through main switch  48  as well. At t 1  main switch  48  turns off. Current flows into the parasitic capacitance of switch  48  until t 2  when its voltage exceeds that at clamp capacitor  52 . At t 2  active clamp switch  50  diode conducts, and transformer  34  leakage inductance sees the voltage at clamp capacitor  52 . Current then rises through transformer  34  leakage inductance as it resonates with clamp capacitor  52 . Shortly after active clamp switch  50  diode conducts, active clamp switch  50  is turned on at t 3 . This allows current to flow back to the leakage inductance of transformer  34  as it resonates with clamp capacitor  52 . This resonant frequency is chosen to be less than the switching frequency to ensure that the current through transformer  34  is rising through out the off time of main switch  48 . The voltage at clamp capacitor  52  varies to maintain charge balance under all operating conditions. At t 4 , active clamp switch  50  is turned off. The leakage inductance of transformer  34  then pulls current through the parasitic capacitance of clamp switch  50 , the parasitic capacitance of switch  48 , and any parasitic capacitances on boost inductor  40  and transformer  34 . This causes the voltage on the parasitic capacitance of switch  48  to decrease until it goes negative and main switch  48  diode turns on at t 5 . Shortly thereafter main switch  48  is turned on at t 6  and the cycle repeats. 
         [0027]    Turn on dissipation is very low as the parasitic capacitance of main switch  48  is already completely discharged at t 6 . Those skilled in the art will recognize that at light loads the energy stored in the leakage inductance of transformer  34  may be insufficient to discharge the parasitic capacitance of main switch  48  and some turn on dissipation will occur. However, conduction losses are generally much lower under these conditions so total dissipation remains low. The value of the leakage inductance of transformer  34  may be varied by changes in transformer  34  construction and by adding an external inductor in series with transformer  34  primary. 
         [0028]    Note that an active clamp improves the operation of a two inductor isolated boost topology shown in  FIG. 3 . The balancing transformer  66  of the embodiment of  FIG. 3  allows a two inductor isolated boost topology to work at very light loads. This is essential for audio amplifier use, where a very light load is a common operating condition. The operation of the embodiment of the invention of  FIG. 3  will now be described in greater detail with reference to the waveform chart of  FIG. 6 . 
         [0029]    At time t 0  switches  48   a  and  48   b  are on. Current is increasing through boost inductors  40   a  and  40   b.  There is no current through the primary of transformer  34 . Switches  50   a  and  50   b  are closed and no current is flowing in clamp capacitor  52 . At time t 1 , switch  48   a  closes. The current in boost inductor  40   a  is diverted through active clamp switch  50   a  into clamp capacitor  52 . At time t 2 , active clamp switch  50   a  turns on under zero voltage conditions, with very little loss. The current in clamp capacitor  52  then swings negative as it resonates with the leakage inductance  76 . As the resonant frequency of clamp capacitor  52  and leakage inductance  76  is less than the switching frequency, leakage inductance  76  and transformer  34  primary current increase at a nearly constant rate. During the t 2  interval the balancing transformer  66  keeps the current from increasing, even though main switch  48   b  is on. In fact, inductors  40   a  and  40   b  are forced to have nearly identical circuits due the action of balancing transformer  66 . At time t 3  active clamp switch  50   a  turns off. The current through leakage inductance  76  is diverted from clamp capacitor  52  to the body diode of main switch  48   a.  At time t 4 , main switch  48   a  is turned on under zero voltage conditions, greatly lowering its switching losses. As the leakage inductance now sees the reflected voltage of the secondary, its current level rapidly declines. However the di/dt is reduced by the leakage inductance, and so the turn off losses in the output diodes  58  are greatly reduced. Also note the triangular shape of the transformer  34  winding currents. This reduces emissions and transformer losses as compared to the more rectangular waveforms of isolated boost converters without an active clamp circuit. 
         [0030]    Although various embodiments of the invention have been shown and described, they are not meant to be limiting. Those of ordinary skill in the art may recognize certain modifications to the embodiments, which modifications are meant to be covered by the spirit and scope of the appended claims.