Abstract:
A synchronous regulator circuit including a transformer having a secondary inductor magnetically coupled to a primary inductor, where the secondary inductor is coupled to control a synchronous power switch. The secondary inductor operates to drive the synchronous power switch using self-regeneration during a flux reversal phase of each cycle. A timing circuit or simple pulse width modulation circuit (PWM) turns on the primary switch and turns off the synchronous switch during a power phase of each cycle, and then turns off the primary switch during the flux reversal phase of each cycle. The flux reversal of the secondary inductor drives the synchronous switch on, thereby achieving synchronous operation without an expensive dual output PWM. The present invention is illustrated using both a buck and a boost topology. A third switch is preferably used to clamp the synchronous switch off during the power phase.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a power supply for driving an electronic load, and more particularly to synchronous regulator circuits using a self-regeneration drive method. 
     DESCRIPTION OF THE RELATED ART 
     DC to DC regulators are generally used to convert an unregulated DC voltage to a regulated DC voltage. Such regulators are widely used in switch mode DC power supplies for generating the appropriate DC voltage signals on the system board and option cards of a computer system. The two must common types of DC to DC regulator topologies include buck and boost topologies. In both buck and boost topologies, an oscillator type circuit such as a pulse width modulator turns on and off a primary power switch driving current from an unregulated source voltage through a choke inductor or transformer to develop an output voltage. In the buck regulator circuit, when the primary switch is turned on, the source voltage is coupled to the load through the inductor and when the primary switch is turned off, the load current flows through a free-wheeling rectifier or diode. The buck regulator is commonly used to step-down the voltage of the unregulated source. In a boost regulator circuit, the primary switch is turned on to apply power to an inductor or transformer for storing energy and is turned off to release the stored energy to the output circuit and load. 
     Synchronous DC to DC regulation is often used to improve the efficiency and performance of DC to DC converters by reducing losses in the power switches or diodes. In general, two switches must be synchronized so that one switch is turned on while the second is off, and vice versa. In particular, a primary switch is turned on while the second switch is turned off during a first half of each cycle to provide power, and then the primary switch is turned off and the synchronous switch is turned on during the second half of the cycle to free-wheel load current or to otherwise release stored energy to the output. Generally, the standard method of achieving an active switch instead of a passive diode in low voltage, high current DC to DC regulators is to use a power metal oxide-semiconductor field-effect transistor (MOSFET). Typically, dual synchronous MOSFETs are driven by a dedicated pulse width modulated (PWM) integrated circuit (IC) having dual synchronized outputs. However, typical PWM ICs having a dual drive capability for driving synchronous switches are relatively expensive. 
     It is desired to achieve the benefits of synchronous DC to DC regulation and to eliminate the need of the relatively expensive PWMs having dual drive outputs. 
     SUMMARY OF THE PRESENT INVENTION 
     A synchronous regulator using a self-regeneration method according to the present invention eliminates the dual drive requirement thereby allowing the use of relatively inexpensive, single output timing circuits. In this manner, a relatively inexpensive off-the-shelf component, such as the NE555 timer, is used rather than a costly dual output PWM IC. In general, the primary switching device is coupled through a primary inductor or coil of a transformer and the synchronous switch is controlled by a secondary inductor magnetically coupled to the primary inductor. A timing circuit or simple PWM turns on the primary switch and turns off the synchronous switch during the first half of each cycle to transfer power to the output in a buck regulator or otherwise to store energy within the transformer in a boost regulator. The timing circuit switches to initiate the second half of the power cycle by turning off the primary switch causing flux reversal of the primary and secondary inductors, where such flux reversal of the secondary inductor turns on the synchronous switch. In this manner, the natural circuit action of flux reversal through the transformer eliminates the need for a second, synchronous output of the PWM and eliminates the problem of cross conduction between the primary and synchronous switching elements. The switching elements are preferably MOSFETs to achieve low power dissipation as compared to a passive device such as a diode. 
     In the preferred embodiment, a feedback path is provided from the output to the timing circuit or PWM to provide stable voltage regulation. The feedback path preferably includes a voltage divider providing a feedback signal to the feedback or modulation input of the timing circuit or PWM. If a timing circuit is used, an error amplifier or shunt regulator is preferably included to complete the feedback loop. 
     In a buck regulator circuit according to the present invention, a timer is used to drive the gate of a primary switch MOSFET. The current path of the primary switch is coupled between the input DC voltage and the primary inductor of the transformer. The timer also drives the gate of a switch element, which is further coupled to the synchronous switch to assure that the synchronous switch is turned off during the power conduction cycle. The current path of the synchronous switch is connected to the primary inductor and its control terminal is coupled to the secondary inductor of the transformer. In this manner, when the output of the timer goes low, the primary switch is turned off and the synchronous switch is turned on through flux reversal of the secondary winding, and the synchronous switch provides a path for free-wheel current through the load. In the preferred embodiment, a third MOSFET is included to clamp the synchronous switch off during the power phase of each cycle. 
     In a boost regulator circuit according to the present invention, a relatively inexpensive PWM with a single output is used to drive two switching elements for further activating the primary power and synchronous switches. In general, the PWM turns on a primary switch for allowing current to flow through the primary inductor of a switching transformer for storing energy. The PWM then switches off the primary switch causing flux reversal of a secondary inductor, which turns on a synchronous switch to provide a current path to the output for transferring stored energy. A third MOSFET is preferably included to clamp the synchronous switch off. Similar to that of the buck converter, separate small signal or double-diffused MOSFETS are also preferably included to control switching action. 
     In either of the disclosed preferred embodiments, synchronous DC to DC regulation is achieved without the use of a relatively expensive dual drive output PWM IC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which: 
     FIG. 1 is a schematic diagram of a buck regulator circuit according to the present invention; and 
     FIG. 2 is a schematic diagram of a boost regulator circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a schematic diagram is shown of a buck regulator circuit 100 implemented according to the present invention. In general, a buck regulator performs step down DC to DC voltage conversion to provide a regulated operating voltage for system boards or planars and option cards. The source voltage is preferably the 5 volt output signal from an AC to DC converter, which is referred to as the +5 V signal. A filter capacitor 102 is coupled between the +5 V signal and ground, which is typically chassis ground, and the +5 V signal is also provided to the drain of an n-channel enhancement type metal-oxide-semiconductor field-effect transistor (MOSFET) Q1. The source of MOSFET Q1 is connected to the drain of another n-channel enhancement type MOSFET Q2, having its source connected to ground. The MOSFETs Q1, Q2 are both preferably the MTP50N05EL, although any suitable power MOSFET would suffice. The source of the MOSFET Q1 is also connected to the dotted terminal of the primary inductor of a transformer T, where the undotted terminal of the primary inductor provides the regulated output voltage, referred to as V OUT . A secondary inductor of the transformer T has its dotted terminal connected to the undotted terminal of the primary inductor. V OUT  is preferably regulated at approximately 3.75 volts and is filtered by a filter capacitor 104 coupled between V OUT  and ground. 
     The undotted terminal of the secondary inductor of the transformer T is connected to the anode of a diode D1, having its cathode connected to one end of a resistor 106. The other end of the resistor 106 is connected to the gate of the MOSFET Q2 and also to the drain of an n-channel double-diffused MOSFET Q3, which is preferably the 2N7000, although any suitable small signal MOSFET or comparable switching device would suffice. The source of the MOSFET Q3 is connected to ground and the gates of the MOSFETs Q1, Q3 receive a PWM signal from a timer circuit U1, which is preferably implemented using an NE555 type timer. The NE555 is a bipolar device, which is preferred for increased drive capability in the range of 100-200 milliamps (ma) as compared to a CMOS device. The timer U1 is coupled to a resistive-capacitive (RC) timing circuit 108 which receives a 12 volt source voltage referred to as +12 V, where the timing circuit 108 preferably establishes the nominal operating frequency of the buck regulator 100, which is typically about 100 KHZ. The timer U1 also has an input referred to as FM which receives a feedback signal V FB  for modifying the duty cycle of the PWM signal to regulate the voltage level of V OUT  in a manner more fully described below. 
     V OUT  is provided to one end of the feedback resistor 110 having its other end connected to one end of a resistor 112, which has its other end connected to ground. The junction between the resistors 110 and 112 is connected to one end of a network compensation capacitor 114 and to the reference input of a shunt regulator U2, having its anode connected to ground. The other end of the capacitor 114 is connected to one end of a resistor 116, having its other end connected to the cathode of the shunt regulator U2 and to one end of a resistor 118. The other end of the resistor 118 receives the +12 V signal. The cathode of the shunt regulator U2 is also provided to one end of a resistor 120, which has its other end providing the V FB  signal and connected to the FM input of the timer U1. 
     Operation of the buck regulator circuit 100 is as follows. The timer U1 asserts the PWM signal as a square wave having a duty cycle substantially controlled by the feedback signal V FB  provided to its FM input. When the PWM signal is asserted high, the MOSFETs Q1, Q3 are turned on so that the +5 V input signal charges the tank circuit formed by the primary inductor of the transformer T and the capacitor 104. The MOSFET Q3 clamps the gate of MOSFET Q2 to ground thereby shutting it off When the PWM signal is asserted low by the timer U1, the MOSFETs Q1, Q3 are turned off causing a flux reversal of the primary and secondary inductors of the transformer T, where the secondary inductor forward biases the diode D1 and drives the gate of the MOSFET Q2 high via the diode D1 and the resistor 106. The MOSFET Q2 thus turns on and conducts, effectively providing a free-wheeling circuit through the diode D1, the current path MOSFET Q2 and V out . When the PWM signal is next asserted high, the MOSFETs Q1 and Q3 again conduct thereby shutting the MOSFET Q2 off and turning on the MOSFET Q1 as described previously. 
     It is seen that the flux reversal of the transformer T causing voltage reversal of the secondary inductor is used to drive or otherwise turn on the MOSFET Q2 through natural circuit action, which also eliminates or otherwise substantially reduces cross-conduction between the MOSFETs Q1 and Q2. Therefore, a separate PWM output signal is not required to drive the MOSFET Q2. The present invention allows the use of relatively inexpensive components, such as the NE555 timer, double-diffused MOSFETS and supporting components. 
     The resistors 110, 112 form a voltage divider of V OUT  which is provided to the reference input of the shunt regulator U2. The reference input of the shunt regulator U2 is compared to an internal reference voltage, which is preferably about 2.5 volts. In this manner, the shunt regulator U2 is preferably operated in the linear mode to maintain its external reference input at approximately 2.5 volts, thereby maintaining V OUT  at approximately 3.75 volts. The timer U1 and the shunt regulator U2 and supporting circuitry combined with the implementation of a secondary inductor are less expensive than a sophisticated PWM IC normally used for synchronous DC to DC converters. The diode D1 is optional, but is preferred in the event of possible ringing of the transformer T1 during flux reversal, which could cause the MOSFET Q2 to oscillate on and off. The diode D1 effectively isolates the MOSFET Q2, so that it remains on during flux reversal in spite of any ringing of the transformer T1. 
     Referring now to FIG. 2, a schematic diagram is shown of a boost regulator circuit 200 according to the present invention. A +5 V signal is provided to one end of resistor 202, having its other end connected to the drain of a double-diffused n-channel MOSFET 204. The MOSFET 204 has its source connected to ground and its gate receiving a signal PWM. The drain of the MOSFET 204 is connected to the gate of another double-diffused n-channel MOSFET Q3, having its source connected to ground and its drain connected to one end of a resistor 206 and to the gate of an n-channel enhancement MOSFET Q2. The other end of the resistor 206 is connected to the dotted terminal of a secondary inductor of a transformer T2, where the undotted terminal of the secondary inductor is connected to the source of an n-channel enhancement type MOSFET Q1. The MOSFETs Q1, Q2 are preferably the 30N03L, although any suitably power MOSFET or comparable power switching device is contemplated. The source of MOSFET Q1 is connected to ground and its gate is connected to the drain of a double-diffused n-channel MOSFET 208, having its source connected to ground and its drain connected to one end of a pull-up resistor 210. The MOSFETS 204, 208 and Q3 are preferably the 2N7002, although any other suitable switching device could be used. The other end of the resistor 210 is connected to the +5 V signal, and the gate of the MOSFET 208 receives the PWM signal. The +5 V signal is coupled through a capacitor 212 to ground and to the undotted terminal of a primary inductor of the transformer T2, where the dotted terminal of the primary inductor is connected to the source of the MOSFET Q2. The primary and secondary inductors of the transformer T2 are magnetically or otherwise mutually coupled. The drain of the MOSFET Q2 provides an output voltage V OUT , which is filtered by an electrolyte capacitor 214 to ground. V OUT  for the boost regulator 200 is preferably regulated at approximately 11.25 volts. 
     The PWM signal is preferably a square-wave type signal asserted by a PWM circuit 216, which is preferably the TL5001 manufactured by Texas Instruments. The VCC input of the PWM 216 receives the +5 V signal and a resistor 218 is coupled between the +5 V signal and the output of the PWM 216. The resistor 218 serves as a feed forward resistor to initiate operation. V OUT  is provided through a resistor/capacitor (RC) feedback circuit comprising resistors 220, 222, 224 and 226 and two capacitors 228 and 230 coupled to the feedback (FB) and compensation (COMP) inputs of the PWM 216 as shown. The resistors 220, 224 form a voltage divider for V OUT  to provide feedback signal V FB  to the FB input of the PWM 216. 
     Operation of the synchronous boost regulator 200 is as follows. The PWM signal is asserted low by the PWM 216 thereby turning off the MOSFETs 204, 208, so that the +5 V signal is provided through the resistors 202, 210 to turn on the MOSFETs Q3, Q1, respectively. When the MOSFET Q3 is turned on, it pulls the gate of the MOSFET Q2 to ground thereby turning it off. The MOSFET Q1 provides a current path to ground from the +5 V signal through the primary inductor of the transformer T2, which stores energy during the power cycle. 
     When the PWM signal is asserted high by the PWM 216, the MOSFETs 204, 208 are turned on thereby turning off the MOSFETs Q3 and Q1, respectively. When the MOSFET Q1 is turned off, the primary and secondary inductors of the transformer T2 reverse polarity causing flux reversal, where the flux reversal of the secondary inductor rams on the MOSFET Q2 through resistor 206. The MOSFET Q2 provides a current path from the +5 V signal through the primary inductor of the transformer T2 to V OUT . The voltage across the primary inductor adds to the +5 V signal to boost the output voltage to approximately 11.25 volts. When the PWM signal is next pulled low, the MOSFETs 204, 208 are mined off thereby taming back on the MOSFETs Q3 and Q1, respectively, as described previously. The resistors 220 and 224 divide the voltage of V OUT  to maintain the V FB  signal at approximately 1 volt. For example, the resistor 220 is preferably 30K ohms and the resistor 224 is preferably 3K ohm thereby properly dividing V OUT . The PWM 216 modifies the duty cycle of the PWM signal to regulate V OUT  at 11.25 volts as desired. It is noted that if the transformer T1 tends to ring during flux reversal, a diode may optionally be inserted in series with the resistor 206 to prevent any oscillation of the MOSFET Q2. The cathode of the optional diode would be connected to the gate of the MOSFET Q2. 
     It is clear that the synchronous boost regulator circuit 200 shown in FIG. 2 obtains all the benefits of synchronous operation through the operation of the MOSFETs Q1 and Q2, and through the use of the secondary or sense inductor of the transformer T2. This allows the use of a relatively simple PWM circuit, such as the TL5001 as shown. Otherwise, the MOSFET Q2 would require a separate synchronized output of the PWM circuit, which in turn would require a more sophisticated and expensive dual output PWM circuit. The MOSFET Q3 assures that the MOSFET Q2 is mined off while the MOSFET Q1 is mined on during energy storage in the transformer T2. 
     Although the method and apparatus of the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the invention as defined by the appended claims.