Abstract:
Presented is a DC/DC converting circuit adapted to convert a DC input voltage to a DC output voltage. The converting circuit uses, as its synchronous rectifier member, a PMOS bipolar power transistor of the PMOS type, and allows it to be turned on by a control logic circuit capable of quickly sensing automatically the difference in electric potential between a conduction terminal and the body terminal of the transistor.

Description:
TECHNICAL FIELD 
     This invention relates to a synchronous CMOS rectifying circuit for step-up devices and to a method of carrying out a DC/DC conversion by means of the rectifying circuit. 
     More particularly, the invention deals with a family of high-efficiency synchronous circuits for switching control that can be operated in the step-up mode even at very low (around 0.8 V) supply voltages and very small idle currents. 
     BACKGROUND OF THE INVENTION 
     Considerable research work is being conducted in the field of synchronous rectifying circuits, specifically for step-up devices, which can be operated at very low (e.g., 0.8 V) supply voltages and very small idle currents. 
     The demand for rectifiers with such features comes especially from those applications where battery-powered apparatuses, such as pagers, cellular phones, portable computers, and more generally long-range portable apparatuses, require that the conversion of a battery voltage to a stabilized voltage of a higher value be carried out in a most efficient manner. 
     A general trend in the design of such rectifiers provides for the use of active switch elements having very low internal resistance and a high switching rate, and the use, as active elements, of field-effect transistors or bipolar transistors in place of the loop-back diodes employed in the past. The class of converters thus produced are known as synchronous rectifying converters, also on account of a control logic being provided therein which is effective to perfectly synchronize the opening and closing of the switch elements and prevent simultaneous activation (cross-conduction) from occurring, as this would result in large power dissipation or unacceptably poor performance. 
     A synchronous rectifying circuit ideally should be capable of operating with very low supply voltages and of minimizing cross-conduction losses as the switch elements are turned on simultaneously. 
     Additional features expected of such an ideal rectifier include the following: it should produce full disconnection of the output load from the electrical source in a shut-down situation and with an output voltage Vout=0; it should dampen out the oscillation that appears as the inductor is fully discharged due to the presence of parasitic capacitances in the switch elements (damped ringing); it should enhance the converting efficiency of the circuit; and it should automatically clamp the internal supply voltage of the circuit to the highest available potential (self-bootstrapping). 
     Shown by way of example in FIG. 1 is a schematic diagram of a prior art synchronous rectifying circuit  20  as applied to a step-up topology. The circuit has an input terminal IN which is supplied a voltage Vin, and has an output terminal OUT which is connected to an electric load. The circuit  20  uses a P-channel Power-MOS  11  for a switch element. A control logic  13  controls the sequencing and times the actuations of a switch  14  and the switch  11  through respective outputs  17  and  16 . By using an active device for the switch element  11 , a smaller voltage drop can be obtained in transferring power to the load. 
     A sensing element  18  adapted to sense the current being transferred to the load is provided for a compare block  12 , which is arranged to control the transfer of power and send a suitable signal to the control logic  13  through an input  15 . 
     The signal from the compare block  12  effectively prevents the Power-MOS  11  from also transferring power from the load to the input IN, thus reversing the current direction and defeating all attempts at improved efficiency. 
     The determination to use a P-channel Power-MOS for a switch element is justified by the low internal resistance of Power-MOSs and the ability to use a voltage-oriented rather than current-oriented drive. Unfortunately, the presence of the Power-PMOS body connection introduces, as shown in FIG. 1, a large-size diode across the input and output terminals of the circuit  20 . At start-up, when the output voltage is still close to zero, this large diode allows an inrush of current whose maximum value can be far above the peak value during steady-state operation and which can detrimental effects on the passive components (e.g., inductor  18 ), unless the latter are provided oversize to accommodate this initial transient phase. 
     An attempt to depress the maximum value of the inrush current by the use of a limiting resistor admittedly would safeguard the components, but also result in unacceptably large power losses. 
     Lastly, the presence of the parasitic diode makes adjusting the output voltage to values below Vin−V BE  (step-down configuration) impracticable. 
     Another approach to providing a highly efficient synchronous rectifier is shown in FIG.  2  and described in European Patent Application No. 97830740.3 by this Applicant, and herein specifically incorporated by reference in toto. 
     Shown in that Figure is a synchronous rectifier  30  which includes a bipolar transistor  7  as a switch element. A rectifier block  2  includes a control logic  5  which is connected to all the terminals  3 ,  4  of the transistor  7  and receives the supply voltage Vin on an input  6 . 
     A second switch element  1  is provided which is formed with MOS technology and incorporates a control logic  10 . 
     While in many ways advantageous, this solution is inadequate to remove all the ringing and minimize the losses from cross-conduction. 
     A further prior approach is shown schematically in FIG.  3 . This is a synchronous rectifying circuit  19  manufactured by Maxim Semiconductors. This circuit cannot be operated at lower voltages than 1.7 V. 
     To summarize, all of the prior art solutions, although providing highly efficient step-up converters, still have limitations and deficiencies, including complex control logic circuitry, inability to operate with low supply voltages, possible cross-conduction side effects, and inability to operate in a step-up/step-down combined mode. 
     Until now, no high-efficiency rectifying circuit, particularly intended for step-up/step-down applications, was available that provided a uniquely simple construction combined with outstanding performance in terms of ringing suppression and full disconnection of the load in the shut-down mode. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention use, as the gist of the synchronous rectifier, a MOS type of power transistor which can suppress the ringing effect and ensure full shutting down of the load. 
     Presented, therefore is a synchronous CMOS rectifying circuit that has an input terminal and an output terminal for converting a DC input voltage into a DC output voltage. The rectifying circuit includes an inductor selectively coupled to the input terminal, and a first electronic switch to selectively control the coupling of the inductor. The rectifying circuit also includes a MOS power transistor for coupling the inductor to the output terminal, and a biasing circuit for biasing a body terminal of the MOS power transistor with one of the voltages that it receives at a first bias input or a second bias input. In one embodiment the first bias input is coupled to the input terminal and the second bias input is coupled to the output terminal. The MOS power transistor has a first conduction terminal connected to a terminal of the inductor, a second conduction terminal coupled to the output terminal of the rectifier, and is driven at a control terminal by a control logic circuit effective to sense a difference in electric potential between at least one conduction terminal of the MOS power transistor and the body terminal of the MOS power transistor. 
     Also presented is a method of converting a DC input signal presented on an input terminal into a DC output signal presented on an output terminal of a converting/rectifying circuit. The method uses a first electronic switch to selectively couple the inductor to the input terminal for transferring energy from the input terminal to the output terminal, and includes a MOS power transistor. The MOS power transistor, that is coupled between the input and the output terminals, is driven by a driving circuit according to a difference in electric potential between at least one conduction terminal and a body terminal of the MOS power transistor, while the body terminal is driven with a voltage that always keeps parasitic diodes within the MOS transistor reverse biased. 
     The features and advantages of the circuits and methods embodying this invention will become apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a DC/DC converter circuit in a step-up configuration, according to the prior art. 
     FIG. 2 is a schematic diagram of a bipolar technology, synchronous rectifying circuit intended for a step-up type of converter, according to the prior art. 
     FIG. 3 is a schematic diagram of yet another prior rectifying circuit. 
     FIG. 4 is a schematic diagram of a CMOS rectifying circuit according to an embodiment of the invention. 
     FIG. 5 is a circuit diagram of one embodiment of a portion of the rectifying circuit in FIG.  4 . 
     FIG. 6 is a wiring diagram of a detail of the portion shown in FIG.  5 . 
     FIGS. 7A to  7 D are plots, on the same time base, of waveforms of voltage signals which appear in the rectifying circuit of a tested embodiment of this invention in the step-up mode of operation. 
     FIGS. 8A to  8 E are plots, on the same time base, of waveforms of voltage signals which appear in the rectifying circuit of a tested embodiment of this invention in a continual mode of operation. 
     FIGS. 9A to  9 E are plots, on the same time base, of waveforms of voltage signals which appear in the rectifying circuit of a tested embodiment of this invention in a discontinual mode of operation. 
     FIGS. 10A to  10 C are plots, on the same time base, of waveforms of voltage signals which appear in the rectifying circuit of a tested embodiment of this invention in a continual mode of operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawing views, specifically to the example shown in FIG. 4, a circuit diagram of a synchronous CMOS rectifier according to an embodiment of this invention is shown generally at  40  in schematic form. 
     The rectifying circuit  40  has an input terminal IN which is applied an input voltage Vin, and has an output terminal OUT where a rectified and stabilized DC output voltage Vout is made available. 
     The circuit  40  includes an inductor L adapted to transfer power from the input terminal IN to the output terminal OUT. A capacitor Cout is provided that is connected between the output terminal OUT and a ground reference GND to store energy as transferred from the inductor L. A first electronic switch  26  is coupled between the inductor L and the ground reference GND, and is adapted for coupling the inductor to the DC input voltage Vin. The circuit  40  also includes a second electronic switch  21  adapted for coupling the inductor L to the output terminal OUT. 
     The first electronic switch  26  is an N-channel MOS power transistor, and is controlled by a control circuit  24  that handles its timing through a logic gate  28  receiving a signal SHDN on one input via an inverter  27 . 
     The second electronic switch  21  has a first conduction terminal coupled to the inductor L at a node  32 , and has a second terminal coupled to the output terminal OUT of the converter  40 . Advantageously, the second switch  21  is a power transistor, preferably of the PMOS type, which has a main conduction path linking the inductor L to the output terminal OUT. The PMOS switch  21  includes parasitic diodes  29 ,  31 . 
     The control circuit  24  also drives the transistor  21  and senses the difference in electric potential between at least one of its conduction terminals, represented by the node  32 , and its body terminal, represented by a bias node X. The control circuit  24  causes the power transistor  21  to conduct by activating a control terminal through a logic gate  25 , which also receives the signal SHDN, when the electric potential Vlx at the node  32  is lower than the potential Vx at the body terminal X. 
     The control circuit  24  has an additional input terminal coupled to the input terminal IN of the converter to sense the electric potential of the input voltage Vin. This additional input terminal allows the control circuit  24  to compare the input voltage Vin with the output voltage Vout and, accordingly, operate correctly in either the step-up or the step-down mode. 
     The circuit  40  can control a synchronous CMOS rectifier architecture, starting from a supply voltage which may be as low as 0.8 V, and thus effect a highly efficient step-up conversion, suppress high-frequency ringing at the inductor, and ensure EMI noise reduction and power off. 
     The circuit  40  only employs N-channel and P-channel CMOS devices of the enhancement type using CMOS/BiCMOS technology. This circuit is specially suitable for incorporation to a step-up DC-to-DC converter. 
     The control circuit  24  functions to synchronize the turning on/off of the power switches  26 ,  21  and to provide a shutdown condition for the circuit  40  with full disconnection of the output load. 
     Further provided within the logic circuit  24  is a ringing suppression circuit portion. In addition, the biasing circuit  22  functions to clamp the internal supply voltage of the whole circuit to the highest electric potential available. 
     Specifically, the circuit  22  receives the input voltage Vin and the output voltage Vout, and is operative to clamp, at the node X and with minimal resistance, the internal voltage Vbias to the highest voltage available in the circuit  40 . This arrangement allows the parasitic body-drain  29  and body-source  31  diodes of the P-channel transistor  21  to be reverse biased at all times, i.e., under any conditions of operation, thereby removing the limitation that all prior synchronous CMOS architectures imposed typically on a minimum Vout. 
     In the instance of a step-up topology with full disconnection of the load in the shut-down condition, upon startup of the circuit  40 , the highest voltage (Vmax) available in the circuit will be: 
     
       
         Vmax =Vin; 
       
     
     but, since Vout=0 at steady state, it is: 
     
       
         Vmax=Vout&lt;Vin. 
       
     
     A possible example of how the logic circuit  24  can be implemented is illustrated by the detailed block diagram of FIG.  5 . 
     A zero crossing comparator  44  is connected in the circuit  24  to turn off the P-channel power transistor  21  when, in a discontinual mode of operation, the inductor energy is completely depleted during the switching cycle. In this case, the voltage across the inductor would be 0 V, and the comparation carried out between the node  32  at a voltage Vlx and a voltage Vbias=Vx generated internally by the biasing circuit  22  and appearing at the node X. This voltage Vbias is given as: 
     
       
         Vbias=max(Vin,Vout). 
       
     
     A wiring diagram of a possible implementation of the zero crossing comparator  44 , having compatible inputs with Vbias, is shown in FIG.  6 . 
     The P-channel transistor  21  is always turned off when the inductor current is zero (e.g., in step-up mode). 
     The circuit  24  also includes a digital circuit portion  56  adapted to generate short pulses on the trailing edge of the gate voltage of the N-channel MOS power switch transistor  26 . This portion  56  comprises a series of two-input logic gates  52 ,  45 ,  46  and a series of inverters  47 ,  48 ,  49 ,  50  for feeding the output signal from the last logic gate in the series back to one of the other logic gate inputs. 
     The short pulses generated by the circuit portion  56  are used for turning on, in a synchronous manner and with minimized cross conduction, the P-channel power transistor  21 , regardless of the value attained by the output of the zero crossing comparator  44  described previously. During the power-off transient of the N-channel transistor  26 , the electric potential Vlx at the node  32  is for a short time higher than the output voltage Vout, so that the zero crossing comparator  44  would issue a wrong instruction not to turn on the P-channel transistor  21 , thereby causing substantial delay in the turning on of this transistor and penalizing the conversion efficiency of the circuit. 
     The logic circuit  24  further includes a drive circuit block  35  which includes a latch adapted to generate synchronized waveforms for driving the control (gate) terminals of the MOS transistors  21  and  26 . 
     The latch  35  receives a signal PWM from an external generator  23  and processes this signal through a series of logic gates. The control signal for the transistor  26  is picked up downstream of a first logic gate  36 . This signal is then transmitted to the other transistor  21  through a pair of logic gates  37  and  38  of the NOR and AND type, respectively. Furthermore, the output signal from the logic gate  38  is applied to an input of the logic gate  36 . The construction of the latch  35  is completed by an inverter  39  and a set of NOR logic gates  41 ,  42  and  43 . The output from the gate  38  is applied to an input of the gate  41  via the inverter  3 , the gate  42  being input signals issuing from the gate  41 , the gate  43  and the logic gate  45  of the pulse generator  56 . The gate  43  receives the output from the comparator  44 , on one of its inputs, and on the other input, receives a signal issuing from the logic gate  52  of the pulse generator  56  which is also applied to one input of the other NOR gate  41 . 
     The construction of the logic circuit  24  further includes a ringing suppression circuit portion  55  which removes high-frequency oscillations that occur, in the discontinual mode of operation, within the node  32  at the electric potential Vlx in consequence of the power-off transient of the second order circuit formed of the inductor and the parasitic capacitances of the power transistors  26  and  21 . The overall effect of the circuit is one of degrading the Q (pole quality factor) of the oscillating circuit, thus heavily damping the free oscillations. 
     The circuit portion  55  includes a logic gate  54  and an output stage  53  consisting of a pair of P-channel MOS transistors that operate to suppress ringing by short-circuiting the inductor. These transistors are provided in a back-to-back configuration in order to prevent their body-drain junctions from being turned on during the power-on time period (Ton) or the power-off time period (Toff) of the waveform PWM that establishes at the node  32 . 
     The graphs shown in FIGS. 7A to  7 D show waveforms of voltage signals which appear in the rectifying circuit  40  in a step-up mode of operation. The graphs shown in FIGS. 8A to  8 D show waveforms of voltage signals which appear in the rectifying circuit  40  in a continual mode of operation. The graphs shown in FIGS. 9A to  9 D show waveforms of voltage signals which appear in the rectifying circuit  40  in a discontinual mode of operation. The graphs shown in FIGS. 10A to  10 C show waveforms of voltage signals which appear in the rectifying circuit  40  in a continual mode of operation. 
     Thus, the proposed circuit distinguishes itself from prior synchronous rectifier architectures, overcomes their limitations and affords the following advantages: a step-up DC-to-DC conversion of very high efficiency (typically 90-95%); deactivation of the P-channel  21  parasitic diode; operability starting from very low supply voltages (0.8 V); suppression of the high-frequency ringing that typifies switching regulators in the discontinual mode of operation; and very low standing current consumption (typically 8 μA). 
     Changes can be made to the invention in light of the above detailed description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all methods and devices that are in accordance with the claims. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined by the following claims.