Abstract:
A control voltage for a synchronous rectifying transistor is generated with the desired anticipation time. The anticipation time is continuously controlled with a closed-loop technique by comparing it with the duration of a reference pulse. The resulting error signal is processed and provides the necessary correction to the MOSFET gate signal to equalize the actual anticipation time to the duration of the reference pulse.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to rectifiers, and more particularly, to a method and relative circuit for controlling a synchronous rectifying transistor, which is an essential component of a switched mode power supply.  
       BACKGROUND OF THE INVENTION  
       [0002]     Rectifiers are used in AC-DC, DC-DC voltage converters, as well as in Switching Mode Power Supplies (SMPS). Typically, they are coupled to a secondary winding of a transformer for providing rectified half-waves to a load to be supplied, as shown in  FIGS. 1-3 .  
         [0003]     Often, properly controlled MOSFETs are used as rectifiers instead of diodes because their power loss is smaller than that of diodes. A MOSFET is often referred to as a synchronous rectifier (SR). A synchronous rectifier is turned on and off by a dedicated control circuit for reproducing a diode voltage-current characteristic. The control signal of a synchronous rectifier is generally derived from a PWM control signal that determines the functioning states of the converter.  
         [0004]     The way the control signal is derived from the PWM control signal depends upon the topology in use and on the presence of voltage isolation in the topology. In a non-isolated switched mode power supply topology, the synchronous rectifier control circuit can obtain the information about the switching transitions (turn-off and turn-on) of the main switch from the main control circuit in a very simple way. In isolated topologies with a primary side control, the absence of a PWM control signal on the secondary side of the isolation barrier makes the generation of control signals for the synchronous rectifiers more difficult.  
         [0005]     If a synchronous rectifier does not function as a diode, the well-known phenomena of cross conduction and shoot-through may occur. In all of these circumstances, one of the switches is forced to a conduction phase in the first quadrant of its voltage-current characteristic. Therefore, switching losses may become predominant, wasting most of the benefits introduced by the reduction of conduction losses by the rectifiers, or may even hinder a correct functioning of the circuit.  
         [0006]      FIGS. 4 and 5  illustrate the turn-off waveforms of the control voltage (gate-source voltage) Vgs and the source current I of the synchronous rectifiers FW and FR after a step variation of the voltage Vs at the secondary winding of a transformer of a sample AC-DC or DC-DC converter, due to the turning on or off of a MOSFET in series to the primary winding of a transformer of a converter.  
         [0007]      FIG. 6  illustrates time characteristics of the gate-source voltage Vgs, the source current I and the drain-source voltage of the synchronous rectifiers FW and FR when the rectifier FW is turned-off a short time before the voltage Vs in the secondary winding is switched high. The time interval from the trailing edge of the control voltage Vgs and the leading edge of the voltage Vs is herein referred as the “anticipation time”. It is desirable that this time not be a null so that the rectifier FW is already off when the rectifier FR is turned on, otherwise the secondary winding could be short-circuited.  
         [0008]     Clearly, the control voltage Vgs of the rectifier FW (and evidently also of the rectifier FR) is generated as a function of the PWM signal that controls the switch in the primary side of the converter, or vice-versa.  
         [0009]     The required timing of the control signals of a synchronous rectifier is shown in  FIG. 7  for a converter formed according to a general switched-mode topology with one switch and only one diode. The conduction phases of the switch and the diode do not overlap each other. The shown dead time intervals prevent contemporary cross-conduction of the switch of the converter and the synchronous rectifier operating as a diode, but they need to be shortened as much as possible to minimize or reduce synchronous rectifier conduction times of the parasitic diode, and the consequent loss of efficiency. In fact, MOSFETs used as rectifying transistors may show poor performance because their parasitic diode (body-diode) is turned on at least during transitions between different states.  
         [0010]     The functioning of the body-diode depends on the timing of the control signal, and in particular, on the turn-off instant of the synchronous rectifier with respect to the time interval in which the diode replaced by a MOSFET is supposed to be in a conduction state A to early turn-off of the synchronous rectifier is susceptible to cause an increase of conduction losses due to the body-diode, in which the whole current would flow. The switching losses caused by the reverse recovery current through the body-diode therefore depend on the current flowing through it at the instant in which the cathode-anode voltage V KA  reverses.  
         [0011]     In isolated topologies, if the control circuit that generates the PWM signal is at the secondary side, the task of controlling synchronous rectifiers may be easily addressed. In fact, the PWM signal available on the secondary side may be used to generate the driving signal for the synchronous rectifiers by adequately delaying edges of the PWM signal for compensating the propagation delays of the control signal transferred to the primary side of the converter through a dedicated coupling device.  
         [0012]     The required timing for this kind of operation is shown in  FIG. 8 , in the more general case of two complementary signals on the secondary side. The control signal of a MOSFET in the primary side of the converter is generated with a slight delay ΔTp 1 , ΔTp 2  from the PWM signal, while the edges of the control signals of the rectifiers FW and FR of the converter of  FIG. 5  are delayed by larger delays ΔT 1  and ΔT 2 , respectively. Even in this case, dead times between drive signals are necessary to prevent an eventual cross conduction between two synchronous rectifiers and between an synchronous rectifier and a switch of the converter.  
         [0013]     However, secondary side control configurations show several drawbacks. One drawback is the need of an auxiliary power supply for the start-up of the converter for a crossing-isolation circuit being able to transfer the PWM signal of synchronous rectifying transistors to switches on the primary side of a converter. Another drawback is in transferring the information regarding the current on the primary switch to the PWM controller in current mode control loops. Therefore, the use of a PWM control signal on the primary side of a converter is mandatory to form switched mode power supplies (SMPS) with high performance in terms of high efficiency, small dimensions and low cost.  
         [0014]     In isolated topologies, if the control circuit that generates the PWM signal is on the primary side of a converter, then the PWM signal cannot be available on the secondary side in a simple, effective and low cost manner. This information may be derived at the secondary side from the output of the isolation transformer of the converter.  
         [0015]     In this case, however, the synchronizing signal at the output of an isolation transformer is the effect of primary main switch commutations. This signal, in fact, is equalized to the PWM signal in a continuous conduction mode (CCM), but it is negatively affected by the parasitic elements of the circuit that distort its waveform.  
         [0016]     The delay in the propagation chain of the signal derived by the PWM signal makes it not suitable to control the synchronous rectifier on the secondary side. This is because the delay control makes the switch and the synchronous rectifier of the converter be in a conduction state at the same time.  
         [0017]     Methods and relative control circuits capable of generating a control voltage for a synchronous rectifier with the desired anticipation time in order to turn-off the synchronous rectifier before a switch of the converter is turned on have been developed in different ways.  
         [0018]     A first technique is described in U.S. Pat. No. 6,418,039. The pulses of a high frequency oscillator (HFO) (more than 100 times the main converter switching frequency) are counted during each switching period of a PWM signal, and the anticipation time is determined by decreasing/increasing the counting obtained during the previous switching period.  
         [0019]     The main advantage of this technique is that the system is very fast to face eventual transient conditions, because the counting is updated at each switching period,  
         [0020]     The main drawback is that the digital counting causes an output jitter, the width of which is twice the period of the HFO. Unfortunately, the minimum anticipation time is determined by the frequency of the HFO, thus the HFO frequency needs to be increased for obtaining low anticipation times. Therefore, this technique is likely to increase power dissipation and requires counters with a relatively large end-scale count.  
         [0021]     An analog method to generate the anticipations of the turn-off transition in the driving timing has been disclosed in the U.S. Pat. No. 5,736,890, implemented in a circuit by SRMOS Inc. According to this method, two different analog ramps and one threshold variable with TON or TOFF are used for generating the anticipation time. Three passive components (capacitors) are needed to generate the ramps and the variable threshold. Two of these capacitors need to be accurate to have a precise anticipation time.  
         [0022]     Stability in time and in temperature is not good because of the presence of passive components. To obtain very short anticipation time values, the capacitors need to be determined very accurately. In case of capacitor value variations due to unpredictable events (temperature changes, capacitor life, etc.) this anticipation time can be lost causing cross conduction problems.  
         [0023]     Another control technique that uses a Phase Lock Loop (PLL) based system for establishing the desired time anticipations of the turn off transitions has been recently implemented by International Rectifier in the integrated circuit IR1175.  
         [0024]     This method is quite complex in terms of design relations needed to obtain the desired control. It requires many external components and has a relatively high pin count to set the parameters that are necessary for correctly implementing the control technique. Moreover, a drawback of this method is a slow response time to switching frequency variations. Should a noise cause a variation of the switching frequency, the PLL would react too slowly to this variation.  
         [0025]     As a consequence, the anticipation times would be inaccurately determined and this would reduce the overall efficiency of the converter. A faster response time may be achieved at the cost of worsening the accuracy with which the anticipation time is determined, which would negatively affect the efficiency of the converter.  
       SUMMARY OF THE INVENTION  
       [0026]     In view of the foregoing background, an object of the present invention is to provide a method and a relative control circuit for generating a control voltage of a synchronous rectifying transistor.  
         [0027]     In accordance with the present invention, a control voltage for a rectifying transistor with the desired anticipation time in generated in a new and effective way. The anticipation time is, in fact, continuously controlled with a closed-loop technique by comparing it with the duration of a reference pulse. The resulting error signal is processed and the MOSFET gate signal is adjusted to equalize the actual anticipation time to the duration of the reference pulse.  
         [0028]     More precisely, the invention provides a method for generating a PWM control voltage for a synchronous rectifying transistor comprising the steps of generating a feedback logic signal that assumes an active logic value when a PWM control voltage of the transistor and the drain-source or collector-emitter voltage of the transistor assume a null logic level.  
         [0029]     A logic pulse signal of a pre-established duration is generated on active edges of the control voltage The PWM control voltage is generated, and the duration of a logically active level of this voltage is determined by the difference between the time durations of the logic signals for equalizing the durations of the logic signals.  
         [0030]     This method may be implemented by a closed-loop control circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     The invention will be described referring to the attached drawings, wherein:  
         [0032]      FIGS. 1, 2  and  3  show sample AC-DC and DC-DC converters in accordance with the prior art;  
         [0033]      FIGS. 4 and 5  illustrate transient turn-off responses of synchronous rectifiers of a converter in accordance with the prior art;  
         [0034]     FIG,  6  depicts transient time-responses of voltages and currents flowing in the synchronous rectifiers of  FIGS. 4 and 5 ;  
         [0035]      FIG. 7  compares time responses of a diode and of a MOSFET used as a synchronous rectifier in accordance with the prior art;  
         [0036]      FIG. 8  shows sample waveforms for the synchronous rectified forward converter of  FIG. 1 ;  
         [0037]      FIG. 9  depicts a first embodiment of a control circuit in accordance with the present invention;  
         [0038]      FIG. 10  depicts sample timing diagrams of waveforms of the main signals of the circuit of  FIG. 9 ;  
         [0039]      FIG. 11  depicts a detailed scheme of a control circuit in accordance with the present invention;  
         [0040]      FIG. 12  depicts sample timing diagrams of waveforms of the main signals for the device shown in  FIG. 11 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     A general block diagram of a control circuit for generating a control voltage of a transistor, to be used as a synchronous rectifier according to the method of the invention is shown in  FIG. 9 , while  FIG. 10  shows sample waveforms of its signals when the logic signals Tfb and Tw are equalized. The N-channel MOSFET M 1  is the synchronous rectifier that is to be controlled. When the converter that includes a synchronous rectifier is working in a continuous mode, the Vin signal is a square wave signal.  
         [0042]     At each trailing edge of the Vg signal, a pulse Tw is generated by the monostable TW P ULSE  G ENERATOR . The block FB S IGNAL  G ENERATOR  generates the feedback logic signal Tfb as a logic NOR between the signals Vin and Vg. The two signals Tw and Tfb are input to a controlled voltage generator DC ERROR that generates a voltage Verr, the level of which is representative of the width difference between the logic pulses Tfb and Tw.  
         [0043]     A logic circuit COMP compares the voltage Verr and a saw tooth waveform Vst synchronized with the voltage Vin that is started, stopped and reset corresponding to edges of the voltage Vint generates an auxiliary PWM control signal Vout that may assume an active logic level only when the saw-tooth voltage Vst is not constant The signal Vout is input to a non-inverting buffer E that controls the MOSFET gate.  
         [0044]     The circuit adjusts the PWM control voltage Vg of the MOSFET for equalizing the pulse Tfb with the pulse Tw. Due to its closed-loop architecture, the circuit controls the MOSEET with the desired anticipation time compensates eventual fluctuations of the duration of the pulse Tfb due to variations of the temperature of the MOSFET or other phenomena whose effects cannot be precisely predicted in advance.  
         [0045]     A detailed circuit scheme of an embodiment of the control circuit that implements the described method is shown in  FIG. 11 , and sample waveforms of its signals are depicted in  FIG. 12  in a sample case in which the logic signals Tfb and Tw are not equalized.  
         [0046]     The drain-source voltage Vin of the synchronous rectifier (the depicted MOSFET) is inverted and the signal  2  is generated. When the signal  2  is at a high logic level, the constant current source charges the capacitor C 1 . When the signal  2  switches low, the voltage on the capacitor C 1  is reset. Therefore, the signal Vst is a saw tooth waveform started corresponding to the trailing edges of Vin, and stopped and reset corresponding to the leading edges thereof.  
         [0047]     To ensure that the voltage Vout is null when the voltage Vin switches high, the depicted comparator compares the voltage Verr on the capacitor C 2  with the saw-tooth voltage Vst, and generates a signal  4 . This is while the AND gate in cascade thereto is input with the signals  4  and  2 , and generates the voltage Vout.  
         [0048]     The auxiliary PWM control signal Vout is input to the buffer E, which generates the PWM control signal Vg of the synchronous rectifying MOSFET.  
         [0049]     The PWM control voltage Vg is logically inverted ( 7 ) and, at each leading edge of the signal  7 , the pulse Tw is generated by the circuit MONOSTABLE. The duration of the pulse Tw represents the desired anticipation time.  
         [0050]     The feedback logic signal Tfb is the logic AND between the signals  7  and  2 . If the width of the signal Tfb is larger than the width of the pulse Tw, this means that the duration of the active phase of the PWM control voltage Vg needs to be increased to equalize Tfb to Tw.  
         [0051]     How the error voltage Verr is generated is evident from  FIG. 11 , which is self-explanatory for any skilled person. When the signal  14  is high, it charges the capacitor C 2  through resistor R A  and diode D A . Charging C 2  will result in an increased voltage Verr. As a consequence, the duration of the active phase of the PWM control voltage Vg is increased The increase of the error voltage Verr depends on the resistance PA and the capacitance C 2  that both determine the gain of the control device.  
         [0052]     Similarly, if the width of the feedback signal is smaller then the width of the pulse Tw, a signal  12  that discharges the capacitor C 2  through the diode D B  and the resistor R B  is generated. The error voltage Verr thus decreases and so does the active phase of the PWM control voltage Vg.  
         [0053]     As it is well know from the of theory feedback controlled systems, the steady state value of the difference between the durations of Tfb and Tw is inversely proportional to the total DC gain of the closed-loop. By choosing appropriately the resistors R A  and R B  and the capacitor C 2 , it is possible to have a relatively large DC gain, and thus it is possible to make very small the difference between the logic signals Tfb and Tw.  
         [0054]     The signal  14  in  FIG. 12  is not null because the signals Tfb and Tw are not equalized, and thus corresponds with the active level of the signal  14 . The error voltage Verr increases according to a charge transient of the R-C circuit composed of the resistor R A  and the capacitor C 2 .  
         [0055]     According to a preferred embodiment of the invention, the capacitors C 1  and C 2  and the resistor R A  are dimensioned for making the saw tooth voltage Vst increase faster than the error voltage Verr, otherwise the signal  4  (and thus almost all signals of the control circuit of  FIG. 11 ) would be corrupted by spurious switching.  
         [0056]     According to an alternative embodiment of the circuit of  FIG. 11 , it is even possible to employ two distinct monostable generators respectively to generate, instead of a single pulse Tw, a pair of pulses Tw A  and Tw B  of different durations, for turning immediately off the MOSFET when the feedback pulse signal Tfb becomes shorter than both pulses Tw A  and Tw B . The pulse Tw A  is equivalent to the pulse Tw and will be used to ultimately generate the signal  14 , while the other pulse Tw B , of shorter duration than the pulse Tw A , will be compared with the pulse Tfb. If the duration of the latter (Tfb) is shorter than the duration of the former (Tw B ), a low-impedance path of the control circuit is enabled for discharging as quickly as possible the capacitor C 2 . As a consequence, the PWN control voltage Vg switches low and the MOSFET is immediately turned off.  
         [0057]     A low-impedance path suitable to this end may be simply composed of a switch for grounding the capacitor C 2 , enabled by a control signal generated by a dedicated logic signal as a logic AND between the pulse signal Tw B  and an inverted replica of the feedback pulse signal Tfb.  
         [0058]     Preferably, the resistance R B  is smaller than R A  for discharging the capacitor C 2  more quickly than how it is charged. In so doing, the off-time of the synchronous rectifier is increased, and this prevents a possible cross-conduction phenomena  
         [0059]     A significant difference between the control circuit of the invention and the circuits of the above-mentioned prior art documents is that the duration of the active phase of the PWM control signal in accordance with the invention is adjusted in a closed-loop manner. Therefore, the desired anticipation time is maintained substantially in any condition. Moreover, the desired anticipation time can be changed as desired by changing the duration of the pulse Tw.  
         [0060]     The method of the invention is substantially analog, and for this reason, the PWM control voltage Vg is not disturbed by high jitter, as in the method disclosed in U.S. Pat. No. 6,418,039. Moreover, the method of the invention is implemented in a control circuit that does not include a PLL, as the above-mentioned integrated circuit IR1175.  
         [0061]     The advantages of the control circuit of the invention are stability, safety, better efficiency, cost and transient response In terms of stability, for example, if the gate capacitance of the MOSEET increases because of temperature or because of its natural fluctuations during the life time of the MOSFET, the discharging time increases and the anticipation time decreases. Even in this case, the method of the invention is effective in keeping constant the anticipation time.  
         [0062]     In terms of safety, by keeping the anticipation time at the pre-established value, the disclosed method prevents the synchronous rectifiers from being incorrectly controlled and from causing a cross-conduction phenomena in the converters,  
         [0063]     In terms of efficiency, better efficiency is provided since the anticipation time is kept always at the same value even with time and temperature changes. The anticipation time may be shorter than in known control circuits of synchronous rectifiers, even down to 10 ns. As a consequence, the time in which the body-diode is in a conduction state is reduced accordingly, thus reducing conduction losses and reverse recovery losses.  
         [0064]     In terms of cost, the control circuit of the invention is straightforward and uses standard low-cost components, while known analogous control circuits exploit complicated PLL techniques or digital counting.  
         [0065]     In terms of a transient response, the worst condition in controlling synchronous rectifiers is when a sudden duty-cycle decrease happens. In known control circuits this situation is likely to vary dangerously the anticipation time. In contrast, if the gain of the control circuit of the invention in discharging the capacitor is much larger than its gain during charging phases, even when the duty-cycle quickly decreases, then the capacitor is quickly discharged.  
         [0066]     Therefore, the anticipation time is immediately increased and the synchronous rectifier is turned off for a longer time, thus preventing effectively any possible malfunctioning The control circuit reacts with only one switching period of delay to fast varying transients This cannot be accomplished using the cited prior methods.  
       REFERENCES  
       [0000]    
       
          1) F. Librizzi, P. Scalia—“STSRX FAMILY: MIXED-SIGNAL ICS TO DRIVE SYNCHRONOUS RECTIFIERS IN ISOLATED SMPS”—STMicroelectronics Application Note—July 2000.  
          2) I. D. Jitaru—“High Efficiency DC/DC Converter”—HFPC Proceedings, April 1994  
          3) M. M. Jovanovic, M. T. Zhang, F, C. Lee—“Evaluation of Synchronous-Rectification Efficiency Improvement Limits in Forward Converter”—IEEE Transaction on Industrial Electronics, Vol. 42, No. 4, August 1995.  
          4) H. P. Yee, S. Sawahata—“A Balanced Review of Synchronous Rectifiers in DC/DC Converters”—PESC 1999.  
          5) M. Madigan, M. Dennis—“50W Forward Converter with Synchronous Rectification and Secondary Side Control”—Unitrode Power Supply Design Seminar 1999-00 Series.  
          6) Yee et ale—U.S. Pat. No. 5,736,890—Method and Apparatus for Controlling Transistors as Rectifiers—Apr. 7, 1998.  
          7) SR-CIC—SRMOC Preliminary Application Note.  
          8) IR1175—International Rectifier Datasheet.