Method of controlling a DC-DC converter

Abstract: A method of controlling a DC-DC converter having means for comparing the output voltage with a low threshold and a high threshold, means injecting a certain minimum electric charge into an inductor of the converter during a charge period of at least a power switch, comprises commanding the beginning of a charge period each time the output voltage drops below the low threshold, progressively increasing the electric charge transferred during a charge period until the output voltage rises to said high threshold, starting from said low threshold upon executing a charge period, measuring and storing the duration of the time interval between two consecutive charge periods, comparing the current time interval with the previously detected and stored time interval, decreasing to said minimum electric charge whenever an increment of the time interval between two consecutive charge periods is detected. A hardware implementation is also described.

Description: FIELD OF THE INVENTION The invention relates to DC-DC converters and in particular to the problem of voltage regulation on variable loads. BACKGROUND OF THE INVENTION Voltage regulation circuits typically provide a certain voltage that is maintained constant by feeding-back the output. In a pulse width modulation (PWM) switching regulator, a square wave drives the control terminal of a switch determining its conduction or cut-off state. By increasing the time interval of conduction of the switch, the output voltage increases and the opposite occurs by increasing the time interval of cut-off. Therefore, the output voltage is controlled by varying the duty cycle of the square wave driving signal. Such a control of the duty cycle is performed by a circuit which continuously compares the output voltage with a reference voltage and adjusts the duty cycle of the square wave in order to keep constant the output voltage. When the switch is a MOS transistor, a non-negligible power is required to charge periodically the gates of the switching transistors. The switching power loss increases as the switching frequency increases, negatively affecting the converter efficiency. On the other side, the requisite of limiting the variations of the voltage applied to the load within a tolerable range does not permit to reduce the switching frequency as much as desired. To maximize the conversion efficiency of a switching regulator, several manufactures implement the so-called PFM (Pulse Frequency Modulation) technique or skip mode. Such a technique consists in controlling the peak value of the current flowing in the inductor while operating in a discontinuous mode. In the commercial device MIC 2177/8/9 provided by Micrel Inc. a control with constant current peaks is realized as far as the load remains relatively low. For high loads, the control made reverts to a traditional PWM control at a fixed switching frequency. This technique does not maximize efficiency because it does not minimize the switching power losses and the power losses in the driver steps of the power MOS that are predominant under low load conditions. A way for reducing the switching power losses is disclosed in the US patent No. 5,568,044, wherein a controller of a DC-DC converter is presented that implements the two different control techniques PWM (Pulse Width Modulation) and PFM (Pulse Frequency Modulation), using the one with lowest power dissipation upon a change of load conditions. OBJECT AND SUMMARY OF THE INVENTION It has now been found and is the object of the present invention, a method of controlling a DC-DC converter through an iterative procedure that makes the circuit operate at optimal conditions by adapting by itself to any variation of the load. The method of the invention can be used together with other known techniques of controlling a DC-DC converter (e.g. PWM and PFM). According to an aspect of this invention, the controller, designed to adjust the switching frequency to be close as possible to the optimal frequency, minimizes switching power losses and driver losses thus maximizing the power being transferred from the supply to the load. The method of the invention of controlling a DC-DC converter, having means comparing the output voltage with a pre-established low threshold and a pre-established high threshold and means injecting a certain minimum electric charge to be transferred into the inductor of the converter during a conduction interval or phase of a power switch, comprises the steps of : beginning a charge period each time the output voltage reaches the low threshold; increasing progressively the electric charge that is transferred during each charge period until the output voltage reaches the high threshold starting from the low threshold in a charge period; monitoring and storing the duration of the off interval of the switch between two consecutive charge periods; comparing the current off interval with the previously stored off interval; decreasing to said minimum value the electric charge being transferred when detecting an increment of the off interval of the switch between two consecutive charge periods. The method of the invention can be implemented by realizing a DC-DC converter according to claim 5. BRIEF DESCRIPTION OF THE DRAWINGS The different aspects and advantages of the invention will result more evident by the description of an embodiment of the invention and by referring to the attached drawings where: Fig. 1 illustrates the mechanism of correction of output voltage changes by varyingly spaced current pulses having different amplitudes being forced through the inductor of the DC-DC converter, according to the method of the present invention; Fig. 2 depicts a control system of the invention based on the use of a digital state machine (DIGITAL ASYNC. CONTROLLER); Fig. 3 is a flow chart of the algorithm executed by the finite state machine at the start-up of the system; Fig. 4 is a flow chart of the regulation algorithm executed by the finite state machine; Fig. 5 is a flow chart of the algorithm of "Low Power Consumption" (LPC) that activates the drivers of the analog portions of the control system, executed by the finite state machine; Fig. 6 shows several possible combinations of current pulses in carrying out a CFM control; Fig. 7 shows the start-up algorithm of the CFM method; Fig. 8 is a circuit diagram implementing the CFM version of the method of the invention; Fig. 9 is a block diagram of the CFM version of the method of the invention. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION The method of the invention is based on an architecture for controlling the output voltage with the maximum efficiency, employing inductor current detection circuits, comparators of the output voltage with pre-established thresholds, and circuit devices measuring the time interval between two consecutive current pulses and a state machine controller managing the converter. To attain the maximum conversion efficiency, an expression of the switching frequency as a function of the load current that minimize the switching losses and driver losses is calculated on the basis of the parameters of the particular application. The converter circuit is thus designed by properly choosing the values of inductance, output capacity of the DC-DC converter and amplitude of the hysteresis window within which the output voltage may vary, such that the controller may be able to establish the switching frequency that minimize power losses in case of a minimum load and of a maximum load. The regulation of the output voltage is ensured by a hysteresis window of regulation of the output voltage of the converter, implemented by a pair of threshold comparators. The control method of the present invention essentially consists in providing periodically to the load a certain quantity of electric charge which is progressively incremented by one or more current pulses that may have different duration and amplitude. In the following description such a provision of a certain electric charge will be indicated as "charge period". The system reaches a steady state condition when with a single charge period a charge suitable to make the output voltage rise from the lower value to the upper value of the hysteresis window using the same current pulse combination is provided. Such a supply of electrical charge, sufficient to bring the output voltage from below the lower threshold to above the upper threshold, can be carried out according to alternative embodiments of the method of the invention. Two alternative ways of implementing such a charge period are described hereinbelow. A first way of realizing the charge period, that may be called PCV (Peak Current Variation), attains the optimal frequency of operation of the converter by progressively incrementing the peak level of the current pulses delivered to the inductor until, in a substantially steady-state of operation, the output voltage rises from the lower threshold to the upper threshold of the hysteresis window of regulation upon forcing a single current pulse through the inductor of the converter. With reference to Fig. 1, when the output voltage reaches the lower threshold value of the hysteresis window, the control logic commands the delivery of a substantially triangular current pulse of peak value I P to the inductor (Inductor Current). If after such a first charge period the output voltage does not reaches the upper threshold of the hysteresis window, one or more triangular current pulses with peak values progressively increased by a certain defined quantity I step will be delivered to the inductor, until the output voltage (Output Voltage) reaches the upper threshold of the hysteresis window. The system waits for the output voltage to become lower than the lower threshold of the hysteresis window and when this occurs it delivers a new current pulse having the same amplitude of the second pulse of the preceding series of delivered current pulses. If this new single pulse is not sufficient to make the output voltage reach the high threshold, one or more current pulses of incremented peak values will be delivered to the inductor. The above algorithm is repeated until a steady state situation is eventually attained whereat, following the delivery of a single current pulse, the output voltage rises from the lower threshold to the upper threshold of the hysteresis window of regulation. An alternative way of realizing the charge period for supplying an equivalent optimal amount of electric charge is to deliver to the load a current pulse or a combination of current pulses having different amplitude and duration, changing the pulse combination provided to the load until the combination making the output voltage rising from the lower threshold to the upper threshold is reached: such an alternative way of realizing the "charge period" may be called CFM (Clamped Frequency Modulation). In other words, a charge period realized in a CFM mode may include the delivery of a number of current pulses, up to a maximum of M pulses, of different amplitudes. A possible example of pulse combinations according to the CFM mode is depicted in Fig. 6. Regulation is carried out by providing initially to the load a certain minimum quantity of electric charge: if the output voltage does not reach the desired value, current pulses whose amplitude is progressively incremented I p1 , I p2 are provided until the output voltage overcomes the upper threshold of the hysteresis window. When the output voltage become lower than the lower threshold of the hysteresis window, a new charge period is started, providing a current pulse having a greater amplitude I p1 : if after such pulse it exceeds the upper threshold of the hysteresis window, the provision to the load is stopped while waiting the output voltage decrease below the value of the lower threshold of the hysteresis window, otherwise a current pulse of amplitude I p2 is provided. After having verified that providing pulses of amplitude Ip1 and Ip2 the output voltage varies from the lower threshold to the upper threshold of the hysteresis window, the load will be controlled by providing during each charge period said combination of the two current pulse of amplitude I p1 and I p2 . The CFM method just described is depicted in Fig. 9, wherein dash-line contours highlight optional functions. After having run the start up algorithm, the circuit of Fig. 8, which implements the CFM method, waits that the output cross the lower threshold of the hysteresis window (UNDER=1) to start a new charge period. If the upper threshold of the hysteresis window has not been reached, the variable N that regulates the amplitude of the current pulses is incremented and a new current pulse is provided to the load. After having carried out a check for verifying the possibility of incrementing further the amplitude of pulses (N>n). If N>n, i.e. the amplitude of pulses cannot be increased, the regulation loop is disabled in favor of a traditional control algorithm, such as a PWM driving with a certain frequency, for example. Once the upper threshold of the hysteresis window has been reached (UP=1), the system returns to provide a current pulse of a certain minimum amplitude (N=0) and a new regulation cycle starts from the beginning. The above described procedures are repeated from the beginning at each decrease of the load that is detected by the controller in the following manner: when the output voltage reaches the upper threshold of the hysteresis window after a charge period, a device (OVERPEAK) measuring the time elapsed from the end of last pulse and the immediately following pulse is activated. Such a value of the time interval is stored and, during the following cycle, is compared with the time elapsed between the two immediately following pulses: if this last interval value is greater than the previously stored value, then the load has diminished and the procedure is repeated from the beginning. On the contrary, if the stored value is greater than the current value, then the load has increased and one of the above described routines will adapt the converter automatically to the new operating conditions. A suitable form of implementation of a circuit for measuring time intervals may consist in generating a linear voltage ramp, for example by way of a capacitor charged by a current generator, that is applied to the negative input of a voltage comparator, to the positive input of which is coupled the output of a DAC2 (Digital to Analog Converter) transforming a datum or digital word N, provided by the controller, in a corresponding analog voltage signal. Initially N is equal to zero and establishes the lowest output voltage of the DAC2. When the analog ramp reaches this threshold set by the controller, the comparator switches from a logic low to a logic high and a counter, contained in the controller, increases N by detecting such a switching edge. In this way, an analog voltage signal greater than the previous one is output by the DAC2 and the comparator switches from high to low. These steps are repeated each time the analog ramp reaches the analog voltage signal set by the DAC2. When the output voltage of the DC-DC converter surpasses the lower threshold of the hysteresis windows, the start-up process terminates, the value N having been so determined. Fig. 2 depicts an hardware embodiment of the invention as applied to a DC-DC "step-down" converter, supplied by a voltage Vin, and essentially constituted by an inductor L, an output capacitor Cout, a pair of power switches, respectively an upper switch N1 and lower switch N2 that in the illustrated example are realized with power MOS transistors, and the respective driving circuits HSDRIVE and LSDRIVE, enabled or disabled by the logic signals CHARGE and NOT CHARGE. The scheme also includes a boot-strap capacitor BOOT, according to a common practice. The control system comprises a pair of voltage comparators, comparing the output voltage with a reference voltage produced by a first digital/analog converter (DAC1) as a function of a given digital datum VCORSELINT, provided by the controller, and producing two logic signals UP and UNDER, respectively. A third comparator ZEROCROSS detects the zero crosses of the current in the inductor L, generating a logic signal ZCR at each event. The controller may be a finite state digital machine DIGITAL ASYNC. CONTROLLER, which preferably may be enabled or disabled by a logic command Enable, allowing the control of the DC-DC converter to be made according to other conventional modes in case that particular load conditions do not allow the PCV or CFM control mode of the invention or for any other reasons. The DIGITAL ASYNC. CONTROLLER receives from outside the information about the amplitude of the hysteresis window of regulation within which the output voltage is permitted to vary, and produce the digital datum VCORSEL INT that is fed to the DAC1, which sets the reference voltage of the UP and UNDER comparators. The current flowing through the upper switch N 1 is read on a sensing resistance in series to the upper switch N1, or to a scaled replica of the power switch, by an operational amplifier CURRENT READER outputting a voltage proportional to current. A fourth voltage comparator OVERPEAK has an input coupled to the output of a second digital/analog converter DAC2 and the other input either, depending on the value assumed by a control signal HCEN produced by the finite state machine, to the output of a linear ramp voltage generator with pre-established slope constituted by an electrical series of a current generator and a capacitor, or to the voltage produced by the operational amplifier CURRENT READER. Such a second digital/analog converter produces a voltage that may assume different values, depending on the digital datum N produced by the DIGITAL ASYNC. CONTROLLER. Such voltage values can be produced, as exemplified in the figure, by a voltage divider through which is forced the current produced by a current generator. Preferably, the asynchronous finite state machine DIGITAL ASYNC. CONTROLLER implements three algorithms, respectively a start-up routine, a regulation routine and "Low Power Consumption" (LPC) routine. A PCV mode start-up algorithm is illustrated in the flow chart of Fig. 3, and CFM mode start-up algorithm is illustrated in Fig. 7. Both carry out the operations necessary to properly enable the analog portions of the control system and to ensure that critical components such as the comparators, for example, reach a steady state operation conditions and a correct bias, before beginning the regulation algorithm. At start-up, the asynchronous finite state machine runs the start up algorithm which is substantially identical for either the PVC method (Fig. 3) or the CFM method (Fig. 7). The finite states machine substantially waits for a predetermined delay from the power-up instant before forcing a first charge period. Making reference to Fig. 3, the asynchronous machine enables the analog portion of the converter, sets the signal HCEN=1, thus so enabling the linear ramp voltage generator and coupling the output of the CURRENT READER from the input of the comparator OVERPEAK, and waits for the output of the comparator OVERPEAK to switch from low to high. At the transition of the signal OVERPEAK, the analog components (voltage comparators) are surely in steady state and correctly biased. An alternative way of generating the initial delay to allow for a correct biasing of the components of the converter is showed in Fig. 7, making reference to the CFM method, wherein the delay is produced by the comparator FLIMITI of Fig. 8 which switches after a pre-established time. At the end of the initial delay, the finite state asynchronous digital machine enables the power drivers (DIGEN=1), establishes the peak value of the current and commands the charge/discharge phases until the output voltage reaches the desired level (UP=1). Once such a level has been attained, the regulation algorithm of the invention, which has been previously described and which is schematically illustrated by the flow chart of Fig. 4, for the PCV embodiment, and in Fig. 9, for the CFM embodiment, is implemented. The "Low Power Consumption" algorithm (LPC), illustrated in the flow chart of Fig. 5, has the function of enabling/disabling the drivers and the analog components, thus allowing a further energy saving. When the LPC signal is high and the finite state machine is executing the regulation algorithm, meanwhile the output voltage is declining toward the lower threshold of the hysteresis window (UNDER=1) the power drivers and the analog components are disabled, except the comparator UNDER. When the voltage has reaches the low threshold, the finite state machine enables again the analog components, sets the signal HCEN=1, thus having the comparator OVERPEAK compare the current in the upper side switch N1 with a pre-established value provided by the DAC2 coupled to the comparator. Similarly to what happens during the start-up algorithm, the leading edge of OVERPEAK signal confirms that the analog components are in a steady state and so it is possible to enable the drivers (DIGEN=1) and to start the regulation algorithm. An example of a circuit implementing the CFM version of the method of the invention is depicted in Fig. 8. It differs from the circuit of Fig. 2 because it uses a signal SYNCRO, indicating a condition of a minimum load, making the rectification of the converter non synchronous (SYNCRO=1), and a signal OVER, indicating a condition of over-elongation of the output (OVER=1). Furthermore, a comparator FLIMITI has the function of establishing a maximum time interval with which pulses must be produced. The comparator FLIMITI is present because in some applications, it may be necessary to limit to a given minimum value the switching frequency in order to avoid that circuits coupled with the DC-DC converter be subject to undesired interferences phenomena. In audio applications for example, it is useful to ensure that the switching frequency do not decrease below about 20kHz in order to prevent interference problems. To ensure this, the method of the invention may be modified in order to force a charge period even if the output voltage has not attained the upper threshold of the hysteresis window, whenever the time interval between two consecutive phases exceeds a certain pre-established time, for example 40 microseconds (equivalent to a switching frequency of 25 kHz). Such a modification can be conveniently implemented both with the circuit of Fig. 8, which has the signal FLIMITI to this goal, and with the finite state asynchronous machine of Fig. 2. In fact such a result can be obtained having the machine of Fig. 2 count the number of OVERPEAK rising edges, compare such a number with a pre-established value Nmax and eventually command a turn-on and turn-off phase even if the output voltage has not yet attained the minimum allowed level, when such a number exceeds the pre-established number Nmax. Given that the rising edges of the OVERPEAK signal are regularly spaced in time, establishing a maximum number Nmax is clearly equivalent to establish a maximum time interval that can elapse between two consecutive pulses, i.e. the minimum frequency of the pulses is inversely proportional to Nmax. The CFM embodiment, complete with functions implementing the above noted frequency control depicted within the dash-line perimeter, is depicted in Fig. 9. After the start-up algorithm, the circuit of Fig. 8 verifies the elapsing of a time of at least 40µs, if so, then this event is stored by way of a flag (FLIMITION=1) and it is verified whether the upper threshold of the hysteresis window has been reached (UP=1). In the negative case, there is no over-elongation (OVER=0) and the usual method of delivering current pulses to the load is carried out. Upon reaching the condition of UP=1, the system checks if there is a condition of minimum load permitting a synchronous rectification of the converter by verifying that N=0 and FLIMITION=1: in fact, if the load is sufficiently low the system will deliver current pulses of minimum amplitude (N=0) and at the allowed maximum time interval of 40µs (FLIMITION=1), therefore it will be necessary to signal to the outside such a condition (SYNCRO=0). If at a restart of a driving cycle the output voltage is not below the lower threshold of the hysteresis window in the established maximum time (FLIMITI=1) and exceeds the upper threshold of the hysteresis window (UP=1), the system signals to the outside (OVER=1) that there is an over-elongation of the output voltage, i.e. the load is being driven at a voltage in excess of the upper threshold of the pre-established hysteresis window.

Claims: A method of controlling a DC-DC converter comprising the steps of : comparing an output voltage of the DC-DC converter with a low threshold and a high threshold; beginning a charge period for an inductor of the DC-DC converter each time the output voltage drops below said low threshold; progressively increasing the electric charge transferred to the induct during the charge period until the output voltage rises to said high threshold, starting from said low threshold upon executing a charge period ; characterised by the steps of : measuring and storing the duration of a new time interval between two consecutive charge periods; comparing the current time interval with the previously detected and stored time interval ; decreasing the duration of the next charge period to correspond to said minimum electric charge when an increase an increase of the new time interval between two consecutive charge periods is detected. The method of claim 1, characterized in that said time interval is further compared with a pre-established maximum value, at the overcoming of which a charge period with said minimum charge is commanded even if the output voltage has not yet attained the low threshold. The method according to one of the preceding claims wherein said electric charge transferred during a charge period is provided to the load with a single current pulse having pre-established duration and amplitude. The method according to one of claims 1 or 2 wherein said electric charge transferred during a charge period is provided to the load with a combination of current pulses having pre-established durations and amplitudes. A DGDC converter comprising a first comparison circuit connected to an output of the DC-DC converter for comparing an output voltage with a low threshold and a high threshold, at least one power switch (N1, N2) for transferring a charge from a supply node to a load connected to the output of the DC-DC converter via an inductor (L), a controller (DIGITAL ASYNC. CONTROLLER) connected to said at least one power switch (N1, N2) for beginning a charge period (CHARGE) for the inductor (L) each time the output voltage drops below the low threshold, characterized in that it further comprises: a monitoring circuit for measuring and storing a new time interval between two consecutive charge periods ; a second comparison circuit connected to said monitoring circuit for comparing the new time interval with a previously detected and stored time interval; wherein said controller progressively increases the charge transferred to the inductor (L) during the charge period until the output voltage rises to the high threshold and decreases a next charge period to correspond to a minimum charge when an increase of the new time interval between two consecutive charge periods is detected. The DC-DC converter according to claim 5, wherein the charge being transferred during the charge period (CHARGE) is based upon a single current pulse having a pre-established duration and amplitude. The DC-DC converter according to claim 5, wherein the charge being transferred during the charge period (CHARGE) is based upon a series of current pulses having pre-established durations and amplitudes. The DGDC converter according to claim 5, wherein the charge period (CHARGE) corresponds to duration of closure of said at least one power switch. The DC-DC converter according to claim 8, where.in said at least one power switch comprises a MOS transistor. The DC-DC converter according to claim 5, wherein said first comparison circuit comprises a first comparator (UP) and a second comparator (UNDER) each having a first input for receiving the output voltage of the DC-DC converter, said first comparator (UP) having a second input for receiving the high threshold and said second comparator (UNDER) having a second input for receiving the low threshold, said first and second comparators (UP, UNDER) having respective outputs connected to respective inputs of said controller (DIGITAL ASYNC. CONTROLLER). The DC-DC converter according to claim 5, wherein said controller (DIGITAL ASYNC. CONTROLLER) comprises a start-up algorithm and a low power consumption algorithm, said controller controlling said at least one power switch for charging said inductor by running the start-up algorithm and the low power consumption algorithm. The DC-DC converter according to claim 5, further comprising a first digital/analog converter (DAC1) having an input connected to said controller for receiving digital data, and an output for providing the high threshold and the low threshold to said first comparison circuit. The DC-DC converter according to claim 5, further comprising: a second digital/analog converter (DAC2) having an input connected to said controller (DIGITAL ASYNC. CONTROLLER) for receiving a digital value therefrom; and a third comparison circuit connected to said second digitavanalog converter for comparing a reference voltage (REF) corresponding to the digital value with either a linear voltage ramp having a pre-established slope or a voltage value proportional to the current charging the inductor through said at least one power switch.

Inventors: BASSANI SIMONE CHRISTIAN, ZAFARANA ALESSANDRO, BASSANI, SIMONE CHRISTIAN, ZAFARANA, ALESSANDRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: H02M3
IPC codes: H02M3