Highly reliable, low dissipation boost circuit

Abstract: A control of the switch-on instant of a power device of a boost-circuit employed in a switching power supply operating in a discontinuous mode is implemented by monitoring the current which actually flows through the "fly-back" inductance. The current is monitored on a sensing resistance and by the use of a comparator a signal is produced for enabling/disabling the transfer to an input of a PWM driving circuit of a switch-on signal produced by a null detector, sensing the voltage induced on a secondary winding coupled to the fly-back inductance. The circuit has a greater immunity to noise and a reduced power dissipation.

Description: The present invention relates to a switching power supply circuit destined to function in a discontinuous mode; i.e. with a momentary nullification of the current through a "fly-back" inductance between successive impulses. More particularly the invention relates to an improved system for controlling the switching-on of a power device whereby besides reducing power consumption, a higher reliability and immunity to noise of the circuit are also achieved. A switching power supply, commonly based on a so- called boost-type circuit utilizing a fly-back inductance, provides an output DC voltage having a value greater than the value of the input voltage, i.e. of the value of the rectified line voltage. If for any accidental cause, the output voltage becomes lower than the rectified line voltage, the circuit operates under anomalous conditions which: under certain circumstances, may lead to failure. Commonly these systems function according to one of two alternative modes of operation as far as the variation of the current which circulates in the fly-back inductance during each switching cycle is concerned. Such an inductance is commonly referred to as a fly- back inductance because alternately during each switching cycle it firstly absorbs energy from the "low" voltage side (i.e. through the terminal connected to the rectified line voltage) during a conducting phase of the power device (transistor), and thereafter, following the switching-off of the power transistor (i.e. conduction taking place through an output diode), it gives back (flies-back) the energy stored in the inductance at the output voltage, releasing it to an utilizer circuit connected thereto. According to a first technique, said of continuous or isteretic mode, the current across the inductance is made to vary between two limit values of the same sign, different from each other by not more than about ten or more percentage points, as referred to the amplitude of the rectified waveform. This first technique is preferred for high-power systems, because, for the same power, the introduction electromagnetic interferences in the line may be contained by employing a relatively inexpensive filter circuit, though requiring a more sophisticated switching circuit. In practice the current flowing through the inductance has a high frequency saw-tooth waveform having a relatively small amplitude (ripple). By contrast, an alternative technique, said of discontinuous mode, is widely employed in power supplies of relatively low power; typically up to one or two hundreds watts. The current that circulates in the fly- back inductance is made to vary, during each switching cycle, between zero and a maximum value which follows the envelope of the rectified waveform. The cost of the relative switching circuit is less than in the preceeding case, while the cost of the circuit necessary for filtering the disturbances which would otherwise be introduced into the network remains economically tolerable because of the relatively low current and power levels. A simplified diagram of a power supply of this latter type is shown in Fig. 1. In the case of this second technique it is possible to optimize the conversion efficiency by exploiting the fact that the power device Pw may be switched-on when the current through the fly-back inductance L is null. The document GB-A-2,213,614, discloses in relation to the schemes of Figures 8 and 9, a boost converter operating at the self-oscillating mode boundary of the continuous/discontinuous mode. The converter employs a sawtooth modulator (27) which is controlled by a secondary winding sensing the energy level of the inductor, the power switch of the converter being operated whenever the energy stored in the inductor falls to zero. In practice the amount of energy that would be lost in the switching power transistor Pw when, upon switching-on, it must dissipate the energy stored in the parasitic capacitance Cp that exists between the circuit node connected to the drain (collector) terminal and ground, may be sensibly reduced. In fact, in the case of the first technique (continuous mode) the drain voltage before the switching-on of the power transistor is equal to the output voltage and because the drain is forced by the switching-on process to ground potential, for each cycle an energy equal to: 1/2 Cp V 2 out , where V out is the output voltage, is dissipated through a discharge of the parasitic capacitance Cp. On the contrary, in the case of the second technique (discontinuous mode) it is possible to reduce this power dissipation. This is often obtained by slightly delaying the switching-on of the power transistor. Instead of switching-on the power transistor as soon as the current flowing through the fly-back inductance toward the output circuit Co through the diode Dr has become null, a certain progress of a resonance process of the inductance L with the parasitic capacitance Cp is available. Therefore, the parasitic capacitance Cp discharges in a nondissipative mode from the output voltage level V out to the input voltage level V in , which is significantly lower than V out . When the power transistor Pw finally switches-on again it must dissipate an energy equal to: 1/2 Cp V 2 in , which is less than that of the case described before, by a proportion given by: V in 2 /V out 2 . Therefore, a switch-on signal is commonly obtained by detecting, on a dedicated secondary winding Ls magnetically coupled to the fly-back inductance L, a condition of null voltage across the secondary winding Ls. A switch-off signal for the power device (transistor) is normally obtained through a comparison between a reference signal, that may be purposely generated within a PWM driving circuit and the current which flows through the power device Pw, which is commonly red on a sensing resistance Rsense; connected in series with the power transistor Pw. When the sensed current reaches a reference threshold the power transistor is turned-off. More in detail, the rectified line voltage V in is substantially present on the node C, while on the other terminal of the fly-back inductance L, i.e. on the D node, a square wave voltage is present that takes a substantially null value when the power transistor Pw is conducting and a V out value (i.e. the output voltage of the power supply) when the power transistor Pw is not conducting. Therefore, the voltage signal that is induced on the secondary winding Ls, i.e. the voltage present on the node A, has a waveform that is enveloped at the top and at the bottom by a sinusoid, as depicted in Fig. 2a. This voltage signal, through the resistance R1, is fed to a clipping circuit composed of the transistor Q1, the bias current generator Ib, the pair of series connected diodes D1 and D2, and the Zener diode DZ1, as shown in Fig. 1. This circuit limits the amplitude of a signal that is a replica of the voltage induced on the secondary winding Ls, on the node B (i.e. output node of the clipping circuit), as represented in Fig. 2b. This limited (clipped) voltage signal is compared with a reference voltage, schematically indicated in Fig. 1 by a battery symbol, by the comparator COMP, capable of producing a switch-on signal for the power device Pw that is sent to an input of a PWM driving circuit. For clarifying better this point, attention is drawn again to the circuit of Fig. 1 and to the signal waveforms shown in Figs. 2a, 2b, 2c and 2d. The minimum value of the sinusoid which envelopes at the top the signal representing the voltage present on the node A (i.e. the voltage induced on the secondary winding Ls) is given by: Vmin = (V out - V in )/K, where K is the L/Ls transformer ratio. When Vmin is lower than the upper threshold of the clipping circuit, the voltage on the node B will shows a "valley", as depicted in Fig. 2b, and this valley expands upon an increase of the V in value. When finally V in becomes greater than V out , Vmin becomes negative, thus determining a situation similar to the one depicted in Fig. 2c. As may be observed in the central zone of Fig. 2c, the valley may grow until completely nullify the singal and as a consequence, the output of the comparator COMP will remain high as depicted in Fig. 2d. In these conditions as depicted in the central zones of the Figs. 2b, 2c, and 2d, it is easily recognised that any noise or spurious spike that may superimpose to the waveform of V in , may simulate spurious transitions of the represented signal. In fact are the positive or negative transitions, as referred to the sense of the magnetic coupling of the secondary winding Ls with the fly-back (primary) winding L, that identify the instant when the voltage across the fly-back inductance drops from V out - V in to zero and that are utilized for generating a switch-on signal for the power transitor Pw. In conclusion of the above discussion of the circuits of the prior art, it may be said that these circuits have a drawback that is represented by the fact that it is possible that a spurious disturbance in the rectified line voltage (input voltage) by transferring itself to the secondary winding Ls, may simulate a transition and therefore cause the switching-on of the power transistor Pw before the current through the fly- back inductance L has effectively dropped to zero. This risk is particularly present in some type of circuits as for example in switching power supplies with a PWM control circuit and where a control upon the power factor is also implemented. In fact, in these circuits the input voltage V in is: relatively unfiltered after rectification, for not reactively loading the line; variable from zero (which makes particularly effective the technique of delaying the switching-on of the power transistor described above) up to a maximum value that is close to the value of the output voltage (which on the other hand makes a spurious switching-on of the power transistor extremely dangerous) for each half-cycle of the line frequency. Particularly critical is the situation that occurs when the rectified line sinusoid reaches its maximum. In fact, in the neighbourhood of these maxima: the input voltage becomes nearly equal to the output voltage (Fig. 2b) or, in case of an r.m.s. value and/or form factor greater than the design maxima, becomes greater than the output voltage (Fig. 2d); during each switching cycle more factors of risk combine together: a) the duration of the conduction period of the power transistor Pw shortens and that of nonconduction longhtens, thus increasing the proportion of a cycle when the occurrence of a spurious switching-on may be possible; b) the current at which the switching-off occurs is relatively high and a spurious switching-on may occur while the fly-back inductance L still stores a relatively high energy; c) the voltage across the fly-back inductance L during the "fly-back" period (power transistor off) is at minimum value and a disturbance may easily induce a spurious sensing of a transition. Notably in these systems; the current flowing in the fly-back inductance has a substantially triangular wave shape (in practice the shape of a nonisosceles triangle of variable aspect) the amplitude of which varies from zero to a maximum that is enveloped by the rectified half-wave of the line voltage (V in ). During the rise; the current flows through the power transistor Pw, during the descent the current flows through the output diode to the utilizer circuit. In these cases, it is important to prevent that the transistor Pw be switched-on again for spurious causes when the current which flows through the fly-back inductance (and the output diode Dr and the utilizer circuit) is at a relatively high level. It is therefore a main objective of the present invention to provide an improved control circuit for the switching-on of the power element of a boost-type circuit, free of the drawbacks of the known circuits, thus making a switching power supply more immune to disturbances and therefore more reliable in its operation. These objectives and advantages are obtained in the control system of the invention by monitoring through a sensing resistance that is purposely connected in series with the output circuit (i.e. with the utilizer circuit and not with the power transistor, as it was commonly the case in the circuits of the prior art) not the current which flows through the power transistor but the current which flows through the fly-back inductance. The control system of the invention prevents the transfer of a switch-on signal, as generated by elaborating the signal present across a secondary winding magnetically, coupled with the winding (primary) of the fly-back inductance, to a Pulse-With-Modulation (PWM) driving circuit, as long as a current flowing through the fly-back inductance is detected. Notably, this does not alter the sensing of the optimum instant for switching-off the power transistor by the PWM driving circuit. In this way, not only the power dissipation associated with the discharge of the parasitic capacitance that is intrinsic to the structure of the power device (transistor), is reduced but also the risks of an anticipated switching-on of the power transistor, which may be caused by noise or spurious events propagating on the line, are eliminated. The circuit object of the present invention is particularly suited when the line voltage has a relatively high form factor or presents noise spikes, as well as in all those instances when the output voltage, because of a certain variation of the load of the power supply, is subjected to a so-called "under-shoot" phenomenon, as related to the design regulated output voltage. The latter being a phenomenon that is practically inevitable when the switching power supply is provided with means for implementing also a function of control on the power factor (PFC). The different aspects and advantages of the invention will be more easily understood through the following description of an embodiment thereof and by referring to the annexed drawings, wherein: Fig. 1 is a basic diagram of a control circuit for the switching-on of a power device according to the prior art as discussed above; Figures 2a , 2b , 2c , and 2d show the wave shapes of signals pertaining to the description of the problems of the circuit of Fig. 1; Fig.3 is a basic diagram of a circuit made in accordance with the present invention. With reference to Fig. 3, a circuit for controlling the switching-on of a power transistor Pw of a boost- type power supply circuit, made according to the present invention, comprises a null detector circuit ⊘-DETECT, a comparator COMP and a logic AND gate. Of course, the circuit for controlling the instant of switching-on of the power transistor and the relative PWM driving circuit may be integrated on a single chip, as indicatively shown in Fig. 3 by the dash line perimeter and by the relevant pins for connecting the integrated circuit to the external components of the power supply circuit. As may be observed, the current IL that flows through the fly-back inductance L of the circuit flows also through the sensing resistance Rsense during all phases of operation of the power supply circuit. Therefore, across the sensing resistor Rsense a voltage Vs proportional to the value of the current IL, is available. Through a comparison of such a voltage Vs with a reference voltage Vref, implemented by the use of the comparator COMP an ENABLE signal is produced that enables or disable the logic AND gate that transfers a signal commanding the switching-on of the power transistor Pw to an input terminal of the pulse width modulation driving circuit PWM. The reference voltage Vref sets the condition for enabling the switching-on of the power device Pw when IL drops below a certain safety value. A switch-on signal is actually produced by the null detector circuit ⊘-DETECT whenever the voltage on the node A becomes null; however this signal is not transferred to the PWM driving circuit until the AND gate is enabled. Of course, the utilizer circuit powered by the power supply of the invention will have as reference common potential a virtual ground potential that is equivalent to the voltage present on the sensing resistance Rsense. In general, this peculiarity of the circuit of the invention will not create any problem.

Claims: A method for controlling the switching-on of a power device (Pw) driven by a pulse width modulation circuit (PWM) of a boost-type switching power supply comprising a fly-back inductance (L) connected between an input node (C) to which a rectified line voltage (Vin) is fed and said power device (Pw), characterized by comprising producing a first switch-on signal whenever the voltage across a secondary winding (Ls) coupled to the winding of said fly-back inductance is null; generating a second switch-on enabling signal whenever the current (IL) in said fly-back inductance (L) drops below a preset level; producing a third logic AND signal of said first and said second signals; feeding said third logic signal to an input of said pulse width modulation circuit (PWM) driving said power device. A circuit for controlling according to claim 1 the switching-on of a power device (Pw) of a boost-type power supply circuit comprising a fly-back inductance connected between an input node (C) to which a rectified line voltage (Vin) is fed and said power device (Pw), a secondary winding (Ls) coupled to the winding of said fly-back inductance (L), a sensing resistance (Rsense) of the current flowing in said fly-back inductance (L), functionally connected between said power device (Pw) and a ground node of the circuit and a pulse width modulation circuit (PWM) driving said power device (Pw), characterized by comprising a null detector circuit (⊘-DETECT) capable of producing on an output node a first logic signal representative of a null value condition of a voltage induced on said secondary winding (Ls); a comparator (COMP) capable of producing on an output node a second logic signal (ENABLE) for enabling the switching-on of the power device (Pw) and having a first input functionally connected to a connection node (Vs) of said sensing resistance to said power device and a second input functionally connected to a reference voltage (Vref); a logic AND gate having a first input connected to the output of said null detector circuit (⊘-DETECT), a second input connected to the output node of said comparator (COMP) and an output connected to an input of said pulse width modulation circuit (PWM); an utilizer circuit powered by said power supply being functionally connected between an output node of said power supply circuit and said connection node between said power device (Pw) and said sensing resistance (Rsense).

Inventors: BORGATO PIERANDREA, DIAZZI CLAUDIO, PIDUTTI ALBINO, BORGATO, PIERANDREA, DIAZZI, CLAUDIO, PIDUTTI, ALBINO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H02M3
IPC codes: H02M3, H02J1, H02M7