PULSE WIDTH MODULATED VOLTAGE CONVERTER FOR GENERATING A PREFERABLY SINUSOIDAL ALTERNATING VOLTAGE

Abstract: A pulse width modulated voltage converter is proposed, which converts a direct voltage to a sinusoidal alternating voltage with low frequency for use as a ringing signal generator for telephone exchanges. The converter includes a power stage comprising a ferrite core transformer (T) with two switches (P<uf'>u, P<ur>u) on the primary side (battery side) and two switches (S<u1>u, S<u2>u) on the secondary side (load side). The switches are controlled by control pulses from a pulse width modulator (PM) with a variable pulse rate so that they are switched with a frequency which is much higher than the frequency of the sinusoidal alternating current and so that a pulsing of the nergy flow between battery and load side is obtained.

Description: PULSE WIDTH MODULATED VOLTAGE CONVERTER FOR GENERATING A PREFERABLY SINUSOIDAL ALTERNATING VOLTAGE ; TECHNICAL FIELD The invention relates to a pulse width modulated voltage converter in¬ tended primarily for generating a sinusoidal alternating voltage of low frequency, the peak value of the converter output voltage being greater than the supply voltage. The proposed converter is used as a signal generator in telephone exchanges, for example, but it can also be used as a suppl .voltage source in a line circuit between telephone subscriber and exchange. BACKGROUND ART In the subscriber stage of a telephone exchange, a ringing signal gener ator is usually included which is common to a plurality of telephone subscribers connected to the exchange. Its function is to provide the receiving subscriber with a ringing signal before connecting a call. For its function, the generator shall generate a sinusoidal alternating voltage which can be added or connected in series to a battery voltage (so-called battery offset) in the line circuit in a plurality of modes depending on different market requirements. The ringing voltage ampli¬ tude and frequency is also subject to market adjustment from 80 V up to 120 V as well as 17, 25 and 50 Hz. The necessary power is about 25 VA for the units and traffic cases normally current. As an example of the prior art, reference may be made to the U.S. Paten 4,056,689. This patent inter alia describes a line circuit in which a current supply circuit is included for feeding a telephone line with current independent of the line length. In the line circuit there is included a ringing generator generating a ringing signal when calling a station from a subscriber, as well as a feed voltage source providing the line with direct current when the subscriber lifts his handset. In the U.S. Patent 4,174,467 there is shown in more detail a ringing signal generator for use in a line circuit. This known generator has a power stage comprising four switches conductive in pairs connected in the speech signal path between the exchange and a subscriber. The switches are controlled with a switching frequency very much higher (15 kHz) than the ringing signal frequency (50 Hz) with the aim of re¬ ducing disturbing overtones in the ringing signal. DISCLOSURE OF THE INVENTION A problem in the construction of ringing signal generators and supply voltage sources for line circuits is to provide desired voltages and currents with the least possible power consumption. Previously known solutions such as a rotating machine or class C oscillator used as a ringing generator have generally too great weight and too great di¬ mensions. In miniaturization attempts it is above all the LF chokes and transformers which take up the greatest space and have the greatest weight. Furthermore, the power components in linear output stages de¬ velop so much heat that they cannot be mounted .much closer together in spite of their physical dimensions being small. The voltage converter in accordance with the present invention contains a transformer which both on the battery and on the load side is con¬ nected to switching devices, e g transistors. The switching devices are thereby controlled so that the direct voltage coming into the battery side is converted to a sinusoidal output voltage and current, i e the output has four-quarter character, whereby the output, can be permitted to be loaded with both resistive as well as reactive load. The frequen¬ cy of the control signals to the switching devices is thereby selected very high in relation to the frequency Ce g the frequency spectrum) of the desired output voltage, whereby the transformer and the subsequent output filter can be given small dimensions. The four-quarter character of the converter power stage results furthermore in that temporary ex¬ cess energy in the output filter can be fed back to the direct voltage source, thus attaining some saving of power here also. The object of the invention is thus to provide a voltage converter for converting DC to AC, which takes up small space and. requires low power for the conversion. The converter in accordance with the invention is thereby characterized in accordance with the characterizing portion of Claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail while referring to the appended drawing, where Fig 1 is a block and circuit diagram showing the basic features of a voltage converter in accordance with the invention; Figs 2a-2d show equivalence. diagrams for the converter (power section ) according to Fig 1 during the different switching sequences; Fig 3 is a time diagram for more specifically explaining the mode of operation of the converter according to Fig 1; Fig 4 is a time diagram of the currents through two transformer winding in the diagram according to Fig 1; and Fig 5 is a -diagram of a possible modulator characteristic of the pulse •width modulator included in the converter according to Fig 1. MODES FOR CARRYING OUT THE INVENTION The basic features of the voltage converter in accordance with the in- vention are apparent from Fig 1. A battery B, which delivers a suitable direct voltage, is connected to an input filter IF. The outputs of the filter IF are connected to the two end points of a first winding of a transformer T. The input filter IF has the task of eliminating the disadvantages occurring -when the supply voltage source of the converter does not have sufficiently low impedance at the switching frequency. The filter shall equalize the pulse currents occurring in the converter so that other equipment (connected to the battery B) is not disturbed and so that the converter feed voltage is not appreciably affected by these pulse currents. The transformer T is. of conventional implementa- tion with a ferrite core, and contains two windings and on one side (called .the primary side below) and two windings , I, on the other side (the secondary side). Via a first switch P,, the winding I- has its end points connected to the filter output according to the above, and via a second switch P the winding l_ has one end point con- nected to said output. In a similar manner, the secondary winding half L, has one end point connected via a third switch S<. to the input of an " CTRI OMPI output filter UF and its other end point directly connected to said in¬ put. The fourth winding half I, has one end point connected to the in¬ put of the output filter UF via a fourth switch S 2 and has its other en point directly connected to said input. The .switches are controllable from the outputs p,, p , s.. and s 2 of a pulse width modulator PM, which will be described in more detail below. The switches P,, P and S-, S may be bipolar transistors, field effect transistors of the MOS type or bipolar type, depending on the application in question. The switch P can also consist solely of a diode, the control .pulses p " from the mod¬ ulator PM then not being utilized- The winding halves --^~ are oriente as indicated by the dot markings in Fig 1.and the ratio of the number of turns * " , I ? "I A can De selected in a suitable mode depending on the desired output voltage level of the load voltage U, obtained from th output filter UF. The block L symbolizes a load which in the ringing signal generator application is a so-called ringing bus, i e a two-wire line containing a plurality of switches (ringing-relays). In the one po sition of these, the ringing signal generator is connected to a telepho set and gives a ringing signal to it. In the other position (the speech position) the telephone is connected to an exchange and the ringing signal generator is .disconnected. Both terminals of the output filter UF are connected to the input of a feedback network N, apart from to the load L. The output voltage U. is attenuated to a suitable level in this network for being suited to the subsequent logic circuits. Since the output voltage U. is floating (it is not related to any reference), suitable relation to a reference po¬ tential (earth) also takes place in the network N. The output voltage U from the network N according to Fig 1 is thus related to earth, c A comparator, for example comprising an operation.amplifier, is connect ed by one input to the output of the feedback network N and by its othe input to the input signal source of the converter. In the case of im¬ plementation of the ringing signal generator according to Fig 1, the « input signal source is a reference oscil-lator -RO, sending a sinusoidal reference signal, the frequency of which is equal to that desired to be obtained for the output voltage U, from the filter UF. In comparing the reference voltage U , with the output voltage from the network N, there is obtained an error voltage U_ across the output of the compara¬ tor JF. This output is connected to the pulse width modulator PM, which sends.control signals from two pairs of outputs p,, p and s , s 2 to th switches P., P and S-, S 2 , respectively. In the pulse width modulator PM, the magnitude of the error voltage U p will determine the pulse- pause relationship (the pulse rate) of the square-wave control signals across the outputs p f , p and s-, s 2 . The pulse rate of these can thus vary from 0 to 100% in response to the error voltage U_, while their frequency is maintained constant, and determined by the frequency f of a clock signal. A pulse width modulator having these properties can be provided by one. ski lied in the art who has available today's assortment of logical circuit components such as operation amplifiers, shift re¬ gisters, flip-flop circuits and AND/OR gates. The detail construction of the modulator. is therefore not dealt with here. In the described embodiments, each clock period 1/f_ is divided, apart from the extreme positions of the modulator, by the pulse width modula¬ tion into two -intervals. During the first one, energy is transported between the load and battery sides according to one of the four opera¬ tion modes described.below. During the second one there is no energy transport. The task of the input and output filters is to equalize this pulsing (form the mean value thereof) so. that the energy flow in load and battery is not notably at variance with the clock frequency f . In this mode the analogue signal which is desired.to be amplified is regenerated from pulse width modulated intermediate form. The function of the converter will now be described in greater detail while referring to Figs 2a-2d, illustrating equivalence diagrams during the different switching sequences, and to Fig 3 illustrating the time diagram of the output voltage U , the reference voltage U ,, the error voltage U, and the control voltage pulses to the switches P , P and S,., S« respectively, for a given loading case,, i e a given reactive load connected to the output filter UF. The pulse width codulator char¬ acteristic is illustrated in the diagram according to Fig 5. This diagram illustrates the pulse rate M of the transmitted control pulses to the switches P,, P and S„, S-, as a function of the controlling er- f r Λ 2 s ror voltage. The modulator is constructed such that when the reference OR£ voltage U , > U and both are positive, the error voltage U p > U p , (cf when t = t in Fig 5). When U , < U and both are positive, then o ia ref c -U_ < U- < 0, (t., < t < t,). When U . < U > Y„, arid both are negative ro T i ret c o then U_ < -U_ , and when U Λ > U and both are negative, then 0 < U , r ro reT c T < U- . Fo At the time t , the reference and load voltages are assumed equal to zero and thereby U_ = 0. During the short ti- e when U grows from 0 to U F , the necessary, regulating deviation is established and the con¬ verter comes into the first operation mode which has a duration up to t... Depending on the load, the error signal U p . rises above U p , as in Fig 3. Within the interval t -t < ., a pulse length modulation of the pul¬ ses to the switches P, and S„ occurs, while the switches P„ and S-, are T P . interrupted (cf Fig 5)', and the equivalence diagram according to Fig 2 is obtained. The switching sequence during the interval t -t- is such that when P, is closed, S is open and vice versa, the switches thus being driven in phase opposition. At the time t.., according to Fig 3, the voltage U_ e -c has sunk so that U - = U and the error, voltage U p will rapidly sink first to zero and thereafter to a negative value, when the reference voltage will continue to be less than the voltage U . The modulation characteristic for 0 < U p < U p according to Fig 5 will therefore be -rapidly run through and the pulse width modulation of the control pulses P and S 2 which, ac¬ cording to Fig 5, are now energized, will be executed. At the time t 2 , as in Fig 3, the load voltage has sunk to zero. For this to be negative, the regulating deviation must be somewhat more negative so that the error voltage U p will be equal to ~ po (corresponding to +U_ at the zero pass ' t = t ), and the third operation mode is initiat¬ ed. During the interval t- j _, t κ there is obtained the equivalence diagram according to Fig 2b. At the time t 2 , both U and U become negative and the error voltage U p decreases towards a greater negative value, (cf the state after the time t ). As in Fig 5, the switches P, and S_ will now be energized and be driven in phase opposition, while the switches P and S.. are riot energized " OMPI (they are interrupted). The equivalence diagram according to Fig 2c is thus obtained. At the time t,, the voltage U will become less negative than U and the error voltage U p will be positive. This results in that U p increase rapidly from -U p to zero, after which the load voltage can be raised towards zero during the interval t j -t,. During this interval, the switches P and S * are energized and switched in- counterphase, pulse width modulation taking place along the portion of the modulation char acteristic according to Fig 5 which lies between 0 < U p < U p . The process in the converter power section will now -be described in de¬ tail with guidance from Figs 2a-2d, which -i llustrate simplified diagrams over the power section, where the input filter has been excluded for the sake of simplicity, and the output filter is included in the block L The battery voltage U„ is a direct voltage, the load voltage U. varies in time with the reference voltage and the currents I. and Io which are ramp- (Fig 4) or trapeziodally shaped pulses in time with the clock fre¬ quency " f .. The interval to-t„n (Fig 2a) During this interval, the switches P- and S.. are energized according to Fig 3. When P , is closed, a current I. flows into the winding at the marking, which magnetizes the transformer so that a given magnetic ener¬ gy is stored (see the diagram in Fig 4), where the duration of the triangular pulse up to the value I corresponds to the closed time of the switch P,. When the switch P, is opened, the switch S 1 closes and the energy stored in the transformer gives rise to a current I to the winding (see Fig 4). The current will flow to the output filter and give a charge to the capacitor in it. The sequence is repeated the next time the switch P, is conductive S is open) and when the switch S_. is conductive (P, is open). A positive current is obtained which succes- sively charges the output filter, there being a positive voltage across * the filter during the whole of.the time interval. -£!TRET, OMFI The inverval t--t 2 (Fig 2b) During this interval the switches P and S 2 are energized, while the switches P, and S.. are not energized. Magnetizing the transformer is done with the current I through the winding from the output filter when the switch S- is conductive (P is open), and discharge of the energy -stored in the transformer gives a current I. in the winding back again to the battery B when the switch P is closed S 2 is open). A negative current is obtained through the output filter, (the load) and a positive voltage. The power direction will be reversed in relation to the previous case. The interval 2 ~t 3 (Fig 2c) During this interval the switches P, and S-, are energized, while the switches P and S., are not energized. The current I. from the battery P i o B flows through the winding when the switch P, is closed S 2 is open and stores magnetic energy in the transformer, cf the state during the interval t -t-. When the switch P, is open and S 2 is closed, a current I will flow, although with a direction opposite to the one according to Fig 1a, depending on the orientation of the winding I.. During the interval t,.-t, there is thus obtained a negative voltage U and a neg¬ ative current I . o The interval t 3 ~t (Fig 2d) During this interval the switches P and S- are energized, while the switches P, and S 2 are not. The magnetization of the transformer is don with the current I through the winding I, from the load side when the switch S. is conductive (P is open) and discharge of the energy stored in the transformer is by the current I. to the battery B. when the switc P is closed (S- is open). A positive current and a negative voltage ar obtained over the load L and energy transfer thus takes place from load to battery side. The time diagram according to Fig 3 applies, as mentioned above, for a given loading case. In the idling case, the times t.. and t, are situate in the middle of the intervals t -t- and t-p-t,, respectively, and are displaced towards t 2 and t, for increasing reactive load. Furthermore, the case is shown where the switches P,, S 1 and P , S 2 are driven in phase opposition, resulting in a so-called fly-back converter being ob¬ tained. It is also possible to drive the switches P,, S. and P , S 2 in phase instead, the converter then forming a forward converter. This typ of converter has the same appearance in respect of- the power stage as i shown in Fig 1 with the exception that the orientation of the transforme windings - will be reversed. The converter output filter UF shall demodulate the pulse width- modulat ed output signal by attenuating tones at the switching frequency and higher. In the fly-back converter, the filter UF is suitably a capacitor connected over the output terminals or by a pi-link with low input im- pedance for the switching frequency. In the forward converter case (in- phase switching) the filter UF is suitably an LC-Link forming a low pass filter with low input impedance for the switching frequency. CM?I

Claims: WHAT WE CLAIM IS: 1 Pulse width modulated converter for. converting a direct voltage t a low-frequency and preferably sinusoidal alternating voltage, the am¬ plitude of. hich is not limited by said direct, voltage level, including a pair of input terminals for connection to the.direct voltage, a pair of output terminals for connection to a load L) and a transformer (T) with four windings ( ,-l 2 , I,, I,), a first and a second controllable switch (P, and P ) being connected. to the end points of the windings (. -.- * l ) on the battery side ("primary side") of the transformer, a third and a fourth controllable switch (S 1 , S 2 ) being connected to the end points of the windings on the transformer load side ("secondary side"), characterized in that a pulse width modulator (PM), in respons to an error signal (U p ) derived from the output voltage (U, ) of the converter and a reference voltage (U ,) corresponding to the desired alternating voltage, is arranged for controlling the switches ( P f , P , S-, S according .to the following: during a first time interval (t -tO the first (PO and the third (SO switches are controlled bx control pulses of constant frequency (f ) higher than the frequency of the out¬ put voltage to an alternatingly conductive and blocked state with a variable pulse rate in response to the magnitude of said error signal (U p ), during a second time interval (t^-t_) the second (P ) and the fourth (SO switches are controlled by said control pulses of constant frequency and variable pulse rate, during a third time interval (t 2 -t,) said first (P f ) and fourth (SO switches are controlled by said control pulses of constant frequency and with variable.pulse rate, and during a fourth time interval (t,-t,) said second (Pr) and third (SO1 switches J are controlled by said control pulses of constant frequency and variabl pulse rate. 2 Pulse width modulated converter according to Claic 1, characteriz in that during each of said time intervals (t,.-t-, etc) both switches (P,, S_. etc) are .controlled in phase opposition, i e the one conducts when the other is blocked, and vice versa. 3 Pulse width modulated converter according to Claic 1, characteriz in that during each of said time intervals (t,.-t 2 , etc), both switches ( P-er S- etc) are controlled in phase, i e both conduct or are blocked simultaneously.

Inventors: HAULIN TORD L
Applicants: ERICSSON TELEFON AB L M, HAULIN TORD L
Application year: 1981
Main classification: H04M19
IPC codes: H02M7, H04M19