PULSE WIDTH MODULATED POWER AMPLIFIER

Abstract: A pulse width modulated power amplifier has a power section containing a transformer (T) with four windings. The windings (l<u1>u, l<u2>u) on the battery side are-each connected by their end points to switch pairs (P<ufa>u, P<urb>u and P<ufb>u, P<ura>u) where each switch pair can conduct current in both directions. The windings (l<u3>u, l<u4>u) on the load side are adapted in the same way, with their end points connected to switch pairs (S<u1a>u, S<u2b>u and S<u1b>u, S<u2a>u), where each pair can conduct current in both directions. Input and output filters (IF and UF) are connected to the battery and load sides, respectively, The output voltage from the output filter (UF) is compared with the incoming signal (U<ur>u) which is to be amplified, and the error signal (U<uf>u) thus formed is caused to control a pulse length modulator (PM), which sends control signals to the switch pairs.

Description: PULSE WIDTH MODULATED POWER AMPLIFIER TECHNICAL FIELD The invention reLates to a puLse width moduLated. power ampLifier ac¬ cording to the preambLe to cLaim 1, for atnpLifying the current and/or voLtage LeveL of an input signaL. The ampLifier can ampLify input sig¬ nals having a frequency range from 0 Hz (direct voltage) up to a maximum frequency related to the clock frequency generated in, or appLied to the ampLifier. The proposed amplv ϊer can be used as a power amplif er for signals, preferably within the audio band and therebelow, and as a transmitting step in.an electronic line connection circuit in telephony. The amplifier will thereby provide the subscriber line with speech modu- Lated DC supply. It can also be used as a ringing signal generator for telephone exchanges. BACKGROUND ART Resistors have been previously used in supplying current to line circuit in telephony, these resistors being connected to a direct voltage source (usually -48 V), speech modulation being executed with.the aid of a transformer (the speech transformer), as wiLL.be seen from the US Patent 4,056,689, for example. Linear amplifiers are used alternatively for the same purpose, e g a so-called class S amplifier described in "Circuits and Systems" IEEE, Volume 7, No 10, December 1975. DISCLOSURE OF THE INVENTION In the current supply of a subscriber line via resistors, power is de- veloped in the form of heat in the current suppLy resistor. The speech transformer must take care of the DC magnetization from the current supply, apart from the low frequency speech band signals, which results in it becoming unnecessarily large and heavy. This transformer is eli¬ minated in Line circuits with linear end stages, but the high power Loss remains, which makes the components unable to.be fitted closer on the circuit board, even if their physical dimensions were to allow it. The introduction of a class D type ampLifier also results in that the prob¬ lem with the speech transformer is eliminated, but as with the above mentioned case, the output voltage is here Limited to the suppLy voltage, thus requiring a DC converter for ringing signal generation, and for current supply in some cases as well. The amplifier in accordance with the.present invention is a variant of the converter described in the Swedish Patent application 8008633-3. The present amplifier contains, as is described in said patent application, a transformer to which con¬ trollable or steerable switching devices are connected on both, battery and load sides. These switching devices are mutually connected and steered so that energy can be transferred from the voltage supply input ("battery side") -to its output ("load side") as well as from the load side to the battery side. Contrary to the switching devices in the con¬ verter according to the above-mentioned patent application, the switch¬ ing devices are two-way, resulting in that the transport of energy takes place in two directions during each clock pulse interval. The energy transfer takes place in different ways depending on the signal polarity, which results in four different operational modes. When the amplifier is in.operation, the switching devices are energized in pairs, one on the battery side and one on the load side. This gives rise to four different combinations, one for each operational mode. The ' energized switches are opened and closed at a fixed switching frequency controlled by a clock signal. The pulse-pause relationship for the ener¬ gized switches is controlled by a pulse width modulator, in turn con¬ trolled by an error voltage derived from the amplifier output voltage and a reference voltage. Energy is thus always transferred (at every instant) in the amplifier between the battery and load sides. Each clock period of the supplied clock signal is divided by the pulse width mo¬ dulation into two time intervals during which the energy transport takes place in each direction. The output and input f lters included in the amplifier have the task of smoothing the pulsing (mean value formation), so that the energy flow, i e the difference between energy fed to, and from battery and load does not vary appreciably in time with the clock frequency. In this mode the analogue signal which is desired to be am¬ plified is regenerated from the pulse width modulated intermediate form. The amplifier in accordance with the invention is characterized accord¬ ing to the characterizing portion of claim 1. -£TJR£ —2z___ \ . ~ V/IPO BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in detail while referring to the attached drawings, where Figure 1 is a block diagram of the basic features of a power amplifier in .accordance with the invention, Figure 2 is a time diagram for more closely explaining the function of the amplifier in Figure 1, and Figure 3 is a circuit diagram of an advantageous embodiment of the in¬ vention in accordance with the invention. MODES FOR CARRYING OUT THE INVENTION The basic features .of the power amplifier in accordance with the inven- tion are apparent from Figure 1. A battery B, with a suitable direct voltage is connected .to an input filter IF. The outputs of the filter IF are connected to both end points of a first winding L_. of a transfoi— mer T. The input filter IF has the task of .eliminating the disadvantages occurring when the supply voltage source of the amplifier does not have sufficiently low impedance at the switching "frequency. The f lter will smooth the pulse flows occurring in the amplifier so that other equip ¬ ment (connected to the battery B) is not disturbed, and so that the . supply voltage of the amplifier is not notably- affected by these pulse currents. The transformer T is of conventional implementation with a ferrite core, and contains two windings L_, and l_ on the battery side, and two windings I,, I, on the load side. The. winding l_. has one end point connected via a first pair of switches P, , P , to the filter out¬ put according to the above, and the winding 1-, has one end point con¬ nected via a second pair of. switches P_ j _ fa/ P _✓ and its second end point directly connected to the output of an input filter IF. In a similar way, the secondary winding half I, has one end point connected via a third pair of switches S, , S^, , and its second end point directly con¬ nected to the input of an output filter UF. The fourth winding half I, has .one end point connected via a fourth pair of switches S^, S^, and its second- end point directly connected to the input of the output filter UF. The switches are controllable or steerable from the outputs ,, , s. and s 2 by a pulse .width modulator PM, which will be OMPI described in detail below. Each of the switches P, , P , P , and S 1 , ^1b' ~2a'-^Zii πιay ^ e b " 'P 0 l- ar transistors, field effect transistors of the MOS -type or bipolar type, depending on the application in question. The winding halves are oriented as indicated by the dot markings in Figure 1, and the ratio of the number of turns ~ _» t ~'- can De selected in a suitable manner depending on the desired output voltage level of the load voltage U, obtained from the output filter UF. The block L symbolizes a load which, in line circuit application, constitute a subscriber Line with associated connected telephone sets. The output of the output-fiIter UF is connected, apart from being con¬ nected to the load, to th_e .input of the feed-back network N, which in its simplest form consists of a conductor. The output from the network N is connected to one input of a comparator JF, which can be an opera¬ tion amplifier. A reference oscillator RO is connected to the other in- put of the comparator JF. The former sends a reference signal or an in¬ put signal, which it is desired to amplify over the amplifier output, i e over the output of the filter UF. In the application under conside¬ ration/ the oscillator RO is omitted and the signal U is a low frequenc speech signal. which is to be amplified in the amplifier. In the comparison of the input voltage U with the output voltage U from the network N there is obtained an error voltage U, at the output of the comparator .JF. This output is connected to the pulse width mo¬ dulator.. PM, to give control signals at eight outputs p,3, pr3, pT.,D, pΓ,D, s 1a' s 1 b ' an( ~ s 2a' s 2 b ' "t ^ lese signals being such that the switch pairs CPra_., Pf,b, ), (Prb, , Pf,a), CS_l,a, S-2,,b) and (S-2 . a, S_l.b_) are energized, as will be seen from the time diagram according to Figure 2. The magnitude of the error voltage U, will determine in the pulse widt modulator PM the pulse rate of the signals consisting of square waves on the outputs p T,3, p , p,, , p . and s.. , S-, , s,., s 2b » The utilization of- the control signals, at the outputs p T-3, pT,,o may possibly not be needed, as shown by the embodiment according to Figure 3. The pulse rate of the 'control sig¬ nals can vary from 0-100%, but their frequency is kept constant and de¬ termined by the frequency f of a clock signal. In the embodiment of the amplifier according to what is set forth below. there have been formed two like switching steps (a and b) by the double up switches.in each .direction, which magnetize the transformer T in eac direction for better utilizing the core material (so-called push-pull connection). By connecting the .a and b steps' switching devices P _ and ra P.. etc, counter-directed, the same result can be achieved as if the transformer contained eight windings instead of the four "illustrated in Figure 1. Since transformer effect i-s utilized in the transformer T, a low-imped¬ ance circuit shall be connected to the battery side at the switching frequency f , and a high - impedance circuit on.the load side. This has been accomplished in. the .amplifier of Figure 1 by a capacitive input on the input filter IF and an inductive input on the output filter UF. Steering the amplifier and its operational modes will now be more closel described with. reference to the time diagram according to Figure 2. At the time t the switch P, is energized and is conductive during an O T3 interval t -t'. During the next part t'-t«. of the half clock pulse pe¬ riod t -t.,, the switch P, is blocked, and the switch.P is conductive. o 1 fa ra During the same half of the clock pulse period t O -t Λ j the switch S.1 on the load side is conductive while the switch S-, is blocked. Remaining switches on both battery and load sides are blocked during the period t -t 1 . During the two phases 1 and 2 (the period t ~t_.) the first switch ing step (the a step) of the ampLifier is energized. During the second half t_.-t ? of the period the switch p,, is energized and is conductive for the time t_--t' and the switch P . conductive for the. time t'-t^- Furthermore, the switch S→, is energized for conductivity during the whole period t_.-t-,. Remaining switches are thereby blocked. As long as the .current I to the output filter UF is positive ( as in Figure 1) the following takes place during each clock period: Phase Conductive switch Energy transfer direction U I o o 1 P fa S 1a Input fi lter- 0utput . f i Iter pos iti ve posit ive 2 P Input f i lter< -0utput fi lter negative ..posit i e ra S 1a 3 P fb S 1b Input f i lter-i»0utput filter posi ti ve posit i ve 4 P rb S 1b Input f i lter*- 0utput fi lter negat ive posi ti ve During two phases 1 and 2 of the clock pulse period, the "a step" is energized and the output filter charged during phase 1 and discharged during phase 2, as. will be seen from the shape of. the curve for the current I , where the ripple has been exaggerated for the sake of cla- rity. The pulse width modulation controls the relationship between the duration of phase 1 and phase 2, and thereby the net energy transport between input, and output filters IF and UF. During the next clock pulse period (phases 3 and 4) the "b step" executes the same func- ion with the difference-that the transformer T is magnetized in the opposite di- rection to maintain flux balance. When the current I to the output filter UF is negative, the sequence during a clock pulse period will be as foLlows: Phase Conductive switch Energy transfer. direction Uo io 11 P rb S 2b Input filters-Output filter postive negative 12 P fb S 2b Input fi lter-_s-Output filter negative negative 13 P ra S 2a Input filters-Output filter positive negative 14 P fa S 2a Input fiIter-^Output filter negative negative During one period of the current I to the output filter UF, the a and b switching steps are energized during half a clock -pulse period each, and are energized alternatingly in time with the double clock pulse frequency since t -t_. = t_,-t, = ... = 1/2f , pulse width modulation tak¬ ing place within these time intervals. During phases 1 and 3 the positiv current I increases, while it decreases during the phases 2 and 4. If the pulse .width modulation is such that the time -for.the phases 2 and 4 increases more than for the phases- 1 and 3, the current I will eventual ly change sign. This will then happen during phase 2 or.4. The pulse width modulator PM will give a steering pulse to the-switches P, and S-,~ (shortly before the time to' .in Figure 2) when Io changes sign from + to -, whereby the current I is taken from one input of the output filter through the closed switch _- , the transformer winding I, and . , back again to the output filter UF. During the first half of the clock pulse period t '-t, 1 the switches P ,, P,. and S-,, will be energized, and during the second half t. j '-t-,* the switches P , ? , and S-, will OMPI be energized. Synchronous operati on and continuous current i s thus en¬ sured if phase 2. (or 4) i s interrupted, when the current I becomes = 0, o whereon phase 12 (or phase 14 as in Figure 2) immediately takes over and continues until the time at which phase 2 (or 4). should have ended. By allowing the pulse width modulator PM, analogous with the above, to control the switch P conductive to P, conductive from phase 11 to phase 1, and the switch P conductive to the switch P,, conductive from ra TO phase 13 to phase 3, a positive current I is-obtained after the zero pass aftec.it has previously been negative (not shown in Figure 2). An advantageous- embodiment of the power step of-the amplifier according to Figure 1 is illustrated in Figure 3, where the feed-back loop has been excluded for the sake of simplicity, (the same as in. Figure 1). The capacitor C forms the input filter IF, while the choke L and capacitor C form the output filter UF. Both switches P , P , have been replaced ra' rb by the diodes D and D. . The switches P__ and P__, a re transi stors T a b fa TO a and T. , which are thus controlled from the output P and P . of the pulse width modulator PM. The control pulses from. oth these outputs may be seen from Figure 2. On the load side,.both switches S.. , S-,, are formed by the diode bridge D 11 ~D 1 , and the transistor l→ , and both switches S ? , S... are formed by the diode bridge D ? _.-D_, and the tran¬ sistor T 2 - The transistor T, is controlled by pulses .frors the output of an OR circuit EK1, both inputs of which - re connected to the control pulse outputs s, and s-,, of the pulse width modulator PM. In a similar manner, the transistor T2 is controlled by the pulse from the OR cir- cuit EK2, both inputs of which are connected to the pulse width modul¬ ator PM control pulse outputs s and s → , . 2a < D The time diagram of Figure 2 is thus appLicable to the amplifier of Figure 3 even if the outputs p . and p,, are not uti lized. The diodes D and D, , corresponding to the switches P and P , , conduct current a b' ra ΓD and are blocked during the same interval as the switches P and P __. ra ΓD During the first. part of the half clock pulse period ~t_. according to " Figure 2 (phase 1), the transistor T 3 is conductive (PT,o is conductive ) as we l l as the t ransi stor T.. . Current then f lows f rom the battery minus pole. By the transformer action, the magnetized transformer drives a current through the winding L,, diode D_,_., transsistor T- and the diode D-y to the choke L to charge it. During the remaining part of the in¬ terval to -t i→ (phase 2) the transistor Ta is blocked.(Tb, is blocked), current from the battery therefore flowing from its minus pole through the diode D 3 into the winding l→I and back again to its plus pole. This gives a discharge current from the choke L to the diodes D^, transis¬ tor T.., diode D_._ and through the winding I,. During phase 3, the tran¬ sistor T,b becomes conductive and a current flows from the battery plus pole through the winding l_. and transistor T, to the battery minus pole. The transformer T→ is magnetized and gives a current through the wind¬ ing I,, diode b-,4. , transistor 1- (which is now conductive) diode D, and through the choke L, which is charged. The same thing happens in prin¬ ciple during phase 4 as during phase 2, i e since both transistors T and T. are blocked and since the transistor T 2 is conductive, the choke will be discharged with a current I through the diodes D--,, D-, an the winding I,, a current on the battery side flowing through the diode D, and winding I- from the battery minus pole to. its plus pole. To simplify the construction of the pulse iπdth modulator PM, control or steering pulses can also be sent to advantage during the time inter¬ vals t_,-t__., t,-t, etc to the switch P__ , i e the transistor T in the 1 3 4 fa a embodiment according to Figure 3, although this switch does not then conduct any current. This gives the advantage that the current I change sign, which takes place when the choke L is discharged during phase 2 or 4, when the transistor T (P- ) is normally blocked, but is brought into a conductive state when I = 0. By control pulses being sent to it durin phase 4 already, the conductive condition in phase 14 can be prepared, i e before I becomes = 0. In a simpler mode, the transistor T, can be prepared for its conductive state in phase 3 by control pulses being sent to the transistor T. during phase 13 (when I < 0). The con¬ trol pulses for both these cases have been shown in Figure 2 by dashed Lines.

Claims: WHAT WE CLAIM IS: 1 A pulse width modulated power amplifier for converting, in re¬ sponse to a controlling input signal, a direct voltage to a low-fre¬ quency output signal from zero Hz to a given upper limiting frequency and with a voltage amplitude which is not limited to said direct voltage level, the amplifier including a pair of input terminals for connection of the direct voltage (U_), a pair of output terminals for connection to a load (L) and a transformer (T) with four windings (l.., I-,, I,, I,)., a first and a second controllable switch (P. and P ) connected to the Ta ra end points of the windings (L, I-,) on the battery side of the trans- former, a third and a fourth controllable switch (S_, , -→_ ~) connected ' 1a' 2a to the end points of- the windings (I,, l.) of the transformer load side to form a first converter step (a), and a pulse width modulator (PM) adapted for steering the switches (P, , P , S_. ,. S_- ) in response to 3 T3 an error signal (U,) derived from the converter output voltage (U.) and the controlling input signal (U ) corresponding to the desired al¬ ternating voltage, so that during a first tiaie interval (t ~t_.) the first and the second switches (Pf,a and Pra) are steered to alternating conductive and blocked states with a given.pulse rate in response to the magnitude of. said error signal, and the third and fourth switches (S Λ and S-, ) are steered to conductive and blocked states during said 1a 2a time interval (t ~t_.), characterized in that in parallel with said first and said second switches (Pt,a, Pra) ther are connected a third and a fourth switch (P,, and P , ) on the battery side, and with reverse con¬ ducting direction, and that in parallel with said first and said second switches (S_. , S- ) on the load side, and with reverse conducting di¬ rection, there are connected a third and a fourth switch S→, , S-,t) to form a second switching stage (b),_the pulse width modulator (PM) be¬ ing adapted for steering, in response to said error signal (U,) during a second time interval (t_.-t-,) subsequent on said first time interval (t -t„), the third and fourth switches (P.,. and P_, ) on the battery O l ID άa side to alternating conductive and blocked conditions with a given pulse rate in response to the size of said error signal (U^), and also for steering the third and fourth switches (S,, , S ? , ) on the load side to conductive and blocked conditions during said second time interval Ct_,-t 2 ). 2 Amplifier according to claim 1, characterized in that a clock signal of given frequency (f ) is arranged for applying to the pulse width modulator (PM), said first and said second time interval (t -t ■ .1 and t^-t^ and the subsequent time intervals (t^-t, etc) each constitut- ing one-half of a clock pulse interval. 3 Amplifier as claimed in claims 1-2, characterized in that said first switch (P T,3) on the battery side is adapted for being steered to a conductive state even during at least a part of said subsequent second time interval (t_.-t_,), when said output signal .(U.) is positive. 4 Amplifier as claimed in claims 1-2, characterized in that said third switch (Pt,b,) on the load side is.adapted for being steered to a conductive state during at least a part of said subsequent second time interval (t^'-t-,'), when said output signal is negative. 5 Amplifier as claimed in claim 1, characterized in that the first and third switches (P, and P,,) on the battery side each comprise a diode (Da and D,)_. that the second and fourth switches (Pra and Pf,b,) on the battery side each comprise a transistor (T and T, ), whereby the control pulses to said first and third switches on the battery side can¬ not be utilized, ' and in that the first and the -third (S_1,a and S-2,.b) as well as the second and the fourth switches (S_ 2a, S_1 . b_) on the load side each comprise a diode -bridge (D_._.-D , and D ? _.-D_,), ea ' ch of which has a transistor (T_. and T-,) -connected to a -diagonal thereof, whereby the control electrodes of ' the transistors are-each connected to the output of an OR circuit (EK1 and EK2) which forms an OR state " between the con¬ trol pulses to said respective first and third as -well as second and fourth switches on the load side.

Inventors: HAULIN TORD LENNART
Applicants: ERICSSON TELEFON AB L M, HAULIN TORD LENNART
Application year: 1981
Main classification: H02M7
IPC codes: H03F3