Abstract:
An apparatus and a method for converting power from a power input to an DC output voltage or current, which apparatus has a serial resonance converter, where a first feedback circuit is connected from the output terminal to an error amplifier, where the apparatus further has a second feedback circuit with at least one first resistor that is connected to a coil and to ground, which second feed back circuit connects the line between the first resistor and the coil and towards an inverting integrator, the output of which is connected through a second capacitor to a second input at a control circuit. As a result, the oscillating frequency is under influence of a signal that depends on the voltage generated in the resistor connected in serial to the coil or transformer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of co-pending application Ser. No. 10/595,706, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for converting power from a power input to an DC output voltage or current, which apparatus comprises a serial resonance converter containing at least two serial coupled semiconductor switches having a common output terminal connected to at least one first coil which coil can be a part of a transformer having a second winding connected to a rectifier means, which rectifier means has its output connected to output terminals, where a first feedback circuit is connected from the output terminal to an error amplifier, which error amplifier is connected to an input at a control circuit, which output is connected over driver means to the input of the semiconductor switches, where the apparatus further comprises a second feedback circuit where the second feedback circuit is leading a signal from the first coil to an input terminal. 
     2. Description of Related Art 
     U.S. Patent Application Publication 2003/043599 describes a DC-DC converter, a regulation method for a DC-DC converter and a switched-mode power supply are proposed. The DC-DC converter comprises an inverter and a primary-side circuit with a transformer whose secondary-side voltage is rectified by at least one rectifier for generating an output DC voltage. To avoid an asymmetrical load, which is in particular exhibited by a different load of the rectifier elements (power semiconductors), an electrical magnitude of the DC-DC converter is measured. This magnitude may, for example, be a primary-side current, a primary-side voltage at a capacitance, or a secondary-side, rectified voltage. From the measurement of the magnitude, a parameter for the symmetry deviation is calculated for which different symmetry measuring methods are proposed. A symmetry regulation arrangement utilizes the drive of the inverter, for example, the duty cycle of the pulse width modulated voltage produced by the inverter to minimize the parameter for the symmetry deviation. This achieves an even distribution of the power over the secondary rectifier elements. 
     The above mentioned document describes regulation of the switching frequency to keep it clearly above the resonant frequency of the resonant arrangement. The circuit is operating in frequency mode, which leads to a lack of linearity in the relationship between output voltage and output power. The focus in the above mentioned patent application is to avoid an asymmetrical load of the rectifying components, whereas the actual invention is to optimize the output stability by linearizing the feedback. 
     U.S. Pat. No. 4,935,857 describes a DC to DC series-parallel resonant converter ( 10 ) having a plurality of switches (Q 1 -Q 4 ) which are switched alternatively between on and off states to cause electrical current to flow alternatively in first and second directions through a series-resonant circuit ( 60 ) including a variable frequency ramp generator ( 28 ) having a reset input (R) for causing an output ramp signal produced at an output to drop to zero in response to each reset signal; a comparator ( 30 ) having an input coupled to the output of the ramp signal generator, a second input for controlling the output DC voltage of the series-parallel resonant circuit and an output which changes level each time the ramp signal reaches the magnitude of the second input; a bistable circuit ( 32 ) having first and second outputs (Q, Q) for respectively outputting first and second signals, the output signals changing in response to a change in the output signal of the comparator coupled to the input; a pulse generator ( 26 ), coupled to the series-parallel resonant circuit for producing an output pulse train with an output pulse occurring each time the flow of current through the series-resonant circuit changes from one of the first and second directions to another of the first and second directions, the output pulses being applied to the reset input of the variable frequency ramp generator to regulate the frequency of the output ramp signal. 
     The focus in the above mentioned patent is to assure that the switching frequency of the converter is held above the resonance frequency of the serial/parallel converter. 
     In an apparatus as described in the opening paragraph, it is known to use an integrated circuit L6598 or the like. This integrated circuit comprises a current controlled oscillator which output is connected over driver means to two inverse output terminals, which are directly connectable to the input of semiconductor switches. The oscillator part in L6598 is also connected to the outside through a connecting terminal where this terminal is connected to an external capacitor that together with two internal current controlled current generators set the frequency. An input signal at the integrated circuit is so connected that changes in current through this terminal lead to control of the frequency. 
     It is achieved that the voltage over the connected capacitor changes in a linear way between two voltage situations. Each time the charge of the capacitor changes its sign in charge current, the oscillator changes its output from a first to a second value, which over the drivers activates and/or deactivates the semiconductor switches. A feedback from the power output is used to control the size of the current used to charge or discharge the capacitor, and thereby, to a change in the frequency of an oscillating system form by extern components. In normal operation, the oscillating frequency oscillates over the resonance frequency of the resonant DC-DC converter, and the first feedback signal leads to a frequency change to a lower frequency nearer the resonance frequency if a higher load is needed. For normal series resonant converter function working above resonant with frequency control, see  FIG. 5   a.    
     When using resonant converters close to resonance frequency, the power gain in the DC-DC converter is highly unlinear with gives big problems in design of the first feedback loop. 
     SUMMARY OF THE INVENTION 
     A primary object of the invention is to improve and stabilize an output voltage or current having a fast response to a change in load by linearization of the power conversion control in the resonance DC-DC converter. 
     This can be achieved with an apparatus or method if modified so that the second feedback circuit is connected to the input terminal of the control circuit, which input terminal is connected to at least one capacitor, which capacitor is controlling the switching frequency, which second feedback circuit comprises at least one first resistor, which first resistor is connected to the second coil and to ground, which second feed back circuit connects the line between the first resistor and the second coil and further towards an inverting integrator, from which inverting integrator output is connected through a second capacitor to a second input at the control circuit. 
     In this way, it can be achieved that the oscillating frequency is under the influence of the signal that depends on the voltage at the resistor connected in serial to the coil or transformer. The voltage at the resistor that is connected to the first coil or transformer depends on the current flowing through the output of the power supply. This means that, at high load, a very powerful signal will be transmitted directly to the input at the oscillator pin in the control circuit. This will change the operation of the circuit into a charge mode operation. As the load on the output is reduced, the influence of the second feedback signal will be reduced, and the influence of the charge mode is reduced and the operation mode changes back into a normal frequency mode of operation. At the start-up of the power supply, there will be no signal at the second feedback circuit, and the whole start-up will take place in normal frequency mode. 
     With charge mode control, the second feedback loop measures and controls how much charge that is flowing through the resistor in each half period of switching. This charge is much more linear dependant to power than frequency. The fact that the change in charge instead of current and frequency is controlled is an important feature. If charge mode control by adding a second loop, is used, there are still some problems at low load. By using a combination of frequency control and charge mode control, this can be solved. 
     The output from the inverting integrator is connected to a serial connection to the second capacitor (Cf), which serial connection comprises a second resistor (Rf) serial coupled to a further third capacitor, which output from the inverting integrator is further connected to a fourth capacitor, which fourth capacitor is connected to ground. 
     The degree of influence can be adjusted by changing the size of the two capacitors, hereby, it can be achieved that the change of operation mode starts its influence on demand, which is defined from the size of the capacitors. The output of the one or two capacitors can be connected to the input terminal of the oscillator part of the control circuit through at least one capacitor and resistor. Hereby, it can be achieved that the signal of the second feedback circuit is reduced to a value that can be used effectively to influence the charging and discharging of the capacitor connected to the oscillator part in the control circuit. This can be important in the design of new power supplies in that a very simple change of a component at a printed circuit board leads to a major change in function of the power supply. 
     Together with an L6598 or similar circuits, the second feedback needs an extra circuit. It contains an inverting amplifier, which output can be connected to the input terminal of the oscillator part through at least one capacitor and one resistor. Hereby, it can be achieved that the signal of the second feedback is inverted and amplified to a value that can be used effectively to influence the charging and discharging of the capacitor connected to the oscillator pin on the control circuit. 
     The output of the inverting amplifier can be connected to a serial connection of a resistor and a further capacitor, which serial connection is coupled in parallel to the capacitor. This can influence the characteristics of the signal that is created as a mix of the output from the inverter and amplifier and from the constant current generators placed inside the integrated circuit. These components generate the automatic change between normal frequency-mode and charge-mode. 
     The invention can also be described as a method for power conversion control in serial resonance switch mode power converters operating in frequency mode at normal operation where a first feedback signal, from the output, is converted to an input to switching means where a second feed back signal is used to influence the charging and discharging of at least one capacitor connected to the oscillating circuit, where by increasing load, the mode of operation is changed into a charge mode control by a second feedback signal, which second feedback signal is based on the actual charging current and thereby change in charge in each half period of switching on the serial resonant resistor. 
     In this way, it is achieved that the start-up of the power converter takes place as usual in frequency mode, and where light load operation also takes place in this mode. However, if the load increases, an automatic change in the direction of operation in charge mode takes place where a voltage change on the serial resonant capacitor(s), depending on the actual current demand of the output, is used as the feedback signal to the control circuit. Full-time operation in charge mode could be critical because power supplies might have problems with starting in charge mode as no feedback signal occurs in the start-up situation and might have problems with stability in light load. This problem is completely solved by letting the start and light load take place in frequency mode, and only use charge mode operation if the output current increases. Under normal operation, a combination of frequency mode and charge mode is possible where direct charge mode operation only takes place at high load. 
     Normal frequency control gives a strong nonlinear conversion, which is known from the state of the art. Charge mode has a better linearity, but at low load, it still has a quite nonlinear conversion. Combined frequency control and charge mode, which is described in this patent application, is highly linear at any load. 
     In the following, the invention is described in detail with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first embodiment of the invention. 
         FIG. 2  shows a alternative embodiment of the invention, and 
         FIG. 3  shows a part of the circuit show at the  FIG. 2   
         FIG. 4  shows same part of the circuit as the  FIG. 3 , but the integrator and inverter are part of an integrated circuit. 
         FIG. 5  shows same part of the circuit as the  FIG. 4 , but the inverter is part of an integrated circuit. 
         FIG. 6  shows same part of the circuit as the  FIG. 5 , but the integrator is part of an integrated circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  describes a switch mode power supply  2  having a power input terminal  4  primarily for DC-power, and output terminals  6 ,  8  between which the power supply can deliver DC-power. Semiconductor switches  10 ,  12  are connected so that, if switch  10  is open, switch  12  is closed. Hereby, the voltage at the connection point  14  between the two semiconductor switches  10 ,  12  changes from zero and up to the input DC-voltage. The point  14  is connected to a first coil  15  from where current is flowing to a coil  16 , which is part of a transformer  18 . The coil  15  can be an integrated part of the transformer  18 . The coil  16  is further connected to a capacitor  19 . The transformer  18  also contains a coil  20 , which is connected to rectifier means, which can be formed as a bridge rectifier  21  having an input terminal  22 . 
     A rectified DC power is delivered at the output  24  towards the output terminals  6 ,  8 , between which a capacitor C-out and a resistor R-load are shown. A feedback signal  26  is connected to the output terminal  6 . The feedback signal  26  is sent to an error amplifier  28 . The now converted feedback signal  29  is led forwards to electrical isolation means  30  which, in practice, is in the form of an optocoupler. This optocoupler is connected to a pin  4  of the integrated circuit L6598. Inside the integrated circuit  4  is an internal power supply comprising a voltage reference connected to the pin  4 . Outside the integrated circuit is the optocoupler connected to the pin  4  through a serial resistor. Also connected to the pin  4  is a resistor that is connected to the ground connection. In this way, all currents between two levels can be generated to flow from the pin  4  depending on the collector voltage on the transistor in the optocoupler  30  so that the feedback signal level defines the current. 
     A current change in the pin  4  leads to a change in size of the current in the constant current generators  42 ,  44 . This leads to a change of the charging and the de-charging speed of the capacitor  46 . As a result, the oscillating frequency is over the switching means  10 ,  12  and the coil  15 . The coil  16  at the transformer and the capacitors  13 ,  19  is changed according to the load. A switching means  41  defines which of the constant current generators  42 ,  44  that are to be active. Both cannot be active at the same time. The common output from the two constant current generators  42 ,  44  is led through a pin  3  at the integrated circuit. The second feedback circuit  50  contains an inverter and amplifier circuit  60 , which is necessary if L6598 have to be used. The output of this inverter and amplifier  60  is connected through a capacitor  62  and a resistor  64  to the pin  3  of L6598, which is connected to an oscillator part of the control circuit  34 . In parallel to the capacitor  62 , a resistor  64  and a capacitor  66  are connected in serial. Furthermore, from the common point of the capacitor  66 , the capacitor  62  and the output from the inverter and amplifier  60  are connected to a capacitor  68 , which is connected to the ground connection. Charging a capacitor  62  leads to a increase or decrease in voltage over the capacitor  62 . In this way, an oscillating voltage is generated at the pin  3  of the integrated circuit. This oscillating signal with a three-angle voltage is led to the input of two comparators and a flip-flop over a line  32 . The output flip-flop  36  switches its output depending on the input of the terminal  32  and on a reference voltage. The output of the flip-flop  36  is connected to driving means  38 ,  40 . Output terminals at the integrated circuit are pin  11  and pin  15 . Pin  11  has the number  45  and pin  15  the number  43 . These are connected to the input of the semiconductor switching means  10 ,  12 . 
       FIG. 2  describes a switch mode power supply  102  having a power input terminal  104  primarily for DC-power, and output terminals  106 ,  108  between which the power supply can deliver DC-power. Semiconductor switches  110 ,  112  are connected so that, if switch  110  is open, switch  112  is closed. Hereby, the voltage at the connection point  114  between the two semiconductor switches  110 ,  112  changes from zero and up to the input DC-voltage  104 . The mid point  114  between the semiconductor switches is connected to a first coil  115  from where current is flowing through a capacitor  117  to a coil  116 , which is part of a transformer  118 . The coil  115  can be an integrated part of the transformer  18 . The coil  116  is further connected to a resistor  119 . The transformer  118  also contains a coil  120 , which is connected to rectifier means, which can be formed as a bridge rectifier  121  having an input terminal  122 . 
     A rectified DC power is delivered at the output  124  towards the output terminals  106 ,  108 , between which a capacitor C-out and a resistor R-load are shown. A feedback signal  126  is connected to the output terminal  106 . The feedback signal  126  is sent to an error amplifier  128 . The now converted feedback signal  129  is led forwards to electrical isolation means  130  which, in practice, is in the form of an optocoupler. This optocoupler is connected to a pin  4  of the L6598 integrated circuit. Inside the integrated circuit  103  is an internal power supply comprising a voltage reference connected to the pin  4 . Outside the integrated circuit is the optocoupler  130  connected to the pin  4  through a serial resistor. Also connected to the pin  4  is a resistor that is connected to the ground connection. In this way, all currents between two levels can be generated to flow from the pin  4  depending on the collector voltage on the transistor in the optocoupler  130  so that, the feedback signal level defines the current. 
     A current change in the pin  4  leads to a change in size of the current in the constant current generators  142 ,  144 . This leads to a change of the charging and the de-charging speed of the capacitor  146 . Hereby, the oscillating frequency is over the switching means  110 ,  112  and the coil  115 . The coil  116  at the transformer and the resistor  119  is changed according to the load. A switching means  141  defines which of the constant current generators  142 ,  144  is to be active. Both cannot be active at the same time. The common output from the two constant current generators  142 ,  144  is led through a pin  3  at the integrated circuit  103 . 
     The second feedback circuit  150  contains an inverter and integrating circuit  160 , which is necessary if an L6598 has to be used. The output  161  of this inverter and integrating circuit  160  is connected through a capacitor  162  to the pin  3  of the L6598, which is connected to an oscillator part of the control circuit  134 . In parallel to the capacitor  162 , a resistor  164  and a capacitor  166  are connected in serial. Furthermore, from the common point of the capacitor  166 , the capacitor  162  and the output  161  from the inverter and amplifier  160  are connected to a capacitor  168 , which is connected to the ground connection. 
     Charging or discharging a capacitor  162  leads to an increase or decrease in voltage over the capacitor  162 . In this way, an oscillating voltage is generated at the pin  3  of the integrated circuit  103 . This oscillating signal is led over a line  132  to the input of two comparators  134  and a flip-flop  135 . The output flip-flop  136  switches its output depending on the input of the terminal  132  and on a reference voltage. The output of the flip-flop  136  is connected to driving means  138 ,  140 . Output terminals at the integrated circuit are pin  11  and pin  15 . Pin  11  has the number  145  and pin  15  the number  143 . These are connected to the input of the semiconductor switching means  110 ,  112 . 
     In operation, the inverter and integrator circuit  160  is able to change the shape of the signal  150  into a signal shape that can be added much better to the signal generated at the capacitor  162  and resistor  164  which are connected to the pin  3  of the integrated circuit  103 . 
     An adjustment of the signal is possible by the parallel coupling of the resistor  164  and the capacitor  162 , and the further capacitor  168  is connected to the ground connection in changing the size of the components. The  164  and the capacitor  166  form a high pass filter, which has an impedance close to resistor  164  in the whole operating area of the converter. The capacitor  166  is only a DC separation capacitor. In this way, the impedance of capacitors  162 ,  164 ,  166  is close to resistor  164  at low frequency operation and close to capacitor  162  at high frequency operation. This gives charge mode control at high load and frequency mode at low load and a soft change between the two modes. 
       FIG. 3  shows a part of the circuit show at the  FIG. 2 . A resistor  219  is connected by line  218  to the coil  116  shown at the  FIG. 2 . The current passing the resistor  219  is changed according to the load. The second feedback circuit  250  contains an inverter and integrating circuit  260 , which is necessary if L6598 have to be used. The output  261  of this inverter and integrating circuit  260  is connected through a capacitor  262  to the pin  3  of L6598, which is connected to an oscillator part of the control circuit ( 134   FIG. 2 ). In parallel to the capacitor  262 , a resistor  264  and a capacitor  266  are connected in serial. Furthermore, from the common point of the capacitor  266 , the capacitor  262  and the output  261  from the inverter and amplifier  260  are connected to a capacitor  268 , which is connected to the ground connection. Charging or discharging a capacitor  262  leads to an increase or decrease in voltage over the capacitor  262 . 
       FIG. 4  shows the same part of the circuit as shown in  FIG. 3 . A resistor  319  is connected by line  318  to the coil  116  shown at the  FIG. 2 . The current passing the resistor  319  is changed according to the load. The second feedback circuit  350  contains an inverter and integrating circuit  360 , which inverter and integrating circuit is now placed inside an integrated circuit. The output  361  of this inverter and integrating circuit  360  is connected through a capacitor  362  to the pin  3  of the L6598, which is connected to an oscillator part of the control circuit ( 134 ,  FIG. 2 ). In parallel to the capacitor  362 , a resistor  364  and a capacitor  366  are connected in serial. Furthermore, from the common point of the capacitor  366 , the capacitor  362  and the output  361  from the inverter and amplifier  360  are connected to a capacitor  368 , which is connected to the ground connection. Charging or discharging a capacitor  363  leads to an increase or decrease in voltage over the capacitor  363 . 
       FIG. 5  shows the same part of the circuit as show in the  FIG. 4 . A resistor  419  is connected by line  418  to the coil  116  shown at the  FIG. 2 . The current passing the resistor  419  is changed according to the load. The second feedback circuit  450  contains an inverter and integrating circuit  460 , which inverter  463  is now placed inside an integrated circuit and the integrator  463  is placed outside the integrated circuit. The output  465  of this inverter and integrating circuit  460  is connected through a capacitor  462  to the pin  3 , which is connected to an oscillator part of the control circuit ( 134   FIG. 2 ). In parallel to the capacitor  462 , a resistor  464  and a capacitor  466  are connected in serial. Furthermore, from the common point of the capacitor  466 , the capacitor  462  and the output  461  from the inverter and amplifier  460  are connected to a capacitor  468 , which is connected to the ground connection. Charging or discharging a capacitor  463  leads to an increase or decrease in voltage over the capacitor  464 . 
     The  FIG. 6  shows the same part of the circuit as show in the  FIG. 5 . A resistor  519  is connected by line  518  to the coil  116  shown in  FIG. 2 . The current passing the resistor  519  is changed according to the load. The second feedback circuit  550  contains an inverter and integrating circuit  560 , which integrator  563  is now placed inside an integrated circuit and the inverter  563  is placed outside the integrated circuit. The output  565  of this inverter  563  and integrating circuit  560  is connected through a capacitor  562  to the pin  3 , which is connected to an oscillator part of the control circuit ( 134   FIG. 2 ). In parallel to the capacitor  562 , a resistor  564  and a capacitor  566  are connected in serial. Furthermore, from the common point of the capacitor  566 , the capacitor  562  and the output  561  from the inverter and amplifier  560  are connected to a capacitor  568 , which is connected to the ground connection. Charging or discharging a capacitor  563  leads to an increase or decrease in voltage over the capacitor  564 .