Abstract:
A flyback-type switched current regulator, comprising first and second coupled windings and a circuit for connecting the first winding to a voltage source as soon as the windings are demagnetized. The regulator further comprises a current sensor that senses the current flowing through the first winding; a switch that disconnects the first winding from the voltage source when the first current in the first winding reaches a reference value, whereby the first winding is switched according to a duty cycle; and a circuit that varies the reference value in accordance with the duty cycle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to switched regulators providing a constant current to a load, for example a battery, with a high efficiency. The present invention more particularly relates to a switched current regulator whose charge should be isolated from the supply source of the regulator, frequently the 220-volt mains. 
     2. Discussion of the Related Art 
     FIG. 1 represents a conventional switched regulator. In this example, the conventional switched regulator is accumulation regulator or &#34;flyback&#34; regulator. Two windings 11 and 12 coupled through a magnetic core 13, but galvanically isolated, are provided for isolating the load R L  from the regulator. The first winding 11 is connected to the voltage source V 1  through a power switch S. Although the first winding 11 and second winding 12 do not constitute a transformer (currents do not simultaneously flow through the two windings), the terminology &#34;first&#34; and &#34;second&#34; is used for the sake of simplification of the description. The second winding 12 is connected to load R L  through a diode D. A filtering capacitor C is connected in parallel to load R L . The diode D is connected so that no current flows through the load while the first winding 11 is connected to voltage V 1 . 
     When switch S is switched on, the first current I 1  in the first winding 11 increases from an initial value, as a function of the values of voltage V 1  and of the inductance provided by the first winding 11. During this time, the second current I 2  in the second winding 12 is zero and the power is stored in the core 13. 
     When switch S is switched off, the first current I 1  is abruptly switched off and the power that was just stored in core 13 is transferred into the second winding 12. The second current I 2  abruptly reaches a peak value equal to the peak current reached by the first current I 1  multiplied by the ratio between the number of turns of the first winding 11 and the second winding 12, when the switch S is switched off. The second current I 2  starts to decrease as a function of the inductance of the second winding 12 and of the voltage across load R L . 
     Switch S is again switched on when the second current I 2  has reached a generally undetermined final value. This final value, that may have been zero since a determined time, determines the initial value from which the first current I 1  starts to increase. 
     The amount of power transferred from the first winding 11 to the second winding 12 depends upon the switching duty cycle of switch S, i.e., the ratio between its on time and the switching duration. 
     To regulate the mean current in the load, a resistor Rs is serially connected to the second winding 12. The voltage across resistor Rs is provided to a regulation circuit 15 which controls switch S. This regulation circuit filters the voltage across resistor Rs to extract therefrom the mean value which is compared with a reference value. The duty cycle of switch S is adjusted so that the mean voltage across resistor Rs (therefore the mean second current) approaches the reference value. 
     A drawback of switched regulators with galvanic isolation between the first winding 11 and the second winding 12, such as the one represented in FIG. 1, lies in the fact that the value to be measured (current, voltage) is found in the section of the regulator housing the second winding 12 whereas the element for adjusting this value, the power switch S, is found in the section of the regulator housing the first winding. As illustrated by dotted lines 16, a galvanic isolation should be provided in the regulation circuit 15, between the portion measuring the voltage across resistor Rs and the portion controlling switch S. This isolation is achieved, for example, via a transformer or a photocoupler. Transformers are expensive and bulky whereas photocouplers are expensive and unreliable over time (many failures in present regulators are caused by photocouplers). 
     Of course, it could be devised to regulate the mean current in the first winding 11, but this approach causes a regulation of the mean power provided to load R L  and not of its mean current which then varies as a function of the supply voltage V 1  and of the output voltage across the load. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a switched current regulator whose regulation circuit is entirely accommodated on the side of the power switch, i.e., the circuit is not connected to the second winding and does not need any isolation element. 
     To achieve this object, the present invention uses a flyback-type switched current regulator comprising a first winding, a second winding coupled to the first winding, a current sensor coupled to the first winding, a connecting means, a disconnecting means and a varying means coupled to a supply voltage. The current sensor senses a first current flowing through the first winding and outputs a current sensor signal. The connecting means connects the first winding to a voltage source when a second current flowing through the second winding is substantially zero. The disconnecting means disconnects the first winding from the voltage source when the first current flowing through the first winding is greater than or equal to a reference value. The first winding is connected and disconnected from the voltage source according to a switching duty cycle. The varying means varies the reference value, corresponding to a duration of the switching duty cycle. 
     In another illustrative embodiment of the invention, a switched current regulator is provided comprising a first winding, a second winding coupled to the first winding, a current sensor coupled to the first winding, a demagnetization sensor coupled to a winding core located between the first winding and the second winding and a connection circuit. The current sensor senses a first current flowing through the first winding and outputs a current sensor signal having a first state when the first current flowing through the first winding is greater than or equal to a reference value. The demagnetization sensor senses a second current flowing through the second winding and outputs a demagnetization sensor signal having a first state when the second current is substantially zero. The connection circuit is responsive to the current sensor signal and the demagnetization sensor signal. It connects the first winding to a voltage source when the demagnetization sensor signal is in the first state and disconnects the first winding from the voltage source when the current sensor signal is in the first state. 
     In another illustrative embodiment of the invention, a method is provided for regulating current in a current regulator having a first winding and a second winding, coupled to the first winding. The method comprises the steps of: (a) determining an amount of first current flowing through the first winding; (b) comparing the amount of first current flowing through the first winding with a reference value; (c) coupling the first winding to a voltage source when the amount of first current in the first winding is less than the reference value; and (d) decoupling the first winding from a voltage source when the amount of first current in the first winding is greater than or equal to the reference value. 
     In another illustrative embodiment of the invention, a method is provided for the self-oscillation of a switched current regulator having a power switch disposed in series with a current sensor between a first winding and a terminal of a voltage supply, a winding core disposed between the first winding and a second winding, a demagnetization circuit coupled to the winding core and a reference switch disposed between a terminal of a current source and ground. The method comprising the steps of: (a) determining a value of a second current flowing through a second winding; (b) closing the power switch when the second current flowing through the second winding is substantially zero; and (c) opening the reference switch when the second current flowing through the second winding is substantially zero. 
     The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 above described, represents an exemplary conventional switched regulator, of the flyback type; 
     FIG. 2 represents an embodiment of a circuit for controlling a power switch in a switched current regulator according to the invention; 
     FIG. 3 represents signal waveforms of the circuit of FIG. 2; 
     FIG. 4 represents an exemplary conventional circuit for detecting demagnetization, which is used in some flyback converters; and 
     FIG. 5 represents an embodiment of the circuit of FIG. 2 which can be easily integrated in &#34;smart power&#34; technology. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 2, a current sensor 20 detects, according to the invention, the current in the first winding 11 of a converter such as the one of FIG. 1. The sensor 20 is, for example, a simple low-value resistor connected, as shown, between the power switch S and the negative terminal of the voltage source V 1 , in this case ground GND. The current information, labeled KI 1 , can also be provided in other ways. For example, when the power switch S is a vertical MOS transistor constituted by a plurality of cells connected in parallel, the current information can be obtained from the current flowing in a small number of cells identical to the cells of the power MOS transistor. 
     The switch S is controlled by the non-inverted output Q of Set/Reset-type flip-flop 22 whose Reset input &#34;R&#34; receives the output of a comparator 24 which compares the current information KI 1  with a reference voltage Vr. The Set input &#34;S&#34; of flip-flop 22 receives the output of a demagnetization detection circuit 26, an example of which is described hereinafter with relation to FIG. 4. The detection circuit is used so that the regulator operates in a &#34;self-oscillating&#34; mode, which regulates the mean current in the second winding by measuring the current at the first winding. 
     FIG. 3 represents exemplary waveforms of the control signal of switch S, the reference voltage Vr, the current information KI 1 , and the second current 12 in the second winding 12. The reference voltage Vr is represented with a slight variation, which will be explained hereinafter with relation to elements of FIG. 4 not described yet. 
     When switch S is turned on, the first current I 1 , and therefore the current information KI 1 , regularly increase mainly as a function of voltage V 1  and of the inductance of the first winding 11. During this period, the second current I 2  is zero. 
     When information KI 1  reaches value Vr, the comparator 24 resets the flip-flop 22, which switches off switch S. The first current I 1 , which has just reached its peak value I 1p , is abruptly canceled while the second current I 2  abruptly reaches its peak value I 2p , which is equal to the peak first current I 1p  multiplied by the ratio n of the number of turns of windings 11 and 12. Thus, the peak value I 1p  is fixed to Vr/K and the peak value I 2p  is fixed to n.Vr/K. The second current I 2  regularly decreases, mainly as a function of the voltage across load R L  and of the inductance of the second winding 12. 
     When the second current 12 is canceled, i.e., when the core 13 located between the first winding 11 and the second winding 12 is demagnetized, the demagnetization detection circuit 26 sets the flip-flop 22, which switches on the switch S, and so on. This operation mode, in which switch S is switched on again as soon as the second current I 2  in the second winding 12 cancels, is referred to as the &#34;self-oscillating&#34; mode. 
     In this self-oscillating mode, assuming that the voltage value across load R L  does not significantly vary during a switching cycle of switch S (which is the case for a battery), the mean value of current I 2  is expressed by: 
     
         I.sub.2m =I.sub.2p (1-r)/2=nI.sub.1p (1-r)/2,              (1) 
    
     where r is the switching duty cycle of switch S. 
     Assuming that the supply voltage V 1  and the reference voltage Vr are constant, the peak value I 1p  of the first current I 1  is reached after a constant time duration after each switching on of switch S. The canceling duration of the second current I 2  varies reversely with respect to the voltage across load R L . The lower this voltage is, the longer the demagnetization duration and the smaller the switching duty cycle r of switch S. Then, according to relation (1), the mean value I 2m  increases when the voltage across the load R L  decreases. A power regulation effect is provided to load R L , instead of a current regulation effect in load R L . 
     To regulate the current in load R L , according to an aspect of the invention, the reference voltage Vr is made to vary proportionally in the same direction as the duty cycle r, for example, 1/(1-r). 
     For this purpose, in FIG. 2, the reference voltage Vr is provided across a capacitor Cr having one terminal connected to ground GND and a second terminal connected to a supply voltage Vcc through a current source Ir. The supply voltage Vcc is a d.c. supply voltage of the regulation circuit and is obtained from voltage V 1 . The reference voltage Vr is also connected to ground GND through a resistor Rr serially connected with a switch S1. Switch S1 is controlled by an inverted output Q *  of flip-flop 22. Thus, switch S1 is switched with a duty cycle 1-r. 
     When switch S is on, switch S1 is off. A current source Ir charges the reference capacitor Cr and reference voltage Vr approaches the d.c. supply voltage Vcc. When switch S is off, switch S1 is on. Then, the reference resistor Rr derives a portion of the current from the current source Ir, and the reference voltage Vr approaches a value RrIr. 
     In practice, the charge and discharge durations of the reference capacitor Cr are selected high with respect to the switching period of switch S. Thus, the reference voltage Vr is practically constant, and is established at: 
     
         Vr=IrRr/(1-r) 
    
     The peak value of the first current I 1  is fixed at: 
     
         I.sub.1p =Vr/K=IrRr/K(1-r).                                (2) 
    
     By combining equations (1) and (2), one obtains: 
     
         I.sub.2m =nRrIr/2K, 
    
     which is a constant value that can be fixed by adjusting any of the values It, Rr or K. 
     FIG. 4 represents an exemplary demagnetization detection circuit 26 that can be used in the regulator of FIG. 2. The circuit comprises a PNP transistor Q1 whose emitter is connected to voltage Vcc through a resistor R1 and whose collector is connected to ground GND. The base of transistor Q1 is connected to the junction between switch S and winding 11 through a capacitor C1. A resistor R2 connects the base of transistor Q1 to voltage Vcc. The demagnetization detection signal is drawn from the emitter of transistor Q1 and inverted (40) before being provided to flip-flop 22. 
     While switched S is on, capacitor C1 is positively charged on the side of the base of transistor Q1 at voltage Vcc. When switch S is off, the voltage across winding 11 is inverted and negatively charges capacitor C1 through resistor R2. When the second current I 2  is canceled, the voltage across the first winding 11 is also canceled. Then, capacitor C1 is discharged through the base of transistor Q1 and resistor R1. Transistor Q1, which was blocked hitherto, becomes conductive and connects the output of the demagnetization detection circuit to ground. 
     FIG. 5 represents an embodiment of a converter according to the invention which is easily integratable in logic wells associated with vertical MOS transistors in &#34;Smart Power&#34; MOS technology. The first winding 11, the current measurement resistor 20, and capacitor Cr are not integrated and are connected to the integrated control circuit through respective pins 51, 52 and 53. The reference current Ir is provided by a source comprising an N channel MOS transistor M1, a resistor R1, a voltage source Vref and an operational amplifier 55. Resistor R1 is connected between the drain of transistor M1 and voltage Vcc. The reference current Ir is drawn from the source of transistor M1. Amplifier 55 controls the gate of transistor M1 and receives at its non-inverting input the drain voltage of transistor M1. The voltage source Vref is connected between the inverting input of amplifier 55 and voltage Vcc. 
     With this configuration, the reference current Ir is equal to Vref/R1. The exact values of resistors R1 and Rr are difficult to assert, but which is important in this case is that the ratio between resistors R1 and Rr which can be accurately selected, because the integrated resistors on a single chip are paired. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereto.