FR2479589A1 - Protected supply circuit for information or video system - has chopper connected to negative temp. coefft. resistance to allow rapid reconnection - Google Patents

Abstract

The protected circuit fed directly from the mains via a diode rectifier bridge, has a current limiting element (L) in series with a chopper and a high value capacitor (C) connected to earth. The current limiting element (L) comprises a negative temp. coefficient resistance with a base material of polycrystalline vanadium dioxide, which exhibits a resistance variation of more than three orders within a narrow temp. range, e.g. +-10 deg.C. Pref. an oxide of tungsten or molybdenum is combined with the vanadium dioxide to alter the transition temp. of the negative temp. coefficient resistance. Pref. the current limiting element (L) is thermally coupled to the chopper (T) which comprises a transistor, a thyristor or a triac, for suppressing oscillation about the transition temp. The circuit allows the supply to be rapidly re-established after a mains integration and permits signal ON-OFF switching.

Description

 The present invention relates to switching power supplies and more particularly their protection against current spikes following the commissioning of these power supplies. Protection against such current peaks is ensured according to the invention by resistors S negative temperature coefficient, commonly called CTN of special design and composition, based on vanadium oxide.

 Switching power supplies are used more and more in computer or video systems and in general in complex electronic assemblies because of the advantages presented by the structure of these power supplies without transformer on the input circuit. A switching power supply essentially consists, starting from the killer dry power supply, of a rectifying bridge composed of diodes or thyristors and of a chopping or chopping member, which is frequently either a transistor or a thyris tor. The chopper is controlled externally by pulses or by a phase shifting circuit and the current which leaves it is partly used to charge a buffer capacitor.

This capacitor, of high value serves as a simple filter of the output voltage, and defines5 in connection with the load of the circuit, the nominal value of the output voltage. For effective filtering of the AC component, the value of the capacitor must be high, which is high, taking into account the voltages used in electronic systems based on transistors and integrated circuits, of the order of 1000 microfarads for 5 volt power supplies. .

When the switching power supply is switched on, the equivalent circuit, seen from the mains, can be shown diagrammatically by a series resistor and a capacitance
C. The resistance r translates all the series resistances of the circuit such as for example passing resistances of the rectifying bridge, the resistance of the transistor or of the thyristor in the conducting state; therefore r is low and of the order of a few ohms. Capacity C represents the output filtering capacity of the power supply.

 After switching on the power supply, the fuel transmitted through the resistor r results in a maximum current equal to the supply voltage divided by the resistor r; this capacity charging current then decreases exponentially, with a time constant equal to rC.

 This system has a certain drawback: the capacitor being discharged before power-up, a large current peak can be measured in the power supply circuit, which can exceed 10 times the value of the average operating current.

 The organization of the switching power supply according to the invention provides for mounting between the rectifier bridge and the chopper a resistance with a negative temperature coefficient, therefore of high value when it is cold, at power-up, and of value weak after it is heated by the Joule effect.

 Such assemblies are already known to those skilled in the art, but in the present case, the resistance with negative temperature coefficient CTN has the particularity of being composed of polycrystalline vanadium oxide, of oxidation state 4: VO2.

 More specifically, the invention relates to a protection device for limiting the overcurrents of starting of a switching power supply, supplied directly by the mains voltage, through a rectifier bridge, comprising a cutting member (T) a capacitor. (C) of high value at the origin of a current peak at power-up, when the capacitor (C) is charging, and and a limiter (L) mounted in series with the cutting member (T) , characterized in that this limiter (L) is constituted by a resistance with negative temperature coefficient (NTC), the material of which is based on vanadium dioxide V02, polycrystalline, which has in a narrow range (+ 10 ° C.) temperature variation of resistance greater than 3 orders of magnitude.

The invention will be better understood from the description which follows, which is based on figures which represent:
- Figure 1: the block diagram of a switching power supply in its general appearance
- Figure 2: the shape of the current after power up, in a first case of switching power supply according to the prior art;
- Figure 3: the resistance characteristic as a function of the temperature of a normal NTC;
- Figure 4: the shape of the current after power up, in a second case of cutting power according to the known art;
FIG. 5: the compared characteristics of resistance as a function of the temperature of three normal CTNs, with polycrystalline vanadium oxide and with monocrystalline vanadium oxide;
- Figure 6: the shape of the current after power-up, at the output of a switching power supply protected by a monocrystalline CSi in V02;
- Figure 7: the shape of the current at the output of a switching power supply according to the invention, protected by a polycrystalline NTC in VO2
- Figure 8: a first model of CTN according to the invention;
- Figure 9: a second model of -CTN according to the invention;
- Figure 10: an example of adaptation of CTN according to the invention to hybrid circuits.

 Figure 1 shows the very simplified block diagram of a switching power supply. This diagram is specific to all power supplies according to known art or according to the invention.

From a mains supply, a rectifier bridge supplies a voltage which is applied on the one hand to the common of the supply and of the circuit, on the other hand to a chopping member called T which is a transistor, a thyristor or a triac, which is controlled either by pulses or by a phase shifting circuit. The current being taken from the current rectified by the bridge, that is to say that it has half-sine waves in full alternation, it is summarily filtered by a capacitor C of high value which delivers on the output
S of the power supply a voltage corresponding to the needs of the circuits supplied. The diagram shows an additional device called L because it constitutes a limiter: this device is intended, as its name suggests, to limit the current draw after powering up.

 A first form of limiter according to the known art has consisted in putting in series with the chopper a resistance R of value significantly greater than the equivalent resistance r of the power supply. This effectively reduces the peak value of the overcurrent, but at the cost of high dissipation.

 FIG. 2 represents the shape of the current as a function of time in such a switching power supply, when the limiter is a resistor R. At power-up from time t1, there is a current draw which again forms a peak very high but less than if there was no limiter. Leak as the capacitor C charges, this current draw decreases and the intensity tends towards the nominal intensity IN, with a time constant equal to (R + r) C. The main drawback lies in the high dissipation by the resistance R in continuous regime, energy dissipation having no subsequent utility.

 To reduce this inrush current in a simple way, the solution currently practiced lies in the use of a CTbi type resistor in place of the R resistor.

 For reasons of simplification and clarification of the text, the acronym CTN will subsequently be called the resistance with a negative temperature coefficient.

 This resistance is mounted in series with the chopper, that is to say that in FIG. 1, the limiter is therefore according to this second known art form, a CTN. The protection is based on the resistance characteristic as a function of the temperature of a -CTN.

Figure 3 gives an example of such a characteristic between room temperature and 100Q C. This variation is expressed using the following relation
R = Ro exp B / T in which is the thermal eoefSicient of the material expressed in degrees Kelvin or in absolute degrees. For example, B is between 2500 and 5000. Ro is a constant, expressed in ohms corresponding to the resistance of the CTN at 04 C. Another characteristic given to define a CTN is its resistance at 250 C
R25 from relationship
R25 -R0 exp B / 298.

R25 can be chosen between 10 ohms and 100,000 ohms approximately.

 For an average coefficient B of 4000, the resistance varies between 200 and 800 in a ratio of approximately 10 and for a coefficient B of 5000 this variation is 18; it is therefore a limited variation.

When the switching power supply is switched on, the CTN is at 250, the temperature taken to be ambient temperature, and therefore has a high resistance value. As the resistance of the chopper, transistor or thyristor, and of the rectifier bridge is low, the starting current which flows through the power supply and the CT1J is given by the expression
220 = 220 / R25 if the supply voltage is 220 volts.

 The power dissipated by the CTN heats it and consequently reduces its resistance. The equilibrium regime corresponds to a temperature of the CTN such that the heat losses balance the absorbed electrical power. To limit dissipation, it is advantageous to take resistors with values as small as possible.

 This implies a temperature coefficient B as high as possible which is generally incompatible with low values of the resistance. So we choose a compromise between these two conditions. The result is currently, according to known art, limited protection against overcurrents. For current embodiments, the choice of the CTN leads to current limitations, such that the maximum current present after switching on the switching power supply does not exceed a continuous value of the order of three times the value of the current in steady state.

 FIG. 4 represents the shape of the overcurrent during the powering up of a switching power supply provided with a NTC according to the known art. The time t being given on the abscissa and the intensity I on the ordinate, it can be seen that this intensity decreases fairly quickly but that it nevertheless has at time t, when the supply is put into service, a still fairly large value.

 The present invention proposes to overcome this drawback by using a NTC made of a composite material based on vanadium dioxide. FIG. 5 represents the comparative curves of resistances as a function of the temperature in the zone of 200 C to 1000 C of three types of CTN. Curve 1 corresponds to a conventional NTC; curve 2 to a CTN in polycrystalline vanadium dioxide and curve 3 to a CTN in monocrystalline vanadium dioxide. Vanadium dioxide has around 650 C, and in a time range erasing on the order of plus or minus 2 or 30 only, an insulating / metal transition which results in monocrystalline samples by a resistivity jump of five orders of magnitude. The amplitude of the jump is lower for polycrystalline samples but is always greater than 3 orders of magnitude.

 The advantage of polycrystalline vanadium oxide is that the curve is slightly more rounded and presents a more gradual transition, which, as will be seen later, is advantageous for switching power supplies.

 The resistance / temperature characteristic of a polycrystalline sample based on vanadium dioxide makes it possible to understand the advantage of this solution compared to protection by a normal NTC.

 Having a higher resistance dynamic is advantageous for use in a switching power supply, but the operating characteristic of these power supplies also depends on the rapid resistance change, i.e. the insulating or semiconductor transition- resistance metal.

 If we first consider the case of a phase change and therefore of frank resistance as it is observed in a single crystal of vanadium dioxide, -curve 3 of Figure 5- the response of the power supply circuit to cutting then presents a point of abrupt discontinuity which is illustrated in FIG. 6.

In FIG. 6, the intensity I is plotted on the ordinate as a function of time t. When the power supply is put into service, when the NTC is still cold, and therefore of high value, an initial current lo flows through the circuit, it decreases as the
CTN heats up and, when it reaches a temperature of 650, and suddenly changes in value, it then goes through a maximum IM before reaching the nominal current. FIG. 6 therefore represents the charging current of the capacitor C of the switching power supply and the additional current peak goes against the desired effect, which explains among other reasons that there is no interest to use CTNs in vanadium dioxide in monocrystalline form.

 On the other hand, when the phase transition is slow and controlled, this effect is favorable to the use of vanadium dioxide CTN in the polycrystalline form -curve 2 of FIG. 5-. This spreading over time of the ignition overvoltage, joined to the fixing of a maximum current level IN as low as possible S power-up, or of a level as close as possible to the DC power level IN of the supply in service, leads to the response characteristic of FIG. 7.

 FIG. 7 therefore illustrates the shape of the current I as a function of the time t at the start-up of a switching power supply protected by a CTN with polycrystalline vanadium dioxide, in which IO represents the intensity at the start-up, IN l nominal service current, and 1M the maximum intensity or peak of overload on power-up: the peak of overload is spread over time and does not have any abrupt discontinuity. This spread is obtained on the one hand by the polycrystalline nature of the material, but also, on the other hand, by the realization of a composition gradient obtained by adding elements of vanadium oxide which shift its transition temperature these elements can be for example molybdenum or tungsten. In addition, the control by the geometry of the samples of the phase change intervenes in the control of the current peak.

 Figures 8 and 9 show two possible forms for the production of vanadium dioxide CTN; Figure 8 corresponding to an axial shape and Figure 9 to a radial shape.

 At the transition, the variation in resistance of these two CTNs results in the nucleation of a zone or of a thread whose conduction is of the metallic type which gradually extends to the whole of the CTN. If we call 4 and 5 the two contact connection connections joined to the body 6 of the C by means of 2 plates or 2 metal caps, the CTN, whether it is according to the configuration in FIG. 8 or according to that of Figure 9, is originally composed of an insulating or semiconductor material denoted SC; when the temperature of the body 6 of the CTN rises, a metal zone denoted M develops and gradually comprises the entire body of the CTN; at this point, the CTN loses most of its initial strength and has reached its transition temperature. In the configuration of FIG. 8, the resistors SC, that is to say the two semiconductor zones which are on either side of the metallic zone, are in series and the overall resistance progressively passes from the resistance of the body in its semiconductor state-the resistance of the body to its metallic state. On the other hand, in the configuration of FIG. 9, the high conduction of the CTN is established more quickly because the metallic zone denoted M unites, as soon as the temperature reaches the transition temperature, the two electrodes 4 and 5. It is obvious to those skilled in the art that the production of CTNs as described is not limited to the forms of FIGS. 8 and 9 which are well known to those skilled in the art. 'art.

 FIG. 10 represents an example of a vanadium dioxide CTN intended for a circuit in hybrid technology. On a substrate 7, a contact-making metallization 8 is deposited and then the body 10 of the cTN is covered by any suitable means covered by a second metallization 9. The conductive metallic zone develops at the interface between the body 10 of C and its lower electrode, and has an upper limit parallel to this interface which depends on the thermal conductivity of the support depending on whether it functions more or less as a radiator of the CTN.

The use of a vanadium oxide NTC in a switching power supply can, depending on the characteristics of the power supply and its original design, present a critical aspect: when the NTC has reached its transition temperature by Joule effect, its pass resistance decreases sharply and the Joule effect being linked to the power 2 of the intensity, the CTN can then no longer be sufficiently heated by the passage of the current, its temperature drops and it crosses its transition temperature in the opposite direction. If the switching power supply is designed in such a way that the CTN can again cross its transition temperature, this will result in a permanent phenomenon of oscillation, the
CTN crossing its transition temperature in one direction then in the other.

 To overcome this drawback, it suffices to have good thermal contact between the CTN for protecting the chopping member, transistor or thyristor, and this member itself which is very generally mounted on a radiator. To do this, the CTN is fixed on the radiator and the thermal contact is ensured by conventional means such as coating, or contact via a silicone grease for example.

The advantages of CTNs with vanadium dioxide V02 polycrystalline, as described according to the invention are the following:
- low resistance due to the conductive region of polycrystalline V02 at moderately high temperatures (70 to 800 C) and simultaneously high resistance at room temperature
- efficient removal of the current peak and progressive charging of the capacitor, which limits overcurrents when the switching power supply is put into service;
- Power dissipated in steady state lower than in the case of conventional NTCs, due to the very low resistance in continuous regime of NTCs according to the invention;
- equilibrium temperature in continuous operation chosen slightly higher than the transition temperature.

 This allows the switching power supply to be available more quickly after a mains interruption or a rapid switch-off. Indeed, the limited cooling time leads to choosing the temperature of the component slightly lower than its transition temperature which it therefore quickly reached during a return to service.

Claims (3)

 1. Protection device for limiting the overcurrents of starting of a switching power supply, supplied directly by the mains voltage, through a rectifier bridge, comprising a cutting member (T), a capacitor (C) of high value, at the origin of a current peak at power-up, when the capacitor (C) charges and a limiter (L) mounted in series with the cutting member (T), characterized in that this limiter ( L) is made up of a resistance with negative temperature coefficient (NTC), the material of which is based on vanadium dioxide V02, polycrystalline, which exhibits in a narrow temperature range (+ 10 C) a resistance variation greater than 3 orders of greatness.  2. Device. protection according to claim 1, characterized in that molybdenum or tungsten, added in the form of oxides to vananium oxide, displace its transition temperature, and create in the CTN resistance of the limiter a composition gradient which decreases correspondingly the current peak at power up.  3. Protective device according to claims 1 or 2, characterized in that the limiter (L) is mounted in thermal coupling with the cutting member (T), which maintains it in temperature and suppresses the oscillations around its temperature of transition.