FR2578365A1 - VOLTAGE TRANSFORMER - Google Patents

Abstract

A VOLTAGE TRANSFORMER, INTENDED FOR EXAMPLE TO OPERATE A COLOR TELEVISION ON A 12V MOTOR VEHICLE ACCUMULATOR, INCLUDES A TR TRANSFORMER WHICH, THROUGH A TL POWER TRANSISTOR, IS CONTROLLED BY AN OSCILLATING ASSEMBLY. THE INVENTION ALLOWS THE ASSEMBLY TO BE PROTECTED AGAINST OPERATING FAULTS, ESPECIALLY AGAINST OVERVOLTAGES AT THE POWER TRANSISTOR.

Description

The present invention relates to a transformer.

  of tension intended to transform a first continuous tension

  in a second higher DC voltage. A transforma-

  This type of voltage tester is necessary, for example, to operate a color television with an accumulator.

  12 V motor vehicle.

  To this end it is known (German patent application

  published mand # 3,017,369) to use a transformer

  voltage in which the accumulator supplies a circuit ca-

  able to oscillate and a power transistor which controls

  dice by the latter, produced in the primary winding of a transformer a periodically interrupted current. The

  tension thus elevated by transformation so as to present

  ter the desired value can then be taken by a

  transformer secondary bearing.

  Such voltage transformers provided

  for the area of use cited, transmit a

  relatively large power of the order of 100 W. This is

  why it is necessary to take additional measures-

  which protect against improper operation and prevent excessive force

  voltages, especially at the power transistor

sance.

  The present invention aims to allow a voltage transformer of the kind described to be protected, in a simple manner in terms of the realization of the

  mounting, against malfunctions and over-

  tensions. This object is achieved according to a first characteristic of the invention, for a voltage transformer intended to transform a first direct voltage into a second

  higher direct voltage and which includes a transforma-

  tor as well as a pilot stage and a transistor controlling the primary winding of the transformer, which elements

  components are supplied with the first continuous voltage

  naked and form a self-oscillating assembly, a winding

  secondary delivering, through a first re-

  blocker working in the direction of go and a conden-

  load sator, the second voltage continues, by the fact that a second rectifier operating in the return direction

  is mounted between another secondary winding and the con-

charge densifier.

  The rectifier intended to produce the voltage of

  higher operation or more rectifiers intended

  born to produce various operating voltages

  argue about the so-called straightening principle. This means that these rectifiers are conductive and bring energy to the connected charge capacitors when the power transistor mounted on the side of the primary is

  also conductive. Circuits presenting this mode of operation

  are also called flux transformers. According to the characteristic of the invention, defined above, there is provided, in addition to the forward rectification, a return rectification which acts on the same charge capacitor as the forward rectification. This can be achieved by corresponding polarization of the rectifier or

  the winding supplying the rectifier. In this respect the

  bearing which feeds the rectifier for the return rectification advantageously acts as a said winding

  demagnetization and has the effect of reducing the energy emma-

  fueled in the transformer. The rectifier for return rectification also has the effect of limiting

  of the return, i.e. during the blocking phase of the

  power sistor, the amplitude of the pulses at the level

  of the winding supplying this rectifier. Thus the ampli-

  pulses at the other windings of the transformer is also limited and it is, therefore, the same for the effective voltage, at the power transistor, during the return time. The device exhibiting this operation in the direction of

  return is also called blocking transformer.

  According to a development of the invention, a conden-

  sator is still connected to the collector of the power transistor. This capacitor limits the slope of the pulse at the collector of the power transistor. To the

  end of the return time this capacitor produced in cooperation

  tion with the inductors an oscillation in the return pulse so that this return pulse presents, at the end of the return time, automatically the

  value zero and that therefore when the transis-

  power tor goes to the conductive state, at the start of the go time H, no unwanted load currents occur.

  Other advantageous features of the invention

  tion consist in that the second rectifier is supplied by a separate secondary winding and in that in the case where the collector current of the power transistor must be relatively high the latter is constituted by

  a Darlington circuit comprising two transistors.

  The assembly on the primary side is made self-oscillating thanks to a feedback loop extending from the power transistor to the pilot stage. A v a r i a n t of the invention consists, for a transformer of tehsion intended to transform a first direct voltage into a second higher direct voltage and comprising a transformer as well as a pilot stage and a transistor

  controlling the primary winding of the transformer,

  which constituent elements are supplied with the first DC voltage and form a self-oscillating assembly, a secondary winding delivering, by means of a first rectifier operating in forward rectification, the

  second continuous voltage, in that the feedback loop

  tion comprises a diode polarized and subjected to an initial voltage so that in the case of an inadmissible increase in the collector voltage of the transistor

  the outward journey ends prematurely and that in the case of an ac-

  more pronounced growth the diode is blocked and the

  oscillating circuit remains in the state where the transistor is blocked

  than. This feedback loop is thus also used to obtain a protective circuit. This will still be

  described in more detail about an exemplary embodiment.

  Another advantageous characteristic of the invention

  tion consists in that the windings supplying the first and second rectifiers are polarized in opposite directions and the rectifiers in the same direction one by

compared to each other.

  A capacitor is advantageously mounted between the collector of the power transistor and a reference point (earth), preferably in parallel with the collector-emitter path of the power transistor and / or

  with the primary winding of the transformer.

  The capacitor is advantageously calibrated in such a way

  the way that the oscillatory shape of the voltage produced

  at the collector by the capacitor and the induc-

  tances active has, at the end of the return, substantially a zero crossing. According to another characteristic of the invention it

  is associated with the pilot stage a self-priming circuit

  oscillating. This makes the mounting possible from

  the rest state, that is to say for example in case of over-

  load or short circuit, switch back automatic-

  lying in working condition. The starting circuit is preferably constituted by a self-oscillating circuit which continues to oscillate even in the event of the transformer stopping

tension itself.

  The pilot stage is advantageously completed for

  form a circuit capable of oscillating which during operation

  normal operation oscillates in synchronism with the working frequency of the voltage transformer and, when the

  diode remains blocked, continues to oscillate at low frequency

  quence (1 to 3 kHz) and serves as a priming circuit.

  The pilot transistor can advantageously be as-

  company yet another transistor, these two transistors form-

mant a bistable circuit.

  The two main variants of the invention,

  above, are applicable independently of each other. However, it is particularly advantageous to combine these two variants since this makes it possible to obtain particularly safe protection against damage due to malfunction. In addition, this gives a high

  yield and a longer battery life

tor.

  An embodiment of the invention, in the-

  which the two main variants mentioned above are combined

  born, is explained in more detail below using the accompanying drawings in which: Figure 1 is a simplified circuit diagram of a voltage transformer according to the invention; Figure 2 shows curves intended to elucidate the operating mode of the assembly of Figure 1; and FIG. 3 represents an exemplary embodiment of

  the invention which has been put to the test on a practical level.

  According to Figure 1 an accumulator G for vehicles

  motor vehicles with a voltage of E = + 12 V supplied

  te, via a pilot stage comprising the transistor T2, the resistors R1, R2, R3, R4 as well as the capacitor C3, the transistor T1 which is produced as a power transistor. The transistor T1 is mounted in

  series with the primary winding P of the transformer Tr.

  This circuit is made self-oscillating thanks to the loop of

  feedback comprising diode D4 and oscillates with a frequency

  frequency of the order of 25 kHz. Secondary winding S1

  delivers, via the rectifier Dl, to the condensa-

  load cell C18 an operating voltage B1 of

  + 80 V for the Ro receiver. The windings S3 and S4 de-

  deliver, through other rectifiers,

  lower value B2 and B3 operating sions. This circuit works according to the so-called forward rectification principle. This means that the rectifier D1 is conductive

  whenever the transistor T1 is also conductive.

  In addition to the charge capacitor C18 is also connected the rectifier D2 which is supplied by the secondary winding S2. This secondary winding S2 is polarized

  in the opposite direction of the winding S1 so that the re-

  trainer D2 operates a so-called back turnaround. The redres-

  sor D2 is therefore conductive each time the transistor T1 is blocked. Transistor T3 forms with transistor T2

a bistable circuit.

  Thanks to the action of the rectifier D2 the voltage applied

  quée at the upper end of the winding S2 can not

  not exceed the constant voltage B1. Pulse voltage-

  applied to winding S2 is therefore limited in

  its amplitude. So all the other impulse voltages

  applied to the transformer Tr, including the voltage

  The voltage applied to the transistor T1 and to the diodes D1 and D2 are limited in amplitude so that ii cannot produce excessively high voltages. Furthermore, it

  an additional energy supply is obtained in the con-

  charge densifier C18. Winding S2 plays the role of a so-called demagnetization winding to reduce energy

  magnetic stored in the transformer Tr. The re-

  training to go used has the advantage that the energy

  effective at the receiver does not go through the induc-

transformer.

  As shown in Figure 2a, it occurs at the-

  calf of the collector of transistor T1 during the recovery time

  turn R, due to the blocking of the transistor, a return pulse. The capacitor C1 serves to limit the slope of the sides of this pulse according to FIG. 2a. The rising edge has a duration of 1 microsecond. At the end of

  return time R the capacitor C1 causes in cooperation

  tion with active inductors, in particular the winding

  elementary P, an oscillation. The tuning is chosen in such a way that the pulse present according to FIG. 2a at the end of the return time substantially the value zero. The falling edge, which corresponds to roughly a quarter of the sinusoidal period, has a duration of approximately 3 to 5 microseconds. When, at the end of this return time R, the transistor Ti is switched on and the voltage across the collector-emitter path must become zero, the voltage Vce therefore presents, thanks to the oscillation, at the end of the return time already substantially zero so that the voltage VA and the current to be passed through diode D2 become zero. If, at the end of the return time R, the voltage Vce was not zero but had a higher value, then the capacitor Cl should be quickly discharged via the transistor Tl, which can be a risk to transistor Tl. The capacitor Cl can

  be mounted, either in parallel with the collector path-

  transmitter, also in parallel with the winding

  primary P. The ratio of the number of turns of the windings-

  elements SI and S2 determine the ratio of the amplitude of the pulse voltage according to FIG. 2a to that of the operating voltage E. In this respect: Vce - E Si

E S2

  Diode D4 forms a feedback loop by which the circuit is self-oscillating. At the start of the go time H the voltage Vce is practically zero, the diode D4 conductive and the voltages VA and VB also almost

  null, the transistor T2 blocked and consequently the tran

  sistor Tl conductor via the resistor Rl. During the going time H the capacitor C3 is charged via the resistor R2 so that the voltage VB increases in the positive direction. At the end of a period of time, which is determined by the time constant R2C3, the base of the transistor T2 is sufficiently positive so that

  the transistor T2 becomes conductive and blocks, by the

  through diode D5, transistor Tl at the end of the go time. As a result of the blocking of the transistor Tl

  produced during the return time R, due to the induc-

  tance of the primary winding P, the so-called return pulse according to FIG. 2a which presents the defined amplitude

  upper. The duration R of the return pulse is essential

  tially determined by the duration of the forward time H and the ratio of the numbers of turns of the windings Sl.1 and S2 according to the equation:

R = H. S2

  o H is determined by the time constant C3-R2. Since the return pulse is produced by the variation of the current through the transistor Tl, this pulse, formed in the manner described by the capacitor C1, drops to zero at the end of the return time R. Then again begins the time to go H, as described above. Thus, the entire primary assembly is made self-oscillating.

  The voltage Vce during the go time H is pro-

  portable to the collector current flowing during this time. In normal operation, the voltage only has low values since the transistor must function as a switch. If the current passing through the transistor T1 comes to present excessively high values as a result of a fault, for example a fault or an overload, then the voltage Vce also increases excessively during the go time H. This is indicated in FIG. 2a by the line L. Under these operating conditions the transistor leaves the state

  saturation and no longer works as a simple inter-

  breaker. During the go time is then produced at the transistor Tl, due to the high voltage available, too high a power. The transistor T1 can thus be endangered. This increase in voltage is detected by the diode D4 and is reflected on the voltage VA which consequently also increases, according to the curve L, more

  quickly than under normal operating conditions

  the. Thus the transistor T2 becomes conductive sooner, that is to say in the course of the go time H, and the transistor T1 is consequently at the same time blocked and protected against too high a power. The time to go H is

  therefore shortened and therefore the frequency of the trans-

  tension trainer is increased in the desired manner.

  The diode D4 remains during the shortened part remaining of the conductive go H time as before. The power supply unit therefore continues to function as such. The voltage Vce, the return pulse according to FIG. 2a and the transmitted power are however limited, by the premature end of the go time H, in such a way that the

  components are not at risk.

  If the applied load still increases, for example in

  case of short circuit, the duration of the go time H and the am-

  pulse width according to Figure 2a become more and more reduced. The voltage Vce ends up varying

  enough that diode D4 remains permanently blocked

  nence. This means that the feedback loop is in-

  broken, the circuit no longer oscillates and remains in the state where the transistor T2 is conductive and the transistor T1 is blocked. Therefore, the mounting is also resistant

short circuits.

  An advantage of the assembly consists in that for the determination of the collector current of the transistor T1 no resistance needs to be provided in the transmitter for the sampling of a voltage proportional to this current. Such resistance would unnecessarily consume power. The value of the current flowing in the transistor T1 is established exclusively from the

  voltage Vce during the time to go.

  The malfunction mentioned, for which the described protective circuits are provided, can occur by

example:

  1) when a high load causes desaturation of the transistor T1; 2) when the core of the transformer Tr is damaged;

  3) when the transistor T1 has too low an ampli

current fication; or

  4) when one of the diodes D1, D2 is blown, that is to say

  say it determines a short circuit.

  - In the absence of other measures, it may happen that, for example after a fault, the circuit remains in the state where the transistor T2 is conductive and the transistor T1 is blocked, and does not restart. This is why the ignition circuit comprising the transistor is also provided.

  T3, the capacitor C4 and the resistors R5, R6. The tran-

  sistor T3 forms with the transistor T2 a bistable circuit

  which alone oscillates with a frequency of around 3 kHz.

  When the transistor T2 is blocked, the transistor T3 is controlled, via the resistor R4 and the capacitor C4, so as to be conductive. When the

  transistor T2 is conductive, the voltage applied to the col-

  reader is almost nil and therefore the transistor T3

  is blocked. As long as the voltage transformer operates-

  does correctly with its nominal frequency, the circuit

  bistable including transistors T2 and T3 oscillates automatically

  at the transformer operating frequency of

  voltage, for example 25 kHz, the transistor T3 -always being

  driver days when transistor T2 is blocked. Under these operating conditions the protective circuit has practically no meaning for operation and simply oscillates in synchronism with the transformer.

  Of voltage. When the assembly remains in the state o the tran

  sistor T1 is blocked, the bistable circuit comprising the transistors T2 and T3 continues to oscillate at a frequency of 3 kHz, which is determined by the resistor R5 and the capacitor C4. This means that the transistor T2 is

  blocked for this frequency and, therefore, the transis-

  tor T1 is controlled so as to be conductive. Thus the voltage transformer would always be restarted periodically, by the ignition circuit comprising the transistors T2 and T3, in order to again trigger the

  voltage transformer operation.

  In FIG. 3, the constituent elements correspond to

  corresponding to those of FIG. 1 are designated by the same figures. Since a relatively high collector current is required for the transistor T1, a Darlington circuit comprising the transistors Tla and Tlb is provided for this power stage. The transistor T2 is also formed by a Darlington circuit comprising the

transistors T2a and T2b.

  In series with the switch S, a battery protective circuit is also provided. This circuit comes into action when the battery voltage drops below the value of 8 V. This circuit consists of a Schmitt rocker comprising the constituent elements T4, D6, DZ, R7, R8, -R9, R10 and Rll. The circuit operates under normal conditions when the voltage VE is greater than 9 V and is switched off when the voltage VE is less than 8 V. In order to reduce the saturation voltage of the transistor Tla the basic current flowing through the transistor Tlb is taken from outlet a and limited to

resistance R27.

  To obtain sufficient switch-off time

  santly short of transistor Tla, transistor T2a must deliver a short base current but whose polarity reverses very quickly. This current passes through the diode

  D27, capacitor C27, diode D5 and transistor T2a.

  This is why the Darlington circuit including the

  transistors T2a and T2b is used. Diode D29 is required

  for the shutdown state and avoids a circu-

  lation of current parallel to the switch S. The diode D5 is used for decoupling between the collector of the transistor T2a and the base of the transistor Tlb so that the voltages which are there

  applied can have the desired appearance.

Claims (9)

CLAIMS i - Voltage transformer intended to transform a first direct voltage (E) into a second higher direct voltage (Bl) and comprising a transformer (Tr) as well as a pilot stage and a transistor (Tl) which controls the winding primary (P), which constituent elements are supplied with the first continuous voltage (E) and form a self-oscillating assembly, a secondary winding (Sl) delivering, via a first rectifier (Dl) operating in the forward direction, and a charge capacitor (C18), the second direct voltage (B1), characterized in that a second rectifier (D2) operating in the return direction is mounted between a secondary winding (S2) and the charge capacitor (C18).   2 - Voltage transformer intended to transform a first direct voltage - (E) into a second voltage   higher continuous (B1) and including a transformer.   (Tr) as well as a pilot stage and a transistor (Tl) which controls the primary winding (P), which constituent elements are supplied with a first direct voltage (E) and form a self-oscillating assembly, a secondary winding (S) delivering, via a first rectifier (Dl) operating in the forward direction, the second direct voltage (Bl), characterized in that the feedback loop comprises a diode (D4)   polarized and subjected to an initial voltage in such a way   re that in the case of an inadmissible increase in the collector voltage (Vce) of the transistor (1) the outward (H)   ends prematurely and that in the case of an increase   even more important of this voltage the diode (D4) is blocked and the oscillating assembly remains in the state where the transistor (Tl) is blocked.   3 - Voltage transformer according to the claim   tion 1, characterized in that the second rectifier (D2) is   powered by a separate secondary winding (S2).   4 - Voltage transformer according to claim 2, characterized in that the windings (Sl, S2)   supplying the first and second rectifiers respectively   sors (D1, D2) are polarized in opposite directions and the rectifiers (D1, D2) are polarized in the same direction one compared to each other.   - Voltage transformer according to claim 1, characterized in that the transistor (T1) consists of a Darlington circuit comprising two transistors (Tla, Tlb).   6 - Voltage transformer according to the claim   tion 1 or 2, characterized in that a capacitor (C1) is mounted between the collector of the transistor (T1) and a point of reference (earth).   7 - Voltage transformer according to the claim   tion 6, characterized in that the capacitor (C1) is mounted in parallel with the collector-emitter path of the transistor (T1).   8 - Voltage transformer according to the claim   tion 6, characterized in that the capacitor (C1) is   mounted in parallel with the primary winding (P).   9 - Voltage transformer according to the claim   tion 6, characterized in that the capacitor (C1) is calibrated so that the oscillatory shape of the voltage (Vce) caused at the collector by the capacitor (C1) and the active inductors present, at the   end of the return time, substantially a zero crossing.   - Voltage transformer according to any one   as in the preceding claims, characterized in that at   the pilot stage comprising a transistor (T2), resistors   these (R1, R2, R3, R4) and a capacitor (C3) is associated with a self-oscillating ignition circuit comprising a transistor   (T3), resistors (R5, R6) and a capacitor (C4).   11 - Voltage transformer according to the claim   tion 10, characterized in that the pilot transistor (T2) is completed so as to form a circuit (T2, T3) capable of oscillating, which in normal operation oscillates in synchronism with the working frequency of the transformer   voltage and, when the diode (D4) remains blocked, conti-   naked to oscillate at a low frequency (1 to 3 kHz) and serves as priming circuit.   12 - Voltage transformer according to the claim   tion 11, characterized in that with the pilot transistor (T2) is associated another transistor (T3) and the two transistors   (T2, T3) form a bistable circuit.