Patent Publication Number: US-5892671-A

Title: Control circuit for a switching transistor using a tapped coil with oppositely wound coil parts

Description:
FIELD OF THE INVENTION 
     The invention is based on a control circuit for a periodically actuated switching transistor. Such a switching transistor is required, for example, in the line output stage for a television receiver or in a switched-mode power supply. 
     BACKGROUND INFORMATION 
     Both line deflection circuits and switched-mode power supplies are increasingly employing higher operating frequencies, for the line deflection for high-definition systems at a line frequency of 32 kHz and for switched-mode power supplies in order to reduce the dimensions of the isolating transformer further. The turn-on and turn-off losses at the switching transistor increase proportionally with the increase in the operating frequency. Efforts are therefore being made to reduce these turn-on and turn-off losses at the switching transistor in order to improve the overall efficiency of the circuit. 
     SUMMARY 
     The present invention is directed to a control circuit for a switching transistor which enables the turn-on and turn-off losses at the switching transistor to be reduced. 
     A control circuit according to the inventive arrangements taught herein comprises a DC voltage source is connected to the base of the switching transistor via a periodically actuated switch element and a coil having a core and having a tap. The two coil windings defined by the tap are wound in opposite senses, and the tap is connected via a diode to earth or to the emitter of the switching transistor. 
     The inventive control circuit for the switching transistor has a number of advantages over known circuits. The circuit is particularly simple because it requires a few passive, inexpensive components. In particular, very rapid and lossless turning off of the switching transistor is achieved. The circuit in this case acts as a current transformer with just one inductor, while a genuine current transformer having a primary winding and secondary winding is not required. The base current during the on phase of the switching transistor is, in a desirable manner, proportional to the collector current of the switching transistor, that is to say it rises with a sawtooth waveform starting from zero during the on phase of the switching transistor. This dynamic matching of the base current to the collector current of the switching transistor reduces the losses at the switching transistor during this time and achieves a better efficiency. The circuit permits, particularly at elevated operating frequencies, a considerable reduction in the losses occurring at the switching transistor. 
     The turns ratio between the coil part connected to the switch and the coil part connected to the base of the switching transistor is preferably about 3:1. This dimensioning ensures an optimum amplitude of the negative base current of the switching transistor during the depletion phase. 
     In a development of the invention, that end of the first coil part which is connected to the switch element is connected to earth via the series circuit formed by a diode and a capacitor. In this case, the capacitor is connected via a resistor to the base of the switching transistor. In this circuit, the capacitor serves to discharge the coil after the depletion phase. In this case, the energy stored in the form of a current in the coil is transferred to this capacitor in the form of a voltage. The voltage occurring across the capacitor serves to switch off the switching transistor. The capacitor therefore advantageously fulfils two tasks, namely the acceptance of the energy in the coil after the depletion phase for the purpose of discharging the coil, and the provision of a negative voltage at the base of the switching transistor for this time. 
     The DC voltage source can also be formed by a rectifier circuit which is fed by a winding of a transformer which is controlled by the switching transistor. The overall circuit can thereby be simplified. A starting circuit may then be required for the purpose of starting the circuit. The switch can be formed by a transistor or a MOSFET. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below with reference to the drawings, in which: 
     FIG. 1 shows an exemplary circuit for the circuit according to the invention; 
     FIG. 2 shows waveforms for explaining the method of operation of the circuit according to FIG. 1; 
     FIG. 3 shows the structure of the coil with the core; and 
     FIG. 4 shows an exemplary embodiment of the circuit according to FIG. 1. 
     FIG. 5 shows an alternative embodiment for the circuit according to FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following symbols are used in the description. 
     ib Base current of the switching transistor 
     ic Collector current of the switching transistor 
     i1 Maximum value of the positive base current 
     i2 Maximum value of the negative base current 
     i2&#39; Base current at the end of the depletion phase 
     n1 Number of turns of the coil part L1 
     n2 Number of turns of the coil part L2 
     UD Breakdown voltage of the diodes 
     Ube Base/emitter forward voltage of the switching transistor 
     U2 Operating voltage generated 
     Us Switching voltage 
     FIG. 1 shows a switched-mode power supply with the switching transistor T, the isolating transformer Tr and the rectifier circuit 4 for generating an operating voltage U2. The isolating transformer Tr preferably contains a plurality of secondary windings for generating operating voltages of varying amplitude and polarity. The switching transistor T is periodically switched on and off by a control or driver circuit. The control circuit contains the voltage source E, the switch S, which is periodically actuated by the switching voltage Us, the coil L having the core K and the two coil parts L1 and L2, which are wound in opposite senses, and a tap 2, which is earthed via the diode D2. The tap 1, which is connected to the switch S, of the coil L is connected to earth via the series circuit formed by the diode D1, the resistor R1 and the capacitor C, the capacitor C being connected via the resistor R2 to the base of the switching transistor T. The base of the latter is connected to earth via the-resistor R3. 
     FIG. 5 shows an alternate embodiment of the circuit according to FIG. 1. In this alternate embodiment, the tap 2 is coupled via the parallel diodes D5 and D6 to the emitter of the switching transistor T. 
     The method of operation of the control circuit is described below, with reference to FIGS. 1 an 2, successively for the different operations during an operating period. 
     On phase of the switching transistor T from t0-t1 
     Let it be assumed that the circuit has zero current and zero voltage at the instant t0 at the beginning of the operating period. At t0, the switch S is closed by the switching voltage Us, with the result that the positive voltage E is present at the tap 1 of the coil L. Consequently, a base current ib begins to flow via the coil L into the base of the switching transistor T, which base current ib rises with a sawtooth waveform on account of the inductance of the coil L and produces a proportional collector current ic through the switching transistor T. As a result of the reciprocal winding of the two coil parts L1, L2, a negative voltage is present at the tap 2 of the coil L, with the result that the diode D2 is switched off during this time. 
     Depletion phase from t1-t2 
     The switch S is opened by the switching voltage Us at the instant t1 at the beginning of the depletion phase. No more current can then flow into the tap 1 of the coil L. The magnetization is now completely concentrated in the coil, the magnetization current initially having to flow further. A large negative base current ib now flows from the base of the switching transistor T via the coil part L2 between the taps 2 and 3 and the diode D2, which is now switched on, to earth. The magnetization changes relatively slowly from the value i2 to the value i2&#39;. The coil L is still not completely discharged, however, at the end of the depletion phase, that is to say at the instant t2. 
     Discharge of the coil L or demagnetization phase 
     The base of the switching transistor T is now finally discharged, i.e. all of the charge carriers are depleted and the collector current ic has decayed to zero. The impedance at the base now rises appreciably, with the result that no more appreciable current can flow into the base. The current still flowing in the coil L must therefore find another path. This current now flows from the capacitor C via the resistor R1, the diode D1, the coil part L1 between the taps 1 and 2 and the diode D2. At the instant t3, the coil L is completely discharged, i.e. the energy stored in the form of the current in the coil L has been transferred to the capacitor C in the form of a negative voltage. 
     Residual time during the operating period 
     During the residual time t3-t4 of the operating period t0-t4, in which all of the currents have decayed to zero, the negative voltage produced across the capacitor C by the demagnetization of the coil L is present at the base of the switching transistor T and ensures that the latter continues to be in the off state. The capacitor C serving for the discharge of the coil L therefore simultaneously makes the negative voltage produced by its discharge available to the base of the switching transistor T as a reverse voltage. At the instant t4, which corresponds to the instant t0, the circuit is thus prepared at the beginning of the next operating period by the closing of the switch S. 
     FIG. 3 shows the core K with the two coil parts L1, L2 designed as layer windings. The core K is designed as a so-called roller-type or mushroom-type core. 
     FIG. 4 again shows the components of FIG. 1. The operating voltage U1 for the switching transistor T is obtained from the power supply voltage UN via the power supply rectifier 4 and the charging capacitor 5. The positive voltage E is obtained by rectification of the pulse voltage present at the winding W by means of the diode D3 and the capacitor 6. The current measuring resistor R4 in the emitter path of the switching transistor T supplies a voltage which is proportional to the collector current ic and, by way of the threshold value circuit 7, causes the switching transistor T to switch off at a specific collector current ic. The switch S of FIG. 1 is formed by the transistor Ts and the switching voltage Us for the transistor Ts is generated by virtue of the fact that the switched-mode power supply illustrated is of free-running design as a result of the winding W. The transistor TT forms a driver circuit for the transistor Ts. 
     In an exemplary embodiment which was tried and tested in practice, the components had the following values: 
     
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Capacitor C:       10      nF
Resistor R1:       47      ohms
Resistor R2:       470     ohms
Resistor R3        470     ohms
Resistor R4        0.2     ohm
Transistor Ts:
              Type BC369
n1:                40      turns
n2:                13      turns
K:            Roller-type or
              mushroom-type core
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