PROTECTIVE CIRCUIT FOR A HACKER TYPE DC CONVERTER

Abstract: A protective circuit for a hacker type DC converter in which a coil (L1), a diode (D1) and a capacitor (C1) are connected in series to the poles of a supply voltage source (U1). Power losses as well as rapid voltage and current variations are reduced by two suitably selected inductive elements (L2, L3) being inserted in series with the diode, and also by these inductive elements (L2, L3) and the diode (D1) being bridged over by a second diode (D2).

Description: PROTECTIVE CIRCUIT FOR A HACKER TYPE DC CONVERTER. TECHNICAL FIELD The invention relates to a protective circuit for a hacker type DC converter in which a series circuit including a coil, a diode and a capacitor is connected to the poles of a supply voltage source, and in which a connection point between the coil and the diode is connected to the end of the capacitor facing away from the diode via a controllable switching element which is adapted for periodically changing between open and closed states. BACKGROUND ART A converter of the kind mentioned above is illustrated, e.g. in DE 2644715, Figure 25. Large power losses occur in such a converter, particularly when converting high voltages and at high power take-off. The losses occur parti¬ cularly in the diode, through which a reverse current occurs each time the switching element closes. Losses in the diode also occur when it is on-state in the forward direction. These so-called on-state losses dominate at comparatively low switching frequencies e.g. at the mains frequency (50-60 Hz). On the other hand, the switching-off losses of the diode dominate when hacking at high frequency, e.g. 20 kHz, particularly in conjunction with high voltages, e.g. 400 volts, and high current strengths. Apart from the dis¬ advantages which the losses intrinsically are, they also result, together with the heavy voltage and current variations occurring in the converter, in that the diode and switch are subjected to large stresses and are easily damaged. The current variations also give rise to radio interference, which can both disturb the apparatus in which the converter is included and other apparatus as well. The known converter according to the above has been provided with a protective circuit for decreasing the power losses and high voltages at the switch. However, it has been found difficult to get a circuit of this kind to function satisfactorily. Furthermore, the losses are increased due to energy that has been stored in coils and capacitors being consumed in resistors. DISCLOSURE OF INVENTION The object of the present invention is to provide a protective circuit for a DC converter of the kind mentioned above, with the aid of which power losses as well as rapid voltage and current variations may be reduced considerably. This is achieved in principle by two suitably selected inductive elements being inserted in series with the diode, and that a second diode is connected in parallel with these and the diode. The characterizing features of the invention are disclosed in the claims. BRIEF DESCRPTION OF DRAWINGS The invention will now be described in detail with reference to the accom¬ panying drawing, on which Figure 1 illustrates an already known DC voltage converter, and Figure 2 illustrates a DC converter with a protective circuit in accordance with the invention. BEST MODE FOR CARRYING OUT THE INVENTION An already known DC converter of the hacker type is illustrated in Figure 1. A coil LI, a diode Dl and a capacitor Cl are connected in series to the poles of a supply voltage source Ul. A load L is connected in parallel with the capacitor. A connection point p between the coil and diode is connected to the negative pole of the voltage source via a controllable switching element T in the form of a transistor. If the transistor is controlled to be alternatingly on-state and off- state, the voltage U2 across the capacitor and load will be higher than the supply voltage Ul, and then the converter functions as a voltage increasing circuit. Since the capacitor Cl will be positively charged on its side facing towards the diode Dl, the diode will be biassed in the reverse direction each time the transistor T becomes on-state. Before the diode blocks in the reverse direction a charge stored during its on-state in the forward direction gives rise to a reverse current during the diode recovery time. The reverse current involves a power loss, which will be considerable for hacking at high frequency, particu- larly in converting high voltages and at high power take-offs. As will be seen from the above the power losses and the heavy voltage and current variations cause the diode and transistor to be subjected to large stresses and the occurrence of radio interference. An embodiment of a DC converter with a protective circuit in accordance with the invention is illustrated in Figure 2, the coil LI, diode Dl, capacitor Cl and load L have their counterparts in Figure 1. The switching element T may comprise a transistor, as with the known converter according to Figure 1. Two further coils L2 and L3, of which the coil L2 is inductively connected to the coil LI, are in series with the coil LI and the diode Dl inserted between the point p and the diode anode. The coil L3 can either consist of an optionally selected inductive element with suitable inductance, or it may be achieved in effect with the aid of the leakage field from the coil L2. All three coils may be provided by a single coil in practice. Since the current through a coil has the tendency of only changing com¬ paratively slowly, there is obtained, due to the coil L3, a considerably weaker and shorter duration reverse current through the diode Dl when the switch T is switched on than in conventional converters of this kind. This gives a lower power loss in the diode and simultaneously a milder current shock through the switch. A diode denoted D2 is in series with a resistor R2 and connects the point p to the capacitor Cl. The task of the diode is to be a path for the current from the coil LI directly after the switch TI is switched off. At the beginning of each switch-off period the coil L3 namely prevents the current from the coil LI from going through the diode Dl. The current path R2-D2 also limits the growth of the voltage across the switch on switching off, and this voltage could otherwise damage the switch. To limit this voltage the switch is also connected in parallel, in a manner known per se, with a circuit consisting of a capacitor C3 in series with a resistor R3, across which a diode D3 is connected. When the switch T is switched off, the current flows first for a short time through the circuit connected in parallel with the switch, the capacitor C3 then being charged. When the current through this decreases, it increases instead through the resistor R2 and diode D2. Finally, the current increases through the diode Dl, which is accelerated by an emk occurring in the coil L2 and generated due to the inductive coupling between the coils LI and L2. By suitable mutual adjustment of the components, the current is transferred entirely from the branch R2-D2 to the branch L2-L3-D1, whereafter it is also retained in this branch. This depends above all on that the emk in the coil L2, at least to a certain extent, can be caused to occur during the whole time the switch is off. The transfer and retention is further facilitated by the resistance of the resistor R2, and if the diode Dl has a lower threshold voltage than the diode D2. The resistor R2 may possibly be excluded. However, it is essential that the current through the diode D2 ceases before the switch T is closed again, since a reversed voltage would otherwise occur in this diode. The capacitor Cl suitably comprises an electrolyte capacitor with relatively high capacitance. A smaller capacitor C2 is connected in parallel with it, but closer to the diode D2 than what the capacitor Cl is situated, from the point of view of impedance. The task of this smaller capacitor is to be a low-impeditive path for high-frequency currents from the branch R2-D2, although it is conceivable to exclude this capacitor. When the switch T is switched on there may be a higher voltage across the diode Dl than across the capacitor Cl, due to that the potential at the diode anode can be lower than the potential at the negative pole of the supply voltage source Ul. This high voltage is reduced by an RC link connected across the diode and consisting of a resistor R4 in series with a capacitor C4. The voltage is also reduced by an attenuation circuit between the diode Dl anode and the negative supply pole. This circuit comprises a diode D4 in series with a capacitor C5, which is connected in parallel with a resistor R5 in series with a Zener diode D5. When the potential at the diode Dl anode becomes lower than the potential at the negative pole of the supply voltage source, the diode D4 becomes on-state and the capacitor C5 is charged. The voltage over the diode Dl is thus substantially limited to the sum of the voltages across the capacitors Cl and C5. The voltage across the capacitor C5 is determined by the Zener voltage of the diode D5, and with suitable values for this and the resistance of the resistor R5 the losses in these components can be limited to modest values. By way of summary it may be said that the losses and stresses in the diode Dl and switching element T may be substantially limited by the circuits shown, while high-frequency interference fields are heavily reduced. Examples of component values from a functioning circuit will be seen from the list below. L:l lmH R2:0,l ohm R5:10 ohms C3:l nF L2: 10f H R3:100 ohms Cl:5 000 j*F C4: InF L3:10 H R4:100 ohms C2:l F C5:0,1JA F D5 zener voltage : 50 V

Claims: CLAIMS 1 Protective circuit for a hacker type DC converter, in which a series circuit including a coil (LI), a diode (Dl) and a capacitor (Cl) is connected to the poles of a supply voltage source (Ul), and in which a connection point (p) between the coil (LI) and the diode (Dl) is connected to the end of the capacitor (Cl) facing away from the diode across a controllable switching element (T), which is adapted to periodically alternate between open and closed states, characterized in that the series circuit between said connection point (p) and the diode (Dl) further includes an inductive element (L3) and a second coil (L2) which is inductively connected to the existing coil (LI) and that a second diode (D2) is connected in a parallel circuit between the connection point (p) and the electrode of the existing diode (Dl) facing away from this point. 2 Protective circuit as claimed in claim 1, characterized in that a resistor (R2) is connected in said parallel circuit in series with the second diode (D2). 3 Protective circuit as claimed in claim 1 or 2, characterized in that the diode (Dl) in the series circuit has a lower threshold voltage than said second diode D2. 4 Protective circuit as claimed in any one of the preceding claims, characterized in that the capacitor (Cl) in the series circuit is connected in parallel with a second capacitor (C2) which from the impedance aspect has a shorter distance to said second diode (D2) than the capacitor Cl in the series circuit has. 5 Protective circuit as claimed in any one of the preceding claims, characterized in that a circuit comprising a capacitor (C3) in series with a resistor (R3) across which a diode (D3) is connected is connected in parallel with the switching element (T). 6 Protective circuit as claimed in any one of the preceding claims, characterized in that an RC link comprising a resistor (R4) in series with a capacitor (C4) is connected in parallel with the diode (Dl) in said series circuit. 7 Protective circuit as claimed in any one of the preceding claims, characterized in that a circuit comprising a diode (D4) in series with two branches connected in parallel, of which one is a capacitor (C5) and the other a resistor (R5) in series with a Zener diode (D5) , is connected between the diode (Dl) electrode facing towards said connection point (p) in said series circuit, and the end of the capacitor (Cl) facing away from this diode (Dl) in said series circuit.

Inventors: EKELUND FOLKE JOSEF BERTIL, LILJA PER OLOF, STAOHL MATTS FILIP, WALD ROLAND JAN ELLERT, EKELUND, FOLKE, JOSEF, BERTIL, LILJA, PER, OLOF, STAOHL, MATTS, FILIP, WALD, ROLAND, JAN, ELLERT
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1985
Main classification: H02H9
IPC codes: H02H9, H02M3