ACTIVE RECTIFIER CIRCUIT

Abstract: The invention relates to power sources, more particularly to rectifier circuits realised with MOSFET-transistors. In the system according to the invention, the sources of MOSFET switching transistors are connected to ground potential, and the secondary windings of the main transformer are connected in between the switching transistor drain and the positive outlet of the power source. The grid control of the MOSFET-transistors is arranged with small power transformers from between the switching transistor drain and the secondary winding of the main transformer, and according to the secondary voltage of said power transformers, the separate grid control circuits switch the MOSFET-transistors rapidly to conductive or non-conductive mode. By employing the arrangement according to the invention, there is achieved an extremely rapid switching and respectively extremely low power losses.

Description: Active rectifier circuit The invention relates to power sources, according to the preambe of claim 1. The invention relates more particularly to rectifier circuits realised with MOSFET- transistors. Figure 1 illustrates a synchronous rectifier circuit according to the prior art. The voltage to be rectified is produced with the secondary windings T1N2 and T1N3 of a mains transformer Tl, and with the windings T1N4 and T1N5 the control voltages for the grids of the MOSFET-transistors. When the voltage of the winding T1N4 effective on the grid of the transistor Ml and the voltage of the winding T1N2 turn to positive, Ml begins to conduct. Current flows first via the internal diode of the transistor Ml, until the control voltage has risen above the grid threshold voltage, whereupon the transistor channel begins to conduct. The voltage loss created in the diode during this period causes power loss, and as a result the transistor is heated. Said period also is relatively long, because the mains voltage frequency is low, and hence the rate of voltage rise on the switching transistor grids is low. The circuit of figure 1 functions as such with low power areas, but with power sources having a higher power than 200 W, the MOSFET:s used in such circuits are heated too much. The losses created in MOSFET-transistors can be reduced by speeding up the control rate, i.e. the rising speed of grid voltage. An increase in the number of turns of the windings T1N4 and T1N5, i.e. an increase in the alternating voltage generated by the windings, would increase the rising speed of the voltage, but the use this method is very limited owing to the low maximum rating of the voltage between the MOSFET power transistor grid and source. Typically the maximum rating of the MOSFET power transistors is a voltage of the order of 20 V between grid and source. The most advantageous form of a control signal would be a square wave generated for instance by a digital oscillator. With this type of control, the losses of a MOSFET remains very small, because then the grid voltage rises so rapidly above the grid threshold voltage, that there is no time for power losses to be created. In a circuit according to figure 1, such a separate control circuit is very troublesome to realise, because the transistor sources are in an alternating potential, and this square- wave-formed control voltage should be created exactly between the source and the grid. It is an object of the invention to realise a low-loss rectifier circuit which is heated remarkably less than in prior art systems. Another object of the invention is to realise a simple MOSFET rectifier circuit, whereby a fast grid control of the switch transistors is obtained. These objects are achieved by changing the place of the secondary windings and the switching transistors. Thus the MOSFET source is no more in a floating potential, and consequently the grid control can be realised without the auxiliary windings T1N4 and T1N5 located in the secondary part of the main transformer Tl . The system according to the invention is characterised in that - the sources of said switching transistors are connected to fixed potential, - the first end of said first secondary winding is connected to the first outlet of the power source, - the source of the first switching transistor is connected to the second outlet of the power source, and - in between the other end of said first secondary winding and the drain of the first switching transistor, there is connected at least a wire; and that - the first end of said second secondary winding is connected to the first outlet of the power source, - the drain of the second switching transistor is connected to the second outlet of the power source, and - in between the other end of said secondary winding and the drain of the second switching transistor, there is connected at least a wire; and in that, that the power source further comprises - at least one switching transistor control circuit (1) which is connected to the grid of at least one switching transistor, - a first control transformer connected in between the drain of said first switching transistor and the other end of said first secondary winding, said first control transformer comprising at least two windings, - a second control transformer connected in between the drain of said second switching transistor and the other end of said second secondary winding, said second control transformer comprising at least two windings; and in that, that - the second winding of said first control transformer is connected to at least one control circuit of a switching transistor, and - the second winding of said second switching transformer is connected to at least one control circuit of a switching transistor. The dependent claims describe further advantageous embodiments of the invention. The invention is explained in more detail below, with reference to a few preferred embodiments described by way of example, and to the appended drawings where figure 1 illustrates a MOSFET rectifier circuit according to the prior art, figure 2 illustrates a preferred embodiment according to the invention, figure 3 illustrates another preferred embodiment according to the invention, figure 4 illustrates a grid control circuit of a MOSFET, suited for the system of figure 2, and figure 5 illustrates a third preferred embodiment according to the invention. Like numbers or symbols for like parts are used in the drawings. The symbol Tx signifies a transformer or toroid x, and the symbol TxNy, where x andy are small integrals, signifies the winding y of said transformer or toroid x. Figure 2 illustrates a preferred embodiment according to the invention. In this embodiment, the sources of both MOSFET-transistors are coupled to ground potential, so that there can be realised a simple grid control circuit resulting in rapid control. The operation of the preferred embodiment illustrated in figure 2 is based on the fact that by means of small transformers T3 and T4, there is formed a control signal of the current generated by the main transformer onto the grid control circuit 1. When the end of the winding T1N2 that is located on the voltage source outlet side is more positive than the other end of said winding, the current passing through the winding T1N2 renders the MOSFET Ml to conductive mode, by means of the windings T3N1 and T3N3 of the toroid T3, and simultaneously it renders the MOSFET M2 to non-conductive mode by means of the windings T4N1 and T4N3 of the toroid T4. When the end of the winding T1N3 that is located on the voltage source outlet side becomes umore positive than the other end of said winding, the current passing through the winding T1N3 renders the transistor M2 to conductive mode by means of the windings T4N2 and T4N3 of the toroid T4, and the transistor Ml to non-conductive mode by means of the windings T3N2 and T3N3 of the toroid T3. The employed transformers T3 and T4 can advantageously comprise a small toroid, through which the wire coming out of the secondary winding of the mains transformer is taken, so that there is only one turn in the primary winding of the transformer T3, T4. According to another preferred embodiment of the invention, the control signal is not obtained with separate control transformers T3, T4, but the control signal is obtained from the third and fourth secondary windings of the mains transformer, as is illustrated in figure 3. Now the windings T1N4 and T1N5 of the mains transformer are coupled to the control circuits 1 of the MOSFET-transistors. An advantageous arrangement of the grid control circuit 1 is illustrated in figure 4. This type of control circuit can advantageously be used together with the preferred embodiment illustrated in figure 2. The control circuit is realised with two complementary bipolar transistors VI and V2, which are connected as emitter followers. When the point-marked end of the winding T3N3/T4N3 is more positive than the emitter-connected end of the transistors VI and V2, the transistor VI is rendered to conductive mode, so that to the outlet of the control circuit and to the grid of the MOSFET Ml, M2, there begins to flow current, whereupon the MOSFET Ml, M2 is switched to conductive mode. When the emitter-connected end of the transistors VI and V2 of the winding T3N3/T4N3 is the more positive, the transistor V2 is rendered to conductive mode, in which case the grid charge of the MOSFET Ml, M2 begins to be discharged. At the same moment, the thyristor coupling formed by the transistors V3 and V4 is triggered to conductive mode, thus remarkably speeding up the discharge of the grid charge of the MOSFET and its switching to non-conductive mode. The diodes Dl - D7 serve as saturation prevention diodes of the transistors VI and V2. When VI is switched to conductive mode, its collector is in a higher potential - for the magnitude of two diode threshold voltages - than its base when VI is not saturated. When the voltage effective in relation to the transistor VI changes polarity, VI is rapidly rendered to non-conductive mode. Saturation prevention diodes reduce the transistor storage time, which forms the longest delay when switching a transistor from conductive mode to non-conductive mode. Figure 5 illustrates a third preferred embodiment of the invention. By applying the arrangement according to the invention, it is also possible to realise a double voltage source, the negative outlet voltage whereof can be obtained by means of a circuit illustrated in figure 2; however, the polarity of the voltages is opposite to said figure 2, and instead of n-type MOSFET-transistors, there are used / type MOSFET- transistors. In this specification, we have described various preferred embodiments of the invention by means of MOSFET-transistors, by way of example. However, for a man skilled in the art it is obvious that in the power source according to the invention, other types of known switching transistors can also be employed. In the appended claims, the terms source, drain and grid signify functionally corresponding members of the switching transistor in question in each particular case. By using the current-controlled active rectification according to the invention, there are achieved extremely low power losses even with high power outputs, because with the grid control circuit illustrated in figure 4, the discharging of the grid charge takes place very rapidly, and consequently the switching and cutting-off times of the MOSFET-transistors are rendered very short. The circuit losses are affected mainly by the resistive losses of the channel resistance of the employed MOSFET- transistors.

Claims: Claims 1. A power source comprising a mains transformer, said mains transformer including a first and second secondary winding, and the power source further comprising two switching transistors for rectifying the current of said secondary windings, characterised in that - the sources of said switching transistors are connected to fixed potential, - the first end of said first secondary winding is connected to the first outlet of the power source, - the source of the first switching transistor is connected to the second outlet of the power source, and - in between the other end of said first secondary winding and the drain of the first switching transistor, there is connected at least a wire; and that - the first end of said second secondary winding is connected to the first outlet of the power source, - the drain of the second switching transistor is connected to the second outlet of the power source, and - in between the other end of said secondary winding and the drain of the second switching transistor, there is connected at least a wire; and in that, that the power source further comprises - at least one switching transistor control circuit (1) which is connected to the grid of at least one switching transistor, - a first control transformer connected in between the drain of said first switching transistor and the other end of said first secondary winding, said first control transformer comprising at least two windings, - a second control transformer connected in between the drain of said second switching transistor and the other end of said second secondary winding, said second control transformer comprising at least two windings; and in that, that - the second winding of said first control transformer is connected to at least one control circuit of a switching transistor, and - the second winding of said second switching transformer is connected to at least one control circuit of a switching transistor. 2. The power source of claim 1, characterised in that said first control transformer further comprises a third winding, which is connected in between the other end of said second secondary winding and the drain of said second switching transistor. 3. The power source of claim 1, characterised in that said second control transformer further comprises a third winding, which is connected in between the other end of said first secondary winding and the drain of said first switching transistor. 4. The power source of claim 1, characterised in that said mains transformer further comprises a third secondary winding, which is connected at least to the control circuit of the first switching transistor. 5. The power source of claim 3 or 4, characterised in that said first outlet of the power source is the positive outlet of the power source, that said second outlet of the power source is the outlet of the ground potential of the power source and that said switching transistors are «-type switching transistors. 6. The power source of claim 3 or 4, characterised in that said first outlet of the power source is the negative outlet of the power source, that said second outlet of the power source is the outlet of the ground potential of the power source and that said switching transistors axep- ype switching transistors. 7. The power source of claim 1, characterised in that said switching transistor is a MOSFET.

Inventors: SAIRANEN MARTTI, SALONEN OLLI, SAIRANEN, MARTTI, SALONEN, OLLI
Applicants: AHLSTROEM OY, SAIRANEN MARTTI, SALONEN OLLI, A. AHLSTROM CORPORATION, SAIRANEN, MARTTI, SALONEN, OLLI
Application year: 1997
Main classification: H02M7
IPC codes: H02M3, H02M7