Power converter using normally on field effect transistors

The present invention relates to a power converter comprising field-effect switching transistors of the normally ON type, said power converter being intended for use, for example, in a variable speed drive, an uninterruptible power supply system or an active filter.

It is known that a power converter comprises a power supply bus provided with a positive line and a negative line, and to which is applied a DC voltage. The power converter also comprises a bus capacitor connected between the positive line and the negative line of the power supply bus and designed to keep the DC voltage constant on the bus. The power converter comprises a number of switching legs, generally three switching legs, connected downstream of the bus capacitor, to the power supply bus. Each switching leg is connected between the positive line and the negative line of the power supply bus and can comprise, for example, two field-effect switching transistors of the normally ON type connected in series on the switching leg. On each leg, a connection mid-point situated between the two transistors is connected to an electrical load. Each transistor is controlled using a control device enabling a control voltage to be applied to it in order to block it. As is known, the transistor control devices are powered by an auxiliary power supply system connected between the positive line and the negative line of the power supply bus.

When the power converter uses field-effect transistors of the normally ON type, it is essential to check on starting that the transistors of the switching legs do not short-circuit the bus capacitor, and thus prevent voltage from being established on the power supply bus and the charging of the auxiliary power supply system. Similarly, when the converter is stopped, it is essential to ensure that the control signals applied to the gates of the transistors do not abruptly short-circuit the bus capacitor before said capacitor is fully discharged.

The aim of the invention is to propose a power converter in which the switching legs or the control signals applied to the gates of the transistors do not short-circuit the bus capacitor respectively when starting up the power converter or when stopping the power converter.

This aim is achieved by a power converter comprising:

a power supply bus provided with a positive line and a negative line, and to which is applied a DC voltage, a bus capacitor connected between the positive line and the negative line of the power supply bus, a number of switching legs connected between the positive line and the negative line of the power supply bus, each switching leg comprising two series-connected transistors and a connection mid-point situated between the two transistors and connected to an electrical load, the transistors of the switching legs being of the field-effect type and each controlled by a gate control device in order to power the electrical load, a first switch connected to the positive line or negative line of the power supply bus, upstream of the bus capacitor, a second switch connected in series with the first switch on the positive line or negative line of the power supply bus, downstream of the bus capacitor, control means for the first switch and the second switch.

According to a particular feature, the first switch is connected in parallel with a limiting resistor on the positive line or negative line of the power supply bus.

According to another particular feature, the control means for the two switches comprise a double-pole relay, each pole corresponding to one of the two switches.

According to another particular feature, the relay is of the electromechanical contactor type.

According to another particular feature, the gate control devices for the transistors are powered by an auxiliary power supply system connected between the positive line and the negative line of the power supply bus.

According to another particular feature, the field-effect transistors of the switching legs are of the normally ON type or normally OFF type.

According to another particular feature, the transistors of the switching legs are of JFET type.

According to another particular feature, the JFET transistors are fabricated from silicon carbide or gallium nitride.

The invention also relates to a variable speed drive comprising a rectifier module, said variable speed drive also comprising a power converter as defined in one of the preceding claims, connected downstream of said rectifier module.

Other features and advantages will emerge from the following detailed description by referring to an embodiment given by way of example and represented by the appended drawings in which:

FIG. 1 represents the power converter of the invention.

The power converter of the invention as represented in FIG. 1 can be used, for example, in a variable speed drive, an uninterruptible power supply (UPS) system or an active filter. It is designed to receive an input voltage from a source 1 and to apply an output voltage for example to an electrical load 2 . In a variable speed drive type application, a rectifier module is used upstream of the converter to rectify the alternating voltage originating from the mains supply.

Referring to FIG. 1 , a power converter typically comprises a power supply bus provided with a positive line 10 and a negative line 11 and to which is applied a DC voltage Vbus. The power converter also comprises a bus capacitor Cbus connected between the positive line 10 and the negative line 11 of the power supply bus and designed to keep the DC voltage Vbus of the power supply bus constant. The power converter also comprises, downstream of the bus capacitor Cbus, a switching module with n phases having, on each phase, 2 n switching transistors. The switching module as represented in FIG. 1 has three phases U, V, W and therefore comprises three switching legs, each connected between the positive line 10 and the negative line 11 of the power supply bus. Each switching leg comprises a top transistor T 2 , T 4 , T 6 and a bottom transistor T 1 , T 3 , T 5 , separated by a connection mid-point M 1 , M 2 , M 3 connected to the electrical load 2 .

Each transistor T 1 -T 6 of the switching module is of the field-effect transistor (FET) type. A field-effect transistor such as, for example, a JFET or a MOSFET, is a known power electronic switch which comprises a control gate (G), the function of which is to allow or prevent the passage of a current between a Drain (D) and a Source (S). Such a transistor is said to be of the normally ON type if the voltage V GS between the Gate and the Source is close to zero. This means that the Drain-Source path is passing or conducting in the absence of control voltage V GS . In the presence of a control voltage V GS between its gate and its source which is negative, the normally ON field-effect transistor is switched OFF. A JFET-type transistor is switched OFF by applying a gate-source voltage V GS , for example at least equal to −15 volts, and a MOSFET transistor is switched OFF with a voltage V GS that is, for example, at least equal to −5 volts. Moreover, the transistor is said to be of normally OFF type if, in the absence of a voltage V GS between gate and source, the Drain-Source path is not conducting. It therefore requires a positive gate-source voltage V GS to be switched ON. For a normally OFF JFET-type transistor, this positive voltage is typically between +1 volt and +3 volts.

The field-effect transistors used in the power converter of the invention will, for example, be fabricated from a high-energy band gap material (also called “wide band-gap material”) such as, for example, silicon carbide or gallium nitride. As is known, a JFET transistor produced from a high-energy band gap material and of normally ON type offers the advantages of being faster to switch, generating fewer conduction losses in the passing state (low resistance R DSon in the passing state), having a better temperature resistance and being smaller. Hereinafter in the description, and in FIGS. 1 and 2 , the transistors T 1 -T 6 used are, for example, of normally ON JFET type. However, it should be understood that the invention can apply to normally OFF type JFET transistors because they have a low control threshold.

Each field-effect transistor T 1 -T 6 of the switching legs is switched OFF by virtue of a specific gate control device CT 1 -CT 6 . Each gate control device CT 1 -CT 6 is powered (A) by virtue of an auxiliary power supply system AUX connected between the positive line 10 and the negative line 11 of the power supply bus and is used to apply a gate voltage V G to the transistor in order to switch the transistor ON or OFF. In addition to the power supply (A), each control device CT 1 -CT 6 receives from a central control system 3 , control signals S 1 to S 6 with pulse width modulation (PWM) that observe a control law executed by the central control system 3 . Each control device CT 2 , CT 4 , CT 6 of the top transistors T 2 , T 4 , T 6 is connected to the connection mid-point M 1 , M 2 , M 3 of the switching leg of its transistor, whereas each control device CT 1 , CT 3 , CT 5 of the bottom transistors T 1 , T 3 , T 5 is connected to the negative line 11 of the power supply bus. Since the control devices CT 1 -CT 6 are already known, they are not detailed in the present application.

Moreover, the converter of the invention comprises, on the positive line 10 of the power supply bus, upstream of the bus capacitor Cbus, a limiting resistor RL also called precharging resistor in parallel with which is connected a first switch I 1 . This limiting resistor is active on starting during the power supply bus charging phase. In a variable speed drive, it can be used to limit the input current through the rectifier module. Once the bus capacitor Cbus is charged, the limiting resistor RL is short-circuited by the first switch I 1 connected in parallel.

The power converter also comprises, on the positive line 10 of the power supply bus, downstream of the bus capacitor Cbus, a second switch I 2 . The two switches I 1 , I 2 are therefore connected in series on the positive line 10 of the power supply bus and positioned either side of the point of connection of the bus capacitor Cbus to the positive line 10 of the power supply bus. This second switch I 2 is used to isolate the switching legs from the power supply bus when the bus capacitor Cbus is being charged.

According to the invention, the two switches I 1 , I 2 are, for example, mutually-synchronized relays controlled by the control means 3 described hereinabove. The relays are, for example, of electromechanical type. In a variant embodiment, the two relays are replaced by a single mechanical relay, for example of the bipolar contactor type 4 . According to the invention, it is in fact possible to re-use the contactor 4 already employed to control the first switch I 1 and to add to it a second pole to control the second switch I 2 . Each pole of the contactor then represents a switch I 1 , I 2 to be controlled. By sending a single control signal S to the contactor 4 , the two switches I 1 , I 2 are then simultaneously controlled.

The power converter of the invention operates as follows:

Before power-up, the bus capacitor Cbus is discharged and the transistors T 1 -T 6 which are normally OFF are ON because no control voltage is applied to their respective gates. The two switches I 1 and I 2 are OFF and the voltage of the auxiliary power supply system AUX is zero.

When the converter is started up, the source 1 applies a DC voltage to the power supply bus causing the bus capacitor Cbus to be charged. The bus voltage Vbus increases. The bus capacitor Cbus is charged via the limiting resistor RL. The two switches I 1 and I 2 remain OFF. Since the second switch I 2 is in the OFF state, the three switching legs are decoupled from the bus capacitor Cbus. This therefore allows the voltage at the terminals of the bus capacitor Cbus and the voltage of the auxiliary power supply system AUX to rise.

Once the bus capacitor Cbus is charged, the auxiliary power supply system AUX has finished starting up and is therefore charged. The auxiliary power supply system AUX is then used to power the control devices CT 1 -CT 6 of the transistors T 1 -T 6 of the switching module. The transistors T 1 -T 6 can therefore be directly switched OFF by their control device CT 1 -CT 6 by applying a control voltage to their respective gates. Once all the transistors T 1 -T 6 are OFF, the two switches I 1 and I 2 are simultaneously switched ON. For this, the control means 3 send the control signal S to the contactor 4 , giving it the command to switch ON the two switches I 1 , I 2 . The limiting resistor RL is then short-circuited via the first switch I 1 and the power supply bus is now connected to the switching legs via the second switch I 2 . The converter can then operate normally.

When the converter is switched OFF, the control means 3 stop the contactor 4 which causes the two switches I 1 and I 2 to be simultaneously switched OFF. The switching legs are then once again electrically isolated from the power supply bus. Before the two switches I 1 , I 2 are fully OFF, the auxiliary power supply system AUX mist guarantee a voltage level that is sufficient to keep the transistors T 1 -T 6 in the OFF state.

According to the invention, the two switches I 1 and I 2 can also be positioned in series on the negative line 11 of the power supply bus, either side of the point of connection of the bus capacitor Cbus to this line.

Obviously, it is possible, without departing from the framework of the invention, to consider other variants and refinements of detail and even to consider the use of equivalent means.