System for balancing the voltage of series-connected semiconductor elements

A circuit for balancing a voltage across a semiconductor element series-connected with other semiconductor elements of the same type may include a comparator configured to compare data representative of a voltage across the semiconductor element with a reference voltage, and a resistive element of adjustable value and configured to be controlled by the comparator.

RELATED APPLICATION

This application claims the priority benefit of French Patent application number 13/61454, filed on Nov. 21, 2013, the contents of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to electronic circuits and, more generally, to a voltage-balancing system intended for series-connected semiconductor devices. The present disclosure more specifically relates to power applications.

BACKGROUND

Frequently, a plurality of semiconductor components, to which are applied, together, a relatively high voltage with respect to the individual voltages that the different components have to withstand, are series-connected. Such is in particular the case for rectifying diodes series-connected between two terminals where a high voltage is applied (typically, several hundreds of volts). Each diode then sees a voltage lower than the total voltage. Ideally, the voltage seen by each diode, or more generally by each semiconductor element, corresponds to the total voltage divided by the number of components. In practice, due to technological dispersions in the forming of semiconductor devices, to manufacturing tolerances, and dispersions due to a temperature difference between each element in the application, there frequently is a voltage imbalance between the different diodes. This may lead to a situation where one of the diodes sees, between its terminals, a voltage which exceeds the voltage that it can withstand.

Systems that include a protection resistor connected in parallel with each diode to achieve a static balance and a capacitor connected for a dynamic protection have already been provided. However, devices based on resistors in parallel with the diodes have the major disadvantage of generating permanent losses through the resistors.

SUMMARY

An embodiment of the present disclosure overcomes all or part of the disadvantages of usual balancing systems. Another embodiment provides an approach for decreasing losses with respect to a system where balancing resistors in parallel with each element are used.

Another embodiment provides a self-adaptive approach. Another embodiment provides an approach that does not use a control signal from the outside.

Thus, an embodiment provides a circuit for balancing a voltage across a semiconductor element series-connected with other semiconductor elements of the same type. The circuit includes a comparator of data representative of the voltage between the terminals of the semiconductor element with a reference voltage. The circuit also includes an adjustable value resistive element that is controlled by the comparator.

According to an embodiment, the reference voltage is representative of the voltage applied to the series association of the semiconductor elements, divided by the number of elements. According to an embodiment, the resistive element is a metal oxide semiconductor (MOS) transistor.

According to an embodiment, the comparator is a first operational amplifier having a first input receiving the data representative of the voltage between the terminals of the semiconductor element and having a second input receiving the reference voltage. According to an embodiment, the reference voltage is provided by a second operational amplifier assembled as a subtractor.

According to an embodiment, the data representative of the voltage across the semiconductor element is sampled from the midpoint of a first resistive dividing bridge connected between the terminals of the semiconductor element. According to an embodiment, the circuit comprises two first terminals intended to be connected between the terminals of said semiconductor element, and two second terminals to be respectively connected to a previous circuit and to a next circuit in the series association of the semiconductor elements.

According to an embodiment, a second resistive dividing bridge connects the two second terminals. The midpoint of the second dividing bridge is connected to a first input of the second operational amplifier having a second input connected, by a second resistive element, to the other second terminal, and by a third resistive element to one of the first terminals.

A system for self-balancing series-connected semiconductor elements includes a circuit such as described above connected in parallel on each semiconductor element. According to an embodiment, the semiconductor elements are diodes.

DETAILED DESCRIPTION

The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those steps and elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the destination of a series association of semiconductor elements has not been detailed, as the described embodiments are compatible with usual applications that use a plurality of series-connected semiconductor devices.

FIG. 1is a simplified representation of an example of a series association of elements or components D1, D2, . . . , Dn-1, Dnhaving a voltage VTapplied between end terminals12,14of their series association. Each device sees, between its terminals, a voltage VDi(i being in the range from 1 to n, and n standing for the number of series-connected components).

The most current applications of such series associations of semiconductor components relate to protection devices, for example, AC/DC or DC/DC voltage conversion devices such as high-voltage switched-mode power supplies capable of being used in fields such as traction, solar inverters, laser control, etc. Thus, devices D1to Dn, for example, are semiconductor diodes but may more generally be any semiconductor component, for example, MOS transistors, insulated-gate bipolar transistors (IGBTs), switches, Schottky diodes, zener diodes, etc.

In an assembly of the type inFIG. 1, technological dispersions between the different components may result in that one or a plurality thereof sees between their terminals a voltage which exceeds the voltage that they can withstand. Taking the example of semiconductor diodes, these are given for a repetitive reverse maximum voltage (VRRM) and a maximum reverse current (IR) corresponding to the maximum instantaneous value which, for voltage VRRM, corresponds to the maximum junction temperature. A maximum reverse voltage (VRM), which corresponds to value VRRMmultiplied by a safety factor k, is then defined.

In known circuits, a resistor (not shown inFIG. 1) is connected in parallel with each diode to statistically balance the voltage, and capacitances are generally also assembled, individually and in parallel on each diode, for dynamic balancing purposes. The resistors and capacitances are sized so that the voltage across each diode of the series association does not exceed value VRN. However, a problem of permanent resistive loss on operation of the circuit arises. Further, the resistors generally have to be sized to take the worst case into account. This results in increasing losses in normal operation.

FIG. 2is a simplified representation of an embodiment of a system where an active circuit Ci(i being in the range from 1 to n) is connected in parallel with each component, in this example, diodes Di(D1to Dn). Each circuit Ciis connected to terminals Kiand Aiof the concerned diode. Since the diodes are in series, anode Aiof a diode of rank i is common (or connected) to cathode Ki+1, of the diode of rank i+1. In the example of direction taken in the drawings, the cathode of diode D1defines terminal12of the application of the high potential of voltage VTand anode Anof diode Dndefines terminal14of the application of the low potential of voltage VT.

Each circuit Ciis connected to circuits Ci−1and Ci+1which are adjacent thereto by its connection to the anode and cathode of the diode associated therewith (except for the first C1and last Cncircuits which are only respectively connected to the next circuit C2and to the previous circuit Cn-1). Further, each circuit Ciis also directly connected to its adjacent circuits Ci−1and Ci+1, with the same exception for the first and last circuits, independently from the connection via its diode. This will better appear from the following description ofFIG. 3.

FIG. 3shows the detailed electric diagram of an embodiment of a circuit Ci.FIG. 4illustrates the connection (or assembly) of n circuits in a series association of n diodes.

Each circuit Cicomprises two terminals21and23to be connected to respective electrodes Kiand Aiof the concerned diode, and two terminals25and27to be respectively connected to circuits Ci−1and Ci+1. Circuit C1has its terminal25connected to its terminal21(and thus to terminal12). Circuit Cnhas its terminal27connected to terminal23(and thus to terminal14).

Although, in the following description of circuit Ci, the case of resistive elements made in the form of resistors will be taken as an example as the “resistors” may take different forms (for example, the form of MOS transistors). Indeed, the values of the resistive elements of circuit Ciare compatible with an embodiment in the form of an integrated circuit.

A resistive dividing bridge formed by two resistors R1and R2series-connected between terminals21and23provides data VFBirelative to the voltage across diode Di(not shown inFIG. 3). Data VFBisampled from midpoint22of this series association is sent onto a non-inverting terminal (+) of an operational amplifier31assembled as an analog comparator with a reference level VREF. The output of amplifier31is connected to a control electrode of a transistor, for example, a MOS transistor M (for example, of type N). Transistor M operates in a linear mode, that is, its gate control signal varies its on-state series resistance (RdsON). Transistor M connects terminals21and23. Comparator31is assembled as an inverter as its output is connected to its inverting input terminal (−) by a resistor R3, and reference level VREFis applied via a resistor R4, preferably of same value as resistor R3. The values of resistors R1and R2are selected according to the supply voltage of the comparator31and to the maximum voltage between terminals21and23(maximum voltage VDicapable of being seen by diode Di).

Assuming that the diodes have identical characteristics (same voltage VRRM), the resistances R1of all circuits Cipreferably have the same value, and the resistances R2of all the circuits preferably have the same value. The same is true, on the one hand, for resistances R3, and on the other hand, for resistances R4.

Reference level VREFis generated by an operational amplifier33assembled as a subtractor and having the function of dividing by number n of diodes Diof the series association voltage VTpresent between terminals12and14, and for each diode Di, referencing this value to its anode (terminal23of circuit Ci). The non-inverting input (+) of the amplifier33is connected to its output24by a resistor R5as well as, by a resistor R6, to junction point26of resistors R7and R8, series-connected between terminals25and27of circuit Ci. The inverting input (−) of amplifier33is connected, by a resistor R9, to terminal27and, by a resistor R10, to terminal23, to reference voltage VREFto anode Aiof diode Di.

Each circuit Ciis connected to the adjacent circuits by its terminals25and27, terminal25of circuit Cibeing connected to terminal27of circuit Ci−1of lower rank and terminal27of circuit Cibeing connected to terminal25of circuit Ci+1of higher rank. Terminal25of circuit C1is connected to terminal12and terminal27of circuit Cnis connected to terminal14.

Assuming that the diodes have identical characteristics (same voltage VRRM), the resistances R7of all circuits preferably have the same value, and the resistances R8of all the circuits preferably have the same value. These values are selected according to the power supply voltage of amplifiers33and to the maximum values capable of being taken by voltages VRRMof the diodes and VT. Resistances R1and R7have the same value and resistances R2and R8have the same value.

Further, resistances R5preferably have the same value, resistances R6preferably have the same value, resistances R9preferably have the same value, and resistances R10preferably have the same value. According to a specific embodiment, the values of resistances R5, R6, R9, and R10are all identical. Resistance R3is preferably much higher than resistance R4(for example, by a factor between 10 and 100) to have a high gain of the system in closed loop.

Circuit Ciis in charge of measuring the voltage across diode Diand of comparing this voltage to total voltage VTdivided by the number of diodes, to deviate, if desired, part of the current through transistor M, and thus balance the different voltages VDiof the series association. The system control results in having voltage VFBitending, for a balanced system, towards voltage VREF.

In case of an imbalance, voltage VFBiis equal to a given ratio of voltage VDiwhile VREFis equal to a given ratio of voltage VT. This ratio is for example equal to R/(R+R′), where R stands for the value of resistances R1and R7, and where R′ stands for the value of resistances R2and R8. Current Ireviin each diode Di, added to current Ioutiin transistor M of circuit Ciassociated therewith, is identical for each diode Di/balancing circuit Ciassociation. The gain of amplifier31and the ratio between current Iouti(and thus resistance RdsON) and gate-source voltage Vgsof transistor M is selected according to the value of total voltage VTand to number n of diodes.

An advantage of the described embodiments is that the power consumption in transistor M of the balancing circuit only occurs in the presence of an imbalance. Indeed, the power consumption due to resistive elements R1and R10and to amplifiers31and32is negligible since the currents are relatively low (typically less than at least 100 times the dissipation for transistor M).

The described embodiments use a power source for amplifiers31and33. The power supply may be extracted from the voltage across the concerned diodes or be sampled from another voltage source of the system.

Various embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. For example, in the case of diodes having nominal characteristics different from one another, the values of the different resistances should be adapted. Further, although reference has been made to a MOS transistor M to deviate the current, the element may more generally be any controllable resistive element, that is, having an adjustable resistance value, for example, a bipolar transistor. Further, the practical implementation of the described embodiments, in particular the sizing of the components of circuits Ci, is within the abilities of those skilled in the art based on the functional indications described above.