Background of the Invention

The invention relates to a method of balancing the voltage distribution over the switches connected in series in a high voltage (HV) switch, wherein a snubber arrangement is associated with each switch. The invention also relates to an HV switch unit.

An HV switch is a switch which is made up a series connection of several semiconductor switches, and capable of switching voltages equal or above 500 V, in particular equal or above 1 kV. The series connection of semiconductor switches may be called a switch stack. All semiconductor switches connected in series and working as one single switch must go into the on and/or off-status simultaneously in order to balance the voltage distribution along the switch stack. Small timing deviations, related to signal propagation delay dispersion, may cause large voltage imbalance and lead to destruction of one or several semiconductor switches and thus the HV switch due to overvoltage or voltage-related stress. The distribution of the voltage on the switches may also be unequal due to different voltages to ground on the bodies of the semiconductor switches and different capacitances between the switches to ground. Unequal voltage distribution can also cause an unequal power loss distribution. This can cause a cooling problem.

The significance of the problem increases with the length of the switch stack, i.e. the number of semiconductor switches, the switching frequency, and switching velocity.

Object of the Invention

It is an object of the invention to provide a method and an HV switch, which overcome the above shortcomings.

Summary

According to a first aspect the invention relates to a method according to claim 1. Disclosed is a method of balancing the voltage distribution over the switches connected in series in an HV switch, wherein a snubber arrangement is associated with each switch, comprising the steps of: a. determine a first quantity related to the snubber arrangement of a first switch, in particular a first voltage across a first snubber energy storage component associated with a first switch, b. determine a second quantity related to the snubber arrangement of a c. second switch, in particular a second voltage across a second snubber energy storage component associated with a second switch, d. compare the first and second quantities, in particular the first and second voltage, d. based on the comparison adjust the drive signal of at least the first switch based on the first quantity. The drive signal of the first switch may be adjusted in a self-controlled manner. Adjustment in a self-controlled manner may include adjustment of the drive signal based on quantities of the HV switch unit without the use of a control. Hence, this term implies that an adjusted drive signal is not generated by a control or a drive signal generator.

The snubber arrangement can be used to determine for example a voltage as a first quantity. Based on the first quantity the control signal for controlling the first switch can be adjusted, thus adjusting the time of switch-on and/or switchoff, i.e. the switching-on and/or switching-off moment.

The drive signal may come from a digital control device, such as a microcontroller, programmable logic device, such as FPGA, or other digital logic circuit. The "adjusting the drive signal" may be done by a drive signal adjuster, which may be a separate electronic circuitry. In this way the drive signal itself could stay constant, i.e. without an individual adjustment. In this way the adjustment may be done very fast and without interference into the logical circuitry or program, which could be disadvantageously slow. In this way the drive signal may also be the same for all switches connected in series and the switching adjustment for one or several, in particular all individual switch(es), may be done by the drive signal adjuster(s). Hence, only one drive signal needs to be generated and there is no need for a control to generate and/or adjust the drive signals for several switches. There is no burden on the control for the generation of several drive signals.

Even voltage distribution among multiple semiconductor switches, which are connected in series to form an HV switch, can be achieved by changing a switching time of an individual switch. Thus, a voltage distribution along the switch stack during each OFF state of the HV switch can be achieved such that each individual switch-stack component does not suffer overvoltage or any excessive voltage-related stress. Thanks to this method of shifting the control signal for semiconductor switches it is possible to influence the voltage distribution along the series connected semiconductor switches. This method can be applied for hard-switching devices (on which a voltage is present before switching-on or/and a current is present before switching-off).

The first and second quantities may be determined for adjacent switches. In this method snubber arrangements, in particular resistor-capacitor (R.C) -snubbers or resistor-capacitor-diode (R.CD) -snubbers, can be provided for each semiconductor switch. Instead of a diode also a different rectifying component may be used. The sum of the voltage of two contiguous snubber arrangements' snubber energy storage components, in particular capacitors, may be divided by a voltage divider, comprising for example resistors. In particular, the voltage may be divided by two. The resistors may be part of the snubber arrangements. By comparing the voltage of the middle of the snubber arrangements' capacitors with the voltage from the voltage divider the control signal may be shifted in time to reach the state, in which snubber capacitors of contiguous switches have the same voltage. This method can be applied to individually shift the drive signal of any number of serial connected semiconductor switches. With this method switching times of each semiconductor may be adjusted in a self-controlled manner to reach an equal voltage distribution for all series connected switches.

Adjusting the drive signal may include delaying or advancing the drive signal. This may include a pulse-width-modulation of the drive signal in order to obtain an adjusted drive signal.

A drive signal may be provided and one of the switches may be driven by the drive signal and the other switches may be driven by an adjusted drive signal formed by adjusting said drive signal. Thus, an equal voltage distribution along all switches can be achieved. The adjusted drive signals for the other switches may be different and individually adjusted for each switch. At the same time, only one drive signal may be generated. This keeps the logical drive signal generation easy. The voltage across a snubber energy storage component associated with a first switch may be determined as first quantity and the voltage across a snubber energy storage component associated with a second switch may be determined as the second quantity. Hence, quantities associated with different snubber arrangements of different switches may be used to adjust the drive signal of one of the switches.

The drive signal of at least one switch may be adjusted based on a quantity related to a voltage limiter provided in the snubber arrangement of the at least one switch. For example, the current of a voltage limiter connected across the capacitor of a snubber arrangement may be determined. Alternatively, the voltage, which drives the voltage limiter, can be determined and the drive signal may be adjusted before the voltage limiter draws current. Use of a voltage limiter allows to reduce power dissipation of the snubber arrangement when the voltage is below the limit value and allows safe operation in burst mode.

If the voltage on a particular switch needs to be reduced, the switch-on time may be slightly advanced, and the switch-off time may be slightly delayed against the switching of the neighboring switches.

The idea of the invention is to introduce small deviations of the switch-on and/or switch-off times of individual switches in the switch-stack making up the HV switch, based on signals available on the local potential level of each switch, in order to compensate propagation delays and minimize the voltage imbalance between the switches.

The control signal used to determine the time adjustment delays may be obtained by: comparing the voltage across the particular switch's snubber capacitor with that on the neighboring switches or detecting the current of a voltage limiter connected across the snubber capacitor of that particular switch or measuring the voltage, which drives the voltage limiter and reacting to its value before the voltage limiter draws current.

If the voltage on a particular switch needs to be reduced, the switch-on time may be slightly advanced and/or the switch-off time may be slightly delayed against the switching of the neighboring switches.

According to a further aspect the invention relates to an HV switch unit according to claim 9. Such a HV switch unit comprises: a. an HV switch comprising several semiconductor switches connected in series, b. connected parallel to each semiconductor switch a snubber arrangement comprising at least one snubber energy storage component, c. at least one drive signal adjuster to adjust the drive signal of at least one switch, d. a feedback arrangement for supplying a quantity related to the snubber arrangement of the at least one switch to the drive signal adjuster.

The drive signal adjuster is configured to e. determining a first quantity related to the snubber arrangement of a first switch, in particular a first voltage across a first snubber energy storage component associated with a first switch, f. determining a second quantity related to the snubber arrangement of a second switch, in particular a second voltage across a second snubber energy storage component associated with a second switch, g. comparing the first and second quantities, in particular the first and second voltage, h. based on the comparison adjusting the drive signal of at least the first switch based on the first quantity.

By adjusting the drive signal even voltage distribution can be achieved. Thus, the individual switches can be protected from overvoltage and voltage stress. A fast HV switch capable of switching high voltages can thus be realized. Known solutions deal with the driver-dispersion-related voltage unbalance instead of the dispersion itself. According to the invention the dispersion is compensated to achieve proper voltage distribution. The compensation works locally at the driver potential. There is no need for extra insulation of additional signals.

As a result, the HV switch unit is configured to switch the switches on and/or off simultaneously. This means that the switches go into the on and/or off-state simultaneously, i.e. the outputs of the switches change their state simultaneously.

The snubber energy storage components of the snubber arrangements of two adjacent switches may be connected in series at a connection point and the connection point may be connected to a comparing component. Thus, quantities related to two different semiconductor switches and/or their associated snubber arrangements can be used for adjusting the drive signal of one of the switches. The comparing component may be configured to determine which of its input signals or quantities is higher and how much it is higher. This is in contrast to a mere comparator which may only determine which of its input signals or quantities is higher.

Resistors of snubber arrangements of two adjacent switches may be connected in series at a second connection point and the second connection point may be connected to the comparing component. Thus, two voltages may be compared and based on the comparison the drive signal may be adjusted.

The comparing component may be integrated in the drive signal adjuster. Thus, the inventive HV switch unit may be implemented with few components.

A voltage limiter may be provided in at least one of the snubber arrangements and a quantity related to the voltage limiter may be supplied to the drive signal adjuster of the switch associated with the at least one snubber arrangement. Use of a voltage limiter allows to reduce power dissipation of the snubber arrangement when the voltage is below the limit value and allows safe operation in burst mode. All switches or all but one switch may be associated with a drive signal adjuster and the same drive signal may be supplied to the one switch and the drive signal adjusters associated with the other switches. The drive signal of the one switch can be considered to be a reference and the signals used to drive the other switches of the HV switch can be adjusted relative to that drive signal to ensure equal voltage distribution and simultaneous switching of the semiconductor switches.

A resistor may be connected between the first connection point and the comparing component.

Further advantages of the invention are evident from the description and the drawing. Likewise, the features mentioned above, and the features mentioned below can, according to the invention, each be used individually or in any combination. The shown and described embodiments are not to be understood as an exhaustive description of the invention, but rather have exemplary character for the description of the invention.

Short description of the figures

Fig. 1 shows a first embodiment of an HV switch unit comprising a comparing component;

Fig. 2 shows a second embodiment of an HV switch unit without a separate comparing component;

Fig. 3 shows a third embodiment of an HV switch unit comprising snubber arrangements and more than two switches connected in series;

Fig. 4 shows a fourth embodiment of an HV switch unit comprising a voltage limiter. Detailed description of the figures

Disclosed is a HV switch unit HVU comprising: a. an HV switch HVS comprising several semiconductor switches SI, S2, Si, Sn connected in series, b. connected parallel to each semiconductor switch SI, S2, Si, Sn a snubber arrangement 10, 10', 12, 12', 14, 14' comprising at least one snubber energy storage component Cl, Cl', C2, Ci, Ci', Cn, Cn', c. at least one drive signal adjuster 8 configured to adjust the drive signal of at least one switch SI, S2, Si, Sn, d. a feedback arrangement for supplying a quantity related to the snubber arrangement 10, 10', 12, 12', 14, 14' of the at least one switch SI, S2, Si, Sn to the drive signal adjuster 8.

All of the above-mentioned properties may also be properties of such a HV switch unit HVU.

Disclosed is further a method of balancing the voltage distribution over the switches SI, S2, Si, Sn connected in series in an HV switch HVS, wherein a snubber arrangement 10, 10', 12, 12', 14, 14' is associated with each switch SI, S2, Si, Sn, comprising the steps of: a. Determining a first quantity related to the snubber arrangement 10, 10' of a first switch SI, b. Adjusting the drive signal 6 of the first switch SI based on the first quantity.

All of the above-mentioned properties may also be properties of such a method.

The method may further comprise: c. determining a second quantity related to the snubber arrangement 12, 12’, 14, 14’ of a second switch SI, S2, Si, Sn, d. comparing the first and second quantities, e. based on the comparison adjusting the drive signal of at least the first switch

SI. Fig. 1 shows an HV switch unit HVU comprising an HV switch HVS comprising two semiconductor switches SI, S2 in a series connection. A first snubber arrangement 10 is associated with the first switch SI and a second snubber arrangement 12 is associated with the second switch S2. Both snubber arrangements 10, 12 comprise a snubber energy storage component embodied as capacitor Cl, C2 and a resistor Rl, R2, as well as a rectifying component DI, D2 embodied as a diode. The resistors Rl, R2 form a voltage divider. The connection point CPI of the snubber energy storage components Cl, C2 is connected to a comparing component 7 and the connection point CP2 of the resistors Rl, R2 is also connected to the comparing component 7.

The resistors Rl, R2 are not connected directly in parallel with the energy storage components Cl, C2 of the respective snubber arrangement 10, 12 but are connected parallel as a strings, i.e. the series connection of the snubber energy storage components Cl, C2 and the series connection of the resistors Rl, R2 are connected in parallel. Thanks to this connection two voltages exist that can be used for further processing: the sum of the voltages on the snubber energy storage components Cl, C2 divided by two by the resistors Rl, R2 at connection point CP2 and a voltage which is a difference in charging voltage of the energy storage components Cl, C2 at connection point CPI. The difference of those two voltages is determined by the comparing component 7 and supplied to a drive signal adjuster 8, which is supplied by a drive signal 3. The drive signal adjuster 8 acts as a pulse shifting device, which controls the switching time of switch SI. This is indicated by a At-arrow.

An optional resistor Rdif may be connected between CPI and CP2 comparing component in order to equal voltages on energy storages components Cl and C2 when HV is switched on or off in long time (like in burst mode or after switching off the power supply)

In order to avoid unequal voltage distribution between the switches SI and S2 the phase of the switching time of switch SI is variable. Switch S2 is controlled directly (without any phase shifting circuit) using drive signal 3, i.e. an unadjusted drive signal. Switch SI is controlled (indicated by arrow 6) by the drive signal adjuster 8, which is controlled by signal 9 created by the comparing component 7 embodied as an error amplifier. The comparing component 7 subtracts the voltage on the energy storage component C2 (at connection point CPI) of the second snubber arrangement 12 associated with the second switch S2 from one half of the voltage on both semiconductor switches SI, S2, i.e. the voltage at connection point CP2.

If the voltage at connection point CPI is higher than the voltage at connection point CP2 it means that the voltage on second switch S2 is too high and it is necessary to decelerate the switching-on moments of switch SI or/and accelerate the switching-off time of switch SI.

If the voltage at connection point CPI is lower than the voltage at connection point CP2 it means that voltage on lower switch S2 is too low and it is necessary to accelerate the switching-on moments of switch SI or/and decelerate the switching-off time of the switch SI.

Connection point CPI is connected with a lower potential of switch SI, which could be an Emitter of an IGBT- or the Source of a MOS-FET transistor.

The drive signal adjuster 8 may be embodied as or comprise a digital signal processor (DSP).

The comparing component 7, the connection points CPI, CP2 and the connections to the drive signal adjuster 8 can be considered to be a feedback arrangement.

The embodiment shown in Fig. 2 largely corresponds to the embodiment of Fig.

1. However, there is no separate comparing component present. A comparing component is implemented in drive signal adjuster 8. The voltage at CP2 is compared directly by drive signal adjuster 8 with the lower potential of switch SI, i.e. the voltage at connection point CPI, with is a ground potential of the drive signal adjuster 8.

Fig. 3 shows an HV switch unit HVU with more than two - in this case n - switches SI, Si, Sn connected in series, where n-1 switches SI, Si are controlled by drive signal adjusters 8 and one switch Sn is driven by a drive signal 3 directly. The drive signal 3 is also supplied to the drive signal adjusters 8 and accelerated or decelerated for each switch SI, Si based on the quantities determined from the snubber arrangements 10 ,12, 14, 10' 12', 14', such that the voltage between terminals 1, 2 is distributed equally among the switches SI, Si, Sn.

If there are more than two semiconductor switches connected in series, it is helpful to connect two snubber arrangements 10, 10', 12, 12', 14, 14' to each switch SI, Si, Sn. The capacitance of each snubber energy storage component Cl, Cl', Ci, Ci', Cn, Cn' can be twice lower or the resistance of each resistor Rl, R.1', R , Ri', Rn, Rn' can be twice higher than for a single snubber arrangement associated with each switch SI, Si, Sn.

The resistors of a snubber arrangement associated with a particular switch are connected to a resistor of a snubber arrangement of an adjacent switch, in this case Rl is connected to Ri, Ri ' is connected to Rn'. If there is no resistor of an adjacent snubber arrangement, the resistor is connected with its own capacitor, see R.1' connected to Cl' and Rn connected to Cn.

Switch Sn is controlled directly by the drive signal 3 while signals for switches SI, Si are shifted in time by drive signal adjusters 8 by comparing voltages on CPI and CP2 as well as on CPi and CPi'.

In the embodiment of Fig. 4 the resistors Rl, R2 are not connected in series. Rather, voltage limiters VI, V2 are provided. As soon as the voltage on Cl or C2 (being a first quantity related to the snubber arrangements 10, 12 respectively) exceeds the threshold voltage of the associated voltage limiter VI, V2, a positive voltage will appear on the associated resistor Rl, R.2, thus influencing the comparison result of comparing components 7, which lead to an adjustment of the drive signal by drive signal adjuster 8.