Equalizer circuit for switches connected in series

An equalizer circuit for switches connected in series between a voltage source (21) and a load (16), each switch (4, 11) being connected in parallel with a respective equalizing capacitor (2, 9) via a respective diode (7, 8). The circuit further includes a transformer (20) having a plurality of windings (6, 13) connected in series with respective diodes (5, 12), with each switch having a respective winding-diode series connection connected in parallel therewith. In this manner, electrical energy initially stored in the capacitors is transferred to be stored in magnetic form by the windings when the switches are closed and is re-used when the switches are opened to accelerate charging of the less charged capacitors until all the capacitors reach the voltage of the most charged capacitor. The invention is particularly applicable to semiconductor switches such as transistors or gate turnoff thyristors connected in series and needing protection against exposure to excessive voltages due to differences in switching times between switches.

The invention relates to an equalizer circuit for switches connected in 
series between a voltage source and a load, each switch being connected in 
parallel with an equalizing capacitor and a diode. 
BACKGROUND OF THE INVENTION 
Electronic switches used in electrical energy converters such as choppers 
or inverters are generally semiconductor components such as thyristors, 
transistors or gate turnoff (GTO) thyristors. 
At present the highest voltage which semiconductor devices are capable of 
withstanding is a few hundreds of volts for transistors and a few 
thousands of volts for thyristors and GTO thyristors. 
When the supply voltage for an electrical energy converter needs to be 
higher than the voltage which the chosen semiconductor devices can 
withstand, a plurality of such devices must be connected in series. 
The preferred application of the present invention lies in connecting 
transistors or GTO thyristors in series. 
There is a major drawback to connecting transistors or GTO thyristors in 
series. Manufacturing tolerances are rather wide for such devices, 
particularly concerning the time lapse between receiving an instruction to 
open the switch and the switch becming effectively open. 
Thus, when two transistors or two turnoff thyristors are connected in 
series without taking any precautions, the faster acting device at turnoff 
is temporarily subjected to the full power supply voltage and is 
destroyed. 
Various circuits exist for eliminating this drawback. In particular, the 
journal "G.E.C. Journal of Science and Technology", volume 47, N.degree. 
3, year 1981, pages 121 and 122 describes one such circuit applicable to 
transistors, and Toschiba's "GTO Application Note," page 17 describes such 
a circuit applicable to GTO thyristors. In each case a capacitor is 
connected in parallel with the switching device. 
However, these circuits still suffer from two other drawbacks. 
Firstly the voltage is not exactly equalized between the two switches since 
the capacitor associated with the faster acting device at turnoff charges 
more than the capacitor associated with the slower turnoff device. As a 
result, both devices in any particular circuit design must have a higher 
specified maximum permissible voltage than would otherwise be the case. 
Secondly, in order to keep this lack of equalization within tolerable 
limits, capacitors of relatively high capacitance must be used. The energy 
stored in the capacitors is thus relatively high, and since this energy is 
dissipated in resistors each time the switches are turned off, efficiency 
is reduced, and an electrical energy converter using such an arrangement 
needs extensive cooling. 
An improvement may be obtained by using transistors or GTO thyristors which 
have been selected for matching turnoff characteristics, and only 
connecting closely matching devices in series. However, this complicates 
manufacture and thus increases the cost of the devices, and also 
complicates maintenance of energy converters made using such matched 
devices. 
Preferred embodiments of the present invention improve equalization of 
voltages across transistors or GTO thyristors connected in series without 
requiring the devices to have matched characteristics, and with reduced 
energy losses compared to prior art circuits. 
SUMMARY OF THE INVENTION 
The present invention provides an equalizer circuit for switches connected 
in series between a voltage source and a load, each switch being connected 
in parallel with a respective equalizing capacitor via a respective diode, 
the improvement wherein the circuit includes a transformer having a 
plurality of windings each connected in series with a respective diode, 
with each switch having a respective winding-diode series connection 
connected in paralle therewith, whereby electrical energy initially stored 
in the capacitors is transferred to be stored in magnetic form by the 
windings when the switches are closed and is re-used when the switches are 
opened to accelerate charging of the less charged capacitors until all the 
capacitors reached the voltage of the most charged capacitor. 
Since the capacitors are connected in parallel with respective ones of the 
switches, the voltages applied across the switches are thus equalized. 
Advantageously, any magnetic energy left over after the equalizing 
capacitors have been charged is returned to the power supply. 
The circuit preferably includes two switches connected in series, but three 
or more may be connected in series, in which case each switch is connected 
in parallel with its own winding, and all the windings form part of the 
same transformer. 
In a specific application of the equalization circuit, an inverter has each 
of its half phases constituted by a series connection of switches 
equalized by a respective equalizer circuit.

MORE DETAILED DESCRIPTION 
FIG. 1 is a diagram of a prior art circuit using transistors 30, 31 as 
switches connected in series. Each transistor has a large value capacitor 
32, 33 connected in parallel therewith via a respective charging diode 38, 
39. Each time the associated switch is closed (turned on) the relatively 
large amount of energy stored in each capacitor is dissipated in a 
respective resistor 34, 35 connected in parallel with the associated diode 
38, 39. 
FIG. 2 is a diagram of a similar prior art circuit using gate turnoff (GTO) 
thyristors 40, 41 as switches connected in series. Each thyristor has a 
large value capacitor 42, 43 connected in parallel therewith. As with the 
transistor circuit of FIG. 1, a relatively large amount of energy is 
dissipated once each cycle at switch turnon. This energy is dissipated in 
respective resistors 44 and 45. 
FIG. 3 is a diagram of a circuit in accordance with the invention. The 
series connected switches 4 and 11 are drawn as mechanical switches, but 
represent semiconductor devices which may be transistors or GTO thyristors 
according to choice. As in the prior art, each switch 4, 11 has a 
respective capacitor 2, 9 connected in parallel therewith via respective 
charging diodes 7 and 8. The capacitors are of equal value and the diodes 
are assumed to be ideal. A first difference from the above-described prior 
art circuits is that both diodes are connected to the common point where 
the switches are interconnected. Respective equal value resistances 3 and 
10 are connected in parallel with the capacitors 2 and 9, and serve to 
provide static voltage equalization across the switches 4 and 11. Unlike 
the prior art circuits described above, each switch 4, 11 also has a 
respective winding 6, 13 of a transformer 20 connected in parallel 
therewith. Each winding is itself connected in series with a respective 
diode to prevent the windings affecting the static equalization provided 
by the resistances 3 and 10. Each winding and diode series connection is 
connected in parallel with the corresponding switch via the charging diode 
7 or 8 which connects the relevant capacitor in parallel with the other 
switch, which is why the diodes 7 and 8 are both connected to the common 
point of the series connection of switches. The windings have the same 
numbers of turns and are assumed to be ideal, ie. perfectly coupled and of 
negligible resistance. 
The switch circuit shown in FIG. 3 is shown, by way of example, as being 
part of a voltage chopper, ie. it is connected in series with a load 16, a 
choke filter 15 and a DC voltage supply 21. The choke 15 and the load 16 
are bypassed by a flywheel diode 14. 
Operation of such a circuit is well known to the person skilled in the art, 
and reference is made directly to the waveform diagrams of FIGS. 4 and 5. 
At a starting instant t1, it is supposed that prior operation of the 
circuit has caused a current I15 to flow through the flywheel diode 14, 
the choke 15 and the load 16. It is also supposed that both switches 4 and 
11 are off (open) and that the capacitors 2 and 9 are charged to 
respective voltages V2 and V9 which are both equal to half the supply 
voltage V, ie. V2+V9=V. 
At this instant t1, the switches 4 and 11 are closed. The current I15 then 
passes through the switches 4 and 11 and the diode 14 ceases to conduct. 
The capacitors 2 and 9 start to discharge, with the capacitor 2 setting up 
a current I2 through both switches 4 and 11 and through the winding 13 in 
series with the diode 12, and with the capacitor 9 setting up a current I9 
through both switches 4 and 11 and through the winding 9 and its series 
connected diode 5. 
At instant t2 the capacitors 2 and 9 are fully discharged. They have 
transmitted their energy to the windings 13 and 6 respectively. This 
energy is stored in the form of a magnetic field and is manifested by the 
winding 6 driving a current I6 through the diode 5, the switch 4, and the 
diode 8, while the winding 13 drives a current I13 through the diode 12, 
the diode 7, and the switch 11. 
Since the windings 6 and 13 have the same number of turns, the currents I6 
and I13 are equal. 
At instant t3, a signal is sent instructing the switches 4 and 11 to turn 
off (to open). Due to dispersion of operational characteristics, the 
switch 4 opens effectively at instant t4 while the switch 11 opens at a 
different instant t5. 
At instant t4 the switch 4 can be considered turned off. The current I4 
(equals I15+I6) which was passing through the switch 4 is interrupted. 
This current is diverted to the capacitor 2 which begins to charge. 
The voltage across the terminals of the winding 6 remains equal to zero 
since the winding 13 to which it is coupled is still short-circuited by 
the switch 11 which is still on. The diode 5 stops conducting and the 
current I6 is interrupted. The capacitor 2 is thus charged by the current 
I15 only. 
Conservation of amp-turns by the windings 6 and 13 increases the current 
I13 which continues to flow through the diode 12, the diode 7 and the 
switch 11. 
At instant t5, the switch 11 opens and the current I11 (equals I15+I13) 
which was passing through the switch 11 is interrupted. This current is 
diverted via the capacitor 9 which begins to charge. 
The voltage across the terminals of the winding 13 can increase since the 
winding 6 to which it is coupled is not short-circuited by the switch 4 
which has been open since instant t4. The diode 12 thus continues to 
conduct and the current I13 continues to flow via the winding 13, the 
diode 12, the diode 7, the diode 8 and the capacitor 9. The capacitor 9 is 
thus charged by a current I15+I13. 
It can thus be seen that the magnetic energy stored in the winding 13 
(which energy is equal to the sum of the energy stored in the windings 6 
and 13 at instant t3, supposing losses to be negligible) serves to charge 
the capacitor 9 at a faster rate that the capacitor 2 from instant t5 
onwards. 
At instant t6, the voltage V9 across the capacitor 9 becomes equal to the 
voltage V2 across the capacitor 2 and the equalization error which existed 
at instant t5 is compensated, which is part of the purpose of the present 
invention. 
At instant t6, the diode 5 starts conducting again and the current I6 
begins to flow again via the diode 5, the capacitor 2, the diode 7 and the 
diode 8. 
Since the voltages V2 and V9 are equal and since the windings 6 and 13 have 
the same number of turns, the currents I6 and I13 are also equal. The 
capacitors 2 and 9 thus both charge at the same rate, ie. the charging 
current I15 +I6 to the capacitor 2 is equal to the charging current I15 
+I13 to the capacitor 9 since I6=I13. 
At instant t7, V2+V9=2.times.V2=2.times.V9=V. The diode 14 thus starts 
conducting again and diverts the current I15 which ceases to charge the 
capacitors 2 and 9. 
At instant t7, the current II6=I13 ceases to charge the capacitors 2 and 9 
since V2=V9 cannot be greater than V because the diode 14 is conductive. 
It can thus be seen that in spite of the time interval which exists between 
t5 and t4 (the different instants at which the switches 4 and 11 open), 
the voltages V4 and V11 across the switches 4 and 11 (which voltages are 
equal to V2 and V9 across the terminals of the corresponding capacitors 2 
and 9) are equalized, and never exceed half the supply voltage V. 
At instant t7, a large part of gthe initial magnetic energy is still stored 
in the windings 6 and 13 and can be seen in the form of a current I6=I13 
flowing through he flywheel diode 14, the winding 13, the diode 12, the 
diode 7, the diode 8, the winding 6, the diode 5 and the voltage source. 
The remaining magnetic energy stored in the windings is thus returned to 
the power supply, and the current I6=I13 decreases linearly as a function 
of time to become zero at instant t8. 
It can thus be seen that the energy stored on the capacitors 2 and 9 at 
instant t1 is stored in the windings 6 and 13 between instants t2 and t5, 
and is then used in part between instants t5 and t7 to recharge the 
capacitors 2 and 9 while ensuring that the voltages V2 and V9 remain 
equal, thereby ensuring that the voltages V4 and V11 remain equal, with 
the remaining energy being returned to the power supply between instants 
t7 and t8. 
At instant t8 the circuit is back in the same condition as at instant t1, 
and another cycle identical to the cycle described can now be performed, 
eg. starting at instant t9. 
FIG. 6 shows a circuit in accordance with the invention applied to a series 
connection of four switches 50, 51, 52, and 53. The windings 60, 61, 62, 
and 63 all have the same number of turns and are all intercoupled on a 
common magnetic circuit, eg. a transformer 64. Static equalizer 
resistances (not shown) are connected in parallel with respective 
capacitors or with respective switches. 
FIG. 7 is a diagram of a circuit in accordance with the invention applied 
to a single phase inverter, with each half phase having two switches 70, 
71 or 72, 73 connected in series. Static equalizing resistances (not 
shown) are connected in parallel with respective capacitors or with 
respective switches. 
The invention extends to the use of any number of switches (but preferably 
an even number) connected in series, and to applications requiring such 
series connections of switches.