Device for Charging and Discharging a Capacitive Load

Abstract

A capacitive load charging/discharging device, including a first capacitor, a down-up converter including a first and a second switching element connected across the first capacitor, wherein a connecting point of the switching elements is connected to a first output terminal of the converter through a main coil. The device further includes an output circuit with a capacitive load arranged between first and second output circuit terminals, which are connected to output terminals of the converter. A discharge circuit is formed with the output circuit, the main coil and the second switching element, including an additional capacitor which is connected to a charging circuit for charging to a specified voltage, wherein the polarity of the voltage corresponds to that of the load voltage in the charged state of the capacitive load.

Claims

1. A device for charging and discharging a capacitive load, comprising: a first capacitor with a first and a second supply terminal, a down-up converter with a first and a second input terminal which are connected to the first and the second supply terminals, respectively, of the first capacitor, wherein the second input terminal of the converter is connected to a ground terminal, the converter comprising a series circuit of a first and a second switching element arranged between the first and the second input terminals, and a main coil connected to a connecting point of the two switching elements such that the connecting point is connected to a first output terminal of the converter through the main coil, and a second output terminal of the converter is connected to the ground terminal, an output circuit corresponding to a capacitive load, the output circuit arranged between a first and a second output circuit terminal of the output circuit, which are connected to the output terminals of the converter a discharge circuit formed with the output circuit, the main coil and the second switching element, the discharge circuit comprising an additional capacitor, and a charging circuit connected to the additional capacitor, the charging circuit charging the additional capacitor to a specified voltage, wherein the polarity of the voltage of the additional capacitor corresponds to the load voltage across the capacitive load in the charged state of the capacitive load.

2. The device as claimed in claim 1, wherein the additional capacitor is arranged between the second switching element and the ground terminal.

3. The device as claimed in claim 2, wherein the charging circuit is a second DC-DC converter.

4. The device as claimed in claim 1, further comprising a filter circuit arranged between the down-up converter and the output circuit, and the additional capacitor is arranged between the filter circuit and the output circuit.

5. The device as claimed in claim 4, further comprising a third switching element arranged between the main coil and the filter circuit, and a fourth switching element arranged between the main coil and the ground terminal.

6. The device as claimed in claim 4, further comprising a charge switching element arranged between a connecting point of the additional capacitor and the capacitive load, and the ground potential.

7. The device as claimed in claim 4, further comprising a flyback converter having an output connected to the first capacitor, the flyback converter comprising a transformer including a first and a second secondary coil, the flyback converter further including a rectifier, wherein output terminals of the second secondary coil are connected to terminals of the additional capacitor, at least one of the output terminals of the second secondary coil being connected to a corresponding terminal of the additional capacitor via the rectifier.

8. The device as claimed in claim 1, wherein the additional capacitor has a capacitance that is at least 10 times greater than a capacitance of the capacitive load.

9. The device as claimed in claim 1, further comprising a protection diode, polarized in a blocking direction and arranged between a connecting point of the additional capacitor and the capacitive load, and the ground potential.

10. The device as claimed in claim 9, further comprising a charge switching element arranged between a connecting point of the additional capacitor and the capacitive load, and the ground potential, wherein the charge switching element is connected in parallel with the protection diode.

11. The device as claimed in claim 1, wherein the output circuit includes a selection switch connected in series with the capacitive load.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The exemplary embodiments of the invention are described in more detail below with the help of figures. Here

[0048] FIG. 1 shows a device for charging and discharging a capacitive load according to the prior art,

[0049] FIG. 2 shows an alternative device for charging and discharging a capacitive load according to the prior art,

[0050] FIG. 3 shows a first embodiment of a device according to the invention for charging and discharging a capacitive load,

[0051] FIG. 4 shows a second embodiment of a device according to the invention for charging and discharging a capacitive load,

[0052] FIG. 5 shows a first development of the second embodiment of a device according to the invention for charging and discharging a capacitive load,

[0053] FIG. 6 shows a second development of the second embodiment of a device according to the invention for charging and discharging a capacitive load, and

[0054] FIG. 7 shows a third development of the second embodiment of a device according to the invention for charging and discharging a capacitive load.

DETAILED DESCRIPTION

[0055] FIG. 3 shows a first embodiment of a device according to the invention with an additional capacitor C.sub.NEG, which is connected in a circuit according to FIG. 1 between the second transistor M2 and the ground potential GND. The additional capacitor C.sub.NEG is charged here by a charging circuit formed of a second switch-mode regulator DCDC2 to a voltage VNEG, which creates a potential that is negative with respect to the ground potential GND at the connecting point of the second transistor M2 to the additional capacitor C.sub.NEG, against which the capacitive load C.sub.PIEZO may always be nearly completely discharged through the main coil L.sub.MAIN, in that the energy stored in the capacitive load C.sub.PIEZO is fed back into the first capacitor C.sub.DCDC. The peak current through the main coil L.sub.MAIN flows, however, through the additional capacitor C.sub.NEG, which requires additional capacitor C.sub.NEG to be of appropriately robust design.

[0056] The additional dissipative discharge path LINEAR DISCHARGE of FIGS. 1 and 2 may now be omitted, which is indicated in that this part of the circuit has been removed.

[0057] The embodiment of FIG. 3 may also be realized in a full-bridge converter according to FIG. 2.

[0058] FIG. 4 shows an alternative, advantageous embodiment of the invention. There, in a circuit topology according to FIG. 1, an additional capacitor C.sub.SHIFT is arranged between the filter circuit C.sub.FILT, L.sub.EMC and the capacitive load C.sub.PIEZO the capacitor C.sub.SHIFT being charged to a voltage VSHIFT, whose voltage is aligned to the voltage at the capacitive load C.sub.PIEZO. A protection diode D.sub.PROT is also provided, connected between the connecting point of the additional capacitor C.sub.SHIFT and the capacitive load C.sub.PIEZO and the ground potential GND. Protection diode D.sub.PROT prevents the capacitive load C.sub.PIEZO being charged negatively when discharged.

[0059] When the capacitive load C.sub.PIEZO is discharged by the down-up converter BUCK/BOOST, a working voltage, raised by the voltage VSHIFT dropped across the additional capacitor C.sub.SHIFT, is created for the down-up converter BUCK/BOOST from the sum voltage of VSHIFT and VPIEZO resulting from the series connection of the capacitive load C.sub.PIEZO and of the additional capacitor C.sub.SHIFT. Through a suitable choice of the level of the voltage VSHIFT at the additional capacitor C.sub.SHIFT, the working point may be adjusted such that the down-up converter BUCK/BOOST may always operate at a defined working point, and generate the development of a current in the main coil L.sub.MAIN. This can be described by equation (11).

[00011]$\begin{array}{cc}\frac{\mathrm{di}}{\mathrm{dt}}=\frac{\mathrm{VPIEZO}+\mathrm{VSHIFT}}{L_{\mathrm{MAIN}}}& \left(11\right)\end{array}$

[0060] In this way, even at load voltages of VPIEZO=0 V, current may be developed in the main coil L.sub.MAIN, and complete discharge of the capacitive load C.sub.PIEZO may occur, even down into the negative voltage range. Since the discharge current of the capacitive load C.sub.PIEZO also discharges the additional capacitor C.sub.SHIFT, it follows that this must support a higher charge than the capacitive load C.sub.PIEZO, so that after capacitive load C.sub.PIEZO has been discharged an adequate voltage VSHIFT is still present across the additional capacitor C.sub.SHIFT.

[0061] Charging the load the opposite way to excessively high negative voltage values may be achieved, for example, through the use of the protection diode D.sub.PROT, which then limits the negative potential at the capacitive load C.sub.PIEZO to the forward voltages of the sum of the voltage V(D_PROT) at the protection diode D.sub.PROT and the voltage V(D_M3) at the selection transistor M3. If the clocked discharge path continues to be driven after the charge has been fully removed from the capacitive load, the protection diode D.sub.PROT effects a “disconnection” of the capacitive load C.sub.PIEZO from the down-up converter BUCK/BOOST, since protection diode D.sub.PROT represents an appropriate bypass current path. Complex detection of the end of discharge is thus unnecessary.

[0062] A further advantage of the topology described above is the implicit charge equilibrium in the additional capacitor C.sub.SHIFT after a complete charge-discharge cycle. The same charge flows both when charging and discharging through the additional capacitor C.sub.SHIFT and the capacitive load C.sub.PIEZO. If therefore the same charge level is reached at the load after a cycle as before the cycle (e.g. 0 μAs), then the charge at the additional capacitor C.sub.SHIFT is again balanced. In the ideal case, therefore, with the exception of the initial charging and any leakage currents that may occur in the additional capacitor C.sub.SHIFT, no further power is needed to supply the additional capacitor C.sub.SHIFT. The charge stored in the capacitive load C.sub.PIEZO may be fully recovered apart from smaller power losses resulting from ESR, RDSon (the drain-to-source “on” resistance) of the switching element and conductive tracks.

[0063] In practical application, the down-up converter BUCK/BOOST with the additional capacitor C.sub.SHIFT may be operated such that the discharge after a cycle is always slightly larger than the charge (by 5-10%), and that a simple recharging of the additional capacitor C.sub.SHIFT is sufficient for balancing.

[0064] The size of the capacitance of the additional capacitor C.sub.SHIFT depends on the maximum voltage swing to be allowed at it, and should be many times the load capacitance. If a voltage swing in practical application at the additional capacitor C.sub.SHIFT of at most 2% of the load voltage is to be achieved, then the capacitance of the additional capacitor C.sub.SHIFT may need to be about 50 times greater than the maximum load capacitance.

[0065] FIG. 5 shows a development of the down-up converter BUCK/BOOST according to FIG. 4. The down-up converter BUCK/BOOST is designed there as a full-bridge converter as in FIG. 2, wherein a third switching element M21 is arranged between the main coil L.sub.MAIN and the filter circuit C.sub.FILT, L.sub.EMC, and a fourth switching element M22 is arranged between the main coil L.sub.MAIN and the ground terminal GND. The series circuit of the third and fourth switching elements M21, M22 formed in this way is connected in parallel with the capacitor C.sub.FILT of the filter circuit C.sub.FILT, L.sub.EMC. A current measuring resistor R.sub.SH1 may be connected between this parallel circuit and the ground terminal GND.

[0066] Initial charging of the additional capacitor C.sub.SHIFT to a defined voltage may be done using a number of methods. In a first embodiment according to FIG. 6, the feed may be made via the clocked down-up converter BUCK/BOOST with an additional charging path with a transistor MPRT.

[0067] If the protection diode D.sub.PROT is replaced by the active switch MPRT, then when the load is not selected by the selection transistor M3 (M3 blocking) and with transistor MPRT conducting, the additional capacitor C.sub.SHIFT may be charged via the existing down-up converter BUCK/BOOST. Since the negative pole of the additional capacitor C.sub.SHIFT is connected to the ground potential GND during the charging phase, the voltage VSHIFT at the additional capacitor C.sub.SHIFT may be regulated by a simple ground-referenced voltage measurement.

[0068] FIG. 7 shows an initial charging circuit for the additional capacitor C.sub.SHIFT with a second secondary winding SW2 of the transformer of the first DC-DC converter DCDC1 which is coupled to this with a fixed winding ratio. The second secondary winding SW2 is connected through a rectifier DGL, which in this case is implemented as a simple diode, to the additional capacitor C.sub.SHIFT.

[0069] If the stray inductances of the transformer are negligibly small, then no specific regulation is required for the charging path for the additional capacitor C.sub.SHIFT, since the auxiliary winding SW2 tracks the intermediate circuit voltage VDC proportionally. The coupled output voltages VDC and VSHIFT are thus determined by the transformer winding ratio. A regulation of the intermediate circuit voltage VDC thus also determines the voltage at the additional capacitor C.sub.SHIFT.

[0070] The device, according to embodiments of the invention, has the advantages that the working range of a DC-DC converter may be extended through simple measures to the extent that it can discharge capacitive loads down to a voltage range of 0 V. The discharge may take place with low losses down to the load voltage of 0 V, and the charge stored in the load may to a large extent be fed back into the intermediate circuit. In addition, through the insertion of a reactive storage element pre-charged to a voltage in series with the load, only reactive power is required during operation, which may almost entirely be recovered.

[0071] The requirements of the electrical series voltage in the second variant of the placement of the additional capacitor C.sub.SHIFT are reduced, since only the filtered and averaged load current, rather than the higher peak current, of the main coil L.sub.MAIN has to be passed through the additional capacitor C.sub.SHIFT. A second, expensive, DC-DC converter to generate a negative auxiliary voltage may thus be avoided.

[0072] The position of the additional capacitor C.sub.SHIFT is independent of the chosen topology, and may thus be used, for example, in the case of half-bridge topologies according to FIG. 1 and in full-bridge topologies according to FIG. 2—and may be inserted at different locations such as, for example, in the high-side path (see FIGS. 4 to 6) or in the low-side path of the load circuit.