DC to DC converter

Details DC to DC converter DC to DC converter

2001-09-25

2002-08-13

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

With voltage multiplication means

C327S534000, C327S536000

Reexamination Certificate

active

06434028

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC to DC converter for raising a voltage supplied from a voltage-generating source and retaining a raised voltage in an arbitrary polarity.
2. Description of the Related Art:
In general, DC to DC converters are classified into a chopper type switching converter, a flyback converter, a forward converter, a charge pump type converter, etc. These types are used in different ways depending on the purpose of use.
The respective types of DC to DC converters may be compared with each other as follows. The chopper type switching converter requires a coil. The flyback converter and the forward converter require a transformer. Therefore, these types are disadvantageous when miniaturization is required, and are expensive. Further, the circuit configuration is complicated as well, and the adjusting operation is also troublesome.
On the other hand, the charge pump type converter requires no large parts such as a coil or a transformer. Therefore, this type converter is advantageously miniaturized, and the circuit can be constituted inexpensively.
A mechanical vibration-electric energy converter, in which a DC to DC converter
200
of the charge pump type is applied, will now be explained with reference to
FIGS. 25
to
27
.
This converter is reported in a literature “A Micropower Programmable DSP Powered using a MEMS-based Vibration-to-Electric Energy Converter” (Rajeevan Amirtharajah et al., M.I.T., 2000 IEEE International Solid-State Circuits Conference).
As shown in
FIG. 25
, the DC to DC converter
200
comprises a pump capacitor Cp, a reservoir capacitor Cr, an inductor L, and a plurality of switching elements SW
1
, SW
2
. Specifically, a first series circuit
202
including the pump capacitor Cp and the reservoir capacitor Cr connected in series, and a second series circuit
204
including the first and second switching elements SW
1
, SW
2
connected in series are connected to one another in parallel. A connection point p
1
of the pump capacitor Cp and the reservoir capacitor Cr of the first series circuit
202
, and a connection point p
2
of the first and second switching elements SW
1
, SW
2
of the second series circuit
204
are connected via the inductor L. Further, a parasitic capacitor Co is connected in parallel to the pump capacitor Cp. A load
206
is connected in parallel to the reservoir capacitor Cr.
As shown in
FIG. 26
, the pump capacitor Cp comprises a comb-shaped movable electrode
210
which is arranged at the center, and comb-shaped fixed electrodes
212
which are fixed on both sides of the movable electrode
210
. The distance d between the movable electrode
210
and the fixed electrode
212
is changed when their comb teeth
210
a,
212
a
make approach to or make separation from each other. Thus, the capacitance is variable.
The operation of the DC to DC converter
200
shown in
FIG. 25
will be explained with reference to a timing chart shown in FIG.
27
. At first, the pump capacitor Cp has the maximum value of the capacitance when the fixed electrode
212
and the movable electrode
210
make approach most closely to each other. It is assumed that the electric charge is stored in the reservoir capacitor Cr with its terminal voltage of Vdd, for example, and no electric charge is stored in the pump capacitor Cp and in the parasitic capacitor Co respectively. Further, both of the first and second switching elements SW
1
, SW
2
are in the OFF state.
At the start of an interval t
1
, when the second switching element SW
2
is turned ON, a ramp current (inductor current i
L
) flows from the reservoir capacitor Cr to the inductor L in the interval t
1
. At the start of a next interval t
2
, when the first switching element SW
1
is turned ON, and the second switching element SW
2
is turned OFF, then the inductor current i
L
is supplied to the pump capacitor Cp in accordance with the energy of the inductor L in the interval t
2
, and the electric charge is stored in the pump capacitor Cp. In accordance with the storage of the electric charge, an output voltage Vc becomes a voltage (V
START
+Vdd) obtained by adding Vdd to the terminal voltage (start voltage V
START
) obtained when the capacitance of the pump capacitor Cp has the maximum value. The change to the voltage (V
START
+Vdd) follows the transient characteristic depending on the time constants of the pump capacitor Cp and the inductor L.
Subsequently, at the start of an interval t
3
, when both of the first and second switching elements SW
1
, SW
2
are turned OFF, then the fixed electrode
212
and the movable electrode
210
of the pump capacitor Cp are controlled in the direction to make gradual separation from each other in the interval t
3
, and the capacitance of the pump capacitor Cp is gradually decreased. In accordance with the change of the capacitance, the output voltage Vc is gradually increased. The interval t
3
comes to end at the point of time when the capacitance of the pump capacitor Cp is minimum, and then a next interval t
4
is started. At the end of the interval t
3
, the output voltage Vc becomes a voltage (Vmax+Vdd) obtained by adding Vdd to the terminal voltage (maximum voltage Vmax) obtained when the capacitance of the pump capacitor Cp has the minimum value.
At the start of the interval t
4
, when the first switching element SW
1
is turned ON, the current flows in the interval t
4
from the pump capacitor Cp to the inductor L. At the end of the interval t
4
, the output voltage Vc becomes Vdd. The change of the output voltage Vc from the voltage (Vmax+Vdd) to the voltage Vdd follows the transient characteristic depending on the time constants of the pump capacitor Cp and the inductor L. However, the voltage arrives at the voltage Vdd for a short period of time as compared with interval t
2
, because the capacitance of the pump capacitor Cp is minimum.
At the start of a next interval t
5
, when the first switching element SW
1
is turned OFF, and the second switching element SW
2
is turned ON, then the energy stored in the inductor L is transmitted to the reservoir capacitor Cr in the interval t
5
. That is, the energy (energy generated by the pump capacitor Cp), which has been increased owing to the increase in voltage in the intervals t
4
, t
5
, is recovered by the reservoir capacitor Cr.
In order to increase the capacitance change of the pump capacitor Cp in the DC to DC converter
200
described above, the following artifices are required.
(1) The gap between the comb teeth
212
a
of the fixed electrode
212
and the comb teeth
210
a
of the movable electrode
210
is decreased.
(2) The thicknesses of the fixed electrode
212
and the movable electrode
210
are increased.
(3) The lengths of the respective comb teeth
212
a,
210
a
of the fixed electrode
212
and the movable electrode
210
are increased.
(4) The numbers of the respective comb teeth
212
a,
210
a
of the fixed electrode
212
and the movable electrode
210
are increased.
However, when the comb teeth
212
a,
210
a
are formed, the isotropic etching such as the wet etching is used. Therefore, when it is intended to decrease the gap between the comb teeth, it is necessary to regulate the etching depth. Then the thicknesses of the fixed electrode
212
and the movable electrode
210
are decreased, and the electrode area is decreased. In such a situation, it is impossible to expect the effect (increase of the capacitance change) to be brought about by decreasing the gap. That is, the artifices (1) and (2) are in a relation of trade-off.
The increase of the vibration frequency of the movable electrode
210
contributes to the raising of the output voltage Vc. However, if the lengths of the respective comb teeth
212
a,
210
a
of the fixed electrode
212
and the movable electrode
210
are increased, or if the numbers of the comb teeth
212
a,
210
a
are increased, then it is impossible to increase the vibration frequency of the movable electrode
210
. That is, the artifices (3) and (4)

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