Electricity: power supply or regulation systems – In shunt with source or load – Using a three or more terminal semiconductive device
Reexamination Certificate
2002-10-15
2003-10-28
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using a three or more terminal semiconductive device
C323S289000
Reexamination Certificate
active
06639388
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a voltage converter (buck converter).
Such a voltage converter is disclosed, for example, in B. Murari, F. Bertotti, G. A. Vignola: “Smart Power ICs—Technologies and Applications”, Springer Verlag, Berlin, 1996, page 287, or in U. Tietze, Ch. Schenk: “Halbleiterschaltungstechnik” [Semiconductor Circuitry], 9th Edition, Springer Verlag, Berlin, 1991, page 564. The construction and the method of operation of such a voltage converter according to the prior art are explained below with reference to FIG.
1
.
The task of a voltage converter is to convert a DC voltage V
1
into a lower DC voltage V
2
for supplying a load R
L
. To that end, in the prior art voltage converter, a series circuit including a switch S, a coil L, and a capacitor C is connected in parallel with the DC voltage source V
1
, the load being connected in parallel with the capacitor C. A diode DI is connected in parallel with the series circuit including the coil and the capacitance C. If the switch S is closed, a current flows from the voltage source V
1
through the coil L to the capacitance C and through the load R
L
and the current through the capacitance rises continuously. The DC voltage V
1
is present across the diode, the diode DI being in the off state with the switch S closed. If the switch S is subsequently opened, the voltage present across the coil L reverses, the current through the coil L maintaining its direction and beginning to decrease. The reversal of the voltage across the coil has the effect that the potential at the node that is common to the switch and the coil decreases. The diode DI is, thereby, turned on and accepts the current flowing from the coil L to the capacitor C and through the load R
L
. The series circuit including the coil L and the capacitor C acts as a low-pass filter and converts the voltage V
1
, which is applied to the series circuit in a clocked manner by the switch S, into a continuous output voltage V
2
, which is lower than the input voltage V
1
. The value of the output voltage V
2
is adjustable by way of the frequency with which the switch is switched on and off and by way of the time duration for which the switch S is respectively open and closed.
What is problematic, in particular, in the case of very high switching frequencies, is that after the opening of the switch S, when the diode DI is in the on state, charge is stored in the pn junction of the diode DI. This stored charge has the effect that even after the closing of the switch S, when the diode DI is supposed to be in the off state, the diode DI is still briefly in the on state until the stored charge has flowed away. The storage leads to switching losses that increase as the switching frequency rises. Moreover, with the diode DI in the on state, the losses brought about at the diode DI are undesirable.
To reduce these losses, the prior art uses, instead of the diode, a field-effect transistor, in particular, a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), which, in a manner driven by a drive circuit, is intended to be in the on state whenever the switch is in the off state. The MOSFETs used for such a purpose have an integrated freewheeling diode that is connected in parallel with the drain-source path of the MOSFET and whose connections correspond to the connections of the diode DI according to FIG.
1
. This freewheeling diode is formed by virtue of the fact that, in conventional MOSFETs, the source zone and the body zone are short-circuited to obtain a high dielectric strength of the FET. Such FETs are in the off state only with application of a forward voltage in the drain-source direction (forward direction), if no drive voltage is present between gate and source, the forward voltage being a positive voltage in the case of n-channel FETs and a negative voltage in the case of p-channel FETs in the drain-source direction. The dielectric strength may have a value of up to a few hundred volts in the case of power FETs. With application of a voltage in the reverse direction, i.e., with application of a negative drain-source voltage in the case of n-channel FETs and a positive drain-source voltage in the case of p-channel FETs, the conventional FETs are already in the on state when the threshold voltage of the freewheeling diode is reached. Such an effect is desired when conventional FETs are used as a replacement for the diode in voltage converters.
To avoid shunt currents, that is to say, currents that flow away through the switch and directly through the FET, the switch and the FET are not permitted to be in the on state simultaneously. Shortly after the opening of the switch, when the FET is supposed to be in the on state but is not yet fully in the on state, the integrated freewheeling diode of the FET accepts the current from the coil until the FET is fully in the on state. The losses incurred at the FET that is fully in the on state are lower than when using a diode in accordance with
FIG. 1
as freewheeling element.
However, in the case of such voltage converters, too, a charge is stored in the freewheeling diode of the FET, the charge having the effect that even after the FET has turned off, the freewheeling diode is still briefly in the on state until the stored charge has flowed away. The affect leads to switching losses that may be considerable, in particular, at high switching frequencies.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a voltage converter that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that reduces the switching losses compared with conventional voltage converters.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a voltage converter, including a pair of input terminals for receiving an input voltage, a series circuit connected in parallel with the pair of input terminals and having a first switch and a low-pass filter having output terminals to be connected to a load, a freewheeling circuit connected in parallel with the low-pass filter, the freewheeling circuit having a second switch, the second switch having a first load path terminal, a second load path terminal, a load path formed between the second load path terminal and the first load path terminal, and a control terminal, and the second switch being a MOS transistor having one of a floating body zone and a source zone, a non-reactive resistor, and a body zone connected to the source zone through the non-reactive resistor.
According to the invention, the freewheeling circuit has a second voltage-controlled switch having a control terminal and a load path formed between a first and second load path terminal, the second switch being configured as a MOSFET whose body zone is formed in a floating fashion, that is to say, is not connected to a defined potential, or is connected to the source zone through a non-reactive resistor. The gate terminal of such a MOSFET forms the control terminal of the second switch, the source and drain terminals form the first and second load path terminals, and the drain-source path of the MOSFET forms the load path of the second switch.
In the case of the MOS transistor used as freewheeling element in the voltage converter according to the invention, the floating configuration of the body zone means that there is no short circuit present between the body zone and the source zone. Consequently, the MOS transistor forming the second switch is in the off state not only with application of a voltage in the forward direction, that is to say, in the case of a positive drain-source voltage in the case of an n-conducting MOSFET and a negative drain-source voltage in the case of a p-conducting MOSFET, but also with application of a voltage in the reverse direction, that is to say, in the case of a negative drain-source voltage in the case of an n-conducting MOSFET and a positive drain-source voltage in the case of a p-conducting MOSFET, if no ga
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Riley Shawn
Stemer Werner H.
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