Circuit arrangement for accurately detecting a direct...

Details Circuit arrangement for accurately detecting a direct... Circuit arrangement for accurately detecting a direct...

1999-06-01

2001-01-23

Wong, Peter S. (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

C363S039000

Reexamination Certificate

active

06178105

ABSTRACT:

FIELD OF AND BACKGROUND OF THE INVENTION
The invention relates to new and useful improvements in circuit arrangements for accurately detecting a direct current derived from clocked electric input values.
Current detection is normally carried out by first converting the current to a proportional voltage, prior to further processing. The traditional method for further evaluation is to evaluate the voltage drop caused by the current to be measured across a measurement resistor. However, as the current levels rise, this results in problems. Depending on the size of the measurement resistor used, this either leads to high power losses, or smaller measurement voltages must be used in order to limit the power loss. This, however, reduces the measurement accuracy.
In one special case, the direct current to be measured is derived from clocked electrical input variables.
FIG. 1
shows the block diagram of a circuit suitable for this purpose, specifically a basic circuit of a so-called pulse controller PS which includes a chopper Z provided with an input DC voltage Ug. The chopper Z uses this to produce a pulsed intermediate voltage Up and a pulsed intermediate current Ip which has, for example, a trapezoidal waveform. The chopper Z is followed by a smoothing device GA for the intermediate voltage Up and for the intermediate current Ip, and this converts the pulsed variables to an output DC voltage Ua and an output direct current Ia. The chopper Z receives a predetermined nominal value Ua* for the actually desired magnitude of the output DC voltage Ua. This is essentially used to influence the frequency or pulse width of the intermediate voltage Up and the intermediate current Ip such that the intermediate voltage Up assumes a desired value.
FIG. 2
shows, inter alia, the basic circuit of a smoothing device GA in the pulse controller PS. This smoothing device GA has a smoothing inductor L with a freewheeling diode Vf, to which the pulsed intermediate voltage Up and the pulsed intermediate current Ip are supplied. The majority of the alternating component of this current is absorbed by a first smoothing capacitor Ca, so that a virtually pure DC voltage Ua is produced at the output of the circuit in
FIG. 1
, and the output direct current Ia corresponds to the mean current Il through the smoothing inductor L.
FIGS. 3A
,
3
B and
3
C show the waveforms of the most important electrical signals in the circuit in FIG.
2
. For example,
FIG. 3A
shows three pulses P
1
, P
2
, P
3
from the waveform of the pulsed intermediate voltage Up at the circuit input.
FIG. 3B
shows the waveforms of the triangular current Il through the smoothing inductor, and of the output direct current Ia. The current Il through the smoothing inductor L is composed of two components. For the duration of each of the pulses P
1
, P
2
and P
3
, the current Il corresponds to the waveform of the pulsed intermediate current Ip while, in the interims between the pulses, it corresponds to the trapezoidal current If through the freewheeling diode Vf.
The circuit in
FIG. 2
also includes a component M
1
which can be used to detect the present magnitude of the output direct current Ia, in a known manner. This first current measurement circuit M
1
includes a first measurement resistor Rs
1
through which the current passing through the smoothing inductor L flows and across which a first measurement DC voltage Um
1
can be detected, as a measure of the magnitude of the output direct current Ia. The already mentioned drawback of considerable power losses in the measurement resistor Rs
1
occurs with this circuit.
A second current measurement circuit M
2
, which is also illustrated in the circuit example in
FIG. 2
, uses a current transformer T to avoid virtually all the power losses in the measurement resistor. The transformation ratio of this current transformer T can be chosen to achieve a sufficiently large pulsed measurement current Im for virtually any given value of the pulsed intermediate current Ip. As a first component element, the current measurement circuit M
2
includes a circuit Ms which is used to detect the pulsed intermediate current Ip and to convert it, as a floating potential, into a pulsed measurement current Im on the secondary of the current transformer T. The pulsed measurement current Im is passed to a first rectifier diode Vs and a second measurement resistor Rs
2
. The pulsed measurement voltage Us appears across this measurement resistor Rs
2
, which effectively forms the output of the current detection circuit Ms.
Since direct current cannot pass through the current transformer T, the measurement current Im for the second measurement resistor Rs
2
is available only in pulsed form. To obtain a continuous measurement signal once again, a so-called peak-value detector Sg is provided downstream of the current transformer T. The circuit in
FIG. 2
shows one example of such a peak-value detector Sg, comprising a second rectifier diode Vg and a parallel circuit formed by a damping resistor Rm and a second smoothing capacitor Cm. A second measurement DC voltage Um
2
is thus available at the output of the second measurement circuit M
2
.
Even though this peak-value detector Sg produces a continuous measurement DC voltage Um
2
from the pulsed measurement current Im, there is nonetheless an associated drawback. Specifically, this measurement DC voltage Um
2
is only approximately proportional to the desired mean value of the current Ip or to the desired value of Ia. The accuracy of this measurement circuit M
2
is also adversely affected by temperature-induced variations in the forward voltage of the second rectifier diode Vg. This will be explained in more detail in the following text with reference to the signal waveforms shown in FIG.
3
C.
FIG. 3C
shows the pulses of the pulsed measurement current Im which flows through the secondary of the current transformer T. This produces a pulsed measurement voltage Us across the second measurement resistor Rs
2
at the output of the current detection circuit Ms. This is described by the relationship Us=Rs
2
*Im. An ideal peak-value detector would convert the pulsed measurement voltage Us into the measurement DC voltage Um*, whose magnitude would correspond to the peak value of the pulsed measurement voltage Us. However, in fact, owing to the non-ideal characteristics of the rectifier diode Vg in particular, a measurement DC voltage value Um
2
is actually produced at the output of the peak-value detector Sg in FIG.
2
.
OBJECTS OF THE INVENTION
It is therefore a first object of the invention to provide a measurement DC voltage whose magnitude corresponds to the mean value of the pulses of the measurement voltage Us in FIG.
3
C. Such a measurement DC voltage is plotted in the form of the signal Um
2
z in FIG.
3
C. Its value is proportional to the mean value of the pulsed measurement current Im during the pulses, and is proportional to the desired value of the output direct current Ia. The error between the output signal Um
2
and the desired value Um
2
z corresponds to the measurement error of the circuit in FIG.
2
.
It is a further object of the invention to provide a current measurement circuit of the type described above which is as simple as possible and which facilitates high-accuracy current measurement. In particular, it is sought to provide such a circuit without a resort to further magnetic components, for example current transformers.
SUMMARY OF THE INVENTION
According to one formulation of the invention, the present invention is directed to an apparatus for detecting an output direct current produced by a pulse controller. The pulse controller includes at least one chopper device forming a pulsed intermediate voltage and a pulsed intermediate current, which are converted by a smoothing device into an output DC voltage and into the output direct current. The detecting apparatus includes: a circuit which performs electrically floating detection and conversion of the pulsed intermediate current into a pulsed measurement voltage; a switching

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