Active filter for reduction of common mode current

Details Active filter for reduction of common mode current Active filter for reduction of common mode current

2001-03-23

2003-10-21

Le, Dinh T. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific identifiable device, circuit, or system

Unwanted signal suppression

C363S037000, C363S047000, C363S040000

Reexamination Certificate

active

06636107

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to filters for electrical circuits and more specifically relates to an active filter for reducing or redirecting the common mode current in switch mode power supplies and particularly for reducing the common mode current and EMI in a PWM motor drive circuit.
BACKGROUND OF THE INVENTION
High-speed switching devices such as bipolar transistors, MOSFETs and IGBT's enable increased carrier frequency for voltage-source PWM inverters, thus leading to much better operating characteristics. High-speed switching, however, causes the following serious problems, originating from a high rate-of-change in voltage and/or current:
a) ground current escaping to earth through stray capacitors inside motors and through long cables;
b) conducted and radiated EMI;
c) motor bearing current and shaft voltage; and
d) shortening of insulation life of motors and transformers.
The voltage and/or current change caused by high-speed switching produces high-frequency oscillatory common-mode and normal-mode currents when the switching device(s) change state because parasitic stray capacitance inevitably exists inside a load, for example, an ac motor, as well as inside the switching converter. Thus, each time an inverter switching event occurs, the potential of the corresponding inverter output terminal moves rapidly with respect to ground, and a pulse of common mode current flows in the d-c link to the inverter, via the capacitance of the heatsink motor cable and motor windings to ground. The amplitude of this pulse of current for a class B (residential) motor drive is typically several hundred millamps to several amps; and the pulse width is typically 250 to 500 ns. For a class A drive (Industrial), and depending on the size of the motor and length of the motor cable, the pulse current amplitude is typically several amperes with a pulse width of 250 ns to 500 ns, to 20 amperes or more with a pulse width of 1 to 2 micro seconds.
The common mode oscillatory currents may have a frequency spectrum range from the switching frequency of the converter to several tens of MHZ, which creates a magnetic field and will produce radiated electromagnetic interference (EMI) throughout, thus adversely affecting electronic devices such as radio receivers, medical equipment, etc.
A number of Governmental restrictions apply to the degree of permissible line current EMI and permissible ground current in certain motor applications. Thus, in Class B residential (appliances), applications, ground current must be kept below from 1 to 20 mA over a frequency range from 0 to 30 kHz respectively (over a logarithmic curve); and conducted line current EMI must be kept below designated values (less than about 60 dB&mgr;V) over a frequency range of 150 kHz to 300 MHZ. For motor drive applications designated as class A Industrial applications, limitations on ground current are less stringent, but line current EMI is still limited over the 150 kHz to 30 MHZ range.
Generally, common-mode chokes and EMI filters, based on passive elements, may not completely solve these problems. Passive filters, consisting of a common mode inductor and “Y” capacitors in the input ac line have been used to filter the common mode current in such motor drive circuits. Passive common mode filters may place limits on the PWM frequency which can be used, are physically large (frequently a major fraction of the volume of the motor drive structure) and are expensive. Further, they are functionally imperfect in that they exhibit undesired resonance which runs counter to the desired filtering action. Further, in general purpose industrial drives, the drive circuit and motor are often connected by cables which are up to 100 meters or more long. The longer the cable, the greater the conducted common mode EMI in the motor cable, and the larger the required size of a conventional passive common mode input filter.
A common-mode transformer with an additional winding shorted by a resistor is known which can damp the oscillatory ground current. Unfortunately, a small amount of aperiodic ground current will still remain in this circuit.
Active filters for control of the common mode current in a pulse width modulated (PWM) controlled motor drive circuit are well known. Such devices are typically described in the paper an Active Circuit for Cancellation of Common-Mode Voltage Generated by a PWM Inverter, by Satoshi Ogasawara et al., IEES Transactions on Power Electronics, Vol. 13, No. 5, (September 1998 and in U.S. Pat. No. 5,831,842 in the names of Ogasawara et al.
FIG. 1
shows a typical prior art active filter circuit or EMI and noise canceller for an a-c motor. Thus, in
FIG. 1
, an a-c source comprising an input terminal L and a neutral terminal are connected to the a-c input terminals of a full wave bridge connected rectifier
40
. While a single phase supply is shown, the principles in this and in all Figures to be described can be carried out with a three-phase or multi-phase input. The positive and negative busses of rectifier
40
contain points A and D respectively and are connected to a three-phase bridge connected PWM controlled inverter
41
, at inverter terminals B and F. The output a-c terminals of the inverter are connected to a-c motor
42
. A filter capacitor
40
a
is also connected across terminals B and F. Motor
42
has a grounded housing connected to ground wire
43
with ground terminal
43
a.
The active filter consists of a pair of transistors Q
1
and Q
2
, connected across the d-c output lines of rectifier
40
with their emitters connected at node E. These define amplifiers which are controlled by output winding
44
of a differential transformer having input windings
45
and
46
connected in the positive and negative output busses of rectifier
40
. The winding polarities are designated by the conventional dot symbols. Winding
44
is connected between the control terminals of transistors Q
1
and Q
2
and the common emitter node E. A d-c isolating capacitor
47
is connected to ground line
43
at node C.
The active filter including capacitor
47
defines a path for diverting the majority of the common mode current which can otherwise flow in the path L or N, A, B, M (motor
42
),
43
,
43
a
and back to L or N; (or in the reverse path when polarity reverses) or in path L or N, D, F, M,
43
,
43
a
(or in the reverse path when polarity reverses). Thus, most common mode current can be diverted, for currents from positive terminal A, in the path B, M, C, E, Q
2
, F, B, for “positive current”, and in the pattern B, M, C, E, Q
1
, B for “negative” current. by the proper control of transistor Q
1
and Q
2
. The path for common mode current flowing into negative terminal D follows the path F, M, C, E, Q
2
, F for “positive” current and F, M, C, E, Q
1
, B for “negative” current. The degree of diversion depends on the current gain of winding
44
and the current gain of Q
2
, for “positive current”, and the current gain of winding
44
and current gain of Q
1
, for “negative” current. In order to obtain a sufficient degree of diversion of the common mode current, the overall current gain of winding
44
and transistors Q
1
and Q
2
must be high.
The sensing transformer
44
,
45
,
46
of
FIG. 1
has been large and expensive in order to provide sufficiently high current gain. It would be very desirable to reduce the size and cost of this transformer without jeopardizing the operation of the circuit. A further problem is that because of the high gain required, this closed-loop circuit has a tendency to produce unwanted oscillation.
Further, it has been found that the transistors Q
1
and Q
2
may not be able to operate in their linear regions over a large enough range within the “headroom” defined by the circuit, thus defeating the active filtering action. The headroom, or the voltage between the collector and emitter of transistors Q
1
and Q
2
is best understood by considering the approximate equivalent circuit of
FIG. 1
, as shown in
FIG. 2
, in which the ground potential at C is the same as

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