ReId of the invention

The present application relatesto active electrical filtersf or suppressing unwanted noise component in medical electrical equipments. More particularly, but not exclusively, the present application dealswith electromagnetic compatibility filtersfor mainslinesor electrical supply lines of electrical medical equipment.

Description of related art

Electromagnetic compatibility (EMC) is an increasingly critical factor in the electric and electronic industry nowadays. A large number of electric circuits and appliances exist, which are liable to generate unwanted Radio Frequency Electro-Magnetic Interference (RF EM I), or to suffer from interferencesgenerated by other circuitsor appliances

The conducted noises and disturbances in electrical equipment are generally dealt with by inserting a low-pass LC filter on the mains supply line of the noise-generating devices or of the noise-sensitive devices. Such filters attenuate the unwanted frequency componentsto a harmless level. Many filter topologies, including the classic " L, " T' and " pi" filter topologies, can be employed.

European Patent application EP1069673 shows an example of a three-phase noise suppression filter comprising passive elements

Passive EM C filters have been proven effective in a number of applications. A shortcoming of thistechnique, however, isthat in order to attain the required attenuation level, high-value capacitors and inductances are needed for thisapplication. The size and cost of the resulting filter are mainly determined by these large components, in particular when high attenuation of common mode noise is needed.

Moreover, the leakage current isdirectly proportional to the capacity of the filter capacitors

Electrical medical equipments have to fulfil special safety requirement, both concerning reliability and immunity to external disturbance and aimed at minimizing the risk of electrical shocksfor patients connected to them. In particular leakage currentsare of much more concern in medical equipment, where a patient may be the recipient of the shock. Even a low level of leakage current may be sufficient to provide a fatal shock to a weakened or unconscious patient, or if the leakage current is applied to internal organs

Current safety regulation impose severe limit on the leakage current in medical equipment. This causes special problemsfor the power supply and the EMCfiltersof those equipments, in that the line-to-ground capacitors, usually denoted as " Y" capacitors, are both an important part of the EMCfilter to function properly and a major source of leakage current.

The leakage current to ground due to the EΞMCfilter can be lessened by reducing the value of the "Y" filtering capacitors. However this affects severely the effectiveness of the filter.

Increasing the inductance value of the filter choke can compensate the lower value of the " Y" capacitor, but on Iy to a certain extent. Larger inductances have larger stray capacitance, which limitsthe attenuation, and are more costly and voluminousthan conventional ones Furthermore, the combination of a large inductance and a small capacity lessensthe stability of the filter and can be the source of unwanted resonant oscillations.

It is an aim of the present invention to provide an EMCfilter for medical electrical equipment exhibiting a low leakage current.

It is also an aim of the present invention to provide an EMCfilter for medical electrical equipment with a production cost lower than that of the known devices

It is another aim of the present invention to provide an EMC filter for medical electrical equipment which is more compact than the known devices.

Brief summary of the invention

These aims are attained by the device which isthe subject of the appended independent claims, optional and additional useful features being introduced in the dependent claims.

Brief Description of the Drawings

Figure 1 representsan EMC passive filter of known type.

Rgure 2 representsa EMCfilter connected to a medical device according to one aspect of the present invention.

Rgure 3 representsan active part of the filter of figure 2.

Rgure 4 showsa simplified diagram of a filter comprising a negative impedance converter according to one aspect of the invention.

Rgure 5 shows a simplified diagram of a filter comprising a current pick-up detector according to one aspect of the invention.

Rgure 6 showsa variant of the filter of figure 5.

Rgure 7 showsanother variant of shunt module for the filter of figure 2.

Detailed Description of the Invention

Figure 1 represents a EM C passive filter of know type, comprising a current-compensated coil 15, an inter-phase capacitor 11 and two "Y" phase-to-ground capacitors 12. The "Y" capacitors 12 are effective in the 5 suppression of common-mode interferencestransmitted on the mains line, yet they are an important source of leakage current.

According to one aspect of the invention, the EMGfilter includes active elementsfor suppressing an unwanted RFEMl component conducted by the mains line, without increasing leakage current, like in conventional

10 pass ve EMCf ilters. With reference to figure 2 an EMCfilter according to the invention com prises a current-compensated coil 15 and an active shunt module 32 presenting a low impedance for the RFEM I component which is shunted to ground and prevented from reaching the medical equipment 31. The effectiveness of the active shunt module may be limited in the

15 upper part of the f requency spectrum, due to the finite response time of electronics. In this case two small "Y" capacitors, introducing a negligible leakage, may be added for improving high frequency attenuation.

The EMCfilter and the medical device represented on figures 1 and 2 are drawn as single-phase devices. It isto be understood, however, 20 that three-phase or multi-phase devices and filters are also possible, and comprised in the scope of the present invention.

An example of an active shunt module 32 is now discussed with , reference to figure 3. This example refersto a three-phase circuit. It would be possible, in the scope of the invention, to modify it for a single-phase 25 circuit. The active shunt 32 actsasa capacitive bypassfor attenuating high frequency voltage components, notably noise, present on the power lines

33a, 33b and 33c. It comprisestwo complementary follower transistors34 for shunting the RFEMI through the impedance 37.

A further example of active shunt module 32 is presented in 30 figure 7. The active module 32 is arranged for keeping the input point 72 at

a potential close to ground potential, at frequencies where the unwanted noise is expected. The capacitors 76a, 76b 76c and 77a, 77b, 77c, form a voltage divider for the supply railsof the active shunt. In thiswaythe active shunt module issupplied with a voltage which is lower than the mains voltage, for example one half of the mainsvoltage. This reducesthe fault current to harmless levels, even in case of a breakdown of some component, and avoidsthe use of fuses, which have large stray inductance. Furthermore the device voltagesin the active shunt are kept to manageable levels, by the capacitive divider.

The example shown above refersto the case of a three-phase mainssupply. The present invention is not limited, however, to this disposition and the presented examples could easily be modified for operation in a single-phase supply line, should the need arise.

Figure 4 represents schematically another variant of an EM G filter according to the present invention. The filter comprisesa Negative

Impedance Converter (NIC) 42 thanksto which a winding of transformers 41 and 44 appear as a large seriesimpedanceto noise present on power lines 45, 40. The circuit of figure 4 allows effective filtering of the EMI to a medical device 31 , by use of a small " Y" capacitor, which contributes negligibly to the leakage current.

Advantageously, in this case, the " Y" capacitors can be totally dispensed with entirely. The NIC-circuit acts as a source of voltage which cancelsthe noise on lines 40 and 45.

Rgure 5 represent another variant of a filter according to the present invention comprising current sensors47 and 47' and an active circuit 48 for injecting a noise correction current in the power lines 45 and 45', in order to cancel the EMI directed to the medical device 31. Isolation between the circuit 48 and the supply lines 54, 45' isprovided by isolation capacitors 49 and 49'. The current sensor 47 comprises, for example a pickup coil, for reading the FFEMI circulating along the supply line 45.

Further, even if not represented, it isto be understood that one of the current sensor could be omitted, and, in a variant, only one of the lines could be sensed. In a further variant a magnetic coil could be coupled with both the power lines 45 and 45' in order to sense, for example, a common-mode noise component.

Figure 6 representsanother variant of the filter of the invention in which the correction signal generated by the active module 49 isinjected in the supply line 45, 45" by meansof a transformer 50 magnetically coupling supply lines 45, 45 ¹ to the correction signal. It isto be understood that several formsof magnetic cores and windingsbay be used for this purpose. In one advantageous variant the transformer 50 constitutes, with respect to the power lines 45, 45', a current-compensated impedance.

In not represented variants of the embodimentsof figuresδ and 6, the current sensors47 and 47' may be replaced by transformersor by pick-up capacitors as vo It age sensors These embodiments of the present invention could also be adapted for a three-phase configuration.

It is an advantage of the above embodimentsthat the circuit 48 is not directly connected to ground, and could, if necessary be completely floated. In this way the unwanted possibility of stray leakage currents is further reduced.

The EΞMCfilter of the invention is preferably included in a system comprising also a medical equipment, connected to a mainssupply line, the EMCfilter providing attenuation of EM I transmitted along said supply line.