WO2002054967A1

WO2002054967A1 - Probe assembly, device and system for rf epilation

Info

Publication number: WO2002054967A1
Application number: PCT/CA2001/000032
Authority: WO
Inventors: Andrew D. Piaskowski
Original assignee: Silhouet-Tone Ltée
Priority date: 2001-01-15
Filing date: 2001-01-15
Publication date: 2002-07-18

Abstract

A probe assembly for MHz range RF epilation with a needle unit. The probe assembly has a coaxial cable input. The probe assembly also has a broadband, low power, impedance matching transformer. The low power transformer has a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core. The probe assembly also has an output conductor connected to the secondary winding, for electrical connection to the needle unit.

Description

PROBE ASSEMBLY, DEVICE AND SYSTEM FOR RF EPILATION

FIELD OF THE INVENTION

The present invention relates to a probe, a device and a system for RF epilation (hair removal) with a needle unit in a MHz range.

BACKGROUND

Referring to Figure 1, there is shown a simplified block diagram of a known RF type hair removal system. The system is composed of a tuned high power RF amplifier 3, a flexible coaxial cable 4, by which RF power is conducted, and a probe or needle assembly 5 by which the RF power is applied to treat the subject 6.

An actuation device or actuator 1 is normally provided for user on-off control of the power applied by the probe to transfer power to the probe and needle assembly 5. The actuator 1 may take form of a foot pedal switch or a hand controlled device.

Under normal operation procedure, the probe or needle assembly 5 is applied to a hair follicle of the subject 6, and after suitable power level settings of the system, the actuator 1 is used to apply RF power to the probe and needle assembly 5.

The RF power amplifier frequency is usually set to 13.56 MHz and in Canada should be designed to operate to Government specifications dictated by: "Industry & Scientific Radio Frequency Generators" Document numbers: ICES-001 (English version) and NMB-001 (French version) . The RF power level and DC electrolysis levels have to be precisely set for specific time intervals. These adjustments are normally programmed in an electronics circuit 2 with control software and have been extensively described in Canadian Patent No. 2,011,693 (GHEDIN) , which discloses a method and an apparatus for removal of unwanted hair by using current modulation. The electronics circuit 2 and the R.F. amplifier 3 are usually contained as one assembly 76, the cable 4 and the needle assembly 5 as another assembly.

The RF power level normally required to provide adequate hair removal can vary from fractions of a Watt to a maximum of 12 Watts at the probe/needle tip. In order to guarantee the maximum level of power transfer, a substantial voltage is necessary at the probe tip/needle, and usually is in the order of 120 V AC RMS at 13.56 MHz. This voltage is required because of the characteristic high impedance of the human body, which is typically 1,200 Ohms at 13.56 MHz.

Typical coaxial cables have a characteristic impedance of about 50 Ohms and are preferred due to their low radiation properties and efficiency of RF power transfer. Most coaxial cables have a characteristic impedance varying from 30 Ohms to 100 Ohms.

The characteristic impedance Z of a cable determines how the power flows through the cable, and in simple terms is defined as the ratio of the voltage to the current as defined by:

I :D

Referring to Figure 2 , the characteristic impedance of a coaxial cable can also be defined in terms of its geometric proportions, for example, the diameter D of its outer conductor 8, the diameter d of its inner conductor 9 and the permeability of the insulation 7 separating the two conductors 8, 9, as given by:

The characteristics of a cable or transmission line can also be represented electrically as a "Pi" or "T" network, as shown in Figures 3A-B.

As hereinabove explained, 120 V of RF is required for hair removal, which means that 120 V has to be generated across the capacitive elements of the cable, whether it is the equivalent "Pi" or "T" network. Cables exhibit a certain distributed capacitance per foot of length and for a "RG174" type cable, the value is in order of 30 pF per foot. This means that for an unmatched transmission line, there are only a few resonant frequency points available (depending on the drive circuit) that can be effectively used for passing RF power.

At RF frequencies, transformation of power is often performed by tuning inductive or capacitive elements, because it is deemed impractical or inefficient to do otherwise. Techniques vary at differing frequencies and are dependent of the power level and application.

Known in the art are US patents Nos. 3,999,552 (HUGGINS);

4,167,187 (BIAGI), 4,224,994 (ROBERTS); 4,372,315 (SHAPIRO); 4,550,728 (RUNYON et al . ) ; 4,566,454 (MEHL et al . ) ; 5,785,708 (BETSILL et al . ) ; 5,971,982 (BETSILL et al . ) ; 5,997,535 (BETSILL et al . ) ; and Canadian Patent Application 2,231,383 (GHEDIN) which disclose devices that use Pi networks or inductive/capacitive networks for tuning the cable impedance to the amplifier and the load, but that have inherent cable length restrictions because there can only be a restricted number of possible lengths of cables that can be used with a particular frequency, and conversely there are only a restricted number of possible frequencies with which a given cable length can be effectively used. The number of frequencies will depend on the drive circuit and performance is greatly dependent of the cable staying tuned at the critical drive frequency.

Referring back to figure 1, a main problem of the prior art system is that, in order for the power amplifier 3 to develop the full voltage for hair removal, i.e. 120 V at a frequency of 13.56 MHz, a large current would have to flow across the distributed capacitance of the cable 4. For example, an 8 feet cable will have a distributed capacitance of 240 pF, which at 13.56 MHz has an impedance of 49 Ohms. The resonant current necessary to sustain 120 V must be 120/49 = 2.45 A. Hence, the power amplifier 3 would require a minimum power rating of 2.45A X 120V = 294 VA (also approximated by P=V²/Zo; i.e. 120 X 120/49 = 294 VA) , which would be 24.5 times larger than the maximum power of 12 Watts that is required.

Furthermore, according to the current state of the art, if one would want to implement such a power amplifier with a transformer for example, it would be cumbersome and too large to be implemented within the probe assembly and would necessitate tuning due to the inductive element of the transformer, which would also limit its operational bandwidth. Furthermore, if one would still want to use a transformer with a closed magnetic core, which could be fitted within the hand held probe and would have acceptable losses, the size of such a transformer would have to be approximately 0.025 in³ and thus for a power of 12 W throughput, the power density required would be 480 W/ in³. The accepted state of the art power density for a transformer using a ferromagnetic core is 100 W/ in³ with acceptable loss, which is probably why no one has implemented a closed core transformer at these high frequencies. Finally, prior art transformer techniques implemented at 5 MHz and above do not usually involve closed core transformers due to material characteristics affecting tuning due to parametric change with frequency, flux density, temperature, etc.

Referring to Figure 4, there is shown an equivalent output circuit with a tuned cable corresponding to a prior art system (the mesh currents are all clockwise) . The resultant frequency response is shown in Figure 5. The solid line represents 50 the voltage at the output, while the dotted line 52 represents the input current to the tuned tank circuit. As can be seen, the output is extremely sensitive to frequency which poses a problem of detuning with time. The high input currents also cause the output coaxial cable to heat up, causing reduction of cable life due to softening of the plastic insulating materials.

In such a system, the cable length has to be exact and the tuning of the amplifier and the output circuit is very difficult and critical for correct operation. In addition, such prior art system suffers from high electromagnetic interference (EMI) .

SUMMARY OF THE INVENTION

An object of the present invention is to provide a more effective probe assembly, device, and system for MHz range RF hair removal, which have low production costs, and which do not suffer from the drawbacks of the prior art.

Another object of the invention is to provide a probe assembly, device and system for MHz range RF hair removal, which have broadband RF characteristics, and which drastically reduce requirements of very high RF power levels contrary to prior art systems.

Another object of the invention is to provide a device and a system for MHz range RF hair removal, which eliminate dependency on output cable characteristics, and which permit to use any cable length.

Another object of the invention is to provide a device and system for MHz range RF hair removal, which do not suffer from cable heating, and short cable life.

Another object of the invention is to provide a device and system for MHz range RF hair removal that are not sensitive to detuning.

Another object of the invention is to provide a device and system for MHz range RF hair removal that is less affected by EMI. Another object of the present invention is to provide a matched impedance transmission line where all inductive and capacitive elements are effectively cancelled, whereas the input and output impedance appear totally resistive.

According to the present invention, there is provided a probe assembly for MHz range RF hair removal with a needle unit, comprising : a coaxial cable input; a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core; and an output conductor connected to the secondary winding, for electrical connection to the needle unit.

According to another aspect of the present invention, there is provided a RF hair removal device for MHz range hair removal with a needle unit, comprising: a probe assembly including a coaxial cable input, a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core, and an output conductor connected to the secondary winding for electrical connection to the needle unit; a RF generator unit including a power supply input, a control circuit coupled to the power supply input and generating a MHz range RF signal, and a tuning circuit coupled to the control circuit and locking the RF signal on a predetermined frequency; and a coaxial cable connected between the tuning circuit of the RF generator unit and the coaxial cable input of the probe assembly, the coaxial cable having an impedance to which an output impedance of the tuning circuit is matched.

According to still another aspect of the present invention, there is provided a RF hair removal system comprising: a probe assembly including a coaxial cable input, a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core, and an output conductor connected to the secondary winding; a needle unit connected to the output conductor; a RF generator unit including a power supply input, a control circuit coupled to the power supply input and generating a MHz range RF signal, and a tuning circuit coupled to the control circuit and locking the RF signal on a predetermined frequency; and a coaxial cable connected between the tuning circuit of the RF generator unit and the coaxial cable input of the probe assembly, the coaxial cable having an impedance to which an output impedance of the tuning circuit is matched.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as its numerous advantages will be better understood by the following non restrictive description of preferred embodiments made in reference to the appending drawings in which:

Fig. 1 is a schematic block diagram of a known hair removal system; Fig. 2 is a perspective view of a known coaxial cable;

Figs. 3A and 3B are circuit diagrams respectively of equivalent T and Pi circuits of the coaxial cable; Fig. 4 is a circuit diagram of an equivalent output circuit with a tuned cable of a prior art system;

Fig. 5 is a graphic showing the typical frequency response of the output circuit with tuned cable of Figure 4; Fig. 6 is a circuit diagram of a MHz range RF hair removal system according to the present invention;

Fig. 7 is schematic view of a printed circuit board output transformer used in the RF hair removal system according to the present invention; Fig. 8 is a section view of a probe assembly with a broadband, low power, impedance matching transformer according to the present invention;

Fig. 9 is a graphic showing input and output voltage and current for two load conditions for a RF hair removal system according to the present invention;

Figs. 10A and 10B are front and side views of a first embodiment of a low power transformer used in the probe assembly according to the present invention; and Figs. 11A and 11B are front and side views of a second embodiment of a low power transformer used in the probe assembly according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 8, there is shown a probe assembly 11 for MHz range RF hair removal according to the present invention. The probe assembly 11 is adapted to be used with a needle unit 23 that is plugged or otherwise connected at one end of the probe assembly 11.

The probe assembly 11 is preferably made up of a sleeve- shaped electrically insulated handle 13 with a first end opening 15 for insertion of a coaxial cable 17. Part of the coaxial cable 17 is inserted inside the handle 13. The handle 13 also houses a broadband, low power, impedance matching transformer 19, which snugly fits inside the handle 13 and has its primary winding Start ' 0 ' to Tap '18' and total winding 58 (see Figs. 10B and 11B) coupled with the coaxial cable 17 through a coaxial cable input 54. The transformer is preferably positioned near the end of the probe assembly 11 so as to maximise power transfer to the probe tip.

The low power transformer 19 has a high inductance and permeability core. The permeability is preferably greater than 100.

Referring to Figures 10A, 10B, 11A and 11B, two exemplary embodiments of low power transformer types that may be used with the probe assembly 11 are shown. The transformers have a ferrite suppression type bead 56 and a power rating of 7 Watts, but this can range substantially from at least 5 to 15 Watts. Ferrite bead transformers made by Fair-Rite ™ provide good results. A special characteristic of RF permeable suppression bead cores is that they are highly resistive and may be wound directly on the core. This is a specific advantage of our particular embodiment.

The low power transformer can be a suppression bead transformer having a volume of material of 0.207 cm³ (16.34 in³) and a power throughput of 8 Watts. The power density of the low power transformer is around 38 Watts/cm³ (3000 Watts/in ) . For an efficiency of 97%, the power dissipated in the core is 0.24 Watts, i.e. 1.15 Watts/cm³ (90 Watts/in³) . These dissipation values under ordinary circumstances would be unacceptable as a design criterion. However, due to the extraordinarily large surface ratio to material volume and small size of the transformer, temperature rise is extremely low. This can be considered a very unorthodox design operating outside of normal design parameters.

The preferred material types for this example are for use with low flux density devices and for EMI suppression low flux density operation to be below 10 MHz and suppression between 30 MHz to 200 MHz (it would not be apparent to persons trained in the art to use these power material types at 13.56 MHz) .

Preferably, for the chosen material types of the low power transformer, the relationship between the inductive reactance vs the frequency maximizes between 10 MHz and 40 MHz.

A preferred bead size has an outer diameter of 5.1 mm, an internal hole of 2.3 mm, and a length of 12.7 mm.

Another feature of the low power transformer is that within the very small volume of the transformer, the voltage can be stepped up to 115 V to 120 V AC RMS at 13.56 MHz. Normally, insulation stresses would be prohibitive, but a choice of material with very high volume resistivity (greater than 10⁵ Ohms cm) prevent field stresses occurring between the wire and the core (because there is no room to put additional insulation) . All power ferrite material have low volume resistivities (e.g. 10³ Ohms cm).

To prevent an enameled wire from moving and failing due to corona effects, a good quality varnish or resin may be used to glue the wire in place (e.g. Krylon ™) . The total inductance of the low power transformer (at 10 KHz and 10 mV) is preferably about 1.4 mH. It is also preferable that the low power transformer 19 have an autotransformer closed magnetic toroidal configuration. The output voltage of the low power transformer is preferably 120 V for efficient hair removal .

The ferrite core transformer is so designed to hide capacitance (or inductive) changes at the other end of the circuit caused by the load, and keeps the voltage/current ratio constant, e.g. 50 Ohms.

Referring back to Figure 8, the handle 13 preferably has an opposite end opening 21 for plug insertion of the needle unit 23 using an output connector 60 which is electrically connected to the secondary winding 62 (see Figs. 10B and 11B) of the low power transformer 19.

To transfer RF power to the probe assembly 11, a flexible coaxial cable is preferably used, otherwise the cable is likely to impede the user. Typical coaxial cables having an appropriate flexibility are RG174U (to MIL-C-17D) or any UL type 1354. These cables have a characteristic impedance of 50 Ohms .

Referring to Figure 6, there is shown a circuit diagram of the output section of a RF hair removal system according to the present invention. The system incorporates a device 25 and the needle unit 23. The device 25 is made of the probe assembly 11 and the coaxial cable 17 described above. The system also incorporates an RF generator unit 27.

The RF generator unit 27 has a power supply input 29 which is coupled to a control circuit 31. The purpose of the control circuit 31 is to transform a DC current from a power supply (not shown) into an AC current signal. A tuning circuit 33 is coupled to the control circuit 31 in order to transfer matched RF power which is at predetermined frequency to the output cable 4. The shape of the RF signal is preferably a sinusoidal signal, but other shapes such as square or triangular can be also used if a tuned circuit is not implemented.

In order for the system to provide maximum power transfer to the probe assembly 11, the output impedance of the generator unit 27 is matched with the impedance of the coaxial cable 17.

In theory, it is possible to adjust the output impedance of the tuning circuit 33 so that it matches exactly the impedance of the coaxial cable 17. However, it is preferable to insert an impedance matching output transformer 35 between the tuning circuit 33 and the coaxial cable 17, so that the output impedance of the generator unit 27 always matches the impedance of the coaxial cable 17. The output transformer 35 can be conveniently used also as a step up (or a step down) transformer depending on the power input.

Referring to Figure 7, the primary and secondary windings 64, 66 of the output transformer 35 may conveniently be printed over adjacent layers of the printed circuit board of the generator unit 27.

Referring to Figure 9, there is shown input and output voltage and current for two load conditions of a RF hair removal system. The top two curves 68 and 70, which are almost superposed in the graph, represent the output voltage response for two "body loads" i.e. 1200 Ohms and 10,000 Ohms. As can be seen, the output response is flat and has a broadband characteristic for differing loads, which is not the case for the for prior art system, as shown in Figure 5.

The bottom two curves 72 and 74 represent the input current for the two differing loads, and as can be seen, it is substantially less than that the one shown in Figure 5.

The hair removal device and system of the present invention have the advantage of using a coaxial cable that is not stressed or heated and which can be of any length. In addition, the system has low EMI. The transformer within the probe assembly provides a broadband characteristic and drastically reduces the requirements of very high RF power levels as found in prior art systems. In the system of the present invention, the power amplifier "sees" a resistive impedance for a large range of cable lengths. This "resistive impedance" is the reflection of the load, i.e. part of the human body, via the transformer arrangement which is not sensitive to tuning.

The operating power of the RF amplifier in the invention is about 24 times less than standard systems. Output amplifier tuning is independent of the output cable and thus insensitive to the output cable length. The present system is not limited to fixed cable length or type of cable as any cable can be matched by design. Only low electromagnetic interference results from the present system since the output amplifier is relatively low. Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.

Claims

1. A probe assembly for MHz range RF hair removal with a needle unit, comprising: a coaxial cable input; a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core; and an output conductor connected to the secondary winding, for electrical connection to the needle unit.

2. The probe assembly according to claim 1, wherein the transformer has an autotransformer configuration.

3. The probe assembly according to claim 1, wherein the core of the transformer has a closed magnetic configuration.

4. The probe assembly according to claim 3, wherein the core of the transformer has a toroidal configuration.

5. The probe assembly according to claim 1, wherein the transformer has a ferrite suppression bead and a power rating substantially from 5 to 15 Watts.

6. The probe assembly according to claim 1, wherein the core of the transformer has a permeability greater than 100.

7. The probe assembly according to claim 1, wherein the transformer has a 120 V step-up transformer configuration.

8. The probe assembly according to claim 1, further comprising a sleeve-shaped electrically insulated handle having a first end opening for coaxial cable insertion, and an opposite end opening for plug insertion of the needle unit .

9. A RF hair removal device for MHz range hair removal with a needle unit, comprising: a probe assembly including a coaxial cable input, a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core, and an output conductor connected to the secondary winding for electrical connection to the needle unit; a RF generator unit including a power supply input, a control circuit coupled to the power supply input and generating a MHz range RF signal, and a tuning circuit coupled to the control circuit and locking the RF signal on a predetermined frequency; and a coaxial cable connected between the tuning circuit of the RF generator unit and the coaxial cable input of the probe assembly, the coaxial cable having an impedance to which an output impedance of the tuning circuit is matched.

10. The RF hair removal device according to claim 9, wherein the RF generator unit further comprises : an impedance matching output transformer coupled between the tuning circuit and the coaxial cable and having an output impedance matching the impedance of the coaxial cable.

11. The RF hair removal device according to claim 10, wherein the RF generator unit includes a multi-layer printed circuit board, and the output transformer has primary and secondary windings printed over adjacent layers of the printed circuit board.

12. The RF hair removal device according to claim 9, wherein the predetermined frequency is substantially 13.56 MHz.

13. The RF hair removal device according to claim 9, wherein the impedance matching transformer of the probe assembly has a power rating substantially from 5 to 15 Watts, and is made of a material having a volume resistivity exceeding 10⁵ Ohms cm.

14. The RF hair removal device according to claim 13, wherein the impedance matching transformer of the probe assembly has a 120 V step-up autotransformer configuration.

15. The RF hair removal device according to claim 9, wherein the probe assembly further comprises a sleeve-shaped electrically insulated handle having a first end opening receiving an end of the coaxial cable, and an opposite end opening in which the needle unit is plugged.

16. A MHz range RF hair removal system comprising: a probe assembly including a coaxial cable input, a broadband, low power, impedance matching transformer having a primary winding coupled to the coaxial cable input, a secondary winding, and a high inductance and permeability core, and an output conductor connected to the secondary winding; a needle unit connected to the output conductor; a RF generator unit including a power supply input, a control circuit coupled to the power supply input and generating a MHz range RF signal, and a tuning circuit coupled to the control circuit and locking the RF signal on a predetermined frequency; and a coaxial cable connected between the tuning circuit of the RF generator unit and the coaxial cable input of the probe assembly, the coaxial cable having an impedance to which an output impedance of the tuning circuit is matched.

17. The RF hair removal system according to claim 16, wherein the RF generator unit further comprises: an impedance matching output transformer coupled between the tuning circuit and the coaxial cable and having an output impedance matching the impedance of the coaxial cable.

18. The RF hair removal system according to claim 17, wherein the RF generator unit includes a multi-layer printed circuit board, and the output transformer has primary and secondary windings printed over adjacent layers of the printed circuit board.

19. The RF hair removal system according to claim 16, wherein the broadband, low power transformer of the probe assembly has a power rating substantially from 5 to 15 Watts.

20. The RF hair removal system according to claim 16, wherein the probe assembly further comprises a sleeve-shaped electrically insulated handle having a first end opening receiving an end of the coaxial cable, and an opposite end opening in which the needle unit is plugged.