Power transfer circuit including a sympathetic resonator.

This invention relates to a circuit for efficiently transferring electrical power across the windings of a transformer, and more particularly to the transfer of power across an air-gap interface.

<u style="single">Background of the Invention</u>

The transfer of electrical power from a primary to a secondary winding of a transformer is a well known task. However, when the primary and secondary windings are separated by an air gap, and when the available power is limited, it is extremely important to maximize power transfer efficiency. Such a situation is encountered in connection with Smart Cards having an air-gap (contactless) interface. In one early design, power was transferred across a contactless interface via capacitive coupling. U. S. Patent 4,480,178 discloses a tuning arrangement designed to enhance such coupling. Nevertheless, greater efficiency was needed, so transformer coupling was used to increase transfer efficiency. U. S. Patent 4,692,604 teaches the use of a flexible magnetic but non-­magnetostrictive core piece for use in connection with an inductor acting as the secondary portion of a power transformer. While significant improvement is achieved, only a limited amount of magnetic material can be used in the available space, and situations arise where only a small battery is available to provide power across the contactless interface.

It is therefore an object of the present invention to improve power transfer efficiency of a transformer whose primary and secondary windings are separated by an air-gap.

It is another object of the present invention to achieve efficient power transfer from a low voltage source.

<u style="single">Summary of the Invention</u>

A circuit for coupling power between a primary winding of a transformer and a secondary winding thereof includes a first capacitor in parallel with the primary winding. This primary tank circuit is connected at one end, to a voltage source; and, at the other end, to ground through a switching circuit that operates at a predetermined frequency.

This circuit is improved by the addition of a third winding, tightly coupled to the primary winding and having a second capacitor in parallel with it. This so-called "sympathetic" tank circuit cooperates with the primary tank circuit to absorb magnetic energy via a common magnetic core, and transfer it to the secondary thereby improving the overall power transfer efficiency.

It is a feature of the present invention that efficient power transfer is achieved in a cost effective manner requiring very few components.

<u style="single">Brief Description of the Drawing</u>

<ul id="ul0001" list-style="none"><li>FIG. 1 is a schematic drawing of a power transfer circuit in accordance with the invention;</li><li>FIG. 2 illustrates various waveforms associated with the circuit of FIG. 1; and</li><li>FIG. 3 is a schematic drawing of a prior art power transfer circuit.</li></ul>

<u style="single">Detailed Description</u>

The prior art power transfer circuit of FIG. 3 is disclosed in Application Serial No. 851,849 filed on April 14, 1986, and functions to deliver power and timing to load resistor R_L. It is understood that R_L represents the load equivalent of circuitry that rectifies, filters, and ultimately dissipates the transferred power. It is further understood that R_L is seldom purely resistive, especially at the frequencies used in the present invention.

A timing signal is presented to terminals 301, 302 and comprises a high frequency sine wave. Transformer 310 steps-up the voltage; its secondary winding is center tapped to split the output into two signals - 180 degrees apart.

The drive circuit of the power amplifier consists of two FETs, 314-­315, arranged as a push-pull, class B amplifier. Tuning capacitors 321, 322 and resistors 316-318 are used for wave shaping.

The push-pull configuration is used to obtain a larger peak-to-peak output swing from the fixed supply than would be possible with a single device amplifier. Theoretically, a peak-to-peak swing of four times the supply voltage can be obtained when the output coil is resonated. Obtaining this output swing is important because it allows the primary to have more turns for the same voltage output at the secondary. This in turn lowers circuit Q, and consequently circuit losses. The peak swing on each FET gate can be as high as 18 volts in the present circuit. This swing is intentionally made high to insure that all devices will turn on hard, thus reducing the variation of "on" channel resistance that might be encountered over various devices if a low drive level is used.

FETs 314, 315 have a V_t of 2 to 4 volts, and an "on" channel resistance of 2.4 ohms max. The gate drives are provided by a center tapped transformer output, with the center tap DC biased at 1.8 volts nominally to reduce deadband during transition intervals. A voltage divider comprising resistors 311, 313 along with filter capacitor 312 provides the necessary bias.

Capacitors 321-322 are used to resonate the primary coil 331. Without definite tuning, the primary would be excited at its self-resonant frequency and produce severe ringing which would create the possibility of false clock pulses appearing on the secondary. Tuning also makes the primary circuit look like a "real" load to the drive circuit, thus greatly reducing reactive current components in the drive and the associated losses. The tuning capacitance is split between capacitors 321-322, each having double the required value of capacitance and placed in series across the primary halves. This provides a smoother and more symmetrical output waveform than a single capacitor placed across the entire primary coil 331.

Returning to FIG. 1, an illustrative embodiment of the invention is disclose A clock signal Vc, operating at the 1.8432 MHz rate, is used to drive FETs 100, 200 through resistors R₁, R₃. One suitable FET is the 2N7000 which is available, for example, from Siliconix and has an "on" resistance of less than 5 ohms. FETs 100, 200 turn on and drive node B substantially to ground through resistor R₂ which has a value of 3.3 ohms in this example embodiment. Resistor R₂ is used to limit the maximum current allowed to flow through primary coil L₁. Two FETs (100, 200) are used in combination with R₂ in order to minimize the effect of variations in "on" impedance between FET devices. This technique improves control over the maximum current limit and hence the maximum power coupled to the secondary. Similarly, capacitor C₃ eliminates the effect associated with wiring inductance in the voltage source which is important at the present current level and frequency.

As the current changes through L₁, its opposition to change (reactance) results in a continual exchange of energy between the 5 volt source and its flux - or magnetic field. The magnetic field energy is absorbed by magnetic core 101 which in turn transfers it to a "sympathetic" tank circuit comprising components L₂, C₂. Although coil L₂ shares magnetic core 101 with coil L₁, it need not be electrically connected to it. L₁ and are connected for convenience in manufacture as shown in FIG. 1. In tnis example embodiment, it is noted that magnetic cores 101 and 102 are separated by an air-gap which makes this circuit particularly useful in a contactless Smart Card application where power and timing need to be transferred across a dielectric interface. Another feature making his circuit attractive is the use of a low voltage (5 volt) source to deliver adequate power to load impedance R_L.

A tuning procedure to be followed in selecting component values is set forth below. Measure the value of L₂ and then select the value of C₂ to achieve unloaded parallel resonance at the clock frequency in accordance with the following equation:<maths id="math0001"><img file="EP0333388A2_D0001.tif" /></maths> measure total inductance L_T = L₁ + L₂ + M_1:2 and then select a value for the total capacitance C_T in parallel with L_T to achieve parallel resonance at the clock frequency according to equation (1).

Select a value for C₁ that satisfys the following equation for series connected capacitors:<maths id="math0002"><img file="EP0333388A2_D0002.tif" /></maths>

Coils L₁ and L₂ are tightly coupled and have an unloaded mutual inductance M_1:2. Example component values are set forth below: C₁ = 1200 pf L₁ =2.0 µH C₂ =3700 pf L₂ =2.0 µH C₃ = 47 µf M_1:2 = 1.25 µH

The primary winding comprises 16 turns of 33 gauge wire; it is center tapped so that each inductor L₁, L₂ has 8 turns. The secondary winding comprises 36 turns of 41 gauge wire.

Referring now to FIG. 2 there is shown a series of waveforms that illustrate the time interrelation of various signals and assist in the understanding of the circuit Clock signal V_c, in this example embodiment is a symmetrical square wave that switches between 0 volts and +5 volts at a 1.8432 MHz rate.

The instantaneous voltage at node B, designated V_b, is driven toward ground when transistor 100 is turned "on". Resistor R₂ and the "on" impedance of transistors 100, 200 allow V_b to fluctuate somewhat in the vicinity of zero volts at this time. When transistors 100, 200 are turned "off", the current through L₁ seeks to continue in the same direction. Accordingly, V_b becomes more positive and delivers current to capacitor C₁. Indeed, when transistors 100, 200 are turned off, the combined primary tank circuits resonate at a frequency determined by the values of the individual components which, in this design, has been selected to be 1.8432 MHz - the clock frequency. At this time, the entire primary circuit is free-oscillating because FETS 100, 200 are turned off.

The instantaneous voltage at node A, designated V_a, has a DC average voltage of +5 volts and fluctuates in a substantially sinusoidal manner. Voltages V_a and V_b combine to optimize power transfer across the air-gap interface to load impedance R_L. Observing the various waveforms of FIG. 3 demonstrates that the tank circuit comprising components C₁, L₁, C₂, L₂, provides one-half of the sine wave drive to the secondary load, and the tank circuit comprising components C₂, L₂ (sympathetic resonator) provides the other half. The resulting primary voltage is sinusoidal with a peak-to-peak value of nearly 40 volts.

The prior art circuit of FIG. 3 has a power transfer efficiency of approximately 10%. By comparison, the inventive circuit of FIG. 1, including a sympathetic resonator, provides a power transfer efficiency in excess of 20%. This circuit is capable of supplying 20 ma across a 0.08 inch air-gap. Average current in the primary is typically 35 ma without the secondary load connected and 85 ma with the secondary load connected - depending on air-gap.

The present invention is most useful in supplying power to a contactless Smart Card. The frequency associated with power transfer is used by the Smart Card as its clock reference. In the FIG. 1 embodiment, L₃ represents the inductive load associated with the transformer secondary. Maximum power transfer is achieved when the secondary circuit is purely resistive which can be accomplished by adding a capacitor in series with resistor R_L. This capacitor is selected to series resonate with L₃ at the power transfer frequency.

Of particular interest, however, is the situation in which the secondary load is reactive. As the secondary coil is brought into the magnetic field of the primary coil, power transfer efficiency drops off due to slight de-tuning caused by a reactive load being reflected into the primary circuit. Such de-tuning provides an important advantage in that it mimmizes certain effects due to misalignment. Although power transfer efficiency between primary and secondary coils decreases as these coils are brought into perfect alignment, the actual amount of power transfer remains substantially constant over a broad range of alignments.

Although a particular embodiment has been disclosed, it is understood that various modifications are possible within the spirit and scope of the invention.

For example, at higher frequencies the need for capacitor C₁ is satisfied by stray capacitance. Further, the magnitude and polarity at the supply voltage and ground may be changed or reversed without detracting from the principles of the invention.