The artificial heart now in use, like the natural heart it is designed to replace, is a four chambered device for pumping blood. Such electrical circulatory assist devices such as total artificial heart or ventricular assist devices generally use a brushless dc motor as their pump. They require 12–35 Watt to operate and can be powered by a portable battery pack and a dc–dc converter.
It would be desirable to transfer electrical energy to these circulatory assist devices transcutaneously without breaking the skin. This technique would need a power supply which uses a transcutaneous transformer to drive use, the motor for the circulatory assist devices. The secondary of this transformer would be implanted under the skin, and the primary would be placed on top of the secondary, external to the body. The distance between the transformer windings would be approximately equal to the thickness of the patient’s skin, nominally between 1–2 cm. This spacing cannot be assumed constant; the alignment of the cores and the distance between them would certainly vary during the operation.
A transformer with a large (1–2 cm) air gap between the primary and the secondary has large leakage inductances. In this application, the coupling coefficient k ranges approximately from 0.1 to 0.4. This makes the leakage inductances of the same order of magnitude and usually larger than the magnetizing inductance. Therefore, the transfer gain of voltage is very low, and a significant portion of the primary current will flow through the magnetizing inductance. The large circulating current through the magnetizing inductance results in poor efficiency.
A dc–dc converter employing secondary-side resonance has been reported to alleviate the problems by lowering the impedance of the secondary side using a resonant circuit .Although the circulating current is lowered, the transfer gain of the voltage varies widely as the coupling coefficient varies .So, advantages characteristics are reduced as the coupling coefficient deviates at a designated value.
In this paper, compensation of the leakage inductances on both sides of the transcutaneous transformer is presented. This converter offers significant improvements over the converter presented in the following aspects.
- High-voltage gain with relative small variation with respect to load change as well as the variation of the coupling coefficient of the transformer—this reduces the operating frequency range and the size of the transcutaneous transformer is minimized.
- Higher efficiency—minimize circulating current of magnetizing inductance and zero-voltage switching (ZVS) of the primary switches, and zero-current switching (ZCS) of the secondary rectifier diodes improves the efficiency significantly, especially at the secondary side (inside the body).
A design procedure allowing for a variable output power as well as a variable air gap and misalignment is presented. The theoretical analysis is verified by an experimental converter which transfers 12–48 Watt through an air gap of 1–2 cm. In addition, the feedback control scheme which processes the secondary sensed signal to the primary switches transcutaneously is presented.
Proposed Energy Transference Scheme
To effectively transfer electric energy through the transcutaneous transformer, a high-voltage gain with small variation and small circulating current through the magnetizing inductance is important. To achieve these requirements, a method of the compensation of the leakage inductances on the primary side as well as the secondary side is proposed, as shown in Figure. In this scheme, two capacitors C1 and C2 are added in series.
In Figure, the square-wave voltage source Vs, the magnetizing inductance LM , and the leakage inductances L11 and L12 are the equivalent values reflected to the secondary side of the transformer. The higher turn ratio requires more windings of the secondary side for a given operating frequency, and the lower turn ratio requires high voltage of the input side. Therefore, the turns ratio of the transformer is considered to be unity in this paper.
Analysis Of The Proposed Scheme
Figure shows a simplified equivalent circuit model of Figure 1 The voltage gain characteristics for the frequency variation can be calculated by applying an approximation method. The load, rectified diodes, and filter in Figure 1 are modelled by a simple equivalent resister Req.
The design specifications are given by the requirements of the output load of the system. Because the power input to the biological heart is approximately 15 W at resting conditions, and 35 W under heavy exercise, the required output power is set from 12 to 48W. The specifications used are
- Vo = 24V
- Iomax = 2.0 A
- Iomin = 0.5 A
Where Vo is the output voltage, Iomax is the maximum output current, Iomin is the minimum output current.
Transformer And Compensating Capacitance
Many researchers have studied the methods to optimize the geometry of the transformer windings to obtain the maximum coupling coefficient. To simplify the task, it is assumed that the size, geometry, and core material of the transformer and the range of air gap and misalignment between them have already been defined. For the transformer windings, the same cores used in series resonant converter were selected to compare the overall performance with proposed scheme.
- Cores: Ferroxcube Pot Core 6656
- 3C8 Ferrite
- OD 2.6 in, thickness 1.1 in
- Air gap: 10–20 mm
- Misalignment: 0–10 mm.
Based on the gain characteristics for the predicted kmin and kmax in Fig. 2.3, a design value Q can be selected. In Region III, higher Q provides a high-gain system with respect to the frequency variation. However, due to the deviation of the leakage inductances and, thus, the normalized frequency for the variation of k, it is desirable to select a lower Q to reduce the sensitivity for the variation. The selection of Q in this design is from two at light load to eight at full load. For minimum size and weight and high-efficiency requirement of the system, the compensating resonant frequency is chosen at 120 kHz. From (19), the required leakage inductance can be determined for a designed Qmin and Fc.
To realize both the high-voltage gain and the minimum circulating current, a method of the compensating leakage inductances on both sides of the transformer is proposed. The properties of the proposed scheme are summarized as follows.
- High-voltage gain and the reduced circulating current.
- A control region of an operating frequency is determined, which realizes the robustness the coupling coefficient as well as the load.
- The minimized configuration of the devices in the thorax is experimented.
- The converter guarantees many advantages because of ZVS of all active switches and ZCS of the rectified diodes, low devices switching loss and stress, and high efficiency.
A design procedure to reduce the effects of the given variations of load and coupling coefficient is established, and the above advantages are experimentally verified.