Wireless Power Transfer and SAR Simulation of Implantable Medical Devices Using Ansys HFSS

Hello everyone, this is Ated from OZEN Engineering. In this video, I'll give you an overview of wireless power transfer and specific absorption rate (SAR) simulation in ANSYS HFSS. In this example, a wireless power transfer system operating at 15 MHz is simulated with the ANSYS head model.

Here, we have two coils: a transmitter, which is external, and a receiver, which is implanted in the head.

Simulating the transmitter or receiver coil by itself in HFSS helps determine its inductance, which is used to calculate the necessary capacitance to achieve resonance at the desired operating frequency. This is the transmitter by itself.

It consists of a copper coil, substrate, a ferrite layer, and a copper layer. If we go to the project manager, we can see the applied boundary conditions. Here, a radiation boundary condition is applied to the airbox, and for the excitation, a 50 ohm circuit port is used.

Additionally, a lumped port is used. We can also plot the coil's self-resonance, which is shown here. You can see that the coil's self-resonance is around 100 MHz, which is higher than the desired operating frequency.

Once the HFSS simulation of the coil is complete, it is linked to a circuit where tuning capacitors are added and tuned to achieve resonance at 15 MHz. So here, I added two tuning capacitors and tuned them using the tuning feature in the circuit.

And here, you can see the return loss, which is better than -30 dB at 15 MHz. Going back to the full HFSS model, a similar coil was used for the receiver with the addition of a plastic enclosure. The coil is now connected to the HFSS model.

If we look at the simulation setup, we have a radiation boundary condition applied to the airbox. And for the excitation, circuit ports are used. This is port 1 and port 2. Here's the simulation setup. The model is first simulated in HFSS.

And then dynamically linked to the circuit where tuning capacitors are added to the design. So if I go to the circuit, this is the circuit schematic with the tuned capacitors.

We have a good resonance at 15 MHz, a return loss better than -19 dB, and an insertion loss of -0.75 dB, which corresponds to an effect of 0.75 dB. The output of the signal is also to an efficiency of 84 percent.

Co-simulation allows the circuit model to control excitation for post-processing of the EM solution. To set 1 W input power in HFSS with a 50 Ohm load, we need to set the AC mag in the voltage source properties of port 1 to 20 V. This is based on the relationship V = square root of 8 * Pmax * R.

So this is a simulation setup. And if I go to the circuit model and set the AC mag in port 1, we have a good resonance at 15 MHz for the HFSS model. This is based on the relationship V = square root of 8 * Pmax * R. And here, you can see that in port 1, the AC mag is set with a setting of 1 W.

So if I double click on port 1 and go to edit sources, edit properties, you can see that ACMAG is set to 20 volts. And after solving the model, to push excitations, we right-click on the model and click push excitations.

Once the updated excitation is pushed back to HFSS, the SAR maximum value and distribution can be calculated. First, an object list is created. Here, under lists, you can see object list 1 that I created and which includes the skin and the skull objects.

Now, if I go to field overlays, right-click on it and go to excitation. And if I go to SAR settings, you can see that I selected the object list. And here, the SAR average over 1 gram of tissue is calculated using the recommended IEC/IEEE method.

We can also check the input power through the edit sources window. So here, you can see that our input power is 1 watt. Now, if I go back to the SAR results and double click on it, you can see our SAR distribution and maximum value.

Here, with 1 watt of input power and 1 gram of tissue mass, the simulated SAR maximum value is 0.4 watts per kilogram. This corresponds to an input power of 4 watts to achieve the US limit of 1.6 watts per kilogram. Similarly, we can plot the SAR average over 10 grams.

The simulated SAR maximum value is 0.082 watts per kilogram. Which corresponds to an input power of 24.4 watts to achieve the European limit of 2 watts per kilogram. This concludes this overview. Thanks for watching and see you in the next video.