A MEMS switch for wireless power transfer

Article By : Marten Seth & Bernard Ryan

A MEMS contact switch can be used to optimize impedance matching in resonant wireless power transfer systems for electric vehicles and other products.

If resonant wireless power transfer (WPT) systems are to fulfill their promise for charging electric vehicles and other high-power applications, there is an engineering issue that must first be satisfactorily addressed.

One of the biggest challenges in optimizing the performance of a resonant WPT system is ensuring that a relatively constant load impedance is presented to the transmitter power electronics at all times. Impedance variances occur when the distance from the receiver to the transmitter changes, for example, or when obstructions come into in the signal path.

Techniques have been developed to address the problem, including large DC/bias power requirements, complex bias circuitry, and large footprint. A new approach uses micro electro-mechanical system (MEMS) contact switches in an impedance matching network that has advantages over these approaches without their limitations.

There are several ways to transfer energy wirelessly from a transmitter to a receiver, and the choice of which one to use depends heavily on the needs of the application. What follows is a quick survey of some of the different approaches for wireless powering, and a discussion of the MEMS-based technique.

Transmission over very large distances is typically achieved either electrically, through the use of radiative electromagnetic transfer, or optically, using lasers. While these techniques can provide high efficiencies, they require sophisticated receiver tracking and an unobstructed path between the transmitter and receiver.

Non-resonant WPT systems likewise offer high transfer efficiencies, though only when transmitter and receiver are fixed and they are separated by a very small gap. A classic example of this type of system is electric toothbrush chargers, which implement inductive charging.

Resonant WPT systems enable high wireless transfer efficiencies as well, but over a wide and variable range of operating distances. This enables a degree of positional freedom not available in non-resonant systems, without the complexity of receiver position tracking.

As the name suggests, resonant WPT systems make use of the physical phenomenon of resonance to transfer electromagnetic energy from a transmit resonator to one or more receive resonators. All resonators are tuned to resonate at the system’s operating frequency. The technique uses a non-radiative method of energy transfer, using the near-field to transfer power to the receive resonator.

There are two common implementations of resonant WPT, resonant inductive coupling and resonant capacitive coupling. Resonant inductive coupling use coils to transfer energy predominately over a magnetic field, while resonant capacitive coupling uses electrodes to transfer the energy largely through an electric field.

In order to maximize the positional freedom of resonant WPT systems, dynamic impedance matching is required. For WPT solutions operating in the high frequency (HF) band, some options for meeting the challenge of variable impedance control include PIN diodes, varactors, and reed relays. However, these techniques have different drawbacks ranging from complicated biasing requirements to low radio frequency (RF) power handling.

Here we describe a MEMS high-power six-channel switch (Figure 1) for WPT applications that provides high-channel density, ultra-low parasitics, and extreme power handling, avoiding some of the drawbacks that hamper other WPT solutions.

Figure 1
The MM3100 six-channel switch handles 200V, switches in less than 10 µs, is rated at more than 3 billion switching operations, and is housed in a 6×6-mm LGA package.

Advances in processing capability and metallurgy have enabled new devices to be fabricated that combine the on-state conductivity of metal contacts with stable mechanical properties, effectively eliminating the long-standing issues with mass producibility and long-term reliability that have kept MEMS switches from entering the market. Menlo Microsystems calls these devices digital micro switches (DMS); the particular DMS we discuss here is the MM3100.

The actuation mechanism of this MEMS switch resembles that of an electromechanical relay, as it opens and closes to make contact, but at a microscopic level. Compared to an electromechanical relay, these devices switch 1,000× faster (less than 10 µs), and are a fraction of the size.

It has extremely low insertion loss (on-resistance configurable from 0.4Ω down to less than 0.1Ω), and even though extraordinarily small, can handle hundreds of watts of RF power and several hundred volts. The device also uses electrostatic switching control, has very low DC power requirements, and requires minimal biasing with no current drain in a steady-state hold operation.

To evaluate the performance of this switch in the application of WPT impedance matching, Menlo Microsystems and Solace Power created circuits and an environment electrically resembling Solace’s Equus system (Figure 2). The Solace WPT approach employs patented resonant capacitive coupling technology that transmits up to 150 W of RF power at 13.56 MHz in either fixed or variable-distance scenarios.

Figure 2 A typical Equus system showing the location of the MEMS switch.

It can be used to power a device or charge it at distances up to about 25 cm (10 in.) from the transmitter and does not heat metal objects in the energy field. It can accommodate misalignment of the transmitter and receiver, making it more flexible than other approaches. The antenna structure can be as thin as a sheet of foil and can be conformal, allowing it to be integrated on the exterior of a host product.

The test evaluated the switch in a variable impedance matching network with a RF transmit power of 75 W. The impedance-matching circuit consists of several fixed reactive elements in conjunction with two variable shunt capacitor branches, each consisting of six parallel capacitors. Two MM3100 switches are used to implement these variable capacitors by switching the capacitors in and out of the circuit at high speed. The matching network attempts to transform the varying wireless link impedance to an optimal impedance Zo = 50 Ω. The transformed impedance loads the transmitter’s power electronics, enabling maximum output power the closer it is to Zo.

There is a direct relationship between the amount of power that can be transmitted and the maximum impedance that loads the matching network. Both the transmit power level as well as the voltage rating of the switch places a limit on the largest impedance that can be matched. With a transmit power level of 75 W, and as the MM3100 in this circuit has a maximum voltage of 141 Vrms, the maximum load impedance is 265 Ω.

[Continue reading on EDN US: Testing]

Marten Seth is a senior systems application engineer at Menlo Microsystems, and Bernard Ryan is a senior electrical researcher at Solace Power.

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