## Magnetically Coupled Multilevel Converter

New Topology for Very High Bandwidth and Efficiency

The envelope tracking voltage modulator developed by Emerson Network Power (now Artesyn) was based on a new technology optimized for an extremely high bandwidth. This purely switched mode, high efficiency, and low cost converter is capable of exceeding the regulation bandwidth of 100 MHz. The efficiency in a typical envelope tracking application is above 90%. This was possible through the use of a new type of converter; a Magnetically Coupled Multilevel Converter with control algorithms implemented in FPGA. Silicon MOSFETS were used so a further increase in the bandwidth could be obtained with GaN devices.

Envelope tracking technology increases the efficiency of RF amplifiers by modulating the supply voltage proportionally to the RF signal.

In this way the amount of energy wasted for the biasing amplifier is reduced and the efficiency goes up. The main problem is that contemporary RF amplifiers require a bandwidth of tens of MHz.

Drawings from TI paper: "RF Front End Power Solutions"

Drawings from TI paper: "RF Front End Power Solutions"

A very high regulation bandwidth was enabled by using a Magnetically Coupled Multilevel Converter. In MCMC all subconverters work in parallel. The incremental output steps are 1/n of the input voltage. There is always only a single power transistor on the path of the current and all subconverters work in parallel at all times. The average switching frequency is well below the regulation bandwidth! The model shown here (actual size) has a peak power of 1kW and an average power of 200W.

The FPGA custom controller performs 50 billion mathematical operations per second, more than half of them on balancing. The main tasks are: sub nanosecond accuracy of each switching transition, shaping a stepping pattern for harmonic control, minimizing the switching frequency and balancing the AC power among all subconverters. The exact instance of every switching transition is controlled via a rotating queue multiphase PWM algorithm.

The main magnetic component has a total effective inductance of 1nH in the forward path which facilitates an extreme bandwidth. At the same time the differential inductance determining the AC ripple current and circulating energy is at least a 1000 times higher. This unusual feature allows the designer to break away from the severe tradeoff between the bandwidth, switching frequency and the AC ripple. A MCMC with the sufficient number of steps for particular a application allows the designer to eliminate LC filter altogether.

A Magnetically Coupled Multilevel Converter may be used in all applications where the voltage is modulated with a high bandwidth and an extreme slew rate is needed. The switching frequency can be low, accuracy of the voltage pattern is high with each switching transition individually controlled for a minimal harmonic impact. Applications may include high power, high fidelity audio amplifiers, high frequency inverters, arbitrary voltage pattern generators, etc. This technology is protected by patents belonging to Artesyn Embedded Technologies but can be licensed on terms suitable for a particular business.

**SEE ALSO ENVELOPE TRACKING PATENTS***Envelope tracking modulator pictures courtesy of Artesyn Embedded Technologies, formerly Emerson Network Power*