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Designing envelope tracking modulator

12 Feb 2015  | Peter Markowski

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While the RF amplifier can now work with full efficiency, there is a price to be paid in the cost, complexity and reduced efficiency of the modulator itself. This approach was realised in some commercial applications, but the most typical architecture is similar to the one proposed by Peter Garde, which is shown in figure 4.

Figure 4: The most popular method for combining switching and linear regulators.

Switching-linear combo for high-end apps
While commercially feasible, the switching-linear combination is most suitable for high-end applications. For widespread applications, eliminating the burden of the linear regulator seems necessary. However, the switching frequency, which according to the rule of thumb has to be at least 10 times higher than the bandwidth, seems impossible.

The conflict between switching frequency and bandwidth can be resolved by using a multi-level converter, which can achieve a small signal bandwidth higher than the average switching frequency. This is possible because the small amplitude regulation can be realised with the switching transition of only one or two of the many subconverters, while other parts continue to deliver the "unchanged portion of the voltage." The high / low bandwidth assignment between subconverters can be continuously rotated, resulting in an equal and moderate burden on switching devices.

The other benefit of a multi-level converter comes from the much higher frequency and lower amplitude of the switching harmonics. As a result, the output filter corner frequency can be raised so high that it does not degrade the overall bandwidth and achieving stability is much easier.

The biggest problem with using a multi-level converter for an envelope tracking application comes from the fact that existing multi-level topologies were devised for high-power, high-voltage applications. As a result, the main current path leads through a large number of power components, thus increasing conduction losses very significantly. In high-voltage applications where conduction losses are low this isn't a problem, but for low-voltage, high-current envelope tracking applications, the resulting efficiency would be unacceptable.

Artesyn Technology developed a new type of multi-level converter designed specifically for this type of application. It consists of a large number of switching cells combined with a proprietary matching impedance network. Like in many multi-level converters, the output voltage is proportional to the number of switching cells that are on at any given time, which creates output in a stepped or staircase-like pattern. A matching impedance network has very low output impedance (about 1 nH), facilitating very high bandwidth. For ET applications, a version with 16 subconverters was chosen as adequate to keep harmonics at an acceptable level with minimum filtering. This was sufficient for a small signal bandwidth of 100MHz and 200 W average / 1000 W peak power to an RF amplifier working with up to 40MHz bandwidth. At the same time, the average switching frequency per phase was only about 10MHz. While still high, it is definitely within the reach of state-of-the-art silicon MOSFET technology, like the SiP2204 used for this design.

Figure 5: New multi-level converter with passive coupling network for low-voltage operation.

To keep the overall efficiency above 90 %, it was necessary to add one more innovation: only a fraction of the total output power was processed through 10MHz switching; the part associated with outputting AC voltage and AC current simultaneously. Most of the DC voltage and current was directed straight from the source to the load, entirely bypassing the switching MOSFETs with an "effective efficiency of 100 %." Concurrently, switching voltage and current were reduced to diminish MOSFET losses. Additional improvement was achieved by the special design of the power stage in the SiP2204, which was optimised for minimal losses at light loads. With all these factors combined, the need for a linear regulator was eliminated.

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