Pablo F. Miaja

An Insight into the Switching Process of Power MOSFETs: An Improved Analytical Losses Model

The piecewise linear model has traditionally been used to calculate switching losses in switching mode power supplies due to its simplicity and good performance. However, the use of the latest low voltage power MOSFET generations and the continuously increasing range of switching frequencies have made it necessary to review this model to account for the parasitic inductances that it does not include. This paper presents a complete analytical switching loss model for power MOSFETs in low voltage switching converters that includes the most relevant parasitic elements. It clarifies the switching process, providing information about how these parasitics, especially the inductances, determine switching losses and hence the final converter efficiency. The analysis presented in this paper yields two different types of possible switching situations: capacitance-limited switching and inductance-limited switching. This paper shows that, while the piecewise linear model may be applied in the former, the proposed model is more accurate for the latter. Carefully-obtained experimental results, described in detail, support the analytical results presented.

Multilevel converter for Envelope Tracking in RF power amplifiers

Envelope Tracking techniques are used to increase the efficiency of modern radiofrequency transmitters. These techniques are based on varying the supply voltage of the amplifiers according to the envelope of the signal being transmitted, thus maximizing their efficiency. Converters suitable for Envelope Tracking applications must provide high tracking bandwidths, along with low output voltage ripple. This paper presents a power supply for Envelope Tracking applications based on a multilevel topology: it targets relatively low voltage systems (12–28 volt range) and medium to high power applications (≫ 50 W peak power). It uses several input voltages to generate a multilevel square waveform which can be easily filtered; it achieves low output voltage ripple, high tracking bandwidths, high efficiency and high output power capabilities. This paper first analyzes its steady-state behaviour; then, a design procedure is provided, along with guidelines for appropriate implementation of the proposed system. A three-level prototype was built to test the system, and its performance is shown in the experimental results section. Finally, conclusions are stated.

A Linear Assisted DC/DC Converter for Envelope Tracking and Envelope Elimination and Restoration Applications

In recent years, there has been a great deal of focus on the development of fast and efficient dc/dc converters in an effort to follow the envelope of communication signals for increasing the efficiency of RF power amplifiers by using techniques such as envelope tracking and envelope elimination and restoration. However, the bandwidth and slew rate required by modern communication signals are higher than the maximum ones achievable by switching dc/dc converters made for this purpose. The slew rate of these switching dc/dc converters can be improved by combining it with the use of a linear stage, thus establishing a trade-off between slew rate and overall efficiency. This paper presents a simple way of combining the use of both a switching dc/dc converter, the multiple input buck converter, and a linear stage to obtain slew rates above 100 V/μs, with a moderate decrease in the overall efficiency [81% efficiency tracking the envelope of an enhanced data rates for GSM evolution (EDGE) standard signal]. Finally, a prototype of the complete system (including the digital control for both stages) is presented in this paper.

Buck-Derived Converters Based on Gallium Nitride Devices for Envelope Tracking Applications

Envelope tracking (ET) is a technique designed to enhance the efficiency of radio frequency power amplifiers (RF PA). It is based on providing the voltage to the RF PA with variations that mimic the shape of the envelope of the communication signal that the RF PA is processing. As the bandwidth of these signals can be around several megahertz, the switching frequency of the switching mode power supply designed for ET applications has to be very high. The good switching characteristics of Gallium Nitride devices makes them suitable for this application. This paper presents two multiphase converters to be used as envelope modulators in envelope tracking applications.

A Linear Assisted Switching Envelope Amplifier for a UHF Polar Transmitter

Spectrally efficient wireless communication standards impose stringent linearity specifications, which would require traditional IQ transmitters to operate with back-offed and power inefficient linear RF power amplifiers (PAs). In order to overcome such a significant limitation, alternative architectures have been proposed, as those based on the envelope elimination and restoration technique. An example of the application of this technique is the polar transmitter. In this paper, a UHF polar transmitter is presented, combining switching and linear stages in the envelope amplifier as to achieve both wide bandwidth and high efficiency, when drain modulating a GaN HEMT Class E RF PA. Several tests, using EDGE, TETRA, and WCDMA standards have been performed with good results.

A linear assisted DC/DC converter for Envelope Tracking and Envelope Elimination and Restoration applications

In recent years, there has been a great effort in the development of fast and efficient DC/DC converters in order to follow the envelope of communication signals for increasing the efficiency of Radio Frequency Power Amplifiers by using techniques such as Envelope Tracking and Envelope Elimination and Restoration. However the bandwidth and slew rate required by modern communication signals is higher than the maximum slew-rate achievable by switching DC/DC converters made for these purpose. The slew rate of these switching DC/DC converter can be improved by combining it with the use of a linear stage, thus establishing a trade-off between slew rate and overall efficiency. This paper presents a simple way of combining the use of both a switching DC/DC converter, the Multiple Input Buck Converter, and a linear stage to obtain slew-rates above 100 V/µs, with a moderate decrease in the overall efficiency (81% efficiency tracking the envelope of an EDGE standard signal). Finally, a prototype of the complete system (including the digital control for both stages) is presented in this paper.

Mismatch-Error Shaping-Based Digital Multiphase Modulator

This paper introduces a digital multiphase modulator (MPM) using mismatch-error shaping techniques based on a digital-to-analog converter view of multiphase power converters. The proposed modulator has relatively simple implementation requiring no explicit pulsewidth modulation and generates control pulses for any number of phases using a single, low-frequency clock. Advantages and disadvantages of the mismatch-error shaping MPM are examined through analysis, simulations, and experimental results. The proposed MPM is particularly well suited for multilevel envelope-tracking amplifiers supplying power to linear RF power amplifiers. Results are shown for an experimental eight-phase buck converter in an envelope-tracking application.

Multiple-input buck converter optimized for accurate envelope tracking in RF power amplifiers

Envelope Tracking techniques are used to increase the efficiency of modern radiofrequency transmitters. These techniques are based on varying the supply voltage of the radiofrequency power amplifier according to the envelope of the signal to be transmitted. This paper presents an Envelope Tracking system based on a multilevel topology that targets relatively low voltage systems (12-28 volt range) and medium to high power applications (> 50 W peak power). The proposed converter achieves low output voltage ripple, high tracking bandwidths, high efficiency and high output power capabilities. An appropriate characterization of the load behavior of the radiofrequency power amplifier allows the system to work in an open loop manner. This paper shows that the envelope tracking system proposed is capable of increasing the efficiency of linear, Class-A and Class-B commercial radiofrequency power amplifiers between 10 and 15%.

Analysis of the switching process of power MOSFETs using a new analytical losses model

The piecewise linear model has traditionally been used to calculate switching losses in switching mode power supplies due to its simplicity and good performance. However, the use of the latest low voltage power MOSFET generations and the continuously increasing range of switching frequencies has made it necessary to review this model to account for the parasitic inductances that it does not include. This paper presents a complete analytical switching loss model for power MOSFETs in low voltage switching converters that includes the most relevant parasitics. It clarifies the switching process, providing information about how these parasitics, especially inductances, determine switching losses and hence the final converter efficiency. The analysis presented in this paper yields two different types of possible switching situations: capacitance-limited switching and inductance-limited switching. This paper shows that, while the piecewise linear model may be applied in the former, the proposed model is more accurate for the latter. Carefully-obtained experimental results, described in detail, support the analytical results presented.

Enhancements of the multiple input buck converter used for Envelope Tracking applications by improved output filter design and multiphase operation

Increasing the efficiency of Radio Frequency Power Amplifiers has growth in importance due to the widespread use of wireless communications. Efficiency enhancement techniques such as Envelope Tracking and Envelope Elimination and Restoration use DC/DC converters to track the envelope of the signals being transmitted. These envelopes often have very high bandwidth and slew rate, challenging the design of fast and efficient tracking converters. Furthermore, traditional second order output filter design techniques applied to tracking converters lead to very high switching frequencies and thus low efficiencies. This paper analyzes the use of higher order filters to improve the dynamic response of tracking converters without increasing the switching frequency, thus maintaining high efficiency operation. This approach can be combined with a multiphase architecture to further increase the achievable bandwidth. Experimental results that demonstrate the effectiveness of the proposed approach are shown using a 2-phase multiple-input buck converter operating in open loop with a fourth order Bessel output filter.