PMIC Seminar - High Frequency DC-DC Converters

PMIC Seminar

The seminar is a regular activity of the Power and Mixed-Signal Integrated Circuit Laboratory, held once a month, hosted by the heads of various research directions in the lab, aimed at enhancing communication and integration among research groups, sharing cutting-edge research dynamics, and summarizing stage achievements. In the last seminar, Senior Brother Yuan Jingyi shared content in the field of high-voltage DC-DC converters, and the theme of this seminar is high-frequency DC-DC converters, hosted by Senior Brother Tang Kui.

Figure1 Seminar Agenda

Integrated Voltage Regulator

Senior Brother Tang Kui, the head of the high-frequency DC-DC converter group, shared relevant background and principles of Integrated Voltage Regulators (IVR) based on the research direction within the group.

Figure2 Applications of Integrated Voltage Regulators

Integrated Voltage Regulators have two significant advantages: (1) high power density; (2) faster transient response.

High power density is due to the fact that Integrated Voltage Regulators do not require external capacitors or inductors, and faster transient response is due to the high switching frequency of the system and the use of smaller inductors.

Figure3 Fast Transient Response in Integrated Voltage Regulators

Regarding the main challenges in research, Senior Brother Tang Kui summarized them into two points: (1) how to improve efficiency; (2) the voltage rating requirements for power transistors under high input voltage. For example, the problems existing in the following circuit are quite obvious.

Figure4 Integrated Voltage Regulator with Voltage Rating Issues

Buck-Boost Converter

Subsequently, Senior Brother Tang Kui introduced the research on Buck-Boost structured DC-DC converters.

Regarding the background introduction, for consumer electronic products, batteries are essential for their normal operation. The energy density of different types of batteries is relatively shown in Figure5. It can be seen that lithium batteries have the highest energy density, making them the most suitable for consumer electronic products like smartphones. However, as usage time increases, the charge of lithium batteries decreases, and the voltage will drop from4.2V to2.8V, as shown in Figure6. However, for the subsequent functional circuits, we hope the battery can provide a stable supply voltage, such as3.4V. Therefore, a simple Buck circuit cannot meet the usage requirements. To extend usage time as much as possible, it is necessary for the power management circuit to have both buck and boost capabilities.

Figure5 Energy Density in Lithium Batteries

Figure6 Charge Variation in Lithium Batteries

As shown in Figure7, this is a traditional boost converter, which can achieve voltage boosting with a conversion ratio of 1/1-D. However, this structure also has obvious disadvantages: the inductor current is 1/1-D of the load current, leading to significant losses in the inductor, while the output current ID being discontinuous can cause large switching noise and significant output voltage ripple.

Figure7 Traditional Boost Converter

[Ref: S.Shin, TPEL 2020]

For traditional boost converters, small signal analysis can be conducted as shown in Figure8. It can be seen that the main pole is related to the load, and there is also a zero in the right half-plane, which greatly increases the design difficulty of the control circuit compensator. Additionally, the presence of the right half-plane zero reduces system stability.

Figure8 Small Signal Analysis of Traditional Boost Converter

[Ref: S.Shin, TPEL 2020]

As shown in Figure9, this is a traditional H-type buck-boost converter, which can simultaneously achieve buck, boost, and buck-boost functions, but it also has obvious disadvantages:

(1) Discontinuous output transmission current;

(2) High switching noise;

(3) The presence of a zero in the right half-plane reduces loop dynamic performance.

These are also urgent problems to be solved and research directions.

Figure9 Traditional Buck-Boost Converter

[Ref: Zheng, ISSCC 2010]

Current Research Work

Finally, addressing the issues with the traditional structures mentioned above, Senior Brother Tang Kui introduced three new topologies, as shown in Figure10: the new topological structures allow the inductor current to flow continuously to the load, while simultaneously achieving buck and boost functions within one topology, expanding the output voltage range. PMIC Seminar - High Frequency DC-DC Converters

Figure 10 New Topological Structures

PMIC

Senior Brother Tang Kui’s sharing sparked enthusiastic discussions, with many junior and senior students asking their questions to the seniors. Thus, this month’s seminar comes to a close, and the next seminar will focus on Envelope Tracking (ET), hosted by Chen Changjin. We welcome students from all grades and majors interested in IC design to come to the fusion building in the north district of the university for交流学习.

PMIC Seminar – High Frequency DC-DC Converters

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