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Flying Capacitor Voltage Balancing Method by Sensing Inductor Current

Fig. 1. Three Level Buck Converter Schematic and control timing diagrams.
Low current non-isolated POL converters are used in a wide range of applications such as telecom, MP3 players, digital cameras, GPS, automotive electronics, laptops, servers, wireless cellular phones, and PDA's. With the desire for smaller size and a greater level of integration these products are being pushed for higher power density. The issues facing higher power density and integration for the standard buck converter are 1) The large passives required for high efficiency and tight regulation, 2). The high switching loss generated at higher switching frequencies, and 3). The ability to integrate semiconductors, minimizing parasitics and increasing efficiency. The three level buck converter (Figure1) can overcome these issues to offer a superior solution to the standard buck converter. For the most beneficial operating mode the flying capacitor C2 is kept at half of the input voltage. To summarize the benefits of the topology:

  1. Inductor size reduced significantly by reducing volt-second balance and doubling the effective frequency across the inductor

  2. Higher efficiency obtained with lower voltage rated devices. Devices see half the input voltage when flying capacitor balanced to 0.5Vin

All previous balancing methods have measured the capacitor voltage directly and balanced the capacitor voltage at the expense of performance by altering one duty cycle (T1,B1) to maintain output voltage and the other (T2,B2) to maintain capacitor balance. This adds complexity, degrades performance, and adds an additional control loop.

The method proposed in by CPES is to regulate the flying capacitor without a separate capacitor balancing loop is by using leading edge modulation current mode control. Current mode control is widely used in fast transient applications because of its ability to have good line regulation and to ease current sharing for multiple phases. Leading edge modulation can be adapted and naturally balance the capacitor in the three level converter by balancing the inductor current. This is possible because the inductor current contains the capacitor voltage information and is balanced when the inductor current is balanced.
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