31.2 A Ripple-Less Buck Converter with Sub -21.94dB EVM for 5G Low Earth Orbit Application

The rise of non-terrestrial networks and 5G NR has brought low earth orbit (LEO) sattelites to the forefront as the next most favored space application. However, due to the constraints of solar systems and the need for efficient long-distance wireless transmission, the power management of LEO systems faces the challenge of low power consumption and guaranteeing high-quality RF circuit transmission (error vector magnitude (EVM)<-21.94dB). To address the shortcoming of the two-step conversion (from high voltage to $5 V$, then $1 V$) in traditional power management systems, batteries in LEO systems with a nominal $21.6 V$ to $33 V$ range need to be converted directly to $1 V$ by a low conversion-ratio (CR) converter (top left of Fig. 31.2.1) for high efficiency. The challenge is to convert directly to $1 V$ and keep suppressing output voltage ripple (Δ VOUT) under 3 %* VOUT=30 mV. Shown in the top middle of Fig. 31.2.1, [1], [2] have proposed a dual-path buck converter to reduce the inductor current by adding a switched-capacitor (SC) path to achieve high efficiency. However, the switching noise due to the charge injection of the SC path will cause a large Δ VOUT(> 20 % × VOUT. in [1], > 6.5 % × VOUT in [2]). For the 64 quadrature amplitude modulation (QAM) in the LEO system, the EVM under the hybrid converter in [2] is $-15.22 dB$ since Δ VOUT=6.5 % × VOUT (top right of Fig. 31.2.1), which deteriorates the performance of RF circuit transmission. [3, 4, 6] have proposed multi-phase hybrid converters to reduce the inductor current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</inf> of each phase for high efficiency and reduce Δ VOUT by current ripple cancellation at C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</inf> (bottom left of Fig. 31.2.1). Δ VOUT is close to zero at $D=50 %$ in [2] and at $D=33 %$ in [3]. However, the current ripple cancellation will become worse when D< 20 % (when phase number ≤ 3). Bottom middle of Fig. 31.2.1 is the current ripple normalized to that of one-phase buck converter. The normalized current ripple of [1] –[4] is greater than 0.8. Therefore, this paper proposes a ripple-less buck converter (RLBC) with an inverted $A C$ current replica (IACCR) circuit (bottom right of Fig. 31.2.1). To further reduce the current ripple to C0 u r, two-phase currents ILa and ILb are summed and inverted by the IACCR to generate IR. The sum of ILa, ILb and IR can supply a near DC current to C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OUT</inf> and reduce the normalized current ripple to zero. Correspondingly, the resultant Δ VOUT< 0.5 %* VOUT and EVM=-28.85 dB can meet the requirements of the 5 G NR LEO system.