A 1.2W 51%-Peak-Efficiency Isolated DC-DC Converter with a Cross-Coupled Shoot-Through-Free Class-D Oscillator Meeting the CISPR-32 Class-B EMI Standard
Dongfang Pan, Guolong Li, Fangting Miao, Wei Sun, Xiaohan Gong, Lele Zhang, Lin Cheng
Abstract
Low electromagnetic interference (EMI) is essential for isolated DC-DC converters that are used in the harsh industrial environments. To pass the CISPR 32 Class-B EMI standard, a 4-layer PCB with a stitching capacitor implemented by the internal layers is commonly required [1], which greatly increases the cost of the system and the effort of PCB layout. Therefore, low-cost circuit techniques that can reduce EMI at the source are highly desirable. As shown in Fig. 14.7.1, input-to-output dipole radiation caused by the common-mode (CM) current I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> across the parasitic capacitance C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</inf> of the isolation barrier is the predominant mechanism of EMI, and the fluctuation of the CM voltages of the transformer at the primary and the secondary side, V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM_PRI</inf> and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM_SEC</inf> , should be suppressed to reduce I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> . V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> _ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SEC</inf> can be kept quiet by adopting a symmetrical full-bridge rectifier in the receiver (RX) side, and thus the amplitude of I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> mainly depends on the fluctuations of V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> _ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PRI</inf> which are determined by the topology of the transmitter (TX). Unfortunately, when using the LC-tank oscillator adopted in [2] and [3], or the leakage-inductance-resonant flyback (LiRF) topology proposed in [4], V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> _ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PRI</inf> suffers from large and quick fluctuations. Frequency hopping technique can be employed to reduce I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> [2], but it will greatly increase the output voltage ripple and the circuit complexity. In [5], an LLCC topology with a multistage pre-driver is proposed to form a more symmetrical structure to reduce I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CM</inf> . However, a costly magnetic-core micro-transformer and two extra external capacitors are needed.