Page 7 - PEN Ebook May 2021

COVER STORY - DESIGN Cover Story - Design

To fulfill those requirements and contribute to a PFC half-bridge, a different control strategy is

greener and safer world through an overall ef- required. In that case, the triangular current
ficiency of 98 percent, it is crucial to achieve mode (TCM) [3] extends the conduction time
super high-efficiency levels in the power factor to revert the inductor current and obtain zero
correction (PFC). In this context, standard PFC voltage switching (ZVS) operation. Nevertheless,
topologies do face a limit, and a trend towards due to the high inductor current ripple, inter-
bridgeless PFC topologies like the continuous leaving of at least two power stages is necessary.
conduction mode (CCM) totem-pole PFC can be Full ZVS operation leads to increased efficiency;
observed [1]. The boost converter in the PFC appli- however, the rise of part count requires a higher
cation has been traditionally controlled in CCM. In switching frequency to increase the power den-

this operation mode, the half-bridge switches op- sity. Furthermore, the control complexity (inter-
erate mainly in hard-commutation, which avoids leaving with variable switching frequency) raises
the possibility of using silicon (Si) superjunction significantly more. When Si SJ MOSFETs are used
(SJ) MOSFETs. If keeping the costs low is the pri- in a half-bridge configuration in CCM operation,
mary driver in the design, Si isolated gate bipolar high efficiency cannot be expected. Instead, even
transistors (IGBT) with antiparallel silicon car- catastrophic failures can happen [4]. The reasons
bide (SiC) diode are the preferred option [2]. are to be found in the very high energy required

to charge and discharge the C OSS of the devices,
However, for high-end applications, if Si SJ MOS- together with the significantly high reverse recov-
Achieving the highest FETs (such as Infineon’s CoolMOS™ transistor ery losses of the intrinsic body diode of the Si SJ

products) are implemented in the totem-pole
MOSFETs (Figure 1).
efficiency in CCM

totem-pole PFC designs

Innovative solutions with Si superjunction (SJ)

CoolMOS™ MOSFETs

By Rafael Garcia, Principal System Applications Engineer, Server and Telecom
SMPS, Infineon Technologies

INTRODUCTION plies (SMPS) are required to achieve this. Inevi-

In server and telecom SMPS applications, the tably, the output power levels of the SMPS have
highest efficiency and power density are not only to reach higher levels while maintaining the same
buzzwords but clear market trends. Given the form factors. This demanding combination ex-
actual situation, there is an essential need for plains why power density is gaining importance.
increased server capacities and fast connectivity Next to performance, system cost down, as well
for real-time data transmission. Highly efficient as modularity, and design simplicity are also in
server and telecom switched-mode power sup- focus. Figure 1: Half-bridge capacitance variation for Si SJ CoolMOS™.

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