Page 7 - PEN eBook March 2022

Cover Story — Semiconductors

The adoption of GaN HEMTs and planar transformers in QR flyback converters helps push the
switching frequency and the power density higher. However, to achieve even higher power density
for ultra-slim charger and adapter designs, soft switching of the switches and recycling of the

transformer leakage energy become indispensable. This inevitably leads to selecting converter
topologies with intrinsic higher efficiency.

This article explains how Infineon’s CoolGaN Integrated Power Stage (IPS) technology can be
applied in active-clamp flyback (ACF), hybrid flyback (HFB), and LLC converter topologies. This way,
designing charger and adapter solutions is made quicker and easier, leading to smaller and lighter
products or products that provide more power from a device of the same size to charge faster or
charge multiple devices from one adapter.

CONVERTER TOPOLOGIES QUALIFIED TO ACHIEVE AN EVEN
HIGHER POWER DENSITY

Some half-bridge topologies such as ACF, HFB, and LLC converters have been proven to be able to
achieve high efficiency, even at a very high switching frequency, due to zero-voltage switching (ZVS)
and zero snubber loss.

ACTIVE-CLAMP FLYBACK (ACF)

Figure 1 shows a typical application example of CoolGaN IPS operated in an ACF converter. In the
ACF topology, the clamp switch provides a path to recover the energy stored in the transformer’s
leakage inductance (L ) when the main switch turns off and the clamp switch turns on. C
lk clamp
and L resonate together through the clamp switch and the transformer, resulting in energy
lk
transfer to the load. This energy recovery increases the system efficiency compared with the
passive-clamp flyback, in which the energy stored in L damps in the traditional RCD clamp circuit.
lk
A well-designed ACF topology operates in soft-switching ZVS condition; therefore, it can run
with a much higher switching frequency
than a QR flyback, which operates in

hard-switching conditions. This helps to
reduce the size of magnetic components,
including the transformer and EMI filters.

The ACF converter consists of a high-side
and a low-side switch, the transformer, a
clamp capacitor (C ), and the output
clamp
stage of the rectifier and capacitors.
Figure 2 shows the typical operating
waveforms that briefly explain the ACF
converter’s operation principle.

Figure 1: Application circuit of the ACF converter

MARCH 2022 | www.powerelectronicsnews.com 7