Feature: Power
switching losses remain, since the voltage across the MOSFET is still substantial.
Efficiency gains when GaN replaces silicon Replacing the silicon MOSFET in the quasi-resonant topology with a GaN HEMT reduces primary-side losses even more. Tanks to the very high electron mobility and very low on-resistance of a GaN HEMT, it can be made much smaller than a silicon MOSFET of the same voltage and current rating. Tis means that its output capacitance will be much lower, and it provides much faster switching edges. Tese faster edges result in much lower switching losses. Te scale of the efficiency improvement to
Figure 3: Drain-voltage waveform of the primary MOSFET during quasi-resonant flyback converter operation
• Resistive losses associated with the Equivalent Series Resistance (ESR) of the input and output capacitors. By adopting good magnetic design
principles, and retaining the diode on the secondary side, it is possible to achieve efficiency around 85% when supplying a full load. Te designer can increase efficiency by a few percentage points by replacing the diode with a low-voltage secondary-side silicon MOSFET. To reduce losses at the primary-side
MOSFET, quasi-resonant flyback is commonly used. Te quasi-resonant flyback AC-DC converter operates in discontinuous conduction mode. Here, the current through the secondary side reaches zero and the diode (or MOSFET) is in its off-state before the primary side switches on; energy is stored in the flyback transformer as the primary inductor current ramps up. During the period when both the
primary and secondary switches are off, relaxation oscillations occur in the drain voltage (Vd
) of the primary MOSFET, due
to the combination of node capacitance and primary inductance. Timing for the turn-on of the primary-side switch can be aligned to the low-voltage troughs of these oscillations. As switching losses are proportional with the
Figure 4: Comparison of the conduction losses and switching losses for a silicon MOSFET and for the PowiGaN switch used in the InnoSwitch3 power stage
square of the drain voltage, switching in this quasi-resonant way results in much lower losses at the primary-side switch. Te drain- voltage waveform for this operation is shown in Figure 3. Te efficiency of a flyback converter using
quasi-resonant switching, a silicon primary MOSFET and a secondary-side synchronous MOSFET, is around 91% at rated load. Although switching the primary-side
MOSFET at the trough of the drain-voltage waveform reduces losses significantly, residual
be gained with GaN HEMTs is demonstrated in the performance ratings of the integrated InnoSwitch3 product line from Power Integrations. In 2019, Power Integrations incorporated its internally-developed GaN technology (PowiGaN) in its InnoSwitch3 product line. Te reduction in primary switch losses
achieved by using PowiGaN technology is shown in Figure 4. Tis performance improvement is achieved because turn-on and turn-off operations using PowiGaN are almost instantaneous. In addition, PowiGaN- enabled devices are much smaller in size than a comparable silicon switch with a similar on-resistance rating. Te InnoSwitch3 is a complete quasi-
resonant flyback stage, a perfect fit for low- cost but high-efficiency and compact flyback
38 November/December 2020
www.electronicsworld.co.uk
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