FEATURE PARTNER FOCUS
High Power, Small Size
Chris Lee, Director of Marketing, Power Integrations, explores how to reduce the size of high efficiency chargers in today’s space constrained small footpint devices
D
elivering the most compact and efficient
US-PD charger has become a requirement for aftermarket vendors eager to grab share in this competitive market. With the functionality of hand-held devices increasing, and rapid charge protocols becoming standard, the need to provide more power is a key deliverable. These opposing requirements have led to a range of solutions being proposed which will optimise the use of PCB circuit space in an offline flyback power supply design - the standard solution for chargers up to 100W. The introduction of GaN power switches
and accurate synchronous rectification has enabled efficiency levels above the 90% point, reducing the requirement for heat-spreaders and heatsinks. The highly variable load conditions experienced with USB-PD charging protocols calls for a wide range in output voltage and current delivery which leads to challenges with operating efficiency. To put this into perspective, a single voltage 65W Notebook adapter can deliver power with up to 95% efficiency, which is relatively flat across load. A design that provides similar power delivery, but supports a USB-PD protocol, can only achieve up to 93% efficiency at full load. The principle cause of efficiency loss in such a design is the wide output voltage range (5V to 20V) which leads to compromises in transformer turns-ratios and widely varying switch duty-cycle which effects transformer flux density. The conventional approach to reducing
power supply size is to leverage the very low switching loss of the GaN switch and increase switching frequency of the power supply. This allows the selection of a physically smaller power transformer by reducing switch on-time per cycle, which in turn reduces the flux density per unit
6 DECEMBER/JANUARY 2021 | ELECTRONICS
A NEW APPROACH FOR REDUCING THE SIZE OF FLYBACK POWER SUPPLIES Apart from the large power transformer, the other significant component in an offline power supply is the input bulk capacitor. Worldwide operation is a universal requirement for rapid-chargers. This typically means an input voltage
core area. As described above, the transformer design is already compromised by the need to meet multiple operating conditions, so increasing switching frequency places an additional burden on the circuit designer. The impact on the main GaN switch
caused by increasing the switching frequency is low, however the standard low-cost RCD primary clamp circuit (required to prevent excess voltage overshoot during switch turn-off) contributes prohibitive losses at high frequency. Active-clamp circuits are often used for high frequency designs to reclaim some of the clamp energy. When paired with an additional high-voltage GaN switch, this approach is effective in reducing power loss and makes a small design viable. Conventional designs
employ complimentary- mode active clamp circuits which require DCM-only operation – a challenge for universal USB-PD designs. This topology must also employ a low value output bulk capacitor to force resonant (ZVS) switching of the flyback primary switch which adds additional complexity to the output filter stage.
Figure 1: Highest practical efficiency of a 65 W single output power supply compared to a variable voltage USB-PD design (3V, 5 V- 9 V-12 V-15 V-20 V) for 65W
range of 90 - 264 VAC, which in turn means that the primary bus voltage will be in the range of 127 to 374 VDC. The bulk capacitor is required to provide energy to the flyback stage for each AC cycle when the line voltage falls. The energy stored by the capacitor to achieve this is proportional to the square of the line voltage. This means that at low-line, to store a
given amount of energy the bulk capacitor must be approximately four times larger than is required for high line operation. For a wide-range power supply, these two factors combine, meaning that the bulk capacitor must be large enough (in capacitance terms) to support low voltage, and also large enough (in voltage terms) to operate safely at the maximum highline voltage. Reducing bulk capacitance too far
results in increased output ripple – a problem for most charger applications, while reducing voltage rating will
Figure 2: Voltage rating and energy storage verses size for the primary bulk capacitor
compromise reliability and dramatically reduce circuit lifetime. Electrolytic capacitor size for a given capacitance is related to the square of the voltage rating. This means that high-voltage, high-capacitance capacitors start to
/ ELECTRONICS
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