April/May 2021

30 POWER SUPPLY DESIGN www.power.com/products/MinE-CAP Issue 2 2021 Power Electronics Europe www.power-mag.com charge up schemes to be adopted. In low- line start-up conditions (VI N < 150 VAC), the MinE-CAP IC performs precisely controlled active charging of C LV . At low-line start-up condition, it is important to pre- charge C LV to support full power capability prior to enabling the InnoSwitch. The MinE-CAP IC controls the internal high- voltage switch as a current source and uses a precise constant current, pulse charging of C LV , see Figure 4. This algorithm allows fast charging of C LV and ensures PSU is able to deliver full power in less than 250 ms from initial AC line connection. In high-line start-up condition (V IN > 150 VAC), the active charging algorithm of C LV is not employed. When selected according to Figure 2, CHV alone can deliver full power converter output power at line voltages above 150 VAC. The InnoSwitch power control IC is therefore enabled immediately using the V pin output signal while C LV is trickle charged at a lower rate until the steady-state C LV voltage is reached. The voltage across C LV is subsequently precisely monitored and recharged as required depending on input line conditions. Designing a 65 W adapter The circuit in Figure 5 shows a 65 W (5 V / 3 A; 9 V / 3 A; 15 / 3 A; 20 V / 3.25 A) USB PD 3.0 compliant adapter using the MinE-CAP IC to maximize power density. The MinE-CAP IC allows for the significant reduction of the physical size of the input bulk capacitors by allowing the use of a smaller (both in size and capacitance) 400 V capacitor paired with a 160 V capacitor. The MinE-CAP IC also eliminated the need for an inrush current limiting thermistor, leading to more saved space and increased efficiency. Together with the InnoSwitch3-Pro IC and low-profile planar magnetics, a form factor of 82 mm x 51 mm x 12 mm was realized (Figure 6). This corresponds to a power density of 21.22 W/in 3 with a system efficiency exceeding 90 %. This design also meets DOE Level 6 and EC CoC 5 average efficiency standards. Fuse F1 isolates the circuit and protects the AC line from excessive current due to component failure. Common mode chokes L1 and L2 along with capacitors C3 and C4 provide common mode and differential mode noise filtering to minimize conducted EMI emissions. The bridge rectifier formed by BR1 and BR2 rectifies the AC line voltage and provides a full-wave rectified DC voltage across the high voltage bulk capacitor, C1. Two bridge rectifiers are used to improve heat dissipation by doubling the rectifier surface area since power loss from two rectifiers is the same as that of a single device. The MinE-CAP IC controls the rate of charge of the 160 V capacitor during start- up; thus, inrush current is mostly dependent on the value of the 400 V capacitor. Since the capacitance of the 400 V capacitor is significantly less when using a MinE-CAP IC, the use of a current limiting NTC thermistor is no longer necessary. When a MinE-CAP IC is used in tandem with the InnoSwitch3, the V pin of the InnoSwitch3 IC is connected directly to the LINE pin of the MinE-CAP IC. Resistors R3 and R5 provide input voltage sensing for both the the MinE-CAP IC and InnoSwitch3 ICs. The MinE-CAP IC uses R3 and R5 primarily to monitor the line voltage and maintain the voltage across the low-voltage bulk capacitor, C2 below its voltage rating when the line voltage is above 100 VAC. In contrast, the InnoSwitch3 uses the current from the LINE pin to determine line under-voltage and over-voltage conditions. During regular operation, the current from the LINE pin follows the current flowing through R3 and R5, so the InnoSwitch3 IC operates as if said resistors are connected directly to the V pin. Resistor R1 is a bleed resistor used to regulate the voltage across C3, while resistor R4 is used by the MinE-CAP IC to sample the voltage at the negative terminal of C2. For this specific design, bypass capacitor C16 is shared by both the BPP pin of the InnoSwitch3 IC and the BYPASS pin of the MinE-CAP IC. The value of C16 is chosen based on the desired current limit of the InnoSwitch3 IC. As with any flyback design using the InnoSwitch3 IC, one end of the transformer primary is connected to the rectified DC bus while the other end is connected to the InnoSwitch3 DRAIN pin. A low-cost RCD snubber formed by diode D3, resistors R9, R10 and R13, and capacitor C6 limits the voltage across the InnoSwitch3’s Drain-Source nodes during turn-off by dissipating the energy stored in the leakage inductance of the transformer. The InnoSwitch3 IC has an internal current source that charges capacitor C16 when AC input is first applied. Once the InnoSwitch3 IC starts switching and during normal operation, bias current is drawn from the auxiliary winding of the Figure 5: Application example of a 65 W power supply featuring GaN MinE-Cap IC and InnoSwitch3-Pro IC