April/May 2021

www.power.com/products/MinE-CAP POWER SUPPLY DESIGN 29 www.power-mag.com Issue 2 2021 Power Electronics Europe designs. MinE-CAP also precisely manages inrush current at AC turn-on, eliminating the need for dissipative NTCs or large slow-blow fuses. Figure 1 illustrates a typical circuit configuration. The input E-CAPs are line voltage when maximum input capacitance is required. To achieve this, MinE-CAP monitors the input rail and voltage across C LV to dynamically engage and disengage this capacitor during every AC line cycle as required to ensure that the supply is designed to accommodate ceramic capacitor characteristics. 400 V electrolytic capacitors are lower cost and when selected according to Figure 2 also provide up to 50 % size reduction compared to traditional designs. A variety of standard input EMI filter configurations can be adopted depending on the form factor of a particular application. The MinE-CAP IC (Figure 3) is designed to partner directly with the InnoSwitch family of power supply ICs with a minimum of external components. The existing InnoSwitch V pin resistor is connected to the MinE-CAP VTOP pin while a resistor connected to the VBOT pin enables C LV voltage monitoring. Input voltage and fault information is transmitted from the MinE-CAP LINE (L) pin to the InnoSwitch V pin with no additional components. The MinE-CAP IC also derives its bias supply directly from the InnoSwitch BPP pin (see application example left). The MinE-CAP IC comprises a digital controller and high-voltage power switch which connected in series with the low- voltage bulk electrolytic capacitor in a power converter. The MinE-CAP IC connects this low-voltage capacitor into the power supply at low input line voltage conditions and disconnects it at high input line voltages. The high-voltage (400 V) capacitor is connected in parallel to support power delivery in high line conditions. The effective input capacitance is equivalent the sum of C LV and C HV at low input line to maintain the same minimum DC voltage to the DC/DC converter stage. At high input line condition the switch is disabled to ensure the voltage across C LV does not exceeded the rated voltage of the capacitor. The MinE-CAP IC also includes a control signal transmitted from the MinE- CAP LINE pin to control the start-up and fault shutdown of an InnoSwitch IC via its V pin. Upon application of AC input, the MinE- CAP controller is in the off-state and the power switch is open. The C LV is not engaged in the circuit and only C HV is charged by the AC input. The IC then performs controlled charging of C LV allowing designs to eliminate the inrush NTC, improving the overall system design by removing a thermal hotspot and increases conversion efficiency (Figure 4). Once the BYPASS (BP) pin reaches regulation the controller waits for the bulk voltage to be above the brown-in threshold (I UV+ ) measured on the VTOP pin. After brown-in, the controller enters a wait state for 20 ms to ensure power supply input voltage levels have stabilized. After that time, the IC samples the bulk DC voltage to determine which of two possible C LV Figure 3: MinE-Cap IC block diagram arranged with a small high-voltage capacitor (C HV typically 400 V) in parallel with a low-voltage capacitor (C LV typically 160 V) connected in series with the MinE- CAP IC. The physical size of the input capacitors is minimized because a high percentage of the input capacitance is 160 V rated rather than 400 V as would normally be used in conventional universal input converters. During steady state-operation MinE-CAP introduces CLV into the circuit at low AC power supply operates smoothly across the entire specified input voltage range. Figure 2 illustrates the recommended range of C HV and C LV values to achieve the required total input capacitance for a given output power. C LV is an electrolytic capacitor while C HV can be selected as an electrolytic or ceramic. Ceramic capacitors in the range of 1 to 5 µF / 400 V (depending on power level) have very low series resistance (ESR) and typically offer the most space saving when the power Figure 4: Charging algorithm for low-line start-up