Tiny ICs minimize electrolytic capacitors, reducing volume of power supply by up to 40 percent

Designing the fast-charging, small adapters consumers are expecting today poses a not unfamiliar challenge to the designers of those devices. How do you deliver loads of power safely and efficiently when the increased component count and complexity of providing sophisticated charging control means a larger adapter? Even a more efficient circuit design requires new ways of critically looking at each component and, perhaps, a bit out of the box.

San Jose-based, high-voltage power conversion IC company Power Integrations’ roadmap to fast charging and high efficiency is paved with their PowiGaN switches, requiring no heat sinks or heat spreaders their recently-announced InnoSwitch3 devices with thermal fold-back. But making an adapter smaller remained a challenge.

“The first thing you normally do is increase the switching frequency, and that’s what some of the solutions on the market do today,” says Andy Smith (title), “That makes for a smaller power transformer but, unfortunately, you also reduce efficiency, so you get more thermal problems. You also add more components because you have to put regenerative clamp circuits into the primary side of the power supply to avoid losing efficiency because of the high switching frequency, causing EMI problems. So it’s not a great way to reduce size.”

PI looked next at the bulk capacitor sitting in the middle of the power supply, which takes up considerable space on the board, and looked at what they could do to reduce that component’s size to make the board smaller.

Enter today’s MinE-CAP IC announcement for high power density, universal input AC-DC converters with applications in smart mobile chargers, appliances, power tools, lighting, and automotive.

By halving the size of the high-voltage bulk, electrolytic capacitors required in offline power supplies, this new type of IC enables a reduction in adapter size of up to 40 percent. The MinE-CAP device also dramatically reduces in-rush current making NTC thermistors unnecessary, increasing system efficiency, and reducing heat dissipation.

Comments Power Integrations’ product marketing director, Chris Lee: “The MinE-CAP will be a game-changer for compact chargers and adapters. Electrolytic capacitors are physically large, occupy a significant fraction of the internal volume, and often constrain form factor options – particularly minimum thickness – of adapter designs. The MinE-CAP IC allows the designer to use predominantly low voltage rated capacitors for a large portion of the energy storage, which shrinks the volume of those components linearly with voltage. USB PD has driven a major market push towards small 65 W chargers, and many companies have concentrated on increasing switching frequency to reduce the size of the flyback transformer. MinE-CAP provides more volume saving than doubling the switching frequency, while actually increasing system efficiency.”

The MinE-CAP leverages the small size and low RDSon of PowiGaN gallium nitride transistors to actively and automatically connect and disconnect segments of the bulk capacitor network depending on AC line voltage conditions. Designers using MinE-CAP select the smallest high-line rated bulk capacitor required for high AC line voltages and allocate most of the energy storage to lower voltage capacitors that are protected by the MinE-CAP until needed at low AC line. This approach dramatically shrinks the input bulk capacitors’ size without compromising output ripple, operating efficiency, or requiring redesign of the transformer.

“So MinE-CAP is very simple,” continues Smith. “It just adds bulk capacitance when required. To show how powerful that is, we’ve got a before and after picture (below). On the left, we’ve got a power supply with a conventional circuit with a big bulk capacitor, 65-watt power supply. That’s the amount of bulk capacitance you need. So you can see it’s using up about somewhere between a quarter and a third of the PCB space. It’s a huge device. On the right is the same power supply with MinE-Cap. What we have is two low-voltage capacitors in parallel and one high-voltage capacitor. You can see it’s using up less than half of the space. In fact, we’ve cleared this big chunk of space here on the circuit board.”

“So we reduced the size of the bulk capacitance by half, but we’ve actually increased the total amount of capacitance. We’ve got 116 microfarads from these caps rather than 100 microfarads from the original design. We’ve actually even increased the capacitance a little bit. So this has allowed us to make a smaller power supply simply by deciding when to add capacitance into the circuit.”

Housed in the miniature MinSOP-16A package, the new devices work seamlessly with Power Integrations’ InnoSwitch family of power supply ICs with minimal external components. MinE-CAP MIN1072M ICs are available immediately from PI offices and franchised distributors and are priced at $1.75 for 10 Ku. Two initial design example reports (DERs) pair the MinE-CAP IC with Power Integrations’ InnoSwitch3-Pro PowiGaN IC, INN3370C-H302. A 65 W USB PD 3.0 power supply with 3.3 V – 21 V PPS output for mobile phone/laptop chargers is described in DER-626, and DER-822 describes a 60 W USB PD 3.0 power supply for USB PD/PPS power adapters using INN3379C-H302.

More information MinE-CAP and downloadable reference designs can be found here: https://www.power.com/products/MinE-CAP.