Minimizing Electrolytic Capacitor Size with Tiny ICs

Article By : Maurizio Di Paolo Emilio

Power Integrations has announced its latest MinE-CAP solution for high power density AC-DC converters with universal input. MinE-CAP technology reduces the size of high-voltage electrolytic capacitors (bulk capacitors), also reducing the overall size of the adapter by up to 40%. The MinE-CAP device also drastically reduces the inrush current, making NTC thermistors unnecessary, thus increasing...

Power Integrations has announced its latest MinE-CAP solution for high power density AC-DC converters with universal input. MinE-CAP technology reduces the size of high-voltage electrolytic capacitors (bulk capacitors),  also reducing the overall size of the adapter by up to 40%. The MinE-CAP device also drastically reduces the inrush current, making NTC thermistors unnecessary, thus increasing system efficiency and decreasing heat dissipation.

Electrolytic capacitors take up a good amount of space in AC/DC power supplies, very often limiting the form factor of an overall battery charger. The goal of Power Integrations is to use low-voltage capacitors for much of the energy storage, thus reducing the volume of these components.

Conventional power conversion solutions reduce the size of the power supply by increasing the switching frequency to allow using a smaller transformer. MinE-CAP aims to not only reduce the size but to avoid the EMI and dissipation challenges associated with high-frequency designs. Applications include smart mobile chargers, household appliances, power tools, lighting, and automotive.

One word: efficiency
The market is in constant search for efficiency. Consumers demand fast charging times, but at the same time, they want smaller chargers that can withstand higher power densities. The output range requires new sophisticated algorithms to keep pace with market demands through dynamic fine voltage and current adjustments.

The implementation of GaN technology has made it possible to reduce heat sinks by fully exploiting its better switching and on-resistance properties. “In addition to GaN, another element to consider when talking about efficiency in power supplies is the switching frequency,” said Andrew Smith, director of training at Power Integrations. He added, “When you want to make power supplies smaller, the conventional approach is to increase switching frequency. So a lot of applications we will see in the marketplace push switching frequency up.”

He also added that “PowiGaN switches increase efficiency — no heatsinks or heat-spreaders required. In the same time, InnoSwitch3 devices introduced thermal fold-back, with no corner-case limits and peak power for rapid charging,” said Smith.

By Increasing switching frequency (> 300 kHz) the size of transformer can be reduced, but this process can create thermal, EMI and efficiency problems for practical flyback implementations making it necessary to add other components to mitigate these effects. “It also means that mechanically it’s hard to build the power supply because you’ve got now many more components,” said Smith.

The increase in switching frequency brings with it the need for additional circuits to reduce snubber and switching losses, thus losing some dimensional advantages that had been created before. “The other component on the primary side (figure 1) that needs consideration is the electrolytic input bulk capacitor” said Smith.

Figure 1: AC-DC power supply with Input Bulk Capacitor. Switching Frequency Dictates Volume of the Transformer Line Voltage Dictates Volume of the Bulk Capacitor. Click to enlarge the image. (Source: Power Integrations)
Figure 1: AC-DC power supply with Input Bulk Capacitor. Switching Frequency Dictates Volume of the Transformer Line Voltage Dictates Volume of the Bulk Capacitor. Click to enlarge the image. (Source: Power Integrations)

He also said, “It’s a big component that controls peak power, and becomes a good candidate to offer a further size reduction for the power supply. What we looked at is a technique to reduce the size of the input capacitor,” said Smith.

He added, “the input voltage and the amount of energy the output needs, dictates how much capacitance is needed. So, as far as size is concerned, it’s all about the input voltage range you’re working from and the amount of output power you’re trying to provide”.

The energy stored in a capacitor is proportional to the square of the applied voltage and the capacitance. “We need less capacitance for the high line (176-264 VAC), and need 4x more capacitance for the low line (90-132 VAC)”, said Smith.

The bulk capacitor must be large enough to withstand high voltage over a wide input power supply range, i.e. 264 VAC, which means approximately one 400 volt capacitor. “The problem with this is that the bulk capacitor for 400 volts is very much larger than a bulk capacitor would be for 160 volts. We have done a side by side comparison 10 uF 400-volt capacitor is about the same size as 100 uF 160-volt capacitor, and 100 uF typically, is the requirement for about a 65-watt power supply. So this is the problem. And this is why a bulk capacitor is so large, it has to support high voltage and provide high capacitance”. Said Smith.

High voltage and high capacitance capacitors are required for a wide input range, making this component large. What Power Integrations has done is to introduce an integrated solution by reducing the size to a minimum.

Capacitance Controller
The MinE-CAP is an intelligent controller that decides whether the input voltage is low enough to add additional (low voltage) capacitance to the circuit. The advantage of this is that we have a small high voltage capacitor and a larger, low voltage one. And this allows you to drastically reduce the amount of space occupied by the bulk capacitor.

“The other advantage of this is that we have now removed most of the capacitance initially seen in the circuit and a reduced inrush current, which is related to the size of the bulk capacitor. So, the power supply sees a much lower inrush current. And this means that we can avoid implementing inrush limiters and other protective circuits on the power supply input stage. So, we can actually increase efficiency,” said Smith.

The inrush current is directly proportional to the size of the bulk capacitor size and, therefore, to the input voltage. A large inrush current leads to more stress on the input rectifier,  consequently good robustness is required to survive the inrush currents. Generally, designers insert an inrush current limiter, a thermistor, or something equivalent on the input stage to limit the inrush current. By reducing inrush by more than 90% with MinE-CAP, it is not necessary to add an inrush current filter, thus increasing efficiency.

The inrush current can be >100 A for short periods of time, creating a strong thermal shock to the rectifier. The thermistor aims to provide a high impedance to its passage, but with MinE-CAP this shock is reduced.

The MinE-CP technology works best between about 25 and 75 watts, adapting very well to the market area where fast charging is required. “We can actually reduce the size of the entire power supply up to about 40% depending on the application”.

Figure 2: MinE-CAP Adds Low-Voltage Capacitors at Low Line Voltage, and Removes them when Voltage Increases. Click to enlarge the image. (Source: Power Integrations)
Figure 2: MinE-CAP Adds Low-Voltage Capacitors at Low Line Voltage, and Removes them when Voltage Increases. Click to enlarge the image. (Source: Power Integrations)

 

Figure 3: MinE-CAP Reduces Size of Bulk Capacitors by 50 %. Click to enlarge the image. (Source: Power Integrations)

The MinE-CAP package provides a good thermal connection while minimizing heat, thus protecting the device. The MinE-CAP benefits from the small size and low RDS (on) of PowiGaN™ gallium nitride transistors to actively and automatically connect and disconnect segments of the Bulk capacitor network depending on AC line voltage conditions. MinE-CAP dramatically reduces the number of high-voltage storage components and protects low-voltage capacitors from mains voltage fluctuations, substantially improving robustness while reducing system maintenance and product returns.

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