In order to improve the efficiency and reduce the size of AC-DC solutions, PI has launched many innovative products in recent years, such as introducing Gan technology into its innoswitch3 series products, so that this series of IC can provide 95% efficiency in the whole load range, and can provide 100W power output without heat sink in the closed adapter.
However, customers' demand for the power adapter requires not only faster charging speed, but also smaller size. They also hope that one adapter can supply / charge a variety of devices, which requires that the adapter can support a variety of fast charging protocols. As a result, the charging control of the power adapter needs to be more intelligent.
Challenges of miniaturization of switching power supply
In order to meet the needs of high power and more intelligent charging control, the number and complexity of power adapter components will inevitably increase. To achieve smaller size, the easiest way to think of is to increase the switching frequency. However, in the process of miniaturization, power design engineers will face two major problems: one is the challenge of temperature rise performance; Second, after the switching frequency is increased to more than 300kHz, it will be difficult to maintain the consistency of output characteristics in mass production due to the tolerance of various components and the dispersion of transformer.
In addition, higher off frequency can reduce the transformer; However, EMI will be worse, so a larger input filter is required; The switching loss will increase accordingly, so it is necessary to increase the active clamping circuit to reduce the loss of buffer circuit and switch; In this way, the board space and cost will increase accordingly.
In fact, in any switching power supply, in addition to the transformer, it is difficult to reduce the size of a filter capacitor. The large filtered electrolytic capacitor is related to the peak output power of the power supply. The larger the peak output power, the larger the capacity of the large capacitor. At the same time, the large capacitor itself is a larger component, This limits the further reduction of switching power supply.
From the process of storing and releasing energy of large capacitance and the energy storage formula of large capacitance shown in Fig. 3, we can know that if the same energy is stored, the required capacitance is small at high voltage input; At low voltage input, the required capacitance is large.
If the 12v multi adapter wants to work in a wide voltage range, it is required that the input filter capacitor not only has a larger capacity (at low voltage), but also has a higher withstand voltage rating (at high voltage). In order to support large capacity and high withstand voltage, the capacitor manufacturer must increase the size of the capacitor, that is, to increase the volume of the large electrolytic capacitor. Therefore, if the power supply wants to be small, it is very difficult.
Is there any better way?
PI's innovative solutions
In order to solve this problem, PI used its many years of experience in the field of AC-DC to think of a new method, which can reduce the switching power supply without increasing the switching frequency. That is to use their new mine cap chip to reduce the volume of large electrolytic capacitor, so as to reduce the volume of switching power supply.
How did that happen? Jason Yan, senior technical manager of power integrations (PI), explained, "in addition to an electrolytic capacitor at the input, we also added a low withstand voltage electrolytic capacitor, which is connected to the powigan switch in mine cap. When the input voltage is relatively low and a larger capacitor is required, we turn on the powigan switch and let the capacitor be used in parallel with the previous high withstand voltage capacitor, so as to ensure the large capacitor required at low voltage. " He also pointed out that mine cap will have a detection pin. Once it is detected that the input bus voltage is greater than the capacitance rating of low withstand voltage, the powigan switch will be disconnected. This method can greatly reduce the size of the input high-capacity capacitor without affecting the output ripple, working efficiency or redesigning the transformer.
It can be seen that mine cap device actually makes use of the small size and low RDSON advantages of PI powigan gallium nitride transistor to actively and automatically connect and disconnect all parts of high-capacity capacitor network according to AC input voltage conditions. In other words, the main function of mine cap is to judge whether it is necessary to connect the capacitors with low withstand voltage and high capacity in parallel to the filter circuit. By optimizing the selection of two capacitors, the volume of the original large electrolytic capacitor is reduced.
In addition, the use of mine cap also brings two benefits: one is to reduce the surge current, which helps to eliminate NTC thermistor, improve system efficiency and reduce heat dissipation; Second, the total capacitance is increased, so that the switching power supply has higher peak power output capacity.
The new device adopts micro minsop-16a package, which can be seamlessly matched with the innoswitch series power IC of power integration. It requires few external components and occupies little board space.
At the same time, Yan Jinguang also showed two new design example reports (DER) of PI, both of which adopt mine cap IC and innoswitch3 Pro Series powigan IC (model inn3370c-h302) of power integration. Der-626 is a 65W USB PD 3.0 power supply with 3.3V - 21V PPS output for mobile phone / laptop charger. The adapter is only the size of a credit card and has a size of 82 × fifty-one × 12mm。
Der-822 is a 60W USB PD 3.0 power supply designed with inn3379c-h302 and suitable for USB Pd / PPS power adapter.
epilogue
Traditional power conversion solutions use smaller transformers by increasing the switching frequency, so as to reduce the size of the power supply. The innovative mine cap IC can not only greatly reduce the overall size of the power supply, but also reduce the number of components, reduce EMI, and avoid the challenge of increasing transformer / clamp loss related to high-frequency design.
It is suitable for ultra wide input voltage range power supply in emerging market applications, especially unstable grid voltage. The advantage is that the mine cap scheme has smaller shape coefficient and is suitable for platform application. Its applications include smart phone chargers, home appliances, power tools, instrument applications, etc.
