Application of Buck and Buck-Boost in Auxiliary Power Supply of Small Household Appliances

In an ACDC power supply, the input voltage is generally 85V-265V AC high voltage from the power grid, while the output voltage is 3.3V, 5V, 12V and other DC low voltages, so a switching power supply is needed to achieve voltage reduction. There are three classic topologies of switching power supplies: Buck, Boost and Buck-Boost, of which Buck and Buck-Boost can both achieve voltage reduction. Synergy Shihui is a subsidiary of Shijian, with rich industry experience and professional technical strength. The FAE of Shihui's Power and New Energy Division combines its own experience to give a detailed introduction on how to select topology circuits and related products in the auxiliary power application of small household appliances.

1. Buck Circuit

Buck circuit is a step-down circuit, Vi=Vls+Vo. Since Vi>Vo, it has a step-down effect.

( 1 ) Switching tube S conduction stage

When the switch is closed, the freewheeling diode D is cut off. Since the input voltage Vi is connected to the energy storage inductor Ls, the input-output voltage difference (Vi-Vo) is added to Ls, causing the current through Ls to increase linearly. In this stage, in addition to supplying power to the load, a portion of the electrical energy is stored in the inductor Ls and the capacitor Cr.

( 2 ) Switching tube S is turned off

When the switch is turned off, Ls is disconnected from Vi, but since the inductor current cannot change suddenly in an instant, a reverse electromotive force is generated on the inductor Ls to maintain the current flowing through it unchanged. At this time, the freewheeling diode D is turned on, and the electric energy stored in the inductor Ls supplies power to the load through the loop formed by D. (The load is mainly supplied by the capacitor C, which is charged at this stage to maintain voltage stability)

2. Buck-Boost Circuit

The polarity of the input voltage and output voltage of the Buck-Boost circuit are opposite.

( 1 ) Switching tube S conduction stage

When the switch is closed, the input voltage returns directly through the inductor L, storing energy in the inductor Ls. At this time, the capacitor Cr discharges to supply power to the load.

( 2 ) Switch tube S cut-off stage

When the switch is turned off, a reverse electromotive force is generated on the inductor Ls, causing the diode D to turn from cutoff to conduction. The inductor supplies power to the load and charges the output capacitor to maintain the output voltage unchanged.

Through the above comparison, we can know that the polarity of the input voltage and output voltage of the Buck-Boost circuit is opposite. Therefore, the biggest difference between Buck and Buck-Boost lies in the polarity of the input and output voltages: Buck is a same polarity topology, that is, the output and input voltages have the same polarity and share a common ground; while Buck-Boost is a reverse polarity topology, that is, the output and input voltages have opposite polarities, and the output terminal provides a negative voltage (relative to the input voltage).

In the power module control part of small household appliances, engineers often use AC switches such as bidirectional thyristors (TRIAC). Using negative voltage to drive the AC switch can achieve higher circuit reliability and compatibility. In systems using AC switches, Buck-Boost that can output negative voltage is often preferred; in systems that do not require negative voltage drive, Buck topology can be selected:

•The input and output of Buck share the same ground, which is more conducive to the multi-level expansion of the system.

• Buck has a higher inductor current utilization rate than Buck-Boost. When the inductor current (I L ) is the same, Buck can output a larger current (I O ).

1. Product Recommendations

Now that we know how to choose a topology, how do we choose a product? Shihui FAE recommends the BP852X series of high-performance switching power supply driver chips from Shanghai Jingfeng Mingyuan (BPS), which supports Buck and Buck-Boost topologies and has a very simple peripheral circuit. It can be applied to low-cost auxiliary power supply designs for small household appliances, such as auxiliary power supply for small household appliances, auxiliary power supply for motor drives, IoT/smart home/smart lighting, and other fields. This series of products has many excellent features, including:

• Integrated Vcc capacitor, freewheeling diode and feedback diode

• Integrated 650V high voltage MOSFET

• Integrated high voltage startup and self-powered circuit

• Low standby power consumption 50mW@230Vac

• Excellent dynamic response speed, small output voltage ripple

• Down-amplitude modulation technology to reduce audio noise

• Frequency Modulation Technology to Improve EMI Performance

Next, Shihui FAE will take the BP85226D of this series as an example to give a detailed introduction.

Figure: Packaging and specifications

Figure: Electrical performance parameter table

Figure: BP85226D application schematic

Picture: Product

Figure: BOM

4. PCB Layout and Suggestions

1. IC-GND can play a role in heat dissipation, and copper can dissipate heat. However, IC-GND is a voltage dynamic point, and the copper area should be as small as possible under the condition of satisfying heat dissipation; at the same time, it should be far away from the input AC terminal to avoid EMI problems caused by capacitive coupling.

2. The output inductor may generate electromagnetic interference. It is recommended to keep it away from the chip FB pin and the input terminal to avoid EMI problems. In order to keep the chip and the voltage moving point away from the AC input terminal, it is recommended to place the input electrolytic capacitor between the chip and the AC input terminal.

3. DRAIN is connected to the input DC bus and is a voltage static point. Copper can be used for heat dissipation. It is recommended that the wiring distance between DRAIN, FB, and IC-GND should be greater than 2mm.

4. Minimize the power loop area to avoid EMI interference and improve system reliability. Taking Buck as an example, it is recommended to reduce the area of ​​the excitation loop composed of input capacitor, built-in MOSFET, inductor, and output capacitor, as well as the freewheeling loop composed of inductor, output capacitor, and freewheeling diode.

5. It is recommended that the power loop routing be wide and short to improve system reliability, such as the routing from the busbar to the GND pin, and the routing from GND to the output capacitor.

6. The feedback loop area and trace length should also be reduced to improve reliability.

7. If the layout cannot fully meet the recommendations in 5 and 6 due to objective factors (such as PCB board shape, etc.), a 100nF (104) chip capacitor can be placed between VOUT and GND close to the pin position to improve system reliability.

Figure: Inductor selection recommendations

Figure: Load Regulation

Figure: Startup waveform

Figure: Output voltage ripple

Figure: Dynamic response

Figure: Conducted EMI

Figure: Surge

Figure: Group pulse test

Figure: ESD test*

* The above test results are obtained under real test conditions in laboratory conditions. The actual product may be different depending on the application environment and batch differences. Jingfeng Mingyuan reserves the right of final interpretation .

Figure: BP852X series product selection table

Summarize

The BP852X series chips of Jingfeng Mingyuan have extremely simple peripheral circuits and ultra-high integration, which can greatly reduce the cost and difficulty of stocking application solutions; rich protection helps to enhance system stability; in addition, the product also has excellent dynamic response, standby power consumption and output adjustment performance, so it is very suitable for auxiliary power supply of small household appliances. Synergy Shihui can provide a full range of products and related technical support.