Universal AC Voltage Step-Down Power Converter – 18V DC Output From 85-265V AC Input

The project described here is an off-line universal AC Voltage input step-down regulator that provides accurate constant voltage (CV) output, outstanding low standby power, high efficiency at light load, and excellent dynamic response based on non-isolated buck topology. The project is built using the AP3928 chip. The project provides 18Vdc output @ 250mA from AC input 85V to 265V AC. The project can be used for many applications such as home appliances, IoT applications, industrial controls, standby power, and auxiliary power.

The AP3928 is a universal AC voltage step-down power switcher, specially designed for home appliances and IoT applications with non-isolated buck solution. The device integrates a 700V high-performance power MOSFET. Coordinating with a single-winding inductor, it uses fewer external components and provides a low bill of materials (BOM) cost solution. The AP3928 can achieve high output current, excellent constant voltage regulation, and high conversion efficiency. The peak current and switching frequency continuously reduce as the load decreases, so the device can achieve excellent efficiency performance at light load and improve overall system efficiency. The AP3928 has multiple protection features to enhance the system’s safety and reliability. The device has overtemperature protection, under-voltage lock function, output short protection, overload protection, and open-loop protection. The AP3928 is available in the SO-8 (Standard) package.

Overload Protection (OLP)

With the increase in load, the peak current and the switching frequency also increase. When the peak current reaches the maximum limitation, and the OFF time is the minimum OFF time, the internal OLP timer begins to count. For avoiding false triggering during start-up and other load transition conditions, OLP activates only when the switching pulses reach the OLP_counting cycles requirement.

Short-Circuit Protection (SCP)

The AP3928 shuts down when the peak current exceeds short-circuit threshold, and the AP3928 resumes operation when the fault is removed. Note that the inductor saturation current should be higher than ISCP to avoid SCP being triggered during start-up. Otherwise, the device may not turn on normally.

Overtemperature Protection (OTP)

The AP3928 integrates an internal overtemperature protection function. The AP3928 shuts down when the inner junction temperature exceeds the thermal shutdown threshold TOTP (+150°C). After exceeding the threshold, the VCC voltage begins to drop, and when VCC voltage drops to VCC_RESTART (4.5V), the internal high-voltage regulator turns on to charge the external VCC capacitor.

Open-Loop Detection (OLD)

When the FB voltage drops below the open-loop detection threshold voltage VOLD, the AP3928 stops working and begins a restart cycle. The open-loop detection is blanked for 64 switching cycles during start-up process. Moreover, if FB pin was floating, output voltage will be fixed to 12V typically. Overshoot Improvement In general, there is no capacitor between FB pin and GNDA (GNDP) pin. But in some cases where strict overshoot is required, we recommend a ceramic capacitor C6 (390pF to 2.2nF) in Figure 6. In addition, adding a C6 can improve output voltage ripple as well.

Features

• Universal 85V to 265V VAC Input
• Output 18V DC
• 250mA Output Current (Rated Up to 500mA)
• Low Standby Power Consumption (<30mW at no load)
• High Light-Loading and Average efficiency can meet DOE and CoC requirement
• Frequency Modulation to suppress EMI to meet EN55022 class B
• Rich Protection including OTP, OLP, OLD, SCP
• Extremely low system component counts small size
• PCB Dimensions 60.80 x 21.75 mm
• 4 x 3mm Mounting Holes

Schematic

Parts List

Connections

Gerber View

Photos

Video

AP3928 Datasheet

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