This article discusses the principles and design methods of a small power multi-output AC/DC switching power supply based on the TOPSwitch series chip. The designed power supply is intended for smart meters with a maximum power output of 10 watts. To minimize the size of the smart meter and reduce the number of PCB layers, it is essential that the power supply module be compact and integrated onto the same PCB as the main control section.
The power supply must meet the following design criteria:
- Output maximum power: 10W
- Input AC voltage: 85 ~ 265V
- Output DC voltages/currents: +5V, 500mA; +12V, 150mA; +24V, 100mA
- Ripple voltage: ≤120mV
Considering the power requirement of 10W and the need for a small form factor, the circuit employs a single-ended flyback topology. This topology is characterized by simplicity, compactness, and cost-effectiveness. It comprises an input filter circuit, a pulse width modulation circuit, a power transfer circuit (including a switch tube and a transformer), an output rectification filter circuit, an error detection circuit (comprising a TL431 chip and peripheral components), and a signal transmission circuit (via isolation optocouplers and resistors).
The power supply is designed as a surface-mount module, with the following specifications:
Control Principle of Single-Ended Flyback Switching Power Supply
Single-ended refers to the fact that the TOPSwitch-II series devices have only one pulse-modulated signal power output terminal and a drain (D). Flyback implies that when the power MOSFET is turned on, electrical energy is stored on the primary winding of the high-frequency transformer. Energy is only transferred to the secondary side when the MOSFET is turned off. Given the high switching frequency of 100 kHz, the high-frequency transformer can quickly store and release energy, resulting in a high-speed continuous output after high-frequency rectification and filtering. This is the fundamental operating principle of the flyback circuit. The feedback loop adjusts the duty cycle by controlling the current at the control terminal of the TOPSwitch device to regulate the output voltage.
TOPSwitch-II Series Chip Selection and Introduction
The drain (D) of the TOPSwitch-II series chip connects to the internal power switching device MOSFET. Externally, it is connected to the main power supply via the load inductor. The internal bias current is supplied by the internal switching high-voltage power supply during startup, with current detection provided. The control terminal (C) serves as both the duty-controlled error amplifier and feedback current input pin, connected to an internal shunt regulator for providing internal bias current during normal operation, while also offering bypass, automatic restart, and a capacitor connection point for compensation functions. The source (S) connects to the source of the MOSFET in the high-voltage power loop, acting as a common and reference point for the primary circuit. The duty cycle of the internal output MOSFET decreases linearly with increasing current at the control pin. The typical control voltage is 5.7V, the limit voltage is 9V, and the maximum allowable current at the control terminal is 100mA.
Temperature compensation is applied to the threshold voltage during design to eliminate variations in drain current due to changes in the drain-source on-resistance with temperature. When the junction temperature of the chip exceeds 135°C, the overheat protection circuit outputs a high-level signal and disables the output. At this point, the control voltage Vc enters a hysteresis adjustment mode, and the Vc-end waveform becomes a sawtooth wave with an amplitude of 4.7V to 5.7V. To restart the circuit, either power off the circuit switch or reduce Vc to below 3.3V, then use the power-on reset circuit to reset the internal flip-flop and restore the MOSFET to normal operation.
Using the TOPSwitch-II series to design a single-chip switching power supply requires fewer external components, and the device's sensitivity to board layout and input bus transients is significantly reduced. This makes the design convenient, stable in performance, and cost-effective.
The choice of chip is primarily based on input voltage and power. According to the design requirements, the input voltage is a wide-range input, and the output power does not exceed 10W, so TOP222G was selected.
Circuit Design
The schematic diagram of this switching power supply is shown in Figure 1. [3]
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