Design of a mobile phone charger with a standby power consumption of only 30mW

The flyback circuit structure is commonly used in the design of low-power adapters. In a flyback design, the mains input is rectified by an isolation transformer before switching at high frequency. For this design structure, if ultra-high efficiency is required, the designer will face the following two major challenges. First, in constant current mode, the load current must be detected and the feedback signal must be transmitted to the primary side; however, sensing the current consumes energy and the feedback path must remain insulated. Second, the low-voltage control and switching circuits on the primary side need their own power supply, but if the required power is obtained from the high-voltage mains, it is easy to cause unacceptable losses. A flyback circuit that can solve these two problems is shown in Figure 1.

The circuit diagram of a 5W CV/CC universal input charger uses LinkSwitch-II devices from Power Integrations as its control and switching elements. The circuit provides 5V, 1A output at 85VAC~265VAC, with constant voltage and constant current accuracy of ±5% and ±10% respectively. In addition to no-load power consumption less than 30 mW, this circuit design can easily meet all current international energy efficiency standards (China (CECP)/CEC/Energy Star EPS v2.0/EU CoC). The design provides overtemperature protection, output short circuit and open loop protection, and meets AC input surge, ESD and EMI specifications.

The LinkSwitch-II device (U1) uses on/off control to regulate the output voltage in constant voltage mode. In addition, the switching frequency is modulated to regulate the output current to achieve constant current characteristics. Both constant voltage and constant current mode control respond to the feedback voltage of the FB input. These feedback voltages come from the secondary bias winding of the transformer (T1). The flux on the T1 bias winding is directly proportional to the flux in the secondary main power winding, so no current sensing elements are required to provide voltage and current information at the load. This saves a lot of power, and the use of the bias winding also eliminates the need for an optocoupler and all other secondary constant voltage/constant current control circuits. This is a transformer tolerance self-compensating control technique that eliminates any control loop compensation circuitry. The use of transformer feedback is a key design element to achieve low power and low cost goals, and is also an innovative technology achieved with LinkSwitch-II.

The second important function of the bias winding is to provide low voltage power feedback to the internal 6V regulator in U1 through D6, C5, R7 and C4. However, during startup, the bias winding needs to obtain power from the primary side to start U1 and trigger oscillation. In the LinkSwitch-II device, this power is provided by an internal JFET (junction field effect transistor); once the switch is established and power is available through the transformer, the JFET is turned off.

At full output load, LinkSwitch-II will switch at a frequency of up to 86 kHz. Under no-load conditions, the switching frequency will drop to 274 Hz, putting the circuit into a semi-quiescent state. The combination of transformer-fed power supply and ultra-wide frequency range control makes the maximum no-load power consumption of the entire circuit only 30 mW.