High efficiency charger power supply with precise control via primary side

The primary side regulation controller (PSR) can accurately control the CV/CC of the charger output at the primary side without the need for a feedback circuit at the secondary side, achieving power saving, high efficiency and low cost. This PSR not only includes a frequency hopping mechanism to reduce EMI, but also includes a power saving mode to reduce power consumption during standby.

Figure 1 shows an example of a flyback converter design using primary-side regulation control. In order to obtain information about the secondary-side output voltage, the PSR controller uses a unique method to detect the waveform on the auxiliary winding of the transformer to obtain the output information of the secondary side for feedback control. Figure 2 shows the main operating waveforms.

For a flyback converter using a PSR controller, operating in discontinuous conduction mode will achieve better output regulation. Therefore, the working principle of the converter is as follows:

When the MOSFET inside the PSR is turned on [ton], the input voltage VIN will be established across the transformer, so the current iP at the primary end of the transformer will rise linearly from zero to ipk.; so ipk. can be derived from equation (1). During this period, the energy at the input end will be stored in the transformer.

Figure 1. Circuit diagram of a flyback converter using PSR control

When the MOSFET is turned off [toff], the energy originally stored in the transformer will turn on the secondary diode and transfer the energy to the load. During this period, the voltage at the output and the forward voltage of the secondary diode will be reflected to the auxiliary winding, so the auxiliary winding voltage VAUX can be expressed as formula (2). At this time, the sampling mechanism inside the PSR will sample the voltage on the auxiliary winding, and the output voltage information will be known as the secondary current decreases. After obtaining the output voltage information, the PSR will compare it with the internal reference voltage VREF to form a voltage loop to control the on-time of the MOSFET to stabilize the constant output voltage.

When the current on the output diode of the secondary side is reduced to zero, the voltage on the auxiliary winding will resonate with the output capacitance COSS on the MOSFET due to the inductance of the transformer until the MOSFET is turned on again.

Figure 2, controller output waveform

Where LP is the inductance of the primary side of the transformer; ton is the on-time of the MOSFET; NAUX/NS is the turns ratio of the transformer auxiliary winding to the secondary winding; VO is the output voltage; and VF is the forward conduction voltage of the secondary output diode.

This sampling method can also obtain the transformer discharge time tdis, as shown in Figure 2. The average current on the secondary output diode is equal to the output current IO. Therefore, the output current IO can be expressed by ipk and tdis as formula (3):

Where tS is the switching period of the PSR controller; NP/NS is the turns ratio between the primary and secondary sides; RSENSE is the primary current sampling resistor.

The actual implementation of a 5W charger has an output specification of 5V/1A. The controller uses FSEZ1216. This PSR controller integrates a 600V high-voltage MOSFET, so it can reduce the interference between the circuit driving the MOSFET and the PCB routing. In order to reduce standby losses, the power saving mode inside the PSR controller will linearly reduce the PWM frequency when lightly loaded to meet the current power saving requirements of power supply specifications; the frequency hopping mechanism improves EMI performance, and the output voltage of the charger will be reduced due to the longer output cable. The internal compensation mechanism can also be used to improve the output voltage regulation capability.

This technology uses the voltage on the auxiliary winding of the primary side of the sampling transformer to achieve constant current and constant voltage regulation at the output end. This advantage can save traditional components such as secondary-side feedback circuits, optical couplers, and secondary-side current detection resistors.