PSR stands for Primary Side Regulator, which refers to primary side feedback control or primary adjustment. This means the error amplifier is integrated within the IC, eliminating the need for a common three-terminal precision voltage regulator like TL431 and an optocoupler like PC817. The advantages, disadvantages, and classifications of PSRs are worth noting. The PSR architecture offers a simpler circuit design, no optocoupler, and excellent CC/CV performance at a lower cost. This makes it particularly popular in low-power chargers and LED constant-current driver circuits. However, in high-power applications, the current PSR control IC's constant current performance lags behind secondary feedback control accuracy, and the cost is comparable to secondary feedback, thus limiting its market appeal. PSR ICs can roughly be categorized into three areas: (1) Whether they have an internal high-voltage MOSFET, such as CR6238T featuring an internal high-voltage MOSFET, whereas OB2520D lacks one; (2) Whether the control mode is digital or analog—examples include IW3620 being digitally controlled and ACT361 being analog-controlled; (3) Operating in CCM or DCM modes—such as LNK406 falling under CCM mode and FSEZ1317 under DCM mode.
Secondly, let’s delve into the working principles. Despite differences among various types of PSR circuits, their fundamental working principles are largely similar. However, certain parameters may vary slightly. Taking FSEZ1317 as an example, its typical application circuit is illustrated in Figure 1. In CV mode, most PSR chips directly sample the voltage from the auxiliary coil. Due to leakage inductance, once the MOSFET switches off, i.e., the secondary diode DR turns on, a spike will occur, affecting voltage sampling. To mitigate this issue, most chips employ a time-delayed sampling method, meaning the voltage on the sampling coil is measured after the MOSFET is turned off for a specific duration, preventing interference from leakage inductance spikes. For FSEZ1317, sampling occurs 4.5 μs after the switch turns off, as depicted in Figure 2. Many power supply ICs, including OB2203, UCC28600, and NCP1377, utilize this delayed sampling technique to obtain more precise sampling values. Additionally, the snubber circuit typically comprises a 1N4007 diode with a recovery time of approximately 2 μs and a 100 Ω resistor in series. This combination helps reduce the ringing caused by leakage inductance and minimizes sampling errors. Some PSR chips incorporate a small capacitor in parallel with the down-sampling resistor to achieve delayed sampling. Based on the transformer's working principle, we derive the equation Np × Ipk = Ns × lpks (assuming the secondary winding has only one turn, Ns). Here, Np, Ipk, Ns, and lpks denote the number of turns in the primary coil, the peak current in the primary coil, the number of turns in the secondary coil, and the peak current in the secondary coil, respectively. When operating in DCM mode, the primary and secondary current waveforms of the switching transformer are shown in Figure 3. The output current lo represents the average value of the secondary current during one duty cycle, i.e., Io = (Td / T) × Ipsk / 2, where T is the working period. Given that Ipks = Np × Ipk / Ns, we find lo = (Td / T) × (Np × Ipk / Ns) / 2. Clearly, since Np and Ns are constants, a fixed current output can be achieved by maintaining Ipk and Td/T values, resulting in a constant current output. Many ICs on the market fix Ipk by limiting the peak voltage on the MOSFET's sampling resistor. Additionally, to prevent current spikes caused by parasitic capacitance, a blanking time is added. The IC internally fixes the Td/T value, such as the OB series ICs having a Td/T value of 0.5. In CC mode, under varying output voltage conditions, PFM mode ensures a fixed Td/T ratio to maintain a stable output current, which is the core principle of achieving constant current. Ensuring the accuracy of the IC's Td/T value and the primary peak current limit accuracy results in higher output current accuracy. These two aspects primarily depend on the IC itself.
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