PSR stands for Primary Side Regulator, which refers to primary-side feedback control or primary-side regulation. This means the error amplifier is integrated inside the IC, eliminating the need for a common three-terminal precision voltage regulator like the TL431 and optocoupler like the PC817. PSR offers several advantages and disadvantages, along with various classifications.
The PSR architecture is simple, doesn't require an optocoupler, and provides excellent CC/CV performance at a lower cost, making it very popular in low-power chargers and LED constant-current driver circuits. However, in high-power supplies, the current PSR control IC's constant current performance is inferior to that of secondary feedback control in terms of accuracy, and the cost is comparable to secondary feedback, limiting its market appeal.
PSR ICs can be broadly categorized into three areas: (1) whether they include a built-in high-voltage MOSFET, such as CR6238T having a built-in MOSFET while OB2520D does not; (2) whether the control mode is digital or analog—examples include IW3620 being digitally controlled and ACT361 being analog-controlled; (3) whether they operate in CCM or DCM mode—LNK406 operates in CCM mode, whereas FSEZ1317 operates in DCM mode.
Now, let’s delve into the working principles of PSR. Although different types of PSR circuits vary slightly, their fundamental working principles remain similar. For instance, consider FSEZ1317 as an example. Its typical application circuit is illustrated in Figure 1 below.
In CV mode, most PSR chips directly sample the voltage from the auxiliary coil. Due to leakage inductance, after the MOSFET switches off, a spike is generated once the secondary diode DR turns on, affecting the voltage sampling. To counteract this, most chips employ a time-delayed sampling method. That is, the voltage on the sampling coil is measured after the MOSFET has been off for a certain duration, thereby preventing interference from leakage inductance spikes. In the case of FSEZ1317, sampling occurs 4.5 μs after the switch is turned off, as shown in Figure 2. Many power supply chips also use this delayed sampling technique in their overvoltage protection circuits, such as OB2203, UCC28600, and NCP1377, as it yields more precise sampling values.
Moreover, snubber circuits typically consist of a 1N4007 diode with a recovery time of about 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 also incorporate a small capacitor in parallel with the down-sampling resistor to achieve delayed sampling.
Based on the transformer's working principle, we can derive Np × Ipk = Ns × IpkS (assuming the secondary coil has only one winding, Ns). Here, Np, Ipk, Ns, and IpkS represent 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 depicted in Figure 3. The output current (Io) is the average value of the secondary current during one duty cycle, i.e., Io = (Td / T) × IpkS / 2, where T is the working period. Since IpkS = Np × Ipk / Ns, then Io = (Td / T) × (Np × Ipk / Ns) / 2. It becomes evident that Np and Ns are constants, and a fixed current output can be achieved by keeping Ipk and Td/T constant, thus enabling a constant current output.
Many ICs 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 Td/T value is fixed internally by the IC, such as in the OB series, which has a Td/T value of 0.5.
In CC mode, under varying output voltage conditions, the IC operates in PFM mode to maintain a fixed Td/T ratio, ensuring a stable output current. This is the basic principle behind achieving constant current. Ensuring the accuracy of the Td/T value of the IC and the primary peak current limit is crucial for obtaining higher output current accuracy. These two factors largely depend on the IC itself.
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