The Kangshu API6PC05 ATX power supply is extensively utilized in Lenovo Yangtian T6900 series desktop computers. Its primary power management chip is UC3843B, while the secondary power chip is TNY268PN. The main power supply features wake-up, over/under voltage, and overcurrent protection chips, identified as PS224U. This power supply also includes a no-load protection mechanism, with a maximum output power of 250W. The actual circuit diagram is illustrated in Figure 1. The internal and external voltage values within parentheses in Figure 1 represent the power supply's offline status, and the load/empty load scenario (by attaching a hard disk). To measure, short the green and black lines and use the DC setting on a 500-type multimeter. Components without specified parameters are chip capacitors and diodes, and the resistance of the chip resistors remains unconverted.
Firstly, let’s discuss the operational process:
1. The input rectification and filtering circuit consists of an EMI filter circuit, which uses a differential mode and common mode inductor and capacitor. After passing through the PFC inductor, the rectified pulsating DC power is smoothed by the filter capacitor C4 to generate a DC voltage of approximately 325V (when the mains supply is around AC230V). This voltage is then used by both the main and auxiliary power sources. The purpose of the EMI is to prevent the electromagnetic interference pulses generated by the power supply from interfering with other electrical devices on the mains via conduction or radiation. Additionally, it prevents the electromagnetic pulses on the mains from disrupting the power supply's own operation.
2. The auxiliary power supply primarily provides a 5VSB standby and wake-up power supply to the computer motherboard. A +5VSB voltage is available as soon as the power supply is connected to the mains. Additionally, it provides a +13V power supply for the main power supply UC3842B.
The auxiliary power supply is made up of a switching power supply chip M5 (TNY268PN), a switching transformer T2, and an optocoupler M7 (SFH615).
The TNY268P series chip is the second-generation enhanced single-chip isolated high-efficiency low-power switching power supply control chip from Power Integration, with a maximum output power of 23W. It internally integrates a 700V power MOSFET, an oscillator (132kHz), a 5.8V regulator, a 6.3V regulator for bypass clamping, a high-voltage switch current source, a current limit regulator, and undervoltage (4.8V), overcurrent, and thermal shutdown (135°C) protection mechanisms. It also includes auto-restart and frequency jitter (with a jitter amount of ±4kHz). The drain voltage directly supplies the energy required for system startup and operation, eliminating the need for transformer bias windings and associated circuitry. The circuit is simple, reliable, and cost-effective.
Its internal functional block diagram and pin function diagram are shown in Figures 2 and 3, respectively. From Figure 2, we can see that the following circuits can turn off the MOSFET: the BP-side undervoltage comparator, overcurrent comparator, overtemperature protection circuit, leading-edge blanking circuit, maximum duty cycle signal Dmax, and EN/W control terminal. These circuits have a logical OR relationship, meaning the MOSFET can be turned off in any one of these ways. The pin functions are detailed in Table 1.
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