The circuit principle of a USB power adapter

At present, portable electronic devices are increasing day by day. They have expanded from simple radios to CDS, mobile phones, VCDS, MP3s, MP4s, portable computers, tablet computers, game consoles, etc. They themselves are not large in size and light in weight. However, traditional power frequency transformers have been gradually phased out due to their disadvantages such as no voltage stabilization function, high internal resistance, large loss and heavy weight. At present, more than 96% of the AC adapters used in conjunction with portable electronic devices are designed as switching power supplies.Here is an introduction to a 2.5W constant current/constant voltage output USB power adapter. Its input voltage is 85V to 265VAC, the output voltage is 5V, and the output current is 0.5A.

5V0.5A

The circuit of the 2.5W (5V0.5A) AC wide-range input mobile phone battery charger designed by USB power adapter is shown in the figure. FR is the fuse resistor. The 85-265V AC power is rectified by VD1 - VD4 bridge, and then passes through the filter composed of inductors L1, C1 and C2 to obtain the DC high voltage Ui. R1 is the damping resistance of L1. By taking advantage of the frequency jitter feature of TNY264, it is possible to meet the international standard for suppressing conducted electromagnetic interference (EMI) with a simple filter and a safety capacitor C8 (referred to as Y capacitor). Even in the most unfavorable case where the capacitive load at the output end is grounded, EMI can still be effectively suppressed by adding a shielding layer to the high-frequency transformer. The clamping protection circuit is composed of diode VD6, capacitor C3 and resistor R2, which can limit the peak voltage applied to the drain when the power MOSFET is turned off within a safe range. When the output current is lower than 500mA, the voltage control loop works, while the current control loop does not function due to the cut-off of transistor VT. At this point, the output voltage is jointly set by the forward voltage drop of the LED in the optocoupler IC2 (LTV817) and the voltage stabilization value of the voltage stabilizing diode VS. Resistor R8 provides bias current to the voltage stabilizing diode, making the stabilizing current Iz of VS approach the typical value. The secondary side voltage is rectified and filtered by VD5, C5, L2 and C6 to obtain an output voltage of +5V.
 

The switching frequency of the USB power adapter is relatively high. During the reverse recovery process after the output rectifier tube VD5 is turned off, switching noise will be generated, which is prone to damage the rectifier tube. Although an RC absorption circuit composed of resistors and capacitors in series is connected in parallel across the VD5, it can suppress the switching noise to a certain extent, the effect is still not ideal. Moreover, it will cause power loss on the resistor. The solution is to connect a magnetic bead in series with the output rectifier filter. The current control loop is composed of transistor VT, current detection resistor R4 and optocoupler IC2. When the output current approaches 500mA, due to the increase in voltage drop across R4, the emitter voltage of transistor VT also rises, and VT enters the amplification region. At this point, the current control loop begins to function, and the output exhibits a constant current characteristic. A short circuit fault occurred at the immediate output terminal, causing the current to suddenly increase and the output voltage to be zero. Due to the total voltage drop across resistors R6 and R4 being approximately 1.2V, the normal operation of the LED in the VT and optocoupler could still be maintained. R3 is the base limiting current resistor.