ZVT soft switching PFC switching regulated power supply circuit

The picture shows a switching regulated power supply circuit using zero voltage conversion (ZVT) soft switching technology power factor adjustment (PFC). Compared with the hard-switching active power factor adjustment (PFC) circuit, this circuit has an additional auxiliary switch tube and resonant circuit. The main switch tube VTr and the auxiliary switch tube VTrl can be controlled by an IC chip - UC3855AN. In the figure, the auxiliary switch tube and the resonant LC circuit work together to enable the main switch tube to achieve ZVT soft switching operation. UC3855AN outputs two pulse trains to control the conduction of the main switch tube and the auxiliary switch tube respectively. The calculation method of its feedback sampling and duty cycle is similar to that of UC3854. The internal structure of the UC3854 monolithic integrated circuit is shown in the figure below. It is mainly composed of a voltage amplifier, an analog multiplier, a current amplifier and a fixed frequency pulse width modulator. In addition, it also includes a gate driver compatible with power MOSFET and a 7.5V voltage reference. , load enable comparator, under-voltage detection and over-current comparator. The UC3854 accurately maintains the sinusoidal ratio of the bus input current without slope compensation of the current. This is because the device's internal reference voltage (7.5V) and the amplitude of the internal oscillator (5.6V) are relatively high, thereby increasing the noise immunity margin. UC3854 can be used in the entire range of input AC voltage from 75 to 275V and frequency from 50 to 400Hz. In order to reduce the power consumption of the bias circuit, UC3854 also features low startup current. The device is packaged in a 16-pin DIP, and there are also surface-mounted products. The pin functions of this device are: Pin l is the "ground" pin, and all voltages inside the device are based on this voltage. Vcc and Vref should be bypassed directly to this point with 0.1µF core or larger ceramic capacitors. The discharge current of the timing capacitor should also return to this point, so the lead from the oscillator timing capacitor to "ground" must be as short as possible. Pin 2 is the peak limit (PK Limit) pin. Its threshold is 0V. When used, connect it to the negative terminal of the current sensing resistor. At the same time, the resistor is connected to the internal reference to compensate the negative current sensing signal to the "ground" potential. Pin 3 is the output terminal of the current amplifier (Current Amplifier Output), which is the output of a wideband operational amplifier that senses the input bus current and sends a current correction signal to the pulse width modulator (PWM). When the PWM needs to output a width-modulated pulse with a duty cycle D=0, the output swing of this pin can be close to zero. Pin 4 is the current sensing negative terminal (Current Sense Minus). It is the negative input of the current amplifier. Since its input port is protected by a diode to ground, the potential of this port should be ensured to be higher than -0.5V during actual use. Pin 5 is the multiplier output and current sensing positive terminal (Multiplier Output). It should be noted that the potential of this pin cannot be lower than -0.5V. Because the output of the multiplier is current, the input impedance of this port is very high, so the current amplifier can be configured as a differential amplifier to suppress ground noise. Pin 6 is the AC current input. The nominal voltage of this port is 6V, so in addition to using a resistor to connect the rectified power frequency bus at pin 6, a resistor should also be used to connect this port to the internal reference. Generally, the value of the latter should be 1/4 of the former, so that the AC distortion of the line current will be minimized. Pin 7 is the voltage amplifier output (Voltage Amplifier Output). This port serves as the output of a voltage amplifier for output voltage adjustment. To prevent output overshoot, it is internally limited to approximately 5.8V. When the voltage connected to this output is lower than 1V, the output of the multiplier will be throttled. Pin 8 is the bus voltage effective value end (RMS Line Voltage). When this port is connected to a voltage that is proportional to the effective value of the input line voltage, changes in line voltage can be compensated. For good control, the voltage of this port should be limited to 1.5-3.5V. Pin 9 is the reference voltage output (Voltage Reference Output). The internal voltage reference outputs an accurate 7.5V reference voltage and 10mA current at this port. When the device's Vcc terminal (Pin 15) or ENA terminal (Pin 10) is low, the reference will remain at 0V. In order to improve the stability of the circuit, a 0.1μF capacitor or pin 9 is generally connected to "ground". Pin 10 is the determination end (Enable). This port is a logic input port. When it is high (2.5V), the PWM output, internal reference and oscillator will be asserted. This port can also release the soft-start clamp, causing the potential of the soft-start port to rise. The acknowledgment terminal can be used as a means of shutting down the circuit under certain fault conditions, or as a method of providing additional delay when starting up. If this port is not used, it must be connected to Vcc through a 100kΩ current-limiting resistor. Pin 11 is the voltage sensing terminal (Voltage Sense). This port is the negative input terminal of the voltage amplifier, which is generally connected to the feedback network or connected to the output of the main circuit of power factor correction conversion through a voltage dividing network. Pin 12 is the multiplier output setting terminal (Oscilator Charging Current and Multi-plier Limit Set). Connecting different resistors to this port and "ground" will adjust the charging current of the oscillator and the maximum output of the multiplier. The output current of the multiplier will not exceed 3.75V divided by the connected resistor value. Pin 13 is the soft start terminal (Soft Start)