How to design the switching circuit controlled by MCU?

The microcontroller can be used to control the switching power supply in computer peripherals such as inkjet printers and laser printers composed of TOPSwitch?FX. The circuit is shown in the figure.

The switching power supply part mainly consists of TOPSwitch?FX (IC1) and optocoupler (IC2). The control circuit includes a microcontroller (MCU), two LTV817A linear optocouplers (IC3, IC4), and push button switch SB. The signal generated is only valid when SB is pressed, and has no effect when raised. There is no need to add an anti-jitter circuit on SB. This is because the soft-start time of the switching power supply (about 10ms) and the reset and initialization time of the MCU can act as a delay, which can avoid the time of jitter interference when SB is pressed; and Only when the switch is pressed for at least the above time, the switching power supply can be turned on through the MCU. This requires that the SB must be pressed to the end, rather than lightly clicked, to ensure that the power starts.

After the MCU completes the reset and initialization, it detects the power-on signal from IC3, and then locks the switching power supply through IC4. The LED light-emitting tubes and phototransistors in the optocouplers IC3 and IC4 are represented by LED3 and VT3, LED4 and VT4 respectively. Now connect LED3 between the control end and the upper end of SB, and connect VT3 to the logic input end of the MCU. Under normal conditions, no current flows through LED3 and IC3 does not work. The logic output of the MCU is connected to the anode of LED4 through the isolation diode VD6 and the resistor R4. VT4 is connected between the M and S terminals. Since the M terminal itself has a current limiting function, VT4 does not need an additional current limiting resistor. CM is the noise-cancelling capacitor of the multi-function terminal.

When the user presses SB for the first time, VD4 is turned on, and the M terminal is connected to the S pole through VD4. TOPSwitch?FX works in the three-terminal mode, and the multi-function terminal (M) and switching frequency setting terminal (F) do not work. , at this time, there is current flowing through LED3, and VT3 sends a start signal to the MCU.

If the switching power supply is initially off (the M terminal is left floating), the power supply will be turned on when SB is pressed for the first time, and the +5V output voltage UCC will provide the working power supply voltage for the MCU. After receiving the start signal, the MCU turns on VT4, keeping the switching power supply in the on state and enabling normal output. When the user presses SB again, a shutdown signal is sent. After receiving the signal, the MCU executes the shutdown program and stops the print head of the inkjet printer in a safe position. Once the shutdown procedure is completed, the MCU turns off VT4, leaves the M terminal floating, and the switching power supply enters the shutdown mode. At this time, TOPSwitch?FX is in a low power consumption state. When UI=230VAC, the chip power consumption is only 160mW. If used as a switching power supply in DVD, the shutdown program can also store the data and setting status in the E2PROM, so that they will not be lost even after power failure.