Circuit diagram explanation: simple switching power supply circuit diagram

Simple switching power supply circuit diagram (I)

Although the voltage regulation accuracy is not high, it can meet general requirements. The circuit is simple, conventional components are used, the cost is extremely low, and the output allows open circuit and short circuit.

After the mains is rectified by D1 and filtered by C1, a DC voltage of about 300V is obtained and applied to the first leg of the transformer (the upper end of L1). At the same time, this voltage is biased by R1 to V1, making it slightly conductive, and current flows through L1. At the same time, the upper end of the feedback coil L2 (the third leg of the transformer) forms a positive voltage, which is fed back to V1 through C4 and R3, making it more conductive and even saturated. Finally, as the feedback current decreases, V1 quickly exits saturation and cuts off. This cycle forms an oscillation, and the required output voltage is induced on the secondary coil L3. L2 is a feedback coil, and it also forms a voltage stabilization circuit together with D4, D3, and C3. When the voltage of coil L3 on C5 increases after being rectified by D6, it also shows that the voltage of L2 on the negative electrode of C3 is lower after being rectified by D4. When it is as low as the voltage value of the voltage regulator tube D3 (9V), D3 is turned on, causing the base of V1 to short-circuit to the ground, turning off V1, and finally reducing the output voltage. In the circuit, R4, D5, and V2 form an overcurrent protection circuit. When the working current of V1 is too large due to some reasons, the voltage transformer generated on R4 is added to the base of V2 through D5, V2 is turned on, the base voltage of V1 drops, and the current of V1 decreases. The theoretical voltage regulation value of D3 is 9V+0.5~0.7V. In actual application, if you want to change the output voltage, just replace D3 with different voltage regulation values. The smaller the voltage regulation value, the lower the output voltage, and vice versa.

Simple switching power supply circuit diagram (II)

24V switching power supply is a type of high-frequency inverter switching power supply. It controls the switch tube through the circuit to conduct high-speed on and off. It converts direct current into high-frequency alternating current and provides it to the transformer for transformation, thereby generating one or more sets of voltages required!

The working principle of 24V switching power supply is:

1. The AC power input is rectified and filtered into DC;

2. The switch is controlled by a high-frequency PWM (pulse width modulation) signal, and the DC is added to the primary of the switching transformer;

3. The secondary of the switching transformer induces a high-frequency voltage, which is supplied to the load after rectification and filtering;

4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.

24v switching power supply circuit diagram

24V overcurrent protection diagram

Simple switching power supply circuit diagram (III)

The circuit is based on the UC3842 oscillator chip, which forms an inverter and rectifier circuit. UC3842 is a high-performance single-ended output current-controlled pulse width modulator chip. The relevant pin functions and internal circuit principles have been introduced, so they are omitted here. The AC220V power supply is introduced through the common-mode filter L1, which can better suppress the high-frequency interference entering from the power grid and radiated from the power supply itself. The AC voltage is filtered by the bridge rectifier circuit and capacitor C4 to become an unstable DC voltage of about 280V, which is used as an inverter circuit composed of the oscillator chip U1, the switch tube Q1, the switch transformer T1 and other components. The inverter circuit can be divided into four circuit parts to explain its circuit working principle.

CL-A-35-24 instrument DC24V switching power supply

1. Oscillation circuit: The main winding N1 of the switching transformer, the drain-source of Q1, and R2 (working current detection resistor) are the paths of the working current of the power supply; the startup circuit of this machine is different from other switching power supplies (the startup circuit is composed of step-down current limiting resistors). The startup circuit is composed of C5, D3, and D4, providing a "transient" startup current. The diode D2 absorbs the reverse voltage, and D3 has a rectifying effect to ensure that the startup current added to the 7th foot of U1 is a positive current; after the circuit starts to oscillate, the N2 self-powered winding, D2, and C5 rectifier filter circuit provide the power supply voltage for the U1 chip. The normal operation of these three links is a prerequisite for the power supply to oscillate.

Of course, the 4-pin external timing components R48, C8 and the U1 chip itself also form part of the oscillation circuit.

Capacitor start-up circuit, when overload or short circuit occurs, the circuit can be in a stable stop oscillation protection state, unlike the resistor start-up circuit, which will reproduce the "hiccup" intermittent oscillation phenomenon. The working current detection is obtained from the resistor R2. When the fault state causes the working overcurrent to increase abnormally, the duty cycle of the PWM pulse output from pin 6 of U1 decreases, and the induction circuit of the self-powered winding of N1 also decreases accordingly. When the supply voltage of pin 7 of U1 is lower than 10V, the circuit stops oscillating and the load voltage is 0. This is because the overcurrent (overload or short circuit) triggers the internal undervoltage protection circuit of U1 to stop the output; when the working current increases abnormally, when the voltage drop on R2 is greater than 1V, the internal latch is activated and the circuit stops oscillating. This is because the overcurrent triggers the internal overcurrent protection of U1 to stop the output.

2. Voltage stabilization circuit: The 24V power supply composed of the N3 winding of the switching transformer, D6, C13, C14 and other components, the reference voltage source TL1, the optocoupler U2 and other components constitute the voltage stabilization control circuit. The U1 chip and the peripheral components R7 and C12 on pins 1 and 2 are also part of the voltage stabilization circuit. In fact, TL1 and U1 form an external error amplifier (relative to the internal voltage error amplifier of U1), which feeds back the voltage change of the output 24V to the feedback voltage signal input terminal of U1. When the 24V output voltage rises, the voltage on pin 2 of U1 rises, the voltage on pin 1 drops, the duty cycle of the output PWM pulse drops, and the output circuit falls back. When the output voltage rises abnormally, when pin 1 of U1 drops to 1V, the internal protection circuit is activated and the circuit stops oscillating.

3. Protection circuit: The U1 chip itself and the 3-pin peripheral circuit constitute an overcurrent protection circuit; the D1, R1, and C9 components connected in parallel on the N1 winding constitute a reverse voltage absorption protection circuit for the switch tube to provide a reverse current path when Q1 is cut off, ensuring the safe operation of Q1; in essence, the voltage feedback signal of the voltage stabilizing circuit can also be regarded as a voltage protection signal - when the feedback voltage amplitude reaches a certain value, the circuit implements a stop-oscillation protection action; the 24V output end is connected in parallel with an overvoltage protection circuit composed of R18, ZD2, and a unidirectional thyristor SCR. When the voltage stabilizing circuit malfunctions and causes the output voltage to rise abnormally, the breakdown of the voltage stabilizing diode ZD2 provides a trigger current for the SCR, and the conduction of the SCR forms a "short-circuit current" signal, forcing the internal protection circuit of U1 to produce an overcurrent protection action, and the circuit is in a stop-oscillation state.

Simple switching power supply circuit diagram (IV)

The typical circuit of a single-ended forward switching power supply is shown in Figure 4. This circuit is similar to the single-ended flyback circuit in form, but the working conditions are different. When the switch tube VT1 is turned on, VD2 is also turned on. At this time, the power grid transmits energy to the load, and the filter inductor L stores energy; when the switch tube VT1 is turned off, the inductor L continues to release energy to the load through the freewheeling diode VD3.

The circuit also has a clamping coil and a diode VD2, which can limit the maximum voltage of the switch tube VT1 to twice the power supply voltage. In order to meet the core reset condition, that is, the flux establishment and reset time should be equal, the duty cycle of the pulse in the circuit cannot be greater than 50%. Since this circuit transmits energy to the load through the transformer when the switch tube VT1 is turned on, the output power range is large and can output 50-200W of power. The transformer used in the circuit has a complex structure and a large volume. For this reason, this circuit is rarely used in practice.