Principle Circuit of Switch-Mode Voltage Regulator – Switch Power Supply Circuit Diagram
1.Basic Circuit
AC voltage is rectified and filtered by a rectifier and filter circuit to become a DC voltage with certain ripple components. This DC voltage is then fed into a high-frequency converter, where it is converted into a square wave with the required voltage. Finally, the square wave voltage is rectified and filtered again to provide the desired DC output voltage.
The control circuit is a pulse width modulator (PWM), which mainly consists of a sampler, comparator, oscillator, PWM, and reference voltage circuits. This part of the circuit is now integrated into various integrated circuits for switch-mode power supplies. The control circuit adjusts the switching time ratio of the high-frequency switching components to achieve a stable output voltage.
2.Single-Ended Flyback Switch Power Supply
A typical circuit of a single-ended flyback switch-mode power supply is shown in the diagram below. The term "single-ended" refers to the fact that the core of the high-frequency transformer operates only on one side of the hysteresis loop. The term "flyback" refers to the fact that when the switch transistor VT1 is turned on, the induced voltage on the primary winding of the high-frequency transformer T is positive on the top and negative on the bottom, causing the rectifier diode VD1 to be in a cutoff state while energy is stored in the primary winding.
When the switch transistor VT1 turns off, the energy stored in the primary winding of transformer T is released through the secondary winding, rectified by VD1, and filtered by capacitor C before being output to the load.
The single-ended flyback switch-mode power supply is a cost-effective power supply circuit, with an output power range of 20-100 W. It can output different voltages simultaneously and has good voltage regulation. The only disadvantage is that the output ripple voltage is relatively high, and it has poor external characteristics, making it suitable for relatively fixed loads.
The maximum reverse voltage endured by the switch transistor VT1 in a single-ended flyback circuit is twice the operating voltage of the circuit, and the operating frequency ranges from 20-200 kHz.
3.Single-Ended Forward Switch Power Supply
A typical circuit of a single-ended forward switch-mode power supply is shown in the diagram below. This circuit is similar to the single-ended flyback circuit in structure, but the operating conditions differ. When the switch transistor VT1 is on, VD2 also conducts, and the energy from the power source is delivered to the load, with filter inductor L storing energy. When VT1 turns off, the inductor L continues to release energy to the load through the freewheeling diode VD3.
The circuit also includes a clamp coil and diode VD2, which limits the highest voltage of the switch transistor VT1 to twice the source voltage. To satisfy the core reset condition, which means the magnetic flux build-up and reset time must be equal, the pulse duty cycle in this circuit cannot exceed 50%. Since this circuit delivers energy to the load through the transformer when the switch transistor VT1 is on, it has a wide output power range and can deliver 50-200 W. The transformer used in the circuit is rather complex, and the overall size is large, making this circuit less commonly used in practical applications.
4. Self-Oscillating Switch-Mode Voltage Regulator
The picture below shows a typical circuit of a self-oscillating switch-mode voltage regulator. This is a switch-mode power supply that uses an intermittent oscillation circuit and is one of the most commonly used basic power supplies.
When the power is connected, resistor R1 provides the starting current to the switch transistor VT1, causing it to start conducting. The collector current Ic increases linearly in inductor L1. This induces a positive feedback voltage in L2, making the base of VT1 positive and the emitter negative, which quickly saturates VT1.
At the same time, the induced voltage charges capacitor C1. As the voltage across C1 increases, the base potential of VT1 gradually decreases, causing VT1 to exit the saturation region. The Ic begins to decrease, and a voltage is induced in L2 that makes the base of VT1 negative and the emitter positive, causing VT1 to turn off rapidly. At this point, diode VD1 conducts, and the energy stored in the primary winding of transformer T is released to the load.
When VT1 turns off, there is no induced voltage in L2, and the DC supply input voltage is fed back through R1 to reverse charge C1, gradually increasing the base potential of VT1, causing it to turn on again and reach saturation. The circuit then continues to oscillate repeatedly.
Similar to the single-ended flyback switch-mode power supply, this circuit uses the secondary winding of transformer T to output the required voltage to the load. In a self-oscillating switch-mode power supply, the switch transistor serves a dual role, both as a switch and as an oscillator, eliminating the need for a separate control circuit. The circuit also has the advantage of isolation between input and output, as the load is connected to the secondary side of the transformer and operates in a flyback mode. This type of circuit is suitable not only for high-power supplies but also for small-power supplies.
5. Push-Pull Switch Power Supply
The typical circuit of a push-pull switch-mode power supply is shown in the diagram below. It is a dual-ended conversion circuit, with the core of the high-frequency transformer operating on both sides of the hysteresis loop. The circuit uses two switch transistors, VT1 and VT2, which alternate between turning on and off under the control of an external excitation square wave signal. The square wave voltage is obtained from the secondary winding of transformer T, and after rectification and filtering, it is converted into the required DC voltage.
The advantage of this circuit is that the two switch transistors are easy to drive. The main disadvantage is that the transistors must withstand twice the peak voltage of the circuit. The output power of the circuit is relatively large, typically in the range of 100-500 W.
6. Buck Converter
A typical circuit of a buck switch-mode power supply is shown in the diagram below. When the switch transistor VT1 is on, diode VD1 is off, and the rectified input voltage charges capacitor C through VT1 and inductor L. This current increases the energy stored in inductor L. When VT1 turns off, inductor L induces a left-negative, right-positive voltage, which, through the load RL and freewheeling diode VD1, releases the energy stored in inductor L and maintains a stable output DC voltage. The output DC voltage is determined by the pulse width applied to the base of VT1.
This circuit uses fewer components and, like the other two circuits introduced below, can be implemented using just an inductor, capacitor, and diode.
7. Boost Converter
The voltage regulation circuit of a boost switch-mode power supply is shown in the diagram below. When the switch transistor VT1 is on, inductor L stores energy. When VT1 turns off, inductor L induces a left-negative, right-positive voltage, which adds to the input voltage. This voltage, through diode VD1, powers the load, resulting in an output voltage greater than the input voltage, thus forming a boost switch-mode power supply.
8. Inverting Switch Power Supply
A typical circuit of an inverting switch-mode power supply is shown in the diagram below. This circuit is also known as a buck-boost converter. Regardless of whether the pulsed DC voltage before switch transistor VT1 is higher or lower than the output voltage, the circuit operates normally.
When VT1 is on, inductor L stores energy and diode VD1 is off. The load RL is powered by the charge stored in capacitor C from the previous cycle. When VT1 turns off, the current in inductor L continues to flow, inducing an upper-negative, lower-positive voltage. This voltage is passed through diode VD1 to power the load while charging capacitor C.
