This article mainly describes the characteristics and advantages and disadvantages of common switching power supply topologies. Common topologies include Buck Buck, Boost Boost, Buck-Boost Buck-Boost, Flyback Flyback, Forward Forward, Two-Transistor Forward, etc.

Ⅰ. The basic pulse width modulation waveform

All of these topologies have something to do with switching circuits. The following is the definition of a basic PWM  waveform:

Ⅱ. The common basic topology

1. Buck

Reduce the voltage of the input to a lower level. Probably the most straightforward circuit. After switching, the inductor/capacitor filter smooths out the square wave. In every case, the output is less than or equal to the input. The input current is irregular (chopped). The output current is smooth.

2. Boost

Increase the voltage of the input. Same as buck, but with inductors, switches, and diodes rearranged. In every case, the output is more than or equal to the input (ignoring diode forward drop). The input current has been softened. The output current is irregular (chopped).

3. Buck-Boost

A different configuration of inductors, switches, and diodes. Combination of the drawbacks of buck and boost circuits. The input current is irregular (chopped). In addition, the output current is not constant (chopping). Although the output is always inverse to the input (due to the polarity of the capacitors  ), the magnitude can be smaller or larger. A "flyback" converter is a buck-boost circuit isolation device (transformer coupling).

4. Flyback

The inductor contains two windings and works as both a transformer and an inductor, similar to a buck-boost circuit. Depending on the polarity of the coil and diode, the output can be positive or negative. The turns ratio of the transformer determines whether the output voltage is larger or less than the input voltage. The simplest of the isolation topologies is this one. By adding secondary windings and circuits, you can have many outputs.

5. Forward

The input current is irregular, while the output current is smooth. The output might be larger or less than the input, in any polarity, due to the transformer.

By adding secondary windings and circuits, you can have many outputs. During each switching cycle, the transformer core must be demagnetized. Adding a winding with the same number of turns as the primary winding is a typical practice. The energy accumulated in the primary inductance during the switch-on phase is released during the switch-off phase via extra windings and diodes, 

6. Two-Transistor Forward

Both switches are active at the same time. The energy stored in the transformer flips the polarity of the primary when the switch is open, causing the diode to conduct. The voltage on each switch never exceeds the input voltage, and the winding tracks do not need to be reset.

7. Push-Pull

To regulate the output voltage, the switches (FETs) are operated out of phase and pulse-width modulated (PWM). Power is transferred in all half-cycles, indicating good transformer core use. Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The FET is subjected to a voltage that is twice that of the input voltage.

8. Half-Bridge

For higher power converters, this is a relatively frequent topology. To modify the output voltage, the switches are operated out of phase and pulse width modulated. Power is transferred in all half-cycles, indicating good transformer core use. The primary winding is also better used than in a push-pull circuit.

Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The input voltage is identical to the voltage applied to the FET.

9. Full-Bridge

For higher power converters, this is the most popular topology. To regulate the output voltage, the switches are powered in diagonal pairs and pulse-width modulated. Power is transferred in all half-cycles, indicating good transformer core use. Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The input voltage is equivalent to the voltage applied to the  FETs. The primary current is half that of a half-bridge at a given power, 

10. SEPIC single-ended primary inductance converter

The output voltage can be higher or lower than the input. The input current is smooth, just like the boost circuit, but the output current is not. Capacitors transport energy from the input to the output. It is necessary to use two inductors.

11. C'uk (patent of Slobodan C'uk)

In this case, the output is inverted. The output voltage can be more or lesser than the input voltage. The input and output currents are both smooth.

Capacitors transport energy from the input to the output. It is necessary to use two inductors. For zero ripple inductor current, inductors can be linked.

Ⅲ. Details of circuit work

The working details of several topologies are explained below.

1. Buck-buck regulator-continuous conduction

The current in the inductor is constant. The average value of its input voltage is Vout (V1). The output voltage is equal to the input voltage multiplied by the switch's duty ratio (D). The inductor current flows from the battery when it is turned on. When the switch is open, the current passes through the diode. D is independent of load current when losses in switches and inductors are taken into account. Buck regulators and their derivatives have a constant output current and a discontinuous input current (chopping) (smoothing).

2. Buck-buck regulator-critical conduction

The inductor current is still continuous; when the switch is turned on again, it "reaches" zero. This is referred to as "critical conduction." The output voltage is still equal to D times the input voltage.

3. Buck-buck regulator-discontinuous conduction

The current in the inductor is 0 for a portion of each cycle in this situation. The average of v1 is still (always) the output voltage. The output voltage is not equal to the input voltage multiplied by the switch's duty ratio (D). D fluctuates with the load current when the load current is below the critical threshold (while Vout remains constant).

4. Boost regulator

The output voltage is always higher (or equal to) than (or the same as) the input voltage. The input current is constant, whereas the output current is variable (as opposed to a buck regulator). In contrast to a buck regulator, the relationship between the output voltage and duty ratio (D) is not as straightforward. In the event of continuous conduction, the following is true:

Vin = 5, Vout = 15, D = 2/3 in this example. D = 2/3, Vout = 15.

5. Transformer work (including the role of primary inductance)

The primary (magnetizing) inductance of a transformer is considered parallel to the primary in an ideal transformer.

6. Flyback transformer

The peak current and stored energy are calculated using the primary inductance, which is low in this case.

7. Forward  Transformer 

Because there is no need to store energy, the primary inductance is high. After the primary switch is turned off, the magnetizing current (i1) flows into the "magnetizing inductance," causing the core to demagnetize (voltage reversal).

Ⅳ. Summary

This article examines the most prevalent switching power conversion circuit topologies currently in use. There are many more topologies, but the majority are combinations or modifications of these. Each topology has its own set of design considerations, such as the voltage provided to switches, the chopping and smoothing of input and output currents, and the winding consumption. Research is required to determine the optimal topology: input and output voltage ranges, current ranges, cost and performance, and size and weight ratios.