Basic pulse width modulation waveform of

These topologies are all related to switching circuits, and the basic pulse width modulation waveform is defined as follows:

Buck 

Features:
Reduce the input to a lower voltage.
Probably the simplest circuit.
The inductor/capacitor filter smoothes the square wave after switching.
The output is always less than or equal to the input.
Input current is discontinuous (chopping)Output current smoothing

Boost 

Features:
Raise the input to a higher voltage
Same as step-down, but the inductors, switches and diodes are rearranged.
The output ratio is always greater than or equal to the input (ignoring the forward voltage drop of the diode)
Input current smoothing
Output current is discontinuous (chopping)

Buck-Boost 

Features:
Another Arrangement Method of Inductors, Switches and Diodes
Combining the disadvantages of buck and boost circuits.
Input current is discontinuous (chopping)
The output current is also discontinuous (chopping)
The output is always opposite to the input (note the polarity of the capacitor), but the amplitude can be smaller or larger than the input.
The "flyback" converter is actually a buck-boost circuit isolation (transformer coupling) form.

Flyback 

Features:
It works like a buck-boost circuit, but the inductor has two windings, which serve as both a transformer and an inductor.
The output can be positive or negative, depending on the polarity of the coil and diode.
The output voltage can be larger or smaller than the input voltage, which is determined by the turns ratio of the transformer.
This is the simplest isolated topology.
Multiple outputs can be obtained by adding secondary windings and circuits.

Forward 

Features:
Transformer coupling form of step-down circuit.
Discontinuous input current and smooth output current.
Because of the transformer, the output can be larger or smaller than the input, and it can be of any polarity.
Multiple outputs can be obtained by adding secondary windings and circuits.
The transformer core must be demagnetized in each switching cycle.
The common practice is to add a winding with the same number of turns as the primary winding.
The energy stored in the primary inductor during the switch-on phase is released through additional windings and diodes during the switch-off phase.

Two-Transistor Forward

Features:
Two switches work at the same time.
When the switch is turned off, the energy stored in the transformer reverses the polarity of the primary and turns on the diode.
Main advantages:
The voltage on each switch will never exceed the input voltage.
There is no need to reset the winding track.

Push-Pull 

Features:
The switch (FET) drives different phases, and performs pulse width modulation (PWM) to adjust the output voltage.
Good utilization ratio of transformer core-power transmission in two and a half cycles.
Full-wave topology, so the output ripple frequency is twice that of the transformer.
The voltage applied to the FET is twice the input voltage.

Half-Bridge 

Features:
Topology structure of higher power converter is very commonly used.
The switch (FET) drives different phases, and performs pulse width modulation (PWM) to adjust the output voltage.
Good utilization ratio of transformer core-power transmission in two and a half cycles. And the utilization ratio of primary winding is better than that of push-pull circuit.
Full-wave topology, so the output ripple frequency is twice that of the transformer.
The voltage applied to the FET is equal to the input voltage.

Full-Bridge 

Features:
The most commonly used topology of higher power converters.
Switches (FETs) are driven in diagonal pairs, and pulse width modulation (PWM) is performed to adjust the output voltage.
Good utilization ratio of transformer core-power transmission in two and a half cycles.
Full-wave topology, so the output ripple frequency is twice that of the transformer.
The voltage applied to FETs is equal to the input voltage.
At a given power, the primary current is half that of the half bridge.

SEPIC single-ended primary inductor converter

Features:
The output voltage can be greater or less than the input voltage.
Like the boost circuit, the input current is smooth, but the output current is discontinuous.
Energy is transmitted from the input to the output through a capacitor.
Two inductors are required.

C 'uk (patent of Slobodan C 'uk)

Features:
Output inversion
The amplitude of the output voltage can be larger or smaller than that of the input.
Both the input current and the output current are smooth.
Energy is transmitted from the input to the output through a capacitor.
Two inductors are required.
Zero ripple inductor current can be obtained by inductive coupling.

Details of the circuit work
Working details of several topologies:
Step-down regulator:continuous conduction, critical conduction and discontinuous conduction.
Boost regulator (continuous conduction)
Working transformer
Flyback transformer
Forward transformer

Buck- buck regulator-continuous conduction

Features:
The inductance is continuous.
Vout is the average value of its input voltage (V1).
The output voltage is the input voltage multiplied by the load ratio (D) of the switch.
When on, the inductor current flows out of the battery.
When the switch is turned off, the current flows through the diode.
Ignore the losses in switches and inductors, and D has nothing to do with the load current.
The buck regulator and its derivative circuit are characterized in that:
The input current is discontinuous (chopping) and the output current is continuous (smoothing).

Buck- buck regulator-critical conduction

The current is still continuous, but it "reaches" zero when the switch is turned on again, which is called "critical conduction". The output voltage is still equal to the input voltage multiplied by d.

Buck- buck regulator-discontinuous conduction

In this case, the current in the inductor is zero for a period of time in each cycle.
The output voltage is still (always) the average value of v1.
The output voltage is not the input voltage multiplied by the load ratio (d) of the switch.
When the load current is lower than the critical value, D changes with the load current (while Vout remains the same).

Boost regulator

The output voltage is always greater than (or equal to) the input voltage.
The input current is continuous and the output current is discontinuous (contrary to the buck regulator).
The relationship between output voltage and load ratio (D) is not as simple as that in buck regulators. In the case of continuous conduction:

Vin = 5
Vout = 15
and D = 2/3
Vout = 15
D = 2/3

Operation of transformer (including the function of primary inductor)

The transformer is regarded as an ideal transformer, and its primary (magnetization) inductance is connected in parallel with the primary.

Flyback transformer

Here, the primary inductance is very low, which is used to determine the peak current and stored energy. When the primary switch is turned off, energy is transferred to the secondary.
 

Forward  converter transforme

The primary inductance is high because there is no need to store energy.

The magnetizing current (i1) flows into the "magnetizing inductor", which makes the magnetic core demagnetize (reverse voltage) after the primary switch is turned off.

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