Switch-mode power supply is a type of power supply widely used in various electronic devices, with its unique advantages and disadvantages. While understanding its working principle and application scenarios, we also need to conduct a comprehensive analysis of its pros and cons. Next, we will delve into the advantages and disadvantages of flyback switch-mode power supply to help you have a more comprehensive understanding of this type of power supply. This article mainly focuses on the characteristics, advantages, and disadvantages comparison of common switch-mode power supply topologies.

Common topologies include Buck step-down, Boost step-up, Buck-Boost step-down/step-up, Flyback, Forward, Two-Transistor Forward, etc.

Topologies

Common basic topologies

I. Basic Pulse Width Modulation Waveform

These topologies are all related to switched circuits.

II. Common Basic Topologies

1. Buck Step-down

Reduces the input to a lower voltage.

Possibly the simplest circuit.

The inductor/capacitor filter smooths the square wave after switching.

The output is always less than or equal to the input.

The input current is discontinuous (chopped).

The output current is smooth.

2. Boost Step-up

Increases the input to a higher voltage.

Similar to the step-down, but with the inductor, switch, and diode rearranged.

The output is always greater than or equal to the input (neglecting the forward voltage drop of the diode).

The input current is smooth.

The output current is discontinuous (chopped).

3. Buck-Boost Step-down/Step-up

Another arrangement of the inductor, switch, and diode.

Combines the disadvantages of step-down and step-up circuits.

The input current is discontinuous (chopped).

The output current is also discontinuous (chopped).

The output is always opposite to the input (note the polarity of the capacitor), but the magnitude can be less than or greater than the input.

The "flyback" converter is actually an isolated (transformer-coupled) form of the step-down/step-up circuit.

4. Flyback

Works like a step-down/step-up circuit, but the inductor has two windings and serves as both a transformer and an inductor.

The output can be positive or negative, determined by the polarity of the coil and diode.

The output voltage can be greater or less than the input voltage, determined by the turns ratio of the transformer.

This is the simplest of the isolated topologies.

Multiple outputs can be obtained by adding secondary windings and circuits.

5. Forward

A transformer-coupled form of the step-down circuit.

Discontinuous input current, smooth output current.

Because of the transformer, the output can be greater or less than the input and 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. A 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 another winding and diode during the switch-off phase.

6. Two-Transistor Forward

Two switches work simultaneously.

When the switches are off, the energy stored in the transformer reverses the polarity of the primary, causing the diode to conduct.

Main advantages: The voltage across each switch never exceeds the input voltage; no need to reset the winding magnetic track.

7. Push-Pull

The switches (FETs) are driven out of phase and Pulse Width Modulation (PWM) is used to regulate the output voltage.

Good transformer core utilization - power is transferred in both half-cycles.

Full-wave topology, so the output ripple frequency is twice the transformer frequency.

The voltage applied to the FETs is twice the input voltage.

8. Half-Bridge

• A very common topology for higher-power converters.

• The switches are driven out of phase and Pulse Width Modulation is used to regulate the output voltage.

• Good transformer core utilization - power is transferred in both half-cycles. And the utilization of the primary winding is better than that of the push-pull circuit.

• Full-wave topology, so the output ripple frequency is twice the transformer frequency.

• The voltage applied to the FETs is equal to the input voltage.

• 9. Full-Bridge

• The most common topology for higher-power converters.

• The switches are driven in diagonal pairs and Pulse Width Modulation is used to regulate the output voltage.

• Good transformer core utilization - power is transferred in both half-cycles.

• Full-wave topology, so the output ripple frequency is twice the transformer frequency.

• The voltage applied to the FETs is equal to the input voltage.

• At a given power, the primary current is half that of the half-bridge.

• 10. SEPIC (Single - Ended Primary Inductance Converter)

The output voltage can be greater or less than the input voltage.

Like the step-up circuit, the input current is smooth, but the output current is discontinuous.

Energy is transferred from the input to the output through a capacitor.

Requires two inductors.

11. C’uk (Patented by Slobodan C’uk)

• The output is inverted.

• The magnitude of the output voltage can be greater or less than the input.

• Both the input current and the output current are smooth.

• Energy is transferred from the input to the output through a capacitor.

• Requires two inductors.

• The inductors can be coupled to obtain zero-ripple inductor current.

• III. Circuit Working Details

• The following explains the working details of several topologies.

• 1. Buck - Step - Down Regulator - Continuous Conduction

• The inductor current is continuous.

• Vout is the average value of its input voltage (V1).

• The output voltage is the input voltage multiplied by the duty ratio (D) of the switch.

• When on, the inductor current flows from the battery.

• When the switch is off, the current flows through the diode.

• Neglecting the losses in the switch and inductor, D is independent of the load current.

• The characteristics of the step - down regulator and its derivative circuits are: discontinuous (chopped) input current and continuous (smooth) output current.

• 2. Buck - Step - Down Regulator - Critical Conduction

The inductor current is still continuous, just "reaching" zero when the switch is turned on again. This is called "critical conduction". The output voltage is still equal to the input voltage multiplied by D.

3. Buck - Step - Down 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 duty ratio (D) of the switch.

• When the load current is below the critical value, D changes with the load current (while Vout remains constant).

4. Boost Step - Up 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 (opposite to the step - down regulator).

The relationship between the output voltage and the duty ratio (D) is not as simple as in the step - down regulator. In the case of continuous conduction:

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

5. Transformer Operation (Including the Role of the Primary Inductance)

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

6. Flyback Transformer

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

7. Forward Converter Transformer

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

• The magnetizing current (i1) flows into the "magnetizing inductance" to demagnetize the core after the primary switch is turned off (voltage reversal).

• Conclusion

• This article reviews the most common circuit topologies in current switch - mode power supply conversion. Besides these, there are many other topologies, but most are combinations or variations of these.

• Each topology involves unique design trade - offs: the voltage applied to the switch, chopping and smoothing of input and output currents, and the utilization of windings.

• To select the best topology, it is necessary to study: input and output voltage ranges, current ranges, cost and performance, and the ratio of size and weight.