This article mainly introduces the characteristics, 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.Basic pulse width modulation waveform
All these topologies are related to switching circuits. The following is the definition of a basic PWM waveform:
2. Common basic topologies
1.Buck
Reduce the input voltage to a lower level. Probably the simplest circuit. After switching, the inductor/capacitor filter smoothes the square wave. In each case, the output is less than or equal to the input. Input current is irregular (chopping). Output current is smooth.
2. Improvement
Increase input voltage. Same as buck , but inductor, switch and diode rearranged. In each case, the output is greater than or equal to the input (ignoring the diode forward voltage drop). The input current has been softened. Output current is irregular (chopping).
3. Buck-Boost
Different configurations of inductors, switches and diodes . Combines the disadvantages of buck and boost circuits. Input current is irregular (chopping). Additionally, the output current is not constant (chopped). Although the output is always opposite to the input (due to the polarity of the capacitor ) , the amplitude can be smaller or larger. A "flyback" converter is a buck-boost circuit isolation device (transformer coupled).
4. Flyback
This inductor contains two windings that act 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 greater or less than the input voltage. The simplest isolation topology is this. By adding secondary windings and circuits you can have many outputs.
5. Forward
The input current is irregular, while the output current is smooth. Due to the transformer, the output may be larger or smaller than the input regardless of polarity.
By adding secondary windings and circuits you can have many outputs. During each switching cycle, the transformer core must be demagnetized. It is typical to add a winding with the same number of turns as the primary winding. The energy accumulated in the primary inductance during the turn-on phase is released through the additional winding and diode during the turn-off phase.
6. Two transistor forward
Both switches are activated simultaneously. When the switch opens, the energy stored in the transformer flips the polarity of the primary, causing the diode to conduct. The voltage on each switch never exceeds the input voltage, and the winding rails do not need to be reset.
7.Push and pull
To regulate the output voltage, the switches (FETs) operate out of phase and are pulse-width modulated (PWM). Power is transferred during all half cycles, indicating good use of the transformer core. Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The FET withstands a voltage twice the input voltage.
8. Half bridge
This is a relatively common topology for higher power converters. To modify the output voltage, the switches operate out of phase and in pulse width modulation. Power is transferred during all half cycles, indicating good use of the transformer core. The primary winding is also better used than a push-pull circuit.
Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The input voltage is the same as 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 supplied in diagonal pairs and pulse-width modulated. Power is transferred during all half cycles, indicating good use of the transformer core. Due to the full-wave topology, the output ripple frequency is twice that of the transformer. The input voltage is equal to the voltage applied to the FET. At a given power , the primary current is half the half-bridge current,
10.SEPIC single-ended primary inductor converter
The output voltage can be higher or lower than the input voltage. The input current is smooth, like a boost circuit, but the output current is not. Capacitors transfer energy from input to output. Two inductors are required.
11. C'uk (Patented by Slobodan C'uk)
In this case, the output is inverted. The output voltage can be higher or lower than the input voltage. Both input and output currents are smooth.
Capacitors transfer energy from input to output. Two inductors are required. To achieve zero ripple inductor current, an inductor can be connected.
three. Circuit working details
The working details of several topologies are explained below.
1. 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 duty cycle (D) of the switch. When the battery is turned on, inductor current flows from the battery. When the switch is open, current flows through the diode. When considering the losses in the switch and inductor, D is independent of the load current. Buck regulators and their derivatives have a constant output current and discontinuous input current (chopping) (smoothing).
2. Buck-buck regulator critical conduction
The inductor current remains continuous; when the switch opens again, it "reaches" zero. This is called "critical conduction." The output voltage is still equal to D times the input voltage.
3. Buck regulator - intermittent conduction
In this case, the current in the inductor is 0 for part of each cycle. The average value of v1 is still (always) the output voltage. The output voltage is not equal to the input voltage times the switch duty cycle (D). When the load current is below the critical threshold (while Vout remains constant), D fluctuates with the load current.
4. Boost regulator
The output voltage is always higher than (or equal to) the input voltage (or the same). The input current is constant, while the output current is variable (as opposed to a buck regulator). Compared to a buck regulator, the relationship between output voltage and duty cycle (D) is not as simple. In the case of continuous conduction, the following conditions hold true:
In this example, Vin = 5, Vout = 15, D = 2/3. D = 2/3, Vout = 15.
5. Transformer operation (including the role of primary inductance)
The primary (magnetizing) inductance of a transformer is considered to be in parallel with the primary in an ideal transformer.
6. Flyback transformer
Peak current and stored energy are calculated using the primary inductance, which in this case is lower.
7. Forward transformer
Since there is no need to store energy, the primary inductance is higher. After the primary switch is closed, the magnetizing current (i1) flows into the "magnetizing inductor", causing the core to demagnetize (voltage reversal).