Electronic Design
Choose The Right Switching Regulator

Choose The Right Switching Regulator

Manufacturers sell different types of switching regulators. The location of the storage elements in reference to the switching elements and their quantities generally determines the type of switching supply configuration, as can be seen in various architectures.

 

Manufacturers sell different types of switching regulators. The location of the storage elements in reference to the switching elements and their quantities generally determines the type of switching supply configuration, as can be seen in various architectures.

1. In the generic buck configuration, the switch controls the current flowing into the inductor.  The inductor stores the energy for the load.
1. In the generic buck configuration, the switch controls the current flowing into the inductor. The inductor stores the energy for the load.

Also known as the step-down converter, the buck converter is the most commonly used switching converter (Fig. 1). It’s used to down-convert a dc voltage to a lower dc voltage of the same polarity.  Although linear regulators can also perform this function, switching buck regulators can do it with higher efficiency.

2. The generic boost configuration steps up the voltage since the inductor is placed prior to the switch.
2. The generic boost configuration steps up the voltage since the inductor is placed prior to the switch.

The boost converter, also known as the step-up converter, takes a dc input voltage and produces a dc output voltage that’s higher in value than the input but of the same polarity (Fig. 2). Linear regulators cannot provide this feature.

The buck-boost or inverting regulator produces a dc voltage that’s above, below, or opposite in polarity to the input (Fig. 3). The negative output voltage can be larger or smaller than the input voltage. There’s usually a limitation in the Vin – (–Vout) magnitude that the regulator can handle. Buck-boost can work with input voltages above and below the output.

3. The generic buck-boost configuration can output a voltage that is either greater or less than the input voltage magnitude, including negative voltages.
3. The generic buck-boost configuration can output a voltage that is either greater or less than the input voltage magnitude, including negative voltages.

The single-ended primary-inductor converter (SEPIC) is similar to a traditional buck-boost converter (Fig. 4). The voltage output can be greater than, less than, or equal to that at its input. The duty cycle of the control transistor controls its output. The SEPIC also is capable of true shutdown. When the switch is turned off, its output drops to 0 V.

4. The generic SEPIC configuration also can provide voltages above or below the input. The duty cycle of the control switch controls this configuration.
4. The generic SEPIC configuration also can provide voltages above or below the input. The duty cycle of the control switch controls this configuration.

The CUK converter’s output voltage can be greater than or less than the input voltage magnitude (Fig. 5). It uses a capacitor as its main energy-storage component. By using inductors on the input and output, the CUK converter produces very little input and output current ripple. And, it has minimized electromagnetic interference (EMI) radiation.

5. The generic CUK configuration can output a voltage that is either greater or less than the input voltage magnitude.
5. The generic CUK configuration can output a voltage that is either greater or less than the input voltage magnitude.

Also known as a charge pump, the switched capacitor regulator uses capacitors as energy storage elements to create a higher or lower voltage (Fig. 6). It can generate arbitrary voltages, depending on the controller and circuit topology. Charge pumps can double, triple, halve, invert, or fractionally multiply or scale voltages such as x3/2, x4/3, and x2/3. It also can provide multiple outputs.

6. The generic switched capacitor converter uses capacitors as storage elements to generate other voltages.
6. The generic switched capacitor converter uses capacitors as storage elements to generate other voltages.

The flyback converter is the most versatile of all the topologies (Fig. 7). It allows for one or more output voltages, some of which may be opposite in polarity. Additionally, it is very popular in battery-powered systems. It provides isolation as well.

7. The generic flyback configuration is similar to a buck-boost converter with the inductor replaced by a transformer. The energy is temporarily stored in a magnetic field in the inductor air gap before it is transferred to the secondary side.
7. The generic flyback configuration is similar to a buck-boost converter with the inductor replaced by a transformer. The energy is temporarily stored in a magnetic field in the inductor air gap before it is transferred to the secondary side.

The forward converter is a buck regulator with a transformer inserted between the buck switch and the load (Fig. 8). It provides both higher and lower voltage outputs as well as isolation. It also might be more energy efficient than a flyback converter.

8. In the generic forward configuration, the energy is transferred directly between the primary and secondary sides.
8. In the generic forward configuration, the energy is transferred directly between the primary and secondary sides.

The push-pull converter is a forward converter with two primaries (Fig. 9). It can generate multiple output voltages, some of which may be negative in polarity. It provides isolation as well. However, it requires very good matching of the switch transistors to prevent unequal ON times.

9. The pairs of switches (transistors) in a generic symmetrical push-pull circuit help to maintain a steadier input current and create less noise on the input line.
9. The pairs of switches (transistors) in a generic symmetrical push-pull circuit help to maintain a steadier input current and create less noise on the input line.

The half-bridge converter is usually operated directly from the ac line (Fig. 10). The switch transistor drive circuitry must be isolated from the transistors, requiring the use of base drive transformers.

10. The primary-side capacitors in a generic half-bridge configuration are used to produce a constant half voltage at their junction, reducing the stress on the switches to only the input voltage.
10. The primary-side capacitors in a generic half-bridge configuration are used to produce a constant half voltage at their junction, reducing the stress on the switches to only the input voltage.

The full-bridge converter provides isolation from the ac line (Fig. 11). The pulse-width modulation (PWM) control circuitry is referenced to the output ground, requiring a dedicated voltage rail to run the control circuits. The base drive voltages for the switch transistors have to be transformer-coupled because of the required isolation.

11. Only the diagonal switches in the generic full-bridge configuration are switched ON simultaneously. This provides full input voltage across the primary winding of the transformer. The polarity of the transformer reverses in each half cycle.
11. Only the diagonal switches in the generic full-bridge configuration are switched ON simultaneously. This provides full input voltage across the primary winding of the transformer. The polarity of the transformer reverses in each half cycle.

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