Most electronic systems rely on regulators to deliver stable direct current (DC) levels. This article explains how regulators operate and reviews the two primary types of regulators used for DC-to-DC conversion: switching and linear. It also compares their advantages and limitations, highlighting common use cases and applications for each.
Regulating and refining DC voltage levels
Electronic systems require specific DC levels for safe and efficient operation. Initially, rectifiers convert alternating current (AC) from the main power supply into DC. DC-TO-DC converters (Figure 1) then refine this output, eliminating fluctuations and delivering precise, targeted DC voltage levels. These converters also adjust voltages from sources such as solar panels, battery banks, and vehicle alternators to align with pre-determined voltage parameters.
Linear and switching regulators facilitate DC-to-DC conversion for various functions, such as step-down (buck), step-up (boost), and step-up/step-down (buck-boost). Linear regulators provide a simple yet limited method for minor voltage reductions (buck), delivering stable output with minimal noise. These regulators don’t support boost or buck-boost configurations due to their lower efficiency, inherent voltage drops, and current output limitations. In contrast, switching regulators offer more flexibility and higher efficiency, accommodating both isolated and non-isolated DC-to-DC conversions, from significant voltage reductions (buck) to boost and buck-boost.
The FETs of switching regulators
Switching regulators use power switches, inductors, and diodes to transfer energy from input to output and regulate voltage. They achieve voltage regulation through the rapid on-and-off toggling of power switches, typically field effect transistors (FETs), which are managed by switching controller integrated circuits (ICs). One key method for regulating these FETs involves pulse width modulation (PWM), where the duration of the ‘on’ state is adjusted to regulate energy transferred to the load.
Within feedback control loops, these ICs, alongside resistors, capacitors, and voltage/current sensors, continuously adjust switch states and PWM duty cycles, ensuring stable and precise voltage output.
Switching regulators can incorporate FETs as discrete components outside the controller — or integrate them in the same IC (Figure 2). The latter configuration broadens applicability across various devices and functions, such as buck and boost converters, inverters, and flyback transformers. IC integration also streamlines design, enabling compact form factors while improving performance and efficiency.
Indeed, switching regulators generally achieve efficiency rates exceeding 90% by using transistors as binary switches, set fully on or off. This configuration reduces voltage drops across the switch (when on) and limits current flow (when off), effectively minimizing power loss. Additionally, switching regulators significantly lower thermal energy dissipation for low input-to-output voltage differentials and moderate load currents.
The non-switching techniques of linear regulators
Linear regulators (Figure 3) precisely adjust voltage and current flow using transistors in the input DC path. Feedback loops control the transistor’s conductive state, effectively stabilizing output voltage despite variations in input voltage or load.
Linear regulators, however, incur power losses, with the transistor dissipating energy as heat when bridging input and output voltage gaps. Inefficiencies are most pronounced in high-power applications, such as powering a 5 V, 10 A load from a 10 V DC source, which results in a 50W loss and 50% conversion rate. To operate effectively, the input voltage must surpass the output voltage by a minimum gap. This difference is known as the dropout voltage.
Notably, low-dropout (LDO) linear regulators increase efficiency in scenarios with minimal input-output voltage differences. Using a feedback mechanism, LDOs adjust their internal resistance to precisely control output voltage — ensuring minimal dropout even when supply voltage marginally exceeds output voltage. LDOs are ideal for applications that require stable regulation and closely matched voltage levels.
Benefits and limitations of switching regulators
Versatile and efficient, switching regulators accommodate both isolated and non-isolated DC-to-DC conversions, from significant voltage reductions (buck) to boost and buck-boost, encompassing flyback, forward converter configurations, and single-ended primary-inductor converters (SEPICs). Effectively minimizing power loss and thermal energy dissipation, switching regulators leverage low quiescent currents (IQs) to maintain operation without load. Switching regulators are implemented in integrated solutions — incorporating FETs directly inside an IC — or in modular designs that use external power metal-oxide-semiconductor field-effect transistors (MOSFETs).
Smartphones, smartwatches, and other advanced wearables rely on switching regulators to extend battery life with optimized power management, low IQs, and precise voltage regulation. Switching regulators play a similarly crucial role in solar inverters and battery management systems (BMS) that require efficient voltage conversion. Additionally, switching regulators in automotive DC-to-DC converters stabilize voltage for advanced vehicle systems, compensating for fluctuations and ensuring consistent power flow. In industrial automation, switching regulators deliver reliable, precise voltage levels to advanced sensors and controllers.
Although versatile and efficient, switching regulators aren’t generally used in sensitive analog circuits or audio applications that demand ultra-low noise and ripple. This is because switching action typically introduces electronic noise and affects performance. Linear regulators, with inherently basic designs and minimal electrical noise, are considered a better fit for these use cases.
Why use a linear regulator?
Linear regulators offer a simple, cost-effective solution for minor voltage drops where efficiency loss is minimal. Effectively dissipating excess voltage as heat, they require only basic connections such as input voltage (VIN), output voltage (VOUT), feedback (FB), and (optionally) ground (GND).
Large voltage differences between input and output significantly increase power losses in linear regulators, reducing their efficiency. Although LDOs function efficiently when voltage differences between input and output are minimal, they can’t boost output voltage above input, invert voltage polarity, and are limited to step-down conversion.
Despite their constraints, linear regulators are a popular choice for analog circuits and audio equipment that demand low noise, rapid transient response, and minimal dropout. Simple, cost-effective linear regulators also target low-power devices without external inductors and minimal thermals, such as basic digital watches, LED drivers, and low-power analog sensor modules used in precision measurement and monitoring.
For communication infrastructure and radio devices, linear regulators ensure minimal supply noise and low output voltage ripple. Additionally, internal feedback loops facilitate rapid responses to load or input voltage alterations without external compensation.
Summary
Switching regulators provide versatility and efficiency for many DC-to-DC conversion configurations, encompassing buck, boost, and buck-boost. In contrast, low-noise linear regulators offer a more limited solution for minor voltage drops where efficiency loss is minimal. Linear regulators are a popular choice for analog circuits, LED drivers, and audio or communications equipment. Switching regulators target a wider range of applications, from sophisticated wearables and automotive systems to industrial automation, solar inverters, and battery management systems.
References
Switching Regulator, Analog Devices
Using a Switching Regulator vs. Linear Regulator for DC-DC Conversion, Cadence
Advantages of Switching Regulators Over Linear Regulation, Analog Devices
Linear vs. Switching Regulators, Renesas
AN-140: Basic Concepts of Linear Regulator and Switching Mode Power Supplies, Analog Devices
