AC/DC, DC/DC Converter Fundamental Guide

First, let's go over the concepts of AC  (alternating current) and  DC  (direct current).

What is AC?

AC stands for Alternating Current.

AC is an electrical current whose magnitude and polarity (direction) change repeatedly over time.

The frequency, measured in  Hz, indicates how many times the current polarity reverses in one second.

What is DC?

DC stands for Direct Current.

The polarity (direction) of a  DC current remains constant over time.

① A DC current is defined as one whose flow polarity (direction) and magnitude do not fluctuate over time.

② While the flow polarity doesn't change, a current whose amplitude varies is also considered DC and is known as ripple current.

Ⅰ. AC/DC Converter

What is an AC/DC Converter?

An AC/DC converter is a device that transforms AC  (alternating voltage) into DC (direct current voltage).

Why is an AC/DC Converter Necessary?

Household and building power supplies typically provide 100V or 200V AC voltage.

However, most electronic equipment operates on DC voltages, such as 5V or 3.3V.

Therefore, the AC  voltage must be converted to DC voltage for these appliances to function.

Some devices, like motors and light bulbs, can operate directly on  AC voltage. However, with the increasing integration of microcontrollers for motor control and the prevalence of energy-saving LED lighting, AC/DC conversion is often still required.

Why is AC Voltage Transmitted?

One might wonder, "If appliances use DC, why not transmit DC in the first place?"

Electricity is generated at power plants, which are often located in remote areas like mountains or coastlines. Transmitting  AC voltage over long distances to urban centers is more advantageous.

Specifically, transmitting AC power at high voltage and low current significantly reduces transmission losses (energy loss).

Since high voltage cannot be used directly in homes, it is stepped down in stages through multiple substations before being converted to 100V or 200V for distribution. This multi-stage voltage transformation process relies on AC, hence the transmission of AC voltage.

Full-wave Rectification and Half-wave Rectification (AC/DC Conversion)

When converting AC (alternating voltage) to DC, two primary methods are used: full-wave rectification and half-wave rectification. Both methods utilize the forward current flow characteristic of a diode.

Full-wave rectification converts the negative voltage component of the input into a positive voltage and then rectifies it into a DC voltage using a diode bridge circuit. Half-wave rectification, on the other hand, eliminates the negative voltage component of the input, rectifying it into a DC voltage.

A capacitor then smooths the resulting waveform through its charging and discharging action, producing a pure DC voltage.

Consequently, full-wave rectification, which utilizes both positive and negative input voltage components, is a more efficient rectification method than half-wave rectification.

Furthermore, the smoothed ripple voltage is influenced by capacitor capacity and load.

Under identical capacitor capacity and load conditions, the ripple voltage from full-wave rectification is lower than that from half-wave rectification, indicating higher stability and better performance.

AC/DC Conversion Methods

AC/DC conversion can be achieved using a transformer method or a switching method.

Transformer Method

This method employs a transformer-type circuit structure, typical of ordinary AC/DC  converters.

[Example of the circuit configuration of the transformer method]

In the transformer method, the AC voltage is first stepped down to a suitable level using a transformer (e.g., from AC100V to AC10V). The winding ratio of the transformer determines this step-down value.

The stepped-down AC voltage is then full-wave rectified by a diode bridge rectifier, converting it into a pulse voltage.

Finally, a capacitor smooths the voltage, outputting a DC voltage with minimal ripple. This is the most common AC/DC conversion method.

[Transformation of the waveform of the transformer method]

Switch Mode

This method utilizes a switching circuit structure for AC/DC conversion.

[Example of circuit configuration of switching method]

In switching mode, the AC voltage is directly rectified by a diode bridge rectifier. Since most residential power is AC100V or AC200V, the diode bridge rectifier must be capable of handling high voltages.

A capacitor then smooths the resulting DC voltage (pulse voltage). High-voltage-resistant capacitors are also required.

The DC voltage is then chopped (switched ON/OFF) by a switching element, and the voltage is stepped down and transferred to the secondary side via a high-frequency transformer. The waveform at this point transforms into a square wave.

Switching elements operate at higher frequencies (e.g., 100kHz) compared to household frequencies (50/60Hz). This high-frequency operation allows for miniaturization and weight reduction of the transformer, which is a significant advantage.

[Waveform transition of switching method]

On the secondary side, a rectifier diode half-wave rectifies the square wave, which is then smoothed by a capacitor to output a DC voltage.

The switching method employs a control circuit to regulate the switching element and produce a stable DC output (e.g., DC12V).

Compared to the transformer approach, the switching technique involves switching elements and control circuits, resulting in a more complex circuit topology. However, the use of a smaller transformer with high-frequency control contributes to equipment miniaturization, a key benefit.

Feedback Control

What is Feedback Control?

In a switching AC/DC converter, feedback control validates the actual output DC voltage and regulates the switching element accordingly to ensure the desired DC output is reliably achieved. This method confirms the output voltage to control the switching element (FB control).

Example of Switching Circuit Structure

Schematic diagram of feedback control.

A switching AC/DC converter rectifies AC electricity to DC voltage using a diode bridge and smooths it with a capacitor. The DC voltage is then chopped (ON/OFF) by a switching element, stepped down by a high-frequency transformer to the secondary side, and smoothed by a capacitor to provide a specified DC value (VDC).

The FB control circuit determines if the actual output voltage value matches or exceeds the target voltage value.

Schematic Diagram of Output Voltage After Smoothing

The switching element's ON time is adjusted to be longer when the actual output voltage is lower than the target voltage, causing the output voltage to increase. Conversely, when the voltage is higher than the target, the control ON time is reduced.

This feedback control circuit continuously monitors the actual output voltage and adjusts the switching element's ON/OFF duration to maintain the target output voltage's stability.

Light Load Mode

What is Light Load Mode?

Light load mode is a strategy to increase efficiency when the output current is low. It is also known as burst mode in DC/DC converters and other electronic devices.

Switching AC/DC and DC/DC converters use ON/OFF switching to achieve voltage chopping and capacitor smoothing, providing a stable output voltage.

However, this switching generates a brief leakage current (through current) during each ON/OFF cycle. The more ON/OFF transitions per unit time, the greater the loss from leakage current and the lower the efficiency.

While Pulse Width Modulation (PWM) control maintains a constant switching frequency and adjusts the ON/OFF time ratio, the number of transitions per unit time remains constant. This leads to constant self-dissipated power and degraded efficiency at light loads due to switching leakage current losses. To address this, Pulse Frequency Modulation (PFM) is employed at light loads to extend and slow down the cycle, reducing the number of ON/OFF transitions per unit time and minimizing losses. This technique is known as light load mode.

PWM Method and PFM Method

The efficiency can be further improved by using PWM and  PFM depending on the situation. For heavy loads (high current usage), constant-frequency PWM control is used, while for light loads (low current usage), variable-frequency PFM control is employed.

■PWM (Pulse Width Modulation): The frequency is constant, and the output control mode is triggered by the input voltage.

■PFM (Pulse Frequency Modulation): This method controls the output component by varying the frequency while keeping the ON time constant (changing the OFF time). Alternatively, the OFF time can be fixed while changing the ON time. Both PWM and PFM are power management systems.

The PFM method offers high efficiency by adjusting the frequency based on the output current, but it can produce intermittent noise during switching, making it difficult to eliminate noise with an unpredictable frequency. Using a PWM approach with a constant frequency simplifies noise management.

Therefore, low-noise PWM and high-efficiency PFM can complement each other. PWM is used for high-frequency driving with high loads (resulting in higher noise), while PFM is used for low loads and lower current consumption to boost productivity.

Ⅱ. DC/DC Converter

What is a DC/DC Converter?

A DC/DC converter is a device that converts direct current (DC) to direct current (DC), specifically by adjusting the voltage level. Electronic components, such as  ICs, often require specific voltage ranges to operate correctly, necessitating voltage conversion.

A "buck converter" reduces the voltage, while a "boost converter" increases it.

Terminology Description

A device that converts direct current to direct current is known as a DC/DC converter.

Following the conversion process, it is commonly referred to as a linear regulator, switching regulator, or other related terms.

Step-down power supply units: Buck converters, Step-down converters.

Voltage boosting power supply: Boost converter, Step-up converter.

Buck-Boost Power Supply: Buck-Boost Converter.

Power supply units that generate negative voltages: Negative voltage converters, inverting converters.

Why is a DC/DC Converter Needed?

An "AC/DC converter," which transforms AC (alternating current) from a wall outlet (e.g., 100V) to DC, is required for devices plugged into an outlet.

This is because most semiconductor components can only operate with DC.

The integrated circuits (ICs) within electronic devices operate at diverse voltage ranges and have specific voltage accuracy requirements.

Supplying an inconsistent voltage can lead to malfunctions or degradation of the device's performance.

Therefore, a "DC/DC converter" is used to convert and stabilize the voltage to the required level.

A voltage regulator is a device that utilizes a DC/DC converter to achieve voltage stabilization.

Ⅲ. Types of Power ICs

Linear regulators and switching regulators are the two primary categories of power ICs.

Linear regulators can only step down output voltages to be lower than the input voltage.

Switching regulators offer four output types, providing greater flexibility:

・Step-down: Output voltage lower than input voltage.

・Boost: Output voltage higher than input voltage.

・Buck-Boost: Output a constant voltage, regardless of the input voltage.

・Reverse: Convert a positive voltage to a negative voltage.

Additionally, switching regulators employ synchronous rectification and asynchronous rectification (diode rectification) methods.

Types of Power ICs

Linear and Switching Regulators

A voltage regulator is a device that uses a DC/DC converter to stabilize voltage.

Voltage regulators are classified into two types based on their conversion method: linear regulators and switching regulators.

Linear Regulator

It is called a "linear regulator" because the relationship between input and output is linear during operation.

As a control element is connected in series between the input and output, it is also referred to as a "series regulator."

Since the control element steps down the voltage, a larger voltage differential between the input and output (greater step-down) results in higher loss and lower efficiency.

Therefore, linear regulators are best suited for low-power applications.

Advantages: Simple circuit, fewer external components, low noise.

Disadvantages: Low efficiency, significant heat generation, only buck conversion capability.

Switching Regulator

In a switching regulator, a switching element (like a MOSFET) is turned ON, allowing power to flow from the input to the output until the output voltage reaches the desired level.

The switching element is then turned OFF, and input power is no longer supplied.

This process is repeated rapidly to adjust the output voltage to a specified value.

Advantages: High efficiency, low heat generation, capable of boost, buck, and negative voltage conversion.

Disadvantages: Requires many external components, design complexity, significant noise.

How Linear Regulators Work

General Pin Configuration

A linear regulator typically has three pins: VIN (input), VO (output), and GND (ground).

Linear regulators with adjustable output also include an FB (feedback pin) for monitoring the output voltage.

Essentially, a voltage stabilizer with a built-in voltage-variable external resistor is known as a fixed-output type.

The internal circuit of a linear regulator is shown in the diagram below.

Internal Circuit

Its operation is analogous to an inverting amplifier circuit. Because the non-inverting pin (FB) of the error amplifier maintains a voltage equal to the reference voltage (VREF), the output voltage value (VO) is determined by the ratio of the two resistors' resistance values (R1 and R2).

VREF = [(R1+R2) / R2] * VO

The output transistor in the diagram is a MOSFET, although bipolar transistors are also used in some products.

Classification of Linear Regulators

Classification by Function

Linear regulators are categorized into positive voltage and negative voltage types based on their function.

Some circuits require a negative power supply, even if they don't use a positive one.

If the power supply is only on the positive side, it cannot handle voltages below ground. By connecting the control transistor to the negative output line, a negative voltage can be achieved.

Classified by Function

They can also be classified into constant voltage and variable voltage types. The fixed type typically has three pins: input, output, and GND, with an integrated resistor for setting the output voltage.

Variable types with a GND reference often have a feedback pin, increasing the pin count to four. Some variable types operate in a floating configuration without a GND pin, in which case they have three pins.

Both the fixed and variable types are further divided into standard and LDO (Low Dropout) categories.

LDO stands for Low Dropout, referring to a linear regulator that operates with a small potential difference between input and output. Standard types require a minimum input-output difference of approximately 2V, whereas LDO can operate with less than 1V.

Ⅳ. What is LDO?

LDO (Low Dropout) is a type of linear regulator that can operate effectively even with a small potential difference between its input and output.

It is also known as a low-saturation linear regulator or a low-loss linear regulator.

There isn't a strict numerical definition for the input-output potential difference of an LDO. Generally, it refers to a voltage stabilizer that can maintain stable operation with a potential difference of less than 1V.

For instance, a standard regulator might not be able to provide a 3.3V supply from a 5V input efficiently. An LDO with a low input-output voltage difference is necessary for ICs requiring a 3.3V supply in such scenarios.

This allows the LDO to use a lower input voltage while still producing the same output voltage as a traditional regulator.

Operating at a low potential difference reduces energy loss and heat dissipation.

Voltage Drop

A transistor is placed between VIN and VO within a linear regulator. The voltage drop is the minimum potential difference between the input and output required for the transistor to operate reliably.

If the voltage difference between the input and output falls below this voltage drop, the transistor struggles to maintain stable operation, and the output voltage may drop.

In this context, the minimum required input voltage for an LDO linear regulator to function is set by (VO + voltage drop). This represents the regulator's minimum operating voltage.

If the input voltage (VIN) falls below this minimum operating voltage, the output voltage cannot be reliably maintained.

Summary: This article provides a fundamental guide to AC/DC and DC/DC converters, explaining the concepts of AC and DC current, the necessity of voltage conversion in electronics, and the different methods of AC/DC conversion like transformer and switch modes. It also details types of power ICs, including linear and switching regulators, and introduces Low Dropout (LDO) regulators as a specialized type of linear regulator.

Frequently Asked Questions

What is the primary difference between AC and DC current?

AC (Alternating Current) has a magnitude and polarity that change repeatedly over time, measured in Hertz (Hz). DC (Direct Current) has a constant polarity and magnitude over time, with no fluctuation in direction.

Why is AC voltage preferred for long-distance power transmission?

AC voltage is preferred for long-distance transmission because it can be easily stepped up to high voltages and low currents, significantly reducing energy loss during transmission compared to DC.

What are the two main methods for AC/DC conversion?

The two main methods for AC/DC conversion are the transformer method, which uses a transformer to step down voltage before rectification, and the switch mode method, which directly rectifies the AC voltage and uses a high-frequency transformer for voltage step-down.

What is the main advantage of a switching regulator over a linear regulator?

Switching regulators offer higher efficiency and generate less heat compared to linear regulators, making them suitable for a wider range of power applications. They can also perform boost, buck, and negative voltage conversions.

What defines a Low Dropout (LDO) regulator?

An LDO regulator is a type of linear regulator that can operate effectively with a very small potential difference between its input and output voltage, typically less than 1V, minimizing energy loss and heat.