Optimizing Switching Power Supply Design with Low Dropout Linear Regulators

破茧佼龙

Optimizing Switching Power Supply Design with Low Dropout Linear Regulators [Copy link]

Optimizing Switching Power Supply Design with Low Dropout Linear Regulators

Power supply is an indispensable component of various electronic devices. The quality of its performance is directly related to the technical indicators of electronic devices and whether they can work safely and reliably. Currently, the commonly used DC regulated power supplies are divided into two categories: linear power supplies and switching power supplies. Since the key components inside the switching power supply work in a high-frequency switching state, the energy consumed by the switching power supply itself is very low. The efficiency of the switching power supply can reach 80%~90%, which is nearly doubled that of the ordinary linear regulated power supply. It has become the mainstream product of regulated power supply. This article introduces a design scheme for optimizing the switching power supply using a low-dropout linear regulator (LDO), and verifies the feasibility of the scheme through experiments.

Basic Principles of LDO

The basic circuit of a low dropout linear regulator (LDO) is shown in Figure 1. The circuit consists of a series adjustment tube VT _, sampling resistors R1 _and R2 _, and a comparison amplifier A.

Figure 1: Basic low-dropout linear regulator circuit.

The sampling voltage is applied to the non-inverting input of comparator A and compared with the reference voltage U _{ref applied to the inverting input. The difference between the two is amplified by amplifier A to control the voltage drop of the series adjustment tube, thereby stabilizing the output voltage. When the output voltage U out} decreases, the difference between the reference voltage and the sampling voltage increases, the drive current output by the comparison amplifier increases, and the voltage drop of the series adjustment tube decreases, thereby increasing the output voltage. On the contrary, if the output voltage U _out exceeds the required set value, the front drive current output by the comparison amplifier decreases, thereby reducing the output voltage. During the power supply process, the output voltage correction is carried out continuously, and the adjustment time is only limited by the reaction speed of the comparison amplifier and the output transistor circuit.

It should be noted that the actual linear regulator should also have many other functions, such as load short-circuit protection, overvoltage shutdown, overtemperature shutdown, reverse connection protection, etc., and the series adjustment tube can also use MOSFET.

LDO selection principles

1. Input-output voltage difference

The input-output voltage difference is the most important parameter of a low-dropout linear regulator. Under the premise of ensuring the stability of the output voltage, the lower the voltage difference, the better the performance of the linear regulator. For example, a 5.0V low-dropout linear regulator can stabilize the output voltage at 5.0V as long as the input is 5.5V.

2. Maximum output current

The power of electrical equipment is different, and the maximum current required by the voltage stabilizer is also different. Generally, the voltage stabilizer with a larger output current has a higher cost. In order to reduce costs, in a power supply system composed of multiple voltage stabilizers, the appropriate voltage stabilizer should be selected according to the current value required by each part.

3. Load Regulation

Load regulation is a very important parameter for many power supply devices. It reflects the ability of the power supply to suppress load interference. The lower the load regulation, the smaller the impact of the output load on the output voltage, and the better the quality of the LDO.

4. Ground current

The ground current I _GND refers to the working current of the voltage regulator provided by the input power supply when the output current of the series adjustment tube is zero. This current is sometimes also called the quiescent current, but this customary name is incorrect when a PNP transistor is used as a series adjustment element. Usually, the ground current of an ideal low-dropout linear regulator is very small.

Figure 2: LDO applied to switching power supply principle.

5. Output capacitor

Typical LDOs require the addition of external input and output capacitors. Using larger capacitors with lower ESR generally improves the overall power supply rejection ratio (PSRR), noise, and transient performance.

Ceramic capacitors are usually preferred because they are low-priced and their failure mode is open circuit, while tantalum capacitors are expensive and their failure mode is short circuit. The equivalent series resistance (ESR) of the output capacitor affects its stability. Ceramic capacitors have lower ESR, which is on the order of 10 mΩ, while tantalum capacitors have ESR on the order of 100 mΩ. In addition, the ESR of many tantalum capacitors varies greatly with temperature, which can adversely affect LDO performance. The specific application of the capacitors needs to be consulted with the LDO manufacturer to ensure proper implementation.

6. Encapsulation

When selecting LDO products, the heat dissipation of the LDO should be considered. For LDOs with large loads, large packages should be selected as much as possible, which is conducive to the stability of LDO performance.

LDO applied in switching power supply design

Following the above principles, this article selects the SG2002 and SG2212 series LDOs produced by Harbin Shengbang Microelectronics Co., Ltd.

The circuit of applying LDO to switching power supply is shown in Figure 2. The dotted line part in the figure is the circuit commonly used in switching power supply.

The circuit can provide +6V/1.5A output voltage/current to LDO. The power supply uses SG2002-5.0XN5/TR, SG2012-3.3XKC3/TR, SG2012-2.5XKC3/TR and SG2012-1.8XKC3/TR to generate +5.0V/0.3A, 3.3/0.4A, 2.5V/0.4A and 1.8V/0.4A voltage/current respectively. The input and output terminals of the LDO chip in the figure are connected with 1uF ceramic capacitors to improve the stability of the LDO. A 0.01uF ceramic capacitor is connected to the BP terminal of each LDO to effectively reduce the output noise of the LDO.

The role of LDO in switching power supply

1. Simplify switching power supply design

The multi-channel output of the switching power supply is generally achieved by adding a high-frequency transformer feedback terminal, which increases the designer's workload during the design process of the switching power supply. Using LDO as the output terminal of the switching power supply can greatly simplify the design of the switching power supply and shorten the development cycle.

2. Improve the load regulation of the switching power supply

LDO is a special chip used to stabilize the power supply voltage. Currently, many companies have designed LDOs with very low load regulation. Applying LDO can significantly reduce the load regulation of switching power supplies.

3. Effectively filter out electromagnetic interference from the switching power supply and reduce ripple output

The prominent disadvantage of the switching power supply is that it generates strong EMI. EMI signals have a wide frequency range and a certain amplitude. Through conduction and radiation, they will pollute the electromagnetic environment and interfere with communication equipment and electronic products. If not handled properly, the switching power supply itself will become a source of interference. LDO has a high power supply rejection ratio and is a low-noise device. Therefore, the application of LDO can effectively filter out the EMI of the switching power supply and reduce the ripple output.

4. Provide overcurrent protection for switching power supply

Although many PWM control chips have overcurrent protection functions themselves, the overcurrent protection function of LDO can improve the safety factor of the switching power supply.

Experimental analysis

Figure 3: Switching power supply load regulation test circuit

The feasibility of this scheme is verified by the following two experiments:

1. Measure load regulation

The experimental circuit is shown in Figure 3. The electronic load pulls out currents from 0mA to 400mA in sequence, and the output voltage of the switching power supply is recorded at each load point. After the test data is processed, the chart shown in Figure 4 can be obtained. This chart fully demonstrates that the excellent load regulation rate of LDO has been completely transplanted to the switching power supply. In other words, LDO greatly improves the load regulation rate of the switching power supply.

2. Output ripple measurement

Connecting an oscilloscope to the LDO input and output of the switching power supply, we can obtain the waveforms shown in Figure 5. Ch ¹ is the output waveform at the LDO input, and Ch ² is the output waveform at the LDO output, that is, the final output waveform of the switching power supply.

As can be seen from the above figure, LDO effectively filters out the EMI signal of the switching power supply. Compared with building a conventional EMI filter, applying LDO is simpler and more reliable.