Sep 14 2020

# LM2596 Circuit: Regulated Power Supply

## I Introduction

### 1.1 What is LM2596?

The LM2596 series of regulators are monolithic integrated circuits. These circuits can provide all the active functions for a step-down (buck) switching regulator, capable of driving a 3-A load with excellent line and load regulation. These devices are also available in fixed output voltages of 3.3 V, 5 V, 12 V, and an adjustable output version.

### 1.2 What is LM2577?

The LM2577 is monolithic integrated circuit that provide all of the power and control functions for step-up (boost), flyback, and forward converter switching regulators. The device is available in three different output voltage versions: 12V, 15V, and adjustable.

Figure 1. LM2596

Catalog

## II Circuit Structure

### 2.1 Power Transformer

We need DC power. By adopting a step-down transformer, the grid AC voltage 220V is transformed into a composite AC power source. After this AC voltage is rectified, the DC voltage required by the electronic device can be obtained.

### 2.2 Rectifier Circuit

Use a single-phase bridge rectifier circuit. By using this circuit, it is possible to convert 50 Hz alternating current with varying direction and magnitude. Here, we need to transform it into direct current in the same direction but still pulsating in magnitude. So what are the benefits of doing this? They are as follows:

• Higher voltage;
• Smaller ripple voltage;
• Make the maximum reverse AC current that the rectifier diode bears flow;
• High utilization rate of the transformer.

### 2.3 Filter Circuit

We can use the properties of the capacitor C (the energy storage element). That is, the voltage across the capacitor cannot change suddenly. In this way, by using an RC filter circuit to filter out most of the pulsating components output by the rectifier circuit, we can get a relatively smooth direct current.

### 2.4 Voltage Regulator Circuit

The rectified and filtered DC voltage does not change with the disturbance of the AC power grid and load. Here, due to the adoption of a large number of high-performance integrated modules, the circuit structure is simplified. The result of this is that, the key part of the power conversion problem is highlighted. Through hard debugging and testing, the overall circuit performance is good and the design purpose can be better achieved.

As a variable voltage stabilizer or current stabilizer source, this power supply can not only be used alone, but also can be placed in other electronic equipment.

## III Power Supply Overview

### 3.1 Introduction to Power Supply

The power supply is the heart of electronic equipment. The quality of the power supply directly affects the reliability of electronic equipment. Since 60% of the failures of electronic equipment come from the power supply, as the basic component of the electronic equipment, the power supply has received more and more attention.

The power supplies used in modern electronic equipment roughly fall into 2 categories: linear regulated power supplies and switching regulated power supplies.

### 3.2 Comparison of 2 Types of Power Supplies

3.2.1 Linear Regulated Power Supply

The so-called linear regulated power supply has the following meaning. The regulator tube in the regulated power supply circuit works in the linear amplification area. After the 220V, 50Hz power frequency voltage is stepped down by a linear transformer, it is rectified, filtered, and stabilized to output a DC voltage.

• High power supply stability and load stability;
• Small output ripple voltage;
• Fast transient response speed;
• The line structure is simple and easy to maintain;
• No switch interference.
• High power consumption and low efficiency, the efficiency is generally only 35~60%;
• Big volume, heavyweight, and cannot be miniaturized;
• Must have a larger capacity filter capacitor.

Among them, the low switching efficiency is an important shortcoming, causing a serious waste of resources. In this context, the switching power supply came into being. Any electronic device needs a DC power supply to supply the circuit to work. Especially electronic products powered by the power grid.

Now that there are grid voltage fluctuations and changes in the working state of the circuit, we need to find ways to adapt. In terms of power supply, it is necessary to have a DC regulated power supply that adapts to this change.

With the development of electronic technology, people have entered in-depth research on how to improve the power conversion efficiency. Thus enhancing the adaptability to the power grid, reduce the volume, and reduce the weight. So the switching power supply came into being.

3.2.2 Switching Regulated Power Supply

• High exchange efficiency. The regulator tube of the switching regulated power supply works in the switch state. Its main advantage is that the exchange efficiency can be as high as 70-95%.
• Low power consumption and high efficiency. Under the excitation of the excitation signal, the transistors alternately work in the on-off switching state. At the same time, the switching speed is very fast, and the frequency is generally about 50kHz. The power consumption of the switching transistor is very small, and the efficiency can be improved to over 80%.
• Small size and lightweight. No bulky power frequency transformer is used in the switching power supply. After the power dissipation on the adjustment tube is greatly reduced, a larger heat sink is omitted.
• Wide voltage regulation range. The output voltage of the switching power supply is adjusted by the duty cycle of the excitation signal. The change of the input signal voltage can be controlled by frequency modulation or width modulation. When the power frequency grid voltage changes greatly, it can still ensure an effective and stable output voltage.

There are also many methods for switching regulated power supplies to achieve voltage regulation. According to actual application requirements, we can choose different types of switching regulated power supplies.

• The circuit is more complicated. The output ripple voltage is high and the transient response is poor. There is also more serious switching interference. Nowadays, difficulties in further promotion and application of switching regulated power supplies are as follows: First, the production technology is difficult; Second, the maintenance is troublesome; Last, the cost is high.
• High requirements for circuit components. The efficiency of the switching power supply is proportional to the switching speed of the switching tube. A switching transformer is used in the switching power supply. The switching transformer can be input by one group, and get multiple outputs with different polarity and size. To further improve efficiency, the operating frequency of the power supply must be increased. However, when the frequency is increased, the requirements would have been further improved.
• Affect the normal operation of nearby electronic equipment. The stabilized power supply has the dual functions of voltage stabilization and filtering. Therefore, the series linear regulated power supply does not produce switching interference. And the ripple voltage output can also be small. However, when the switch tube in the switching regulated power supply is in the on-off state, things have changed. Its alternating voltage and output current will produce strong spike interference and resonance interference through the components. These interferences will enter the mains grid and affect the normal operation of nearby electronic devices.

### 3.3 The Power Supply Selection

According to the comparison, we choose a linear power supply.

Most electronic circuits require a DC power source to work. The DC power supply is the guarantee of the working state of electronic circuits and the provider of energy. But most fixed power supplies allow the output voltage to vary within ±10%, which still cannot meet the requirements of some circuits. So we designed a power supply with adjustable output or allowing a larger range of variation.

The performance of this power supply: On the one hand, it is designed and made with all integrated circuits and has the function of automatic short-circuit and overload protection. On the other hand, it has high accuracy, stable current performance, and continuous adjustment. Besides, it can be used for multiple experimental power.

## IV Performance Indicators

### 4.1 Working Mode

The output voltage can be adjusted and changed throughout the current range. This power supply can also work in a DC source mode within a certain range.

Let's introduce the work produced by a netizen.

First, the 18V AC output from the transformer is converted into 20V DC through a rectifier filter circuit. As shown below.

Figure 2. Rectifier Filter Circuit

And then the adjustable step-down regulator circuit composed of LM2596-ADJ outputs a voltage of 1.25 to 19V. The circuit is shown in the figure below. Adjusting RW1 can change the output voltage, and increasing the input voltage can increase the output. The maximum supply voltage is 45V.

Figure 3. LM2596 Adjustable Step-down Regulator Circuit

Then through the step-up voltage regulator circuit composed of LM2577, the output voltage of LM2596 is increased by 1 to 11 times. That is to say, if RW1 is adjusted to make LM2596 output 3V, RW2 in LM2577 is adjusted to output voltage of 3 to 33V. The figure of the circuit is shown below. Changing the resistance of RW2 and the input voltage of LM2577 can output a voltage of 3 to 100V.

Figure 4. LM1577 Step-up Voltage Regulator Circuit

Under the condition of the input voltage of 220V, 50HZ and the voltage range of +15%～-20% of the modified regulated power supply:

• The adjustable range of output voltage is +1.25V～input voltage;
• The maximum output current is 5A;
• The voltage adjustment rate is not more than 0.5%;
• The highest input voltage and output ripple voltage at full load are not more than 10mV.

DC-DC converter, under the condition of input voltage +2.6V～+18V:

• The output voltage is adjustable from +2.6V to 100V;
• The maximum output current is 3A;
• Voltage regulation rate is not more than 1.5%, the load regulation rate is not more than 3%;
• The power supply also has over-current and short-circuit protection functions. After eliminating the short-circuit fault, it will automatically return to the normal state. Anti-reverse connection, diodes form a voltage indicator.

### 4.2 PCB and Printed Circuit Board

The pictures below are the PCB drawn by a netizen and the printed circuit board.

Figure 5. PCB and Printed Circuit Board