Master these skills to easily operate DC-DC circuits

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Master these skills to easily operate DC-DC circuits [Copy link]

DC-DC can always be seen in electronic products. Today, I will share some relevant knowledge points about DC-DC.

Concept and characteristics

DC-DC refers to Direct Current. It is a device that converts electrical energy of one voltage value into electrical energy of another voltage value in a DC circuit.

For example, a converter can convert a DC voltage (5.0V) into another DC voltage (1.5V or 12.0V). We call this converter a DC-DC converter, or a switching power supply or a switching regulator.

A DC-DC converter is generally composed of a control chip, an inductor, a diode, a transistor, and a capacitor.

When discussing the performance of a DC-DC converter, it is not possible to judge its quality by focusing only on the control chip. The characteristics of the components in the peripheral circuit and the wiring method of the substrate can change the performance of the power supply circuit, so a comprehensive judgment should be made.

The use of DC-DC converters is conducive to simplifying power supply circuit design, shortening the development cycle, achieving optimal indicators, etc., and is widely used in power electronics, military industry, scientific research, industrial control equipment, communication equipment, instrumentation, switching equipment, access equipment, mobile communications, routers and other communication fields and industrial control, automotive electronics, aerospace and other fields.

With the characteristics of high reliability and easy system upgrade, the application of power modules is becoming more and more extensive. In addition, DC-DC converters are also widely used in mobile phones, MP3, digital cameras, portable media players and other products. In terms of circuit type classification, it belongs to the chopper circuit.

Its main feature is high efficiency: compared with the linear regulator LDO, high efficiency is a significant advantage of DC-DC. Usually the efficiency is above 70%, and the highest efficiency can reach above 95%. The second is the wide voltage range.

Modulation mode:

1. PFM (Pulse Frequency Modulation)

The switching pulse width is constant, and the output voltage is stabilized by changing the frequency of the pulse output. The PFM control type has the advantage of low power consumption even when used for a long time, especially when the load is small.

2. PWM (Pulse Width Modulation)

The switching pulse frequency is constant, and the output voltage is stabilized by changing the pulse output width. The PWM control type has high efficiency and good output voltage ripple and noise.

Generally speaking, the performance differences of DC-DC converters using PFM and PWM are as follows:

How to select the frequency of PWM and the duty cycle of PFM. The PWM/PFM conversion type implements PFM control when the load is small, and automatically switches to PWM control when the load is heavy.

Architecture Classification

There are three common DC-DC principle architectures:

1. Buck (step-down DC/DC converter)

2. Boost (step-up DC/DC converter)

3. Buck-Boost (step-up/step-down DC/DC converter)

Design skills and main technical parameters selection requirements

The DC-DC circuit design should at least consider the following conditions:

The range of external input power supply voltage and the size of output current.

The DC-DC output voltage, current, and maximum power of the system.

1. Input/output voltage

The recommended operating voltage range of the device should be followed and the actual voltage fluctuation range should be considered to ensure that it does not exceed the device specifications.

2. Output current

The continuous output current capability of the device is an important parameter. This parameter should be taken into consideration when selecting the device, and a certain margin should be reserved.

The selection of this parameter also needs to evaluate the instantaneous peak current and heat generation of the circuit, and be determined comprehensively to meet the derating requirements.

3. Ripple

Ripple is an important parameter to measure the output voltage fluctuation of the circuit. Pay attention to the light load and heavy load ripple. Generally, the light load ripple is larger. Pay attention to whether the light load ripple in nuclear power and other occasions exceeds the requirements. Test the load in various scenarios in actual tests. Usually use an oscilloscope with a bandwidth of 20M for testing.

4. Efficiency

We need to pay attention to both light load and heavy load. Light load will affect standby power, and heavy load will affect temperature rise. Usually, the efficiency of 10mA at 12V input and 5V output is generally above 80%.

5. Transient response

The transient response characteristic reflects whether the system can adjust in time to ensure the stability of the output voltage when the load changes dramatically. The output voltage fluctuation is required to be as small as possible, generally below 10% of the peak-to-peak value.

In practice, it is important to select the feedback capacitor according to the recommended value. Common values are 22pF to 120pF.

6. Switching frequency

The commonly used switching frequencies are mostly above 500kHz. There are also higher switching frequencies of 1.2M to 2M. Since the high frequency increases the switching loss, the IC heat dissipation design must be better, so it is mainly concentrated in 5V low-voltage input and small current products. The switching frequency is related to the selection of inductors and capacitors, and other issues such as EMC and noise under light load are also related to it.

7. Feedback reference voltage and accuracy

The feedback voltage should be compared with the internal reference voltage, and the external feedback voltage divider resistor should be used to output different voltages. The reference voltage of different products may be different, such as 0.6~0.8V. Please pay attention to adjusting the feedback resistor when replacing.

The feedback resistor should have a 1% accuracy and should be selected according to the manufacturer's recommendation. Generally, do not choose a resistor that is too large to avoid affecting stability.

The accuracy of the reference voltage affects the output accuracy. The common accuracy is below 2%, such as 1%~1.5%. The cost of products with high accuracy will be different. Choose according to your needs.

8. Linear stability and load stability

Linear stability reflects the stability of output voltage when input voltage changes. Load stability reflects the stability of output voltage when output load changes. Generally, 1% is required, and the maximum should not exceed 3%.

9. EN level

The high and low levels of EN must meet the device specification requirements. Some ICs cannot exceed a specific voltage range. When dividing the voltage with resistors, pay attention to timely shutdown and consider the maximum range of voltage fluctuations.

Due to the need for timing control, this pin will increase capacitance, and for level adjustment and shutdown discharge, a resistor to ground is also required.

10. Protection performance

There must be overcurrent protection OCP, overheat protection OTP, etc., and they must be able to self-recover after the protection conditions disappear.

11. Others

Requirements include soft start, thermal resistance and packaging, and the operating temperature range must be able to cover high and low temperatures.

General principles for device selection

· universality

High cost performance

Easy procurement and long life cycle

Compatible and replaceable

Resource conservation

Derating

Easy to produce and normalize

Requirements for peripheral device selection

1. Input capacitor

The withstand voltage and input ripple requirements must be met. Generally, the withstand voltage requirement is 1.5 to 2 times the input voltage. Note that the actual capacity of the ceramic capacitor will decrease with the bias of the DC voltage.

2. Output capacitor

It must meet the requirements of withstand voltage and output ripple. Generally, the withstand voltage requirement is 1.5~2 times

The relationship between ripple and capacitance:

3. BST capacitor

According to the recommended value in the specification, generally 0.1uF-1uF. The withstand voltage is generally higher than the input voltage.

4. Inductor

Different output voltages require different inductances; note that the temperature rise and saturation current must meet the margin requirements, generally more than 1.2 times the maximum current (or the saturation current of the inductor must be greater than the maximum output current + 0.5*inductor ripple current). Usually, the appropriate inductance value L is selected so that ΔIL accounts for 30% to 50% of the output current. Calculation formula:

5. VCC capacitor

Take the value as required by the specification, it cannot be reduced, nor too large, and pay attention to the voltage resistance.

6. Feedback capacitor

The values are determined according to the specifications. Different chips from different manufacturers have different values, and different output voltages will also have different requirements.

7. Feedback resistor and EN voltage divider resistor

The value must be taken according to the specification, with an accuracy of 1%.

PCB Design Requirements

The input capacitor is placed near the chip input Vin and power PGND to reduce the presence of parasitic inductance. Because the input current is discontinuous, the noise caused by parasitic inductance has an adverse effect on the chip's withstand voltage and logic units. Add vias to the capacitor ground to reduce impedance.

The power loop should be as short and thick as possible, with a small loop area and less noise radiation. SW is a noise source, so keep the area as small as possible while ensuring the current, and keep it away from sensitive and susceptible locations. For example, the inductor should be close to the SW pin and away from the feedback line. The output capacitor should be close to the inductor, and a ground via should be added to the ground terminal.

The VCC capacitor should be placed between the VCC pin of the chip and the signal ground of the chip, as close as possible to one layer without vias.

FB is the most sensitive and easily disturbed part of the chip, and is the most common cause of system instability.

The FB resistor connected to the FB pin should be as short as possible and placed close to the IC to reduce noise coupling; the voltage divider resistor under FB is usually connected to the signal ground AGND;

Keep away from noise sources, SW, inductor, diode (asynchronous buck); FB routing is wrapped with ground;

The FB of the current load should be taken at the far end of the load, and the feedback capacitor routing should be taken nearby.

The BST capacitor trace should be as short as possible and not too thin.

Chip heat dissipation should be carried out according to design requirements, and vias should be added underneath for heat dissipation as much as possible.

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