DC-DC Circuit Design Concepts and Features
Updatezeit: 2022-05-30 17:56:43
DC-DC refers to direct current power (Direct Current). It is a device that changes the electrical energy of one voltage value to another voltage value in a DC circuit. For example, if a converter can convert a DC voltage (5.0V) to another DC voltage (1.5V or 12.0V), we call this converter a DC-DC converter, a switching power supply, or switching regulator.
DC-DC converters generally consist of a control chip, an inductor coil, a diode, a triode, and a capacitor. When discussing the performance of a DC-DC converter, it is not possible to judge the merits of the controller chip alone. The characteristics of the components of the peripheral circuit and the substrate's wiring method can change the power supply circuit; therefore, a comprehensive judgment should be made.
Using a DC-DC converter is conducive to simplifying the design of power supply circuits, shortening the development cycle, achieving the best indicators, etc. It is widely used in power electronics, military industry, scientific research, industrial control equipment, communication equipment, instrumentation, switching equipment, access equipment, mobile communication, routers and other communication fields, industrial control, automotive electronics, aerospace, and other areas. With high reliability and easy system upgrade characteristics, power supply modules are increasingly widely used. In addition, DC-DC converters are also commonly used in cell phones, MP3, digital cameras, portable media players, and other products. It belongs to the chopper circuit in the circuit type classification.
Its main feature is high efficiency: compared with the LDO of a linear regulator, high efficiency is the significant advantage of DCDC. Usually, the efficiency is over 70%, and the high efficiency can reach over 95%. The second is the wide adaptive voltage range.
A: Modulation method
1: PFM (pulse frequency modulation method)
The switching pulse width is certain, and the output voltage is stabilized by changing the pulse output frequency. The PFM control type has the advantage of low power consumption even for extended use, especially for small loads.
2: PWM (Pulse Width Modulation)
PWM control type is highly efficient and has good output voltage ripple and noise.
Usually, the performance differences of DC-DC converters with two different modulation methods, PFM and PWM, are as follows.
PWM frequency, PFM duty cycle selection method. PWM/PFM conversion type implements PFM control at small loads and automatically switches to PWM control at heavy loads.
Three common principle architectures
Boost (Boost DC/DC converter)
Buck-Boost (boost type DC/DC converter)
BUCK circuit working principle detailed explanation
Volt-second balance principle: In the inductor's stable state, the positive volt-second product at both ends of the inductor is equal to the negative volt-second product. The volt-second product at both ends of the inductor must be balanced within one switching cycle.
When the switch is on: the input voltage, Vin, is added to the input of the LC filter, and the current on the inductor rises linearly with a fixed slope. The following figure
When the switch is turned off: Since the current on the inductor cannot change abruptly, the energy stored in the inductor is released to the load, and the inductor current is renewed through the diode, and at this stage, the current waveform is a slope with a negative slope. The following diagram
3. Design techniques and main technical parameters selection requirements
DC-DC circuit design should consider at least the following conditions.
A. The range of external input power supply voltage, the output current size.
B. DC-DC output voltage, current, and power maximum of the system.
The device should select the recommended operating voltage range of the device, and the actual voltage fluctuation range should be considered to ensure that the device specifications are not exceeded.
Input &Output Voltage：Vin/Vout
The continuous output current capability of the device is an important parameter to refer to when selecting this parameter and to retain a certain margin.
The selection of this parameter also assesses the circuit's instantaneous peak current and heat generation to determine a comprehensive and meet the derating requirements.
Ripple is an important parameter to measure the output voltage fluctuation of a circuit. Pay attention to light load and heavy load ripple; generally, light load ripple is large. Pay attention to whether soft load ripple in nuclear power and other scenarios exceed the requirements. The actual test under various scenarios under load. Usually, choose oscilloscope 20M bandwidth to test.
To focus on both light load and heavy load conditions. The light load will affect the standby power; the heavy load affects the temperature rise. Usually, look at the 12V input and 5V output under 10mA efficiency, generally 80% or more.
Transient response characteristic response to the load changes dramatically when the system can be adjusted in time to ensure the stability of the output voltage. The output voltage fluctuation must be as small as possible; generally, less than 10% of the peak-to-peak value required.
The current feedback capacitor should be selected according to the recommended value. The standard value is 22p to 120pF.
Most of the common switching frequencies are above 500kHz. A higher switching frequency of 1.2M to 2M is also available. The high-frequency switching losses increase IC thermal design to be good, so mainly concentrated in 5V low-voltage input small current products. Switching frequency is related to the choice of inductors and capacitors. Other issues such as EMC and noise under light load are also related to it.
Feedback Voltage &output accuracy：Vref
Feedback voltage to be compared with the internal reference voltage, with the external feedback divider resistor, and the output of different voltages. When replacing the attention to adjust the feedback resistor, other products will have different reference voltages, such as 0.6 ~ 0.8V.
Feedback resistor to choose 1% accuracy, just according to the manufacturer's recommendation, generally do not choose too large, so as not to affect the stability.
Reference voltage accuracy affects the accuracy of the output. The common accuracy of 2% or less, such as 1% ~ 1.5%, high precision products will differ in cost. Choose as needed.
Linear stability responds to input voltage variation and output voltage stability. Load stability responds to the output voltage stability of the output load variation. The general requirement is 1%, and the maximum should not exceed 3%.
EN high and low levels to meet the device specifications. Some ICs can not exceed a specific voltage range; the resistor voltage divider pays attention to meeting the timely shutdown and considers the voltage fluctuations in the maximum range to meet.
Due to the need for timing control, the pin will increase the capacitance, level regulation and shutdown discharge, and resistance to the ground.
To have overcurrent protection OCP, overheat protection OTP, etc., and protection after the disappearance of conditions can be self-recovery.
Requirements are a soft start, thermal resistance, and packaging; use temperature range to cover high and low temperatures, etc.
4. General principles of device selection
Easy to procure long life cycle
Compatible and replaceable
Ease of production and normalization
5. Requirements for peripheral device selection
a. Input capacitor: to meet the voltage withstand and input ripple requirements. Input capacitor: to meet the needs of voltage withstand and input ripple. Note that the actual capacity of the porcelain chip capacitor will be reduced with the influence of DC voltage bias.
b. Output capacitor: to meet the requirements of withstand voltage and output ripple. Generally, the withstand voltage is 1.5~2 times.
The relationship between ripple and capacitance：
c.BST capacitance: according to the recommended value in the specification. Generally 0.1uF-1uF. The withstand voltage is generally higher than the input voltage.
d. Inductor: Different output voltage requires different inductance; note that the temperature rise and saturation current should meet the margin requirement, generally more than 1.2 times the maximum current (or the inductor's saturation current must be greater than the maximum output current + 0.5*inductor ripple current). Usually, choose the appropriate inductance value L so that ΔIL accounts for 30% to 50% of the output current. Calculation formula.
e. VCC capacitance: according to the specification requirements to take the value, it can not be reduced and not too large; pay attention to the withstand voltage.
f. Feedback capacitance: take the value according to the specification, different manufacturers' chips take different values, and the different output voltage will also have different requirements.
g. Feedback resistor and EN voltage divider resistor: required to take the value according to the specification, precision 1%.
6. PCB design requirements
a. Input capacitors are placed close to the chip's input Vin and power PGND to reduce the presence of parasitic inductors because the input current is not continuous, the noise caused by parasitic inductors on the chip's voltage withstand and logic unit caused by adverse effects. Capacitor ground terminal increase over the hole to reduce impedance.
b.SW is the noise source; ensure the current while keeping the area as small as possible, away from sensitivity and susceptible to interference. For example, the inductor is close to the SW pin and away from the feedback line. The output capacitor is close to the inductor, and the ground terminal increases the ground over the hole.
c. VCC capacitor should be placed close to the chip's VCC pin, and the chip's signal ground between, as far as possible in a layer, does not have a hole.
FB is the most sensitive and easily disturbed part of the chip and is the most common cause of system instability.
1. FB resistor connected to the FB pin may even be short, placed close to the IC to reduce noise coupling; FB under the voltage divider resistor is usually related to the signal ground AGND;
2. away from noise sources, SW points, inductors, diodes (non-synchronous buck); FB alignment wrapped around the ground.
3. FB of the high current load is taken at the far end of the load, and the feedback capacitor alignment should be taken nearby.
d. BST capacitor alignment as short as possible, not too thin.
e. Chip heat dissipation according to the design requirements. Try to increase the bottom of the over-hole heat dissipation.
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