How to select DC-DC Buck IC?
Time:2023-07-13
Views:857
The operation of an electronic system requires the support of an efficient and reliable power supply system. Converting energy from different power sources (such as mains and batteries) into power rails required for various loads in electronic circuits requires the rational use of various components to build a complete power architecture, which is what "power management" is to accomplish.
As we all know, according to the needs of power conversion, power management includes three main scenarios: AC to DC (AC-DC), direct current DC (DC-DC) and direct current AC (inverter), of which the application of DC-DC should be particularly extensive.
DC-DC converter (or voltage regulator) is generally composed of control circuit, switching tube (diode or transistor), inductance coil, capacitor and other components, which according to the signal provided by the feedback circuit, through the control of the switching device "switch" action to stabilize the output voltage at the required voltage level. Therefore, it is also called "DC-DC switching power supply" to distinguish it from other voltage regulator circuits.
With different DC-DC switching power supply topologies, three types of operations can be realized, such as buck, boost and lift, especially the buck DC-DC application scenarios are more abundant - where the required voltage is lower than the available voltage in the system, the buck converter will be used; In some more complex distribution architectures, multiple DC-DC buck conversion operations are also used to improve efficiency and simplify the system.
Today, we will focus on talking about how to choose buck DC-DC power management devices.
Different buck DC-DC types
Because of the step-down conversion, step-down DC-DC is often compared to linear regulated power supplies (Ldos) when the power supply is designed. In comparison, LDO circuit is simple, low cost, and has the characteristics of fast load response, low output ripple and low noise. Buck DC-DC has unique advantages in high efficiency, support wider input operating voltage and greater voltage drop, support larger current and power, and flexible output voltage.
Therefore, when making specific technology selection, it is necessary to comprehensively evaluate efficiency, cost, noise, performance and so on to make correct decisions. Sometimes, the advantages of the two can also be combined in the power supply design, such as in order to improve the accuracy of the switch DC-DC, to overcome its loud shortboard, will be optimized by adding LDO in the DC-DC back end.
However, in general, the application of buck DC-DC devices is more extensive, and the types of products are more diverse, which makes engineers spend more attention when doing component selection, and carefully evaluate different types of buck DC-DC converters.
Before the selection, it is necessary to have a relatively fresh understanding of the types of buck DC-DC devices. According to the different dimensions, buck DC-DC can usually be divided into different types.
Voltage modulation system
Step-down DC-DC can be divided into PFM (pulse frequency modulation) and PWM (pulse width modulation) according to the difference in its control mode.
The working principle of PFM is to maintain a fixed switching pulse width, adjust and control the output voltage by changing the frequency of the pulse output and make it reach a stable state. The advantage of this method is that it has high efficiency at light load (because there is no need to increase power at light load, the switching frequency is low, and the switching loss is reduced), while the disadvantage is that the changing frequency makes it difficult to eliminate noise by filtering, and it is easy to cause interference to other circuits.
As the name suggests, PWM refers to the frequency of the switching pulse is certain, by changing the pulse output width to control the output voltage method, it has a better output voltage ripple and noise, but because the frequency is constant, heavy load and light load when the switching times are the same, so the light load when the switching loss is relatively high.
There are also some power management devices that combine the advantages of the two control methods, using PWM mode in heavy and steady state, and switching to PFM mode in light load, which is undoubtedly beneficial to the overall performance of the power system. The choice of control method in the specific application should be carefully weighed according to the design requirements.
Output feedback mode
In order to maintain voltage stability, the buck DC-DC converter feeds the output back to the control circuit. According to the way of output feedback, it can be divided into voltage mode control, current mode control and hysteresis control.
In voltage mode, the feedback loop feeds back the output voltage signal, which is also the basic way. It has the advantages of simple control, good noise resistance and short on-time. The disadvantage is that the phase compensation circuit is more complex.
Based on the optimization of voltage mode control, the current mode control is born. Its design idea is to detect circuit inductance or transistor current instead of voltage signal acquisition. Compared with voltage mode, it has the advantages of high stability, simple phase compensation circuit and fast load transient response. The disadvantage is that the current detection is more sensitive, so the design has higher requirements for the noise processing of the feedback loop.
Hysteresis control is an output feedback method developed for CPU, FPGA and other power applications that have higher requirements for high-speed load transient response. It has the advantages of rapid transient response, high stability of feedback loop and no phase compensation. The disadvantage is that the switching frequency will produce changes, large jitter, and the need for capacitors with large ESR to detect ripple.
Variable voltage rectifier mode
In addition to the differences in voltage modulation and output feedback modes mentioned above, step-down DC-DC converters also differ in transformer rectification modes between asynchronous rectification and synchronous rectification.
Asynchronous rectification refers to the switch of the upper transistor in the circuit to control the turn-on/turn-off of the lower rectifier diode, so that the current flows or does not flow to the diode. This circuit is simple and reliable, and is widely used in industrial equipment and other applications.
The main difference between synchronous rectifier and asynchronous rectifier is that the lower diode in the asynchronous rectifier architecture is replaced with a transistor, so the loss of the output switch can be effectively reduced and higher efficiency can be achieved, but the circuit will be more complex because of the need to ensure the synchronization of the transistors on both sides.
It can be seen that different types of buck DC-DC converters have their own advantages and shortcomings, and fully understand these characteristics in order to make the right choice according to the actual design needs.
Interpretation of key parameters of buck DC-DC
After understanding the type of buck DC-DC converter, as an important step in selection, it is necessary to carefully consult the various parameters of the device to determine whether it meets the requirements of the design specification. To sum up, the following parameters need our special attention when buck DC-DC selection.
1. Input/output voltage
This is the basic parameter of buck DC-DC, which determines whether it meets the requirements of power conversion design. A wide input voltage range and flexible output voltage are essential features of a well-designed buck DC-DC converter.
2. Output current
The continuous output current capacity of a buck DC-DC determines the amount of power it can provide and is also an important parameter. The selection should retain enough margin to avoid damage to the device caused by excessive current.
3. Efficiency
Efficiency is the advantage of buck DC-DC converters, but also a hard indicator to measure their performance, many design optimization is ultimately to serve the improvement of efficiency. It should be noted that the selection should pay attention to the efficiency performance of light load and heavy load at the same time.
4. Switching frequency
A higher switching frequency usually means that peripheral components such as smaller output capacitors and inductors can be used in the circuit, which is more advantageous for the miniaturization of the design. However, higher frequencies also bring greater switching losses and noise problems, which need to be carefully dealt with.
5. Transient response
This characteristic reflects whether the system can adjust in time and ensure the stability of the output voltage when the load changes dramatically. The architecture of buck DC-DC devices and their output capacitance performance (capacity and ESR) will affect the transient response performance.
6. Output ripple
This is an important parameter to measure the voltage fluctuation of the buck DC-DC output, and is also a key indicator of the quality of voltage regulation. In general, the light load ripple is larger.
7. Linear stability and load stability
Linear stability refers to the stability of the output voltage when the input voltage changes, while load stability reflects the stability of the output voltage when the output load changes, they are expressed in percentages, and the smaller the number is naturally the better.
8. Static current
The so-called static current refers to the current generated by each analog circuit submodule during no-load decompression of DC-DC, which will bring static loss, so the reduction of static current is very key to the improvement of efficiency. Whether the static current can be lowered is also a touchstone for the design and process level of power management IC manufacturers.
9. Protection function
The integration of OCP overcurrent protection and OTP overheat protection in the design is an important part of ensuring the reliable and stable operation of buck DC-DC. These protection functions are usually triggered by specific external conditions and can recover when the conditions disappear.
10. EMI suppression
The operating mode of the switch control determines that the buck DC-DC will produce more noise, so in some noise sensitive applications, it is necessary to consider adding effective EMI suppression measures.
Understanding the above parameters will also master the key points of buck DC-DC device selection. Based on this, comparing the characteristics of the device with the design requirements should be able to quickly lock in the desired material.
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