Basic knowledge of switching power supply (i)

1.1 Types of switching power supplies

1.1.1 Linear regulator, the so-called linear regulator, which is what we say the Ldo, generally have two characteristics:

? The transmission element works in the linear region, it does not have a switching transition;

? For buck conversions only, you will rarely see the boost application.

1.1.2 Switching Regulator

? Transmission device switches (FET), fully connected and completely cut off at each cycle;

? Contains at least one energy storage element, such as: inductor or capacitor;

? Multiple topologies (buck, boost, buck-boost, etc.)

1.1.3 charge pump, generally in some small current applications

? Transmission device switches (e.g. FET, transistor), some are fully conductive, while others work in the linear region;

? In the process of electric energy conversion or energy storage, only capacitors, such as some voltage doubler circuits, are used.

Q: Why should I use a switching regulator in some cases? Why not use an Ldo and a charge pump?

We know that all energies will not vanish in a vacuum, and the energy of loss will eventually be transmitted in the form of heat,

In this way, engineers will have a great challenge in design, for example, if the loss is eventually transmitted in the form of heat, then

The circuit needs to add a larger heat sink, the result is the volume of the power supply becomes larger, and the efficiency of the machine is very low.

If switching power supply in switch mode, it can not only improve efficiency, but also reduce the design difficulty of thermal management.

We can cite an example to compare the efficiency and volume of linear and switching power supplies:

From their efficiency, a 12V input, 3.3v/2a output of the power supply, if using a linear regulator to achieve the

, it output efficiency is only 28%, and with switching power supply to do, its output efficiency can reach more than 90%. So

The efficiency of the linear power supply at high input voltage, low output voltage is very low, it is only suitable for some input and transmission

The low pressure difference. The advantages of using switching power supplies In these cases are obvious. The loss of the linear regulator

Consumption is 17.4W, the switching regulator's loss is only 0.73W, these losses will eventually be transmitted in the form of heat, the device

Operating temperature = device temperature rise + ambient temperature, temperature rise = Thermal resistance x loss case: If the device's thermal resistance θ=35℃/w

To calculate the LDO temperature rise =35℃x17.4w=609℃, switching regulator temperature rise =35℃x0.73w=25.55℃.

Visible, the switching regulator can work in the 60~70℃ ambient temperature is also no problem, and the Ldo in this case,

Fever is very serious, it must be reduced its thermal resistance, and the size of the thermal resistance depends on the heat dissipation area, the greater the heat dissipation area, the heat

The smaller the resistance, the LDO needs a large heat dissipation area (e.g.) to reduce its thermal resistance to obtain a lower temperature rise.

The red label places are a 2.5W Ldo and a 6W switching power supply, each of which has a 2.4 difference in power

, but the area of the switching power supply is only 1/4 less than the Ldo, which means the loss of switching power supply is greatly reduced, can

withstands higher thermal resistance and reduces the area of heat dissipation.

Again, if the pressure difference between the input and output is low, the Ldo can be used, but the differential pressure is more

In large cases, switching power supplies are recommended. Of course, the switching power supply also has its disadvantage, its output will have noise, ringing,

And the Ldo does not. In some cases the load is very sensitive to the voltage of the power supply, it can be behind the switching power supply

Add a level Ldo. For example, we want to turn 5V into 1.2V, if there is a direct ldo, the efficiency may be only 20%,

But we can change the 5V switch power supply to 1.5V, and then use the Ldo to turn 1.5V into 1.2V, so that the efficiency will be high,

is a more optimized design.

1.1.5 Summary: Switching power vs linear regulator

(1) Switching power supply

① to increase voltage (boost)

② and Reducing voltage (buck) and even inverting the

③ with high efficiency and power density

(2) Linear regulator

① can only achieve buck

② output voltage is relatively more stable

is a simplified buck switching power supply, in order to facilitate the analysis of the circuit, the first part of the feedback control is not added.

State one: When the S1 is closed, the input energy is C1 from the capacitor, the s1→ inductor l1→ capacitor c2→ load RL

Power supply, at this time the inductor L1 also in the storage of energy, can be added to the L1 on the voltage: Vin-vo=l*di/dton.

State two: When the S2 is turned off, the energy is no longer obtained from the input, but through the continuation circuit, from the inductor L1 storage

Energy → capacitance c2→ load rl→ diode D1, at this time can get the formula: l*di/dtoff= Vo, finally we can get

Out of Vo/vin=d, and Vo is always less than Vin, because the duty-d≤1.

The role of each device:

1, the input capacitor (C1) is used to stabilize the input voltage;

2, the output capacitor (C2) is responsible for the output voltage stability;

3. Clamp diode (D1) provides a current path for the inductor when the switch is open-circuit;

4. The inductor (L1) is used to store the energy that will be transmitted to the load.

Basic knowledge of switching power supply (i)