Methods to measure and reduce DCDC converter output ripple

Latest update time：2021-03-17

Reads：

In recent years, DC/DC converters have developed very rapidly. Due to their high efficiency, small size, and high reliability, they are widely used. In DC voltage, there are often AC components superimposed on the DC stability, which is ripple. Ripple easily generates harmonics in electronic systems, thereby reducing the efficiency of the converter, interfering with the logical relationship of digital circuits, interfering with the normal operation of the system, and bringing hidden dangers to the normal operation of the entire electronic equipment. Therefore, it is very necessary to measure and reduce the ripple through reliable testing methods before the DC/DC converter is installed in electronic equipment.

Sources of DCDC converter output ripple

IDEAL BUCK CONVERTER

Actual BUCK converter

After considering PCB parasitic parameters and component parasitic parameters, high-frequency oscillation and switching frequency ripple will occur.

Output ripple noise

SCT2A10 DEMO 48V input 12V output 600mA load, 22uF output ceramic capacitor ripple results

Sources of low frequency ripple

Low frequency ripple is generated by inductor ripple current and output capacitor impedance

Different Types of Capacitors - Aluminum Electrolytic Capacitors

The ESR of aluminum electrolytic capacitors generally ranges from several hundred milliohms to several ohms, while that of ceramic capacitors generally ranges from tens of milliohms. Tantalum capacitors are between aluminum electrolytic capacitors and ceramic capacitors.

Aluminum capacitors are made of aluminum foil that is grooved and oxidized, then rolled with an insulating layer, and then dipped in electrolyte solution. The principle is chemical. The charging and discharging of capacitors rely on chemical reactions. The response speed of the capacitor to signals is affected by the charge in the electrolyte. The movement speed limit of ions is generally used in filtering situations with lower frequencies (below 1M). The ESR is mainly the sum of the aluminum resistance and the equivalent resistance of the electrolyte, and the value is relatively large. The electrolyte of aluminum capacitors will gradually evaporate, causing the capacitance to decrease or even fail. The evaporation rate accelerates as the temperature increases. For every 10 degrees increase in temperature, the life of the electrolytic capacitor will be halved. If the capacitor can be used for 10,000 hours at room temperature of 27 degrees, it can only be used for 1,250 hours at 57 degrees. Therefore, try not to place aluminum electrolytic capacitors too close to heat sources.

Different types of capacitors - ceramic capacitors

Ceramic capacitors rely on physical reactions to store electricity, so they have a very high response speed and can be applied to high-G applications. However, ceramic capacitors also show great differences due to different media. The best performance capacitor is made of C0G material, which has a small temperature coefficient. However, the dielectric constant of the material is small, so the capacitance value cannot be too large. The worst performance material is Z5U/Y5V. This material has a large dielectric constant, so the capacitance can reach tens of microfarads. However, this material is seriously affected by temperature and DC bias (DC voltage will polarize the material and reduce the capacitance). Let's take a look at how C0G, X5R, and Y5V capacitors are affected by ambient temperature and DC operating voltage. It can be seen that the capacitance of C0G basically does not change with temperature, the stability of X5R is slightly worse, and the capacity of Y5V material becomes 50% of the nominal value at 60 degrees.

It can be seen that when the Y5V ceramic capacitor with a voltage of 50V is applied at 30V , the capacity is only 30% of the nominal value . Ceramic capacitors have a big disadvantage, which is that they are fragile. Therefore, it is necessary to avoid collisions and try to stay away from places where the circuit board is prone to deformation.

Different Types of Capacitors - Tantalum Capacitors

Tantalum capacitors are like a battery in terms of principle and structure. On the right is a schematic diagram of the internal structure of a tantalum capacitor:

Tantalum capacitors have the advantages of small size, large capacity, fast speed, low ESR, etc., and the price is relatively high. What determines the capacity and voltage resistance of tantalum capacitors is the size of the raw material tantalum powder particles. The finer the particles, the larger the capacitance can be obtained, and if you want to obtain a larger withstand voltage, you need thicker Ta2O5, which requires the use of tantalum powder with larger particles. Therefore, it is very difficult to obtain tantalum capacitors with high withstand voltage and large capacity of the same size. Another thing that needs attention about tantalum capacitors is that tantalum capacitors are prone to breakdown and short-circuit characteristics, and have poor surge resistance. It is possible that a large instantaneous current causes the capacitor to burn out and form a short circuit. This needs to be considered when using ultra-large-capacity tantalum capacitors (such as 1000uF tantalum capacitors).

Filter capacitor location

If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic capacitors at the same time, place the electrolytic capacitors closest to the power supply to protect the tantalum capacitors. The ceramic capacitor is placed behind the tantalum capacitor. This will give you the best filtering effect.

Output ripple of different types of capacitors with the same capacitance value

22uF output ceramic capacitor ripple result 9.3mV

The output capacitor is a 22uF electrolytic capacitor, the result is 228mV

High frequency ripple sources

Noise sources:

High di/dt loops and parasitic inductances on the loops;

Noise occurs at the edges where the SW level changes.

How to couple to the output:

Parasitic capacitance, the inductor has a capacitance of tens of pF ;

There are parasitic capacitances in the parts of the PCB that are close to each other .

■ ■ ■ ■ ■

Measurement of DCDC converter output ripple

Measurement of output ripple

Before looking for ways to reduce the output ripple, we need to ensure that we measure the accurate ripple.

Wrong measurement methods will result in a ripple that is larger than the actual value, leading to aggressive ripple elimination methods at additional costs, such as more expensive output capacitors, such as higher switching frequencies.

Example of poor ripple measurement method (large ground loop)

Example of improved ripple measurement method (small ground loop)

Example of good ripple measurement method (coaxial cable)

Comparison of different measurement methods

Conclusion: It is best to use coaxial cables for ripple measurement to ensure accuracy. If not, minimize the ground loop to ensure measurement accuracy.

Reduction of DCDC converter output ripple

Methods to reduce ripple

• Low frequency ripple

• Inductor and switching frequency selection

• Output capacitor selection

• Multi-stage filter circuit

• High frequency noise

• Component layout

• Input filter capacitor selection

• PCB layout and layering setup

Low frequency ripple reduction - inductor and output capacitor

The low-frequency ripple mainly depends on the inductor ripple current and the output capacitor impedance, so it can

•Reduce inductor ripple current

•For the same inductance, increasing the switching frequency has the disadvantage of increasing switching losses.

•For the same switching frequency, increasing the inductance has the disadvantage of increasing the solution size

•Reduce output capacitor impedance

•The disadvantage of using low ESR and low ESL capacitors is increased cost

•The disadvantage of using multiple capacitors in parallel is that it increases the board area and may increase the cost.

Low frequency ripple reduction - two-stage filtering

For some special applications, such as the test and measurement industry, which are very sensitive , extremely low output voltage ripple is often required, such as 0.1% output voltage ripple. For this requirement, it is often necessary to add a second-stage LC filter circuit, as shown in the figure below. When designing the second-stage LC filter, it is mainly necessary to ensure circuit stability and sufficient ripple suppression capability. In order to ensure sufficient phase margin, a resistor is often connected in parallel to both ends of L1 to reduce the Q value and improve the phase margin.