Generation, measurement and suppression of switching power supply ripple

Switching power supply ripple generation

Our ultimate goal is to reduce the output ripple to a tolerable level. The most fundamental solution to achieve this goal is to avoid the generation of ripple as much as possible. First of all, we must understand the types and causes of switching power supply ripple.

The figure above shows the simplest topology of a switching power supply - a buck step-down power supply.

As the switch turns on and off, the current in the inductor L also fluctuates around the effective value of the output current. Therefore, a ripple with the same frequency as the switch will also appear at the output end. This is what is generally referred to as ripple. It is related to the capacity and ESR of the output capacitor. The frequency of this ripple is the same as that of the switching power supply, which is tens to hundreds of kHz.

In addition, SWITCH generally uses bipolar transistors or MOSFETs. No matter which type, there will be a rise time and fall time when it is turned on and off. At this time, a noise with the same frequency or odd multiples of the rise and fall time of SWITCH will appear in the circuit, generally tens of MHz. Similarly, at the moment of reverse recovery, the equivalent circuit of diode D is a series connection of resistance, capacitance and inductance, which will cause resonance and generate noise.

The frequency is also tens of MHz. These two types of noise are generally called high-frequency noise, and the amplitude is usually much larger than the ripple.

If it is an AC/DC converter, in addition to the above two types of ripple (noise), there is also AC noise, the frequency of which is the frequency of the input AC power supply, which is about 50-60Hz. There is also a common mode noise, which is caused by the equivalent capacitance generated by the use of the casing as a heat sink for the power devices of many switching power supplies.

Switching Power Supply Ripple Measurement

basic requirements:

Using an oscilloscope with AC coupling

20MHz bandwidth limit

Unplug the ground wire of the probe

1. AC coupling is to remove the superimposed DC voltage to obtain an accurate waveform.

2. Opening the 20MHz bandwidth limit is to prevent interference from high-frequency noise and prevent erroneous measurement results. Because the high-frequency component has a large amplitude, it should be removed during measurement.

3. Remove the grounding clip of the oscilloscope probe and use the grounding ring to measure in order to reduce interference. Many departments do not have grounding rings. If the error is allowed, the grounding clip of the probe can be used directly for measurement. However, this factor should be considered when judging whether it is qualified.

Another point is to use a 50Ω terminal. The Yokogawa oscilloscope document states that the 50Ω module removes the DC component and accurately measures the AC component. However, few oscilloscopes are equipped with this special probe. In most cases, the standard 100KΩ to 10MΩ probe is used for measurement. The impact is not clear yet.

The above are the basic precautions when measuring switching ripple. If the oscilloscope probe is not directly in contact with the output point, it should be measured using a twisted pair or 50Ω coaxial cable.

When measuring high-frequency noise, use the full passband of the oscilloscope, which is generally from several hundred megahertz to GHz. The rest is the same as above. Different companies may have different test methods. In the final analysis, the first thing is to know your test results. The second thing is to get the approval of the customer.

About Oscilloscope:

Some digital oscilloscopes cannot measure ripples correctly due to interference and memory depth. In this case, the oscilloscope should be replaced. In this regard, although the bandwidth of the old analog oscilloscope is only tens of megabytes, it performs better than the digital oscilloscope.

Switching power supply ripple suppression

Switching ripple exists both theoretically and practically. There are three common ways to suppress or reduce it:

1. Increase inductance and output capacitor filtering

According to the formula of switching power supply, the current fluctuation in the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance value. Therefore, increasing the inductance value and output capacitance value can reduce the ripple.

The figure above is the current waveform in the switching power supply inductor L. Its ripple current △I can be calculated by the following formula:

It can be seen that increasing the L value or increasing the switching frequency can reduce the current fluctuation in the inductor.

Similarly, the relationship between output ripple and output capacitance is: vripple = Imax / (Co & TI mes; f). It can be seen that increasing the output capacitance value can reduce the ripple.

Usually, aluminum electrolytic capacitors are used for output capacitors to achieve large capacity. However, electrolytic capacitors are not very effective in suppressing high-frequency noise, and their ESR is relatively large, so a ceramic capacitor is connected in parallel next to it to make up for the shortcomings of aluminum electrolytic capacitors.

At the same time, when the switching power supply is working, the voltage Vin at the input end does not change, but the current changes with the switch. At this time, the input power supply cannot provide current very well, and usually a capacitor is connected in parallel near the current input end (for example, near SWITcH for BucK type) to provide current.

After applying this countermeasure, the BUCK type switching power supply is as shown below:

The above method has limited effect on reducing ripple. Due to volume limitations, the inductor cannot be made very large; increasing the output capacitance to a certain extent will have no obvious effect on reducing ripple; increasing the switching frequency will increase the switching loss. Therefore, this method is not very good when the requirements are strict.

For information about the principles of switching power supplies, etc., you can refer to various switching power supply design manuals.

2. Secondary filtering, that is, adding another LC filter

The LC filter has a more obvious effect on suppressing noise ripple. Selecting appropriate inductors and capacitors to form a filter circuit based on the ripple frequency to be removed can generally reduce the ripple well.

However, in this case, the sampling point of the feedback comparison voltage needs to be considered. (As shown in the figure below)

If the sampling point is selected before the LC filter (Pa), the output voltage will decrease. Because any inductor has a DC resistance, when there is current output, there will be a voltage drop on the inductor, causing the output voltage of the power supply to decrease. And this voltage drop changes with the output current.

The sampling point is selected after the LC filter (Pb), so that the output voltage is the voltage we want. However, this introduces an inductor and a capacitor into the power system, which may cause system instability. There are many materials about system stability, so I will not write about it in detail here.

3. After the switching power supply output, connect to LDO filter

This is the most effective way to reduce ripple and noise. The output voltage is constant and there is no need to change the original feedback system. However, it is also the most expensive and power-hungry method.

Any LDO has an indicator: noise suppression ratio. It is a frequency-dB curve, such as the curve of LT3024 from Linear Technology Corporation in the figure on the right.

After passing through the LDO, the switching ripple is generally below 10mV.

The following figure is a comparison of the ripple before and after LDO:

Comparing the curve on the top and the waveform on the left, we can see that the LDO has a very good suppression effect on the switching ripple of several hundred kHz. However, in the high frequency range, the effect of the LDO is not so ideal.

To reduce ripple. The PCB layout of the switching power supply is also very critical, which is a very difficult problem. There are specialized switching power supply PCB engineers. For a simple reference, you can refer to AN1229: SIMPLE SWITCHER PCB Layout Guidelines of National Semiconductor (there is a translated Chinese summary online).

For high-frequency noise, due to the high frequency and large amplitude, the post-filtering has a certain effect, but the effect is not obvious. There are special studies in this area. The simple way is to connect a capacitor C or RC in parallel with the diode, or connect an inductor in series.

4. Connect capacitor C or RC to the diode

The figure above is the equivalent circuit of a practical diode. When the diode is turned on and off at high speed, parasitic parameters must be considered. During the reverse recovery of the diode, the equivalent inductance and equivalent capacitance become an RC oscillator, generating high-frequency oscillations. In order to suppress this high-frequency oscillation, a capacitor C or an RC snubber network must be connected in parallel across the diode. The resistor is generally 10Ω-100Ω, and the capacitor is 4.7pF-2.2nF.

The value of the capacitor C or RC connected in parallel with the diode must be determined through repeated experiments. If it is not selected properly, it will cause more serious oscillation.

If the high frequency noise is strictly required, soft switching technology can be used. There are many books dedicated to soft switching.

5. Connect an inductor after the diode (EMI filtering)

This is also a common method to suppress high-frequency noise. According to the frequency of noise generation, choosing the right inductor component can also effectively suppress noise. It should be noted that the rated current of the inductor must meet the actual requirements. This is a relatively simple method and will not be explained in detail.

summary

The above is a summary of the switching power supply ripple. It would be better if some waveforms could be added. Although it may not be complete, it is sufficient for general applications. Regarding noise suppression, not all of them may be applied in practice. The important thing is to choose the appropriate method according to your own design requirements, such as product volume, cost, development cycle, etc.