A complete analysis of ripple, noise and harmonics in switching power supplies

A complete analysis of ripple, noise and harmonics in switching power supplies

Ripple

Ripple: It is a noise signal containing periodic and random components attached to the DC level. It refers to the peak value of the AC voltage in the output voltage under the rated output voltage and current. In a narrow sense, ripple voltage refers to the industrial frequency AC component contained in the output DC voltage.

Noise

Noise: For the nominal noise in electronic circuits, it can be generally considered that it is a general term for all signals other than the target signal. Initially, people called those electronic signals that caused noise from audio equipment such as radios as noise. However, the consequences of some non-target electronic signals on electronic circuits are not all related to sound, so people gradually expanded the concept of noise. For example, those electronic signals that cause white spots on the screen are also called noise. It can be said that all signals in the circuit except the target signal, regardless of whether it affects the circuit, can be called noise. For example, ripple or self-excited oscillation in the power supply voltage can have an adverse effect on the circuit, causing the audio device to emit AC sound or cause the circuit to malfunction, but sometimes it may not lead to the above consequences. For such ripple or oscillation, it should be called a noise of the circuit. There is also a radio wave signal of a certain frequency. For the receiver that needs to receive this signal, it is a normal target signal, but for another receiver it is a non-target signal, that is, noise. The term interference is often used in electronics, and sometimes it is confused with the concept of noise. In fact, there is a difference. Noise is an electronic signal, while interference refers to a certain effect, which is an adverse reaction to the circuit caused by noise. The existence of noise in the circuit does not necessarily mean interference. In digital circuits. It is often observed with an oscilloscope that some small spike pulses mixed with normal pulse signals are not expected, but a kind of noise. However, due to the characteristics of the circuit, these small spike pulses will not affect the logic of the digital circuit and cause confusion, so it can be considered that there is no interference.

When a noise voltage is large enough to interfere with the circuit, the noise voltage is called interference voltage. The maximum noise voltage applied to a circuit or a device while it can still maintain normal operation is called the anti-interference tolerance or anti-interference degree of the circuit or device. Generally speaking, noise is difficult to eliminate, but we can try to reduce the intensity of noise or improve the anti-interference degree of the circuit so that the noise does not cause interference.

Harmonics

Harmonics: refers to the amount of electricity contained in the current whose frequency is an integer multiple of the fundamental wave. Generally, it refers to the amount of electricity generated by the remaining current greater than the fundamental frequency after the Fourier series decomposition of periodic non-sinusoidal electricity. In a broad sense, since the effective component of the AC power grid is a single frequency of the power frequency, any component different from the power frequency can be called a harmonic.

Causes of harmonic generation: Since the sinusoidal voltage is applied to the non-linear load, when the current flows through the load, it is not linearly related to the applied voltage, and the fundamental current is distorted to form a non-sinusoidal current, that is, harmonics are generated in the circuit. The main non-linear loads are UPS, switching power supplies, rectifiers, frequency converters, inverters, etc.

Today, this article mainly explains the ripple and noise in switching power supplies.

Compared with linear power supplies, the most prominent advantage of switching power supplies (including AC/DC converters, DC/DC converters, AC/DC modules and DC/DC modules) is the high conversion efficiency, which can generally reach 80% to 85%, and as high as 90% to 97%. Secondly, the switching power supply uses a high-frequency transformer to replace the bulky power frequency transformer, which not only reduces the weight but also reduces the volume, so the application range is getting wider and wider. However, the disadvantage of the switching power supply is that the output ripple and noise voltage is large because its switch tube works in a high-frequency switching state, which is generally about 1% of the output voltage (the lowest is about 0.5% of the output voltage). The ripple and noise voltage of the best product is also tens of mV; while the adjustment tube of the linear power supply works in a linear state, there is no ripple voltage, and the output noise voltage is also small, and its unit is μV.

Briefly introduce the causes and measurement methods, measurement devices, measurement standards and measures to reduce ripple and noise of switching power supplies.

Causes of ripple and noise

The switching power supply does not output a pure DC voltage, but contains some AC components, which are caused by ripple and noise. Ripple is the fluctuation of the output DC voltage, which is related to the switching action of the switching power supply. In each on-off process, the electric energy is "pumped" from the input end to the output end, forming a charging and discharging process, which causes the output voltage to fluctuate, and the fluctuation frequency is the same as the switching frequency. The ripple voltage is the peak-to-peak value between the peak and the trough of the ripple, and its size is related to the capacity and quality of the input and output capacitors of the switching power supply.

There are two reasons for the generation of noise. One is generated by the switching power supply itself; the other is the interference of external electromagnetic fields (EMI), which can enter the switching power supply through radiation or enter the switching power supply through the power line.

The noise generated by the switching power supply itself is a high-frequency pulse train, which is caused by the sharp pulses generated at the moment of switch on and off, also known as switching noise. The frequency of the noise pulse train is much higher than the switching frequency, and the noise voltage is its peak-to-peak value. The amplitude of the noise voltage is largely related to the topology of the switching power supply, the parasitic state in the circuit and the design of the PCB.

The waveform of ripple and noise can be seen by using an oscilloscope, as shown in Figure 1. The frequency of the ripple is the same as the frequency of the switching tube, while the frequency of the noise is twice that of the switching tube. The sum of the peak-to-peak value of the ripple voltage and the peak-to-peak value of the noise voltage is the ripple and noise voltage, and its unit is mVp-p.

Figure 1 Waveform of ripple and noise

Measurement method of ripple and noise

Ripple and noise voltage is one of the main performance parameters of the switching power supply, so how to measure it accurately is a very important issue. At present, the method of measuring ripple and noise voltage is to use a wide-band oscilloscope, which can accurately measure the ripple and noise voltage value.

Due to the wide variety of switching power supplies (different topologies, operating frequencies, output powers, different technical requirements, etc.), but all manufacturers use oscilloscope measurement methods, only the measurement devices are not exactly the same, so each factory has its own standards for measuring different switching power supplies, that is, corporate standards.

The block diagram of the device for measuring ripple and noise with an oscilloscope is shown in Figure 2. It consists of a switching power supply to be tested, a load, an oscilloscope and measurement connections. Some measuring devices also have inductors, capacitors, resistors and other components welded on them.

Figure 2 Oscilloscope measurement block diagram

From Figure 2, it seems that there is no difference from other waveform measurement circuits, but in fact the requirements are different. The requirements for measuring ripple and noise voltage are as follows:

● To prevent the intrusion of electromagnetic field interference (EMI) in the environment, so that the output noise voltage is not affected by EMI;

● To prevent EMI interference that may be generated in the load circuit;

● For small switching module power supplies, since there is no output capacitor or the output capacitor is small, appropriate output capacitors should be added during measurement.

To meet the first requirement, the measurement connection should be as short as possible, and twisted pair (to eliminate common mode noise interference) or coaxial cable should be used; general oscilloscope probes cannot be used, and special oscilloscope probes are required; and the measurement point should be at the output end of the power supply. If the measurement point is on the load, it will cause a huge measurement error. To meet the second point, the load should use a resistive dummy load.

It often happens that when users measure the ripple and noise performance indicators of the switching power supply or module power supply they bought, they find that it does not meet the indicators in the product technical specifications and greatly exceeds the performance indicator requirements in the technical specifications. This is often caused by the user's inappropriate measurement device, inappropriate measurement method (selection of measurement points) or the use of a general measurement probe.

Several measurement devices

1 Twisted pair measurement device

The twisted pair measurement device is shown in Figure 3. A 300mm (12-inch) long twisted pair consisting of #16AWG wire gauge is connected to the +OUT and -OUT of the switching power supply under test, and a resistive dummy load is connected between +OUT and -OUT. A 4TμF electrolytic capacitor (tantalum capacitor) is connected to the end of the twisted pair, and then an oscilloscope with a bandwidth of 50MHz (some corporate standards are 20MHz) is input. When connecting the measurement point, one end should be connected to +OUT and the other end to the ground plane.

Figure 3 Twisted pair measurement device

It should be noted here that the end of the twisted pair ground wire should be as short as possible and clamped on the ground wire ring of the probe.

2 Parallel line measurement device

The parallel line measurement device is shown in Figure 4. In Figure 4, C1 is a multilayer ceramic capacitor (MLCC) with a capacity of 1μF, and C2 is a tantalum electrolytic capacitor with a capacity of 10μF. The sum of the voltage drops of the two parallel copper foil strips is less than 2% of the output voltage value. The advantage of this measurement method is that it is closer to the actual working environment, and the disadvantage is that it is easier to pick up EMI interference.

Figure 4 Parallel line measurement device

3 Special oscilloscope probe

Figure 5 shows a special oscilloscope probe directly connected to the wave test power supply. There is a ground wire ring on the special oscilloscope probe, the tip of the probe contacts the positive pole of the power supply output, and the ground wire ring contacts the negative pole (GND) of the power supply, and the contact must be reliable.

Figure 5 Oscilloscope probe connection method

By the way, it is suggested that the universal probe of the oscilloscope cannot be used, because the ground wire of the universal oscilloscope probe is not shielded and is long, which is easy to pick up interference from the external electromagnetic field, causing a large noise output. The larger the dotted area, the greater the impact of interference, as shown in Figure 6.

Figure 6 Universal probe is prone to interference

4 Coaxial cable measurement device

Here are two coaxial cable measurement devices. Figure 7 shows that the output end of the power supply under test is connected to the R and C circuits and then connected to the AC input end of the oscilloscope through the input coaxial cable (50Ω); Figure 8 shows that the coaxial cable is directly connected to the power output end, and a 0.68μF ceramic capacitor and a 47Ω/1w carbon film resistor are connected in series at both ends of the coaxial cable before being connected to the oscilloscope. The connection of the T-shaped BNC connector and the capacitor and resistor is shown in Figure 9.

Figure 7 Coaxial cable measurement device 1

Figure 8 Coaxial cable measurement device 2

Figure 9 Connection of T-shaped BNC connector and capacitor and resistor

Measurement standard of ripple and noise

The above introduces a variety of measurement devices. If different measurement devices are used for the same power supply under test, the measurement results will be different. If the same standard measurement device can be used for measurement, the measurement results will be comparable.

Figure 10 Measurement standard measurement device

The national standard stipulates that two capacitors C2 and C3 should be connected in parallel less than 150mm from the positive and negative ends of the power supply under test. C2 is a 22μF electrolytic capacitor and C3 is a 0.47μF film capacitor. The connection end of these two capacitors is connected to a load and a 50Ω coaxial cable not longer than 1.5m. The other end of the coaxial cable is connected to a 50Ω resistor R and a 4700pF capacitor C1 in series and then connected to an oscilloscope. The bandwidth of the oscilloscope is 100MHz. The connecting wires at both ends of the coaxial cable should be as short as possible to prevent picking up radiated noise. In addition, the longer the line connecting the load is, the greater the measured ripple and noise voltage will be. In this case, it is necessary to connect C2 and C3. If the ground wire of the oscilloscope probe is too long, the ripple and noise measurement cannot be accurate.

In addition, the test should be carried out under greenhouse conditions, and the power supply under test should input normal voltage and output rated voltage and rated load current.

Measures to reduce ripple and noise voltage

In addition to switching noise, the AC power input in the AC/DC converter is full-wave rectified and capacitor filtered, and the current waveform is pulsed, as shown in Figure 11 (Figure a is a full-wave rectifier and filter circuit, and b is a voltage and current waveform). There are high-order harmonics in the current waveform, which will increase the noise output. A good switching power supply (AC/DC converter) adds a power factor correction (PFC) circuit to the circuit to make the output current approximate a sine wave, reduce high-order harmonics, and increase the power factor to about 0.95, reducing pollution to the power grid. The circuit diagram is shown in Figure 12.

Figure 11 Switching power supply rectifier waveform

Figure 12 Switching power supply PFC circuit

The output ripple and noise voltage of the switching power supply or module are related to the topology of its power supply, the design of each part of the circuit and the PCB design. For example, the use of a multi-phase output structure can effectively reduce the ripple output. The switching frequency of today's switching power supplies is getting higher and higher; the lowest is tens of kHz, generally hundreds of kHz, and the highest can reach more than 1MHz. Therefore, the frequency of the ripple voltage and noise voltage generated is very high. The simplest way to reduce ripple and noise is to add a passive low-pass filter to the power supply circuit.

1 Measures to reduce EMI

Metal casing can be used for shielding to reduce external electromagnetic field radiation interference. In order to reduce electromagnetic interference input from the power line, an EMI filter is added to the power input end, as shown in Figure 13 (EMI filter is also called power filter).

Figure 13 Switching power supply with EMI filtering

2 Use capacitors with good high-frequency performance and low ESR at the output end

It is best to use aluminum or tantalum electrolytic capacitors with high molecular polymer solid electrolyte as output capacitors. They are characterized by small size and large capacitance, low ESR impedance at high frequency, and large allowable ripple current. It is most suitable for high-efficiency, low-voltage, high-current step-down DC/DC converters and DC/DC module power supplies as output capacitors. For example, a high molecular polymer tantalum solid electrolytic capacitor is 68μF, and its maximum equivalent series resistance (ESR) at 20℃ and 100kHz is 25mΩ, and the maximum allowable ripple current (at 100kHz) is 2400mArms. Its size is: 7.3mm (length) × 4.3mm (width) × 1.8mm (height), and its model is 10TPE68M (chip or package).

The ripple voltage ΔVOUT is:

ΔVOUT=ΔIOUT×ESR (1)

If ΔIOUT=0.5A, ESR=25mΩ, then ΔVOUT=12.5mV.

If ordinary aluminum electrolytic capacitors are used as output capacitors, the rated voltage is 10V, the rated capacitance is 100μF, the equivalent series resistance is 5.0Ω at 20℃ and 120Hz, and the maximum ripple current is 70mA. It can only work at around 10kHz and cannot work at high frequencies (frequencies above 100kHz). Increasing the capacitance is also ineffective because it has become an inductive characteristic when it exceeds 10kHz.

For some power supplies with switching frequencies between 100kHz and several hundred kHz, the use of multilayer ceramic capacitors (MLCC) or tantalum electrolytic capacitors as output capacitors is also effective, and its price is much lower than that of polymer solid electrolyte capacitors.

3 Use frequency synchronization with the product system

To reduce output noise, the switching frequency of the power supply should be synchronized with the frequency in the system, that is, the switching power supply uses the frequency of the external synchronous input system to make the switching frequency the same as the system frequency.

4 Avoid mutual interference between multiple module power supplies

There may be multiple module power supplies working together on the same PCB. If the module power supplies are not shielded and are close together, they may interfere with each other and increase the output noise voltage. To avoid this mutual interference, shielding measures can be used or they can be kept away appropriately to reduce the interference caused by their mutual influence.

5 Add LC filters

For To reduce the ripple and noise of the module power supply, LC filters can be added to the input and output of the DC/DC module, as shown in Figure 14. The left figure of Figure 14 is a single output, and the right figure of Figure 14 is a dual output.

Figure 14 Adding LC filter to DC/DC module

The capacitance values of the VIN and VOUT terminals of the 1W DC/DC module at different output voltages are listed in Tables 1 and 2. It should be noted that the capacitance cannot be too large to cause starting problems. The resonant frequency of LC must be staggered with the switching frequency to avoid mutual interference. L uses a μH pole and its DC resistance must be low to avoid affecting the output voltage accuracy.

Table 1 and Table 2

6 Add LDO

Adding a low-dropout linear regulator (LDO) after the output of the switching power supply or module power supply can greatly reduce the output noise to meet the needs of circuits with special requirements for noise (see Figure 15), and the output noise can reach the μV level.

Figure 15 Add LDO to the power supply

Since the voltage difference of LDO (the difference between input and output voltage) is only a few hundred mV, the standard voltage can be output when the output of the switching power supply is slightly higher than the LDO by a few hundred mV, and its loss is not large.

7 Add active EMI filter and active output ripple attenuator

The active EMI filter can attenuate common mode and differential mode noise between 150kHz and 30MHz, and is particularly effective in attenuating low-frequency noise. At 250kHz, it can attenuate 60dB common mode noise and 80dB differential mode noise, and the efficiency can reach 99% at full load.

The output ripple attenuator can reduce the output ripple and noise of the power supply by more than 30dB in the range of 1 to 500kHz, and can improve dynamic response and reduce output capacitance.

Many people often appear hundreds of mV or hundreds of mV when testing ripple and noise, which is much larger than the ripple value provided in the manual. This is mainly caused by incorrect testing methods. Several misunderstandings about ripple testing.

Myth 1: The selection of test bandwidth, the larger the bandwidth, the more accurate the test

This belief is incorrect. The frequency of the output ripple is the same as the switching frequency of the power supply, and the switching frequency is currently generally from tens of KHZ to several MHZ. In addition, the interference caused by the switching device is also less than 20MHZ. The bandwidth is limited to 20MHZ, which is also to avoid the external high-frequency noise from affecting the ripple test. In general, the module manual will mention the oscilloscope test bandwidth selected by the module when testing the ripple. Usually there is no special description, and the bandwidth of the ripple test is generally set to 20MHZ. Currently, the oscilloscopes on the market have a 20MHZ bandwidth limit function.

Myth 2: Selection of test method

The selection of test methods is currently controversial. The same module will get different results using different test methods. At present, there are four popular methods in the industry: leaning test method, twisted pair method, parallel line method, and 50-ohm coaxial cable test. Their purpose is only one, which is to test the output ripple of the module in a real and objective way. Due to various objective factors, users generally use the wire-throwing method, which is to take the oscilloscope probe and the ground wire clip and directly connect them to the output pin of the module for testing. This method cannot be said to be incorrect, but it will bring great differences to the test results, generally reaching hundreds or hundreds of millivolts of ripple.

The ground wire length of the oscilloscope probe is about 13cm, and its own inductance is about 80nH. The common-mode current will generate a certain amount of non-negligible spike voltage on the ground wire clip. In actual testing, the ground wire clip usually appears in a ring, so it is easy to receive space radiation. The loop formed by the test terminal and the ground wire clip works like an antenna. The larger the area of the ground wire loop, the greater the noise obtained during the switching process, which affects the correct test of the ripple. In order to reduce the impact caused by the excessive length of the ground wire clip, the probe should be directly against the two ends of the output pin, so that the area of the ground wire loop where the signal and the ground are connected is very small. This is the leaning test method. When testing, remove the ground wire clip and probe cap of the oscilloscope probe, and test directly on the output pin. If the output pin spacing is slightly larger, the oscilloscope probe cannot be directly placed on it. You can use a homemade ground wire ring for testing, as shown in the figure below.

(Left) Direct testing with ground wire clip (Right) Testing with the ground wire test method

For some specific occasions that require low ripple output, a specific design solution is required. The wire throwing method can also obtain relatively small ripples. Xi'an Weijing Electronic Manufacturing Co., Ltd. has launched two high-reliability military power supply modules with input of 16VDC-40VDC, output of 5VDC, 12VDC, 15VDC, ±5VDC, ±12VDC, ±15VDC, six output voltages, output power of 15W, built-in input filter and low ripple output. One adopts all-metal airtight packaging, and the other adopts a pentahedral metal structure with excellent thermal conductive potting glue. Both modules use the line-swinging method to test the 20MHZ bandwidth, and the ripple is 20-50mv. In addition, the module can pass the requirements of CE102 in GJB151-97 without an external filter.