ripple test method

power supply ripple are roughly divided into two types: one is voltage signal measurement method; The other is current signal measurement. Voltage signal measurement ripple refers to the use of an oscilloscope to measure the AC ripple voltage signal superimposed on the DC voltage signal.

1?

ripple is defined as the AC component superimposed on the DC stability in the DC voltage or current. It mainly has the following harm: 1.1. It is easy to generate harmonics on electrical appliances, and harmonics will produce more harm; 1.2. Reduce the efficiency of the power supply; 1.3. Strong ripple will cause surge voltage or current The generation of electricity causes the burning of electrical appliances; 1.4. It will interfere with the logical relationship of digital circuits and affect their normal operation; 1.5. It will cause noise interference, make the image equipment and audio equipment not work properly

2. ripple and ripple coefficient can be expressed by effective value or peak value, or by absolute quantity and relative quantity. The unit is usually: mV. For example, a power supply works in a regulated state, its output is 12V5A, and the measured effective value of ripple is 10mV. This 10mV is the absolute amount of ripple, while the relative amount, that is, ripple coefficient = ripple voltage/output voltage = 10mv/12V = 0.12%.

3. Ripple

3.1. Taking the bandwidth of 20M oscilloscope as the limiting standard, the voltage is set to PK-PK (there are also effective values), remove the clip and ground wire on the oscilloscope control head (because the clip and ground wire form a loop, like an antenna receiving noise and introducing some unnecessary noise), use a grounding ring (it is also possible not to use a grounding ring, but the error generated by it must be considered), A 10UF electrolytic capacitor and a 0.1UF ceramic capacitor are connected in parallel to the probe, and the probe of the oscilloscope is used for direct testing. If the oscilloscope probe is not directly in contact with the output point, it should be measured by twisted pair or 50 Ω coaxial cable.

4. switching power supply ripple The output ripple of switching power supply mainly comes from five aspects: 4.1. Input low frequency ripple; 4.2. High frequency ripple; 4.3. Common mode ripple noise caused by parasitic parameters; 4.4. UHF resonant noise generated during switching of power devices; 4.5. Ripple noise caused by closed-loop regulation control.

5. Power Supply Ripple Test

ripple is an AC interference signal superimposed on a DC signal and is a very important standard in power supply testing. Especially for power supplies for special purposes, such as laser power supplies, ripple is one of its fatal points. Therefore, the test of power ripple is extremely important.

power supply: one is voltage signal measurement; The other is current signal measurement.

Generally, voltage signal measurement can be used for constant voltage sources or constant current sources with little ripple performance requirements. For constant current sources with high ripple performance requirements, it is best to use current signal measurement.

voltage signal measurement ripple refers to measuring AC ripple voltage signal superimposed on DC voltage signal with oscilloscope. For constant voltage sources, the test can directly measure the voltage signal output to the load with a voltage probe. For the test of a constant current source, the voltage waveform across the sampling resistor is generally measured by using a voltage probe. During the whole test process, the setting of oscilloscope is the key to sampling the real signal.

is a TDS1012B oscilloscope equipped with a voltage measuring probe. The following settings are required before measurement.

1. Channel settings: the channel coupling method. Ripple is an AC signal superimposed on a DC signal. Therefore, if we want to test the ripple signal, we can remove the DC signal and directly measure the superimposed AC signal. Broadband limit: turn off the probe: first select the voltage probe. Then select the attenuation ratio of the probe. It must be consistent with the attenuation ratio of the actual probe used, so that the number read from the oscilloscope is the real data. For example, if the voltage probe used is placed in × 10 gear, the probe option here must also be set to × 10 gear.

2. Trigger setting: Type: Edge Source: The channel that you have selected. For example, if you are going to test with the CH1 channel, you should select CH1. Slope: Up. Trigger mode: if the ripple signal is observed in real time, select' auto' trigger. The oscilloscope will automatically follow the changes of the actual measured signal and display it. At this time, you can also display the measurement value you need in real time by setting the measurement button. However, if you want to capture the signal waveform during a certain measurement, you need to set the trigger method to' normal' trigger. At this time, you also need to set the trigger level. Generally, when you know the peak value of the signal you are measuring, set the trigger level to 1/3 of the peak value of the measured signal. If you don't know, the trigger level can be set slightly smaller. Coupling: DC or AC..., AC coupling is generally used.

3. Sample length (seconds/grid): The sample length is set to determine whether the required data can be sampled. When the set sampling length is too large, the high-frequency components in the actual signal will be missed; when the set sampling length is too small, only the part of the measured actual signal can be seen, and the real actual signal cannot be obtained. Therefore, in the actual measurement, you need to rotate the button back and forth and observe carefully until the displayed waveform is a true and complete waveform.

4. Sampling method: You can set the sampling method as required. For example, if it is required to measure the P-P value of ripple, it is best to choose the peak measurement method. The number of samples can also be set according to actual needs, which is related to the sampling frequency and sampling length.

5. Measurement: By selecting the peak value of the corresponding channel, the oscilloscope can help you display the required data in time. At the same time, you can also select the frequency, maximum value, root mean square value, etc. of the corresponding channel.

, through reasonable setting and standardized operation of oscilloscope, the required ripple signal can be obtained. However, during the measurement process, attention must be paid to prevent other signals from interfering with the oscilloscope probe itself, so as not to make the measured signal untrue.

Ripple value measured by current signal measurement refers to measuring AC ripple current signal superimposed on DC current signal. For constant current sources with high ripple index requirements, that is, constant current sources with small ripple ratio, more real ripple signals can be obtained by direct current signal measurement method. Unlike the voltage measurement method, a current probe is also used here. For example, continue to use the above oscilloscope, add a current amplifier and a current probe. At this time, only by clamping the current signal output to the load with the current probe, the current measurement method can be carried out to measure the ripple signal of the output current. Like the voltage measurement method, the setting of the oscilloscope and the current amplifier is the key to sampling the real signal during the whole test process.

In fact, when measuring in this way, the basic settings and usage of the oscilloscope are the same as above. The difference is that the probe settings in the channel settings are different. Here, it is necessary to choose the current probe mode. Then, select the ratio of the probe, which must be the same as the ratio set by the amplifier, so that the number read from the oscilloscope is the real data. For example, if the ratio of the amplifier used is set to 5A/V, then this item of the oscilloscope also needs to be set to 5A/V. As for the coupling mode of the current amplifier, when the channel coupling of the oscilloscope has been selected as AC coupling, either AC or DC can be selected here.

, it should be noted that when using this method, you need to turn on the oscilloscope before turning on the current amplifier. Also, remember to demagnetize the current probe before use.

In addition, measuring power ripple itself has certain skills. Figure 1 gives an example of improper use of an oscilloscope to measure power supply ripple. In this example, several errors occurred. First, an oscilloscope probe with a long ground wire was used. The second is to allow the loop formed by the probe and the ground wire to be close to the power transformer and switching element. Finally, it allows additional inductance to be formed between the oscilloscope probe and the output capacitor. As a result, the problem is that the picked-up high-frequency components are carried in the measured ripple waveform.

in the power supply, there are many high-speed, high-voltage and current signal waveforms that are easily coupled into the probe, including magnetic field coupling from the power transformer, electric field coupling from the switching node, and common mode current generated by the transformer's interwinding (interwinding) capacitance.

the correct measurement technology can effectively improve the ripple measurement results.

Figure 1: Improper ripple measurement results in poor results.

First of all, the upper bandwidth limit of ripple is usually set to avoid picking up high-frequency noise that exceeds the upper bandwidth limit of ripple. An appropriate upper bandwidth limit should be set for the oscilloscope used for measurement.

Secondly, the antenna formed by the grounding long lead can be removed by removing the "hat" of the probe. As shown in Figure 2, we wrap a short wire around the probe ground lead and connect it to the power supply ground. The side benefit of this is to shorten the length of the probe exposed to high-intensity electromagnetic radiation near the power supply, thereby further reducing high-frequency pickup.

Finally, in the isolated power supply, the real common mode current is generated by the current flowing in the probe ground lead, which causes a voltage drop between the power supply ground and the oscilloscope ground, which is manifested as ripple. To suppress this ripple, the common mode filtering problem needs to be carefully considered in the power supply design.

In addition, winding the oscilloscope lead on the iron core can reduce this current, because this will form a common mode inductance that does not affect the differential voltage measurement, but can reduce the measurement error caused by the common mode current.

Figure 2 shows the ripple voltage measurement results obtained for the same circuit using the improved measurement technology. It can be seen that the high-frequency spikes have been almost eliminated.

Figure 2: Four simple improvements greatly improve the measurement results.

In fact, when the power supply is integrated into the system, the power supply ripple performance will be even better. There is almost always a certain amount of inductance between the power supply and the rest of the system. The inductance may be formed by wires or etched lines on the printed circuit board, and there will always be an additional bypass capacitor as a power supply load near the chip, both of which form a low-pass filtering effect and further reduce power supply ripple and/or high frequency noise.

to take an extreme example, a filter composed of a short line with an inductance of 15nH and a bypass capacitor with a capacity of 10 μF has a cutoff frequency of 400kHz. This example means that high frequency noise can be greatly reduced. The cut-off frequency of the filter is many times lower than the ripple frequency of the power supply, which can effectively reduce the ripple. Smart engineers should try to make use of it during testing.

1. Classification and characteristics of AC stabilized power supply:

power supply capable of providing a stable voltage and frequency is called AC stable power supply. At present, most domestic manufacturers do the work of AC voltage stability. The following is a brief description of the classification characteristics of AC stabilized power supplies in the market.

parameter adjustment (resonance) type

this kind of regulated power supply, the basic principle of voltage regulation is LC series resonance, and the early magnetic saturation regulator belongs to this category. Its advantages are simple structure, no numerous components, high reliability, wide voltage range, strong anti-interference and overload resistance. The disadvantages are high energy consumption, high noise, heavy weight and high cost.

on the basis of the principle of magnetic saturation and the "magnetic amplifier adjustable electronic AC regulator" (I. e. 614 type) that has been popular in my country in the 1950 s are both AC regulators of this principle.

auto-coupling (variable ratio) adjustment type

1. Mechanical voltage regulation type, that is, the carbon brush is driven by a servo motor to move on the winding sliding surface of the autotransformer to change the ratio of Vo to Vi to realize the adjustment and stability of the output voltage. This kind of voltage regulator can range from a few hundred watts to a few kilowatts. It is characterized by simple structure, low cost, and low output waveform distortion; however, due to the sliding contact of the carbon brush, it is easy to generate electric sparks, causing the brush to be damaged and even burned and fail; and the voltage adjustment speed is slow.

2. Change the tap type, make the autotransformer into multiple fixed taps, and use relays or thyristors (solid state relays) as 10 switches to automatically change the tap position to stabilize the output voltage.

The advantages of this type of voltage stabilizer are simple circuit, wide voltage stabilization range (130V-280V), high efficiency (≥ 95%) and low price. The disadvantage is that the voltage stabilization accuracy is low (8 ~ 10%) and the working life is short. It is suitable for families to supply power to air conditioners.

high-power compensation-purification regulator (including precision regulator)

it uses compensation link to stabilize the output voltage and is easy to realize microcomputer control.

its advantages are good anti-interference performance, high voltage stabilization accuracy (≤ 1%), fast response (40~60ms), simple circuit and easy work

shit. Disadvantages are: with computer, program-controlled switch and other nonlinear loads have low-frequency oscillation phenomenon; input side current distortion is large, the source power factor is low; the output voltage has a phase shift to the input voltage. For units with high requirements for anti-interference function, it is appropriate to apply it in cities. When the computer supplies power, it must be used with a voltage stabilizer about 2-3 times of the total power of the computer. Because it has the advantages of voltage stabilization, anti-interference, fast response speed, moderate price, etc., it is widely used.

switching type AC regulated power supply

it is applied to high-frequency pulse width modulation technology, the difference from general switching power supplies is that its output must be an AC voltage with the same frequency and phase as the input side. Its output voltage wave types include quasi-square wave, ladder wave, sine wave, etc. The uninterruptible power supply (UPS) on the market drains the storage power supply and charger, which is a switching type AC stabilized power supply with good voltage stability, strong control function and easy realization of intelligence. It is a very promising AC stabilized power supply. However, due to its complicated circuit and high price, its promotion is slow.

2. Types and Selection of DC Stable Power Supply:

DC stable power supply can be divided into chemical power supply, linear stable power supply and switching stable power supply according to habits. They have different types respectively:

chemical power supply

the dry batteries, lead-acid batteries, nickel-cadmium, nickel-hydrogen, and lithium-ion batteries we usually use belong to this category, each with its own advantages and disadvantages.

With the development of science and technology, intelligent batteries have been produced; in terms of rechargeable battery materials, American developers have discovered a kind of iodide of manganese, which can be used to produce cheap, compact, long discharge time, and still maintain performance after multiple charges. Good environmentally friendly rechargeable battery.

linear stable power supply

linear stable power supply is that its power device regulating tube works in the linear area and stabilizes the output by the voltage drop between the regulating tubes. Due to the large static loss of the adjusting tube, a large radiator needs to be installed to dissipate heat. And because the transformer works at power frequency (50Hz), it is heavier.

The advantages of this kind of power supply are high stability, small ripple, high reliability, easy to make multi-channel, and continuously adjustable output. Disadvantages are large size, bulkiness, and relatively low efficiency. There are many kinds of stable power supplies. From the output nature, they can be divided into stabilized power supply and stabilized current power supply, and stabilized current (bistable) power supply that integrates stabilized voltage and stabilized current. Judging from the output value, it can be divided into fixed-point output power supply, band switch adjustment type and potentiometer continuously adjustable type. From the output indication, it can be divided into pointer indication type and digital display type.

switching type DC stabilized power supply

a kind of stable power supply different from linear regulated power supply is switching type DC regulated power supply. Its circuit types mainly include single-ended flyback type, single-ended forward type, half-bridge type, push-pull type and full-bridge type. The fundamental difference between it and a linear power supply is that its transformer does not work at power frequency but at tens of kilohertz to several megahertz. The function tube does not work in the saturation and cut-off zone, that is, the switching state; the switching power supply is named after it.

switching power supply has the advantages of small size, light weight, stability and reliability. Disadvantages: Compared with linear power supply, ripple is relatively large (generally ≤ 1% VO(P-P), good can achieve more than ten mV(P-P) or less). Its power can range from several watts to several kilowatts. The price is 3 yuan-more than 100,000 yuan/watt. The following is a classification of several switching power supplies:

①, AC/DC power supply

This kind of power supply is also called primary power supply. It obtains energy from the power grid and obtains a DC high voltage through high-voltage rectification and filtering. The DC/DC converter obtains one or several stable DC voltages at the output end. The power ranges from several watts to several kilowatts. All products are used in different occasions. There are many specifications and models of this type of product, and the primary power supply (AC220 input, DC48V or 24V output) in the communication power supply according to user needs is also this.

class.

(2) DC/DC power supply is also called secondary power supply in communication system. It is a DC input voltage provided by primary power supply or DC battery pack, and one or several DC voltages are obtained at the output end after DC/DC conversion.

③. Communication power supply

communication power supply is essentially a DC/DC converter power supply, but it is generally powered by DC -48V or -24V, and uses a backup battery as a backup of DC power supply to convert the DC power supply voltage into the working voltage of the circuit. Generally, it is divided into three types: central power supply, layered power supply and single board power supply, with the latter having the highest reliability.

④. Radio power supply

radio station is AC220V/110V, and the output is DC13.8V. The power depends on the power of the supplied station. There are products in several amps and several hundred amps. In order to prevent the power failure of AC power grid from affecting the work of the radio station, a battery pack is required as a backup, so this kind of power supply not only outputs a 13.8V DC voltage, but also has the function of automatic conversion of battery charging.

⑤. Module power

With the rapid development of science and technology, the requirements for power reliability and capacity/volume ratio are getting higher and higher, and the module power supply shows its superiority more and more. It has high working frequency, small size, high reliability, easy installation and combined expansion, so it is more and more widely used. At present, although there are corresponding modules produced in China, the failure rate is high due to the failure of the production process to catch up with the international level.

Although the cost of DC/DC module power supply is relatively high at present, from the perspective of the overall cost of the long application cycle of the product, especially the high maintenance cost and goodwill loss caused by system failure, it is still cost-effective to choose this power module. What is worth mentioning here is the Roche converter circuit. Its outstanding advantage is the simple circuit structure, the ripple value of high efficiency and output voltage and current is close to zero.

⑥. Special power supply

high voltage and small current power supply, high current power supply, 400Hz input AC/DC power supply, etc. can be classified as such and can be selected according to special needs. The price of switching power supply is generally 2-8 yuan/watt. The price of special small power and high power power supply is slightly higher, reaching 11-13 yuan/watt.