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Dongguan led waterproof power supply, switching power supply ripple measurement
Author:AdminDate:2013-9-28View:3456

Dongguan led waterproof power supply, switching power supply ripple measurement

Basic requirements: the use of oscilloscope AC coupling, 20MHz bandwidth limit, wirepull probe

1, the AC coupling is superimposed DC voltage is removed, get accurate waveform.

2, open the 20MHz bandwidth limit is to prevent the interference of high frequency noise, preventing measure error results. Because the high frequency components oflarger amplitude, measurement time should be removed.

3, remove the ground clip of oscilloscope probe, using ground loop measurement, in order to reduce the interference. Many departments have no ground ring, if the error is allowed to directly probe for measuring grounding clip. But in the qualified to judge whether to consider this factor.

Another point is to use the 50 termination. Henghe oscilloscope data says, 50 Ω module is to remove DC components, accurate measurement of AC components. But there are few oscilloscope with the special probe, in most cases is measured using the standard 100K ohm to 10M ohm, effect of temporary not clear

The above is the basic measure of attention switching ripple. If the oscilloscope probeis not in direct contact with the output, should use the twisted pair or coaxial cable,measuring 50 way.

In the measurement of high frequency noise, the use of oscilloscope all pass, usuallyseveral hundred megabytes to GHz level. The other with the same.

There are different testing methods may be different companies. In the final analysisfirst to clear their own results. Second to get customer approval.

On the oscilloscope:

Some digital oscilloscope because of interference and storage depth, cannot be accurately measured. Should the replacement oscilloscope. This sometimes althoughanalog oscilloscope bandwidth old only tens of megabytes, but performance than thedigital oscilloscope. The Tektronix Inc has dedicated to separate measurements of the two kind of ripple (noise) software, can look at the reference 5. Similarly,grounding on the oscilloscope, knowledge power test, also can look at the.

Suppression of switching power supply ripple

For switching ripple, both theory and practice exists. Usually to inhibit or reduce itsapproach has three:

1, increase the inductance and the output filter capacitor

According to the formula of switching power supply, current fluctuations in size andinductive inversely proportional to the value, the output ripple and output capacitanceis inversely proportional to. So to increase the inductance and the output capacitance value can reduce the ripple.

Similarly, the relationship between the output ripple and output capacitors: the vripple=Imax/ (Co × f). As can be seen, increase the output capacitance value can reduce the ripple.

The usual practice, for the output capacitor, the use of aluminum electrolytic capacitor in order to achieve the purpose of large capacity. But the electrolytic capacitor effect in suppressing high frequency noise is not very good, but ESR is relatively large, so will the next to it in parallel with a ceramic capacitor, to make up for the deficiency of aluminum electrolytic capacitor.

At the same time, switch power supply, voltage Vin input current is constant, butchanges with the switch. When the input power is not a good way to provide current,usually near the current input (taking BucK as an example, is near SWITcH), shunt capacitor to provide current.

The above approach is limited to reduce the ripple effect. Because of size restrictions, inductance will not do much; the output capacitor is increased to a certain extent, to reduce the ripple has no obvious effect; increasing the switching frequency,and increase the switching loss. So in the more stringent requirements, this method is not very good. On the principle of the switch power, can refer to all kinds of switching power supply design manual.

2, the two stage filter, is coupled with a LC filter

Inhibitory effect of LC filter to the noise ripple obviously, according to the ripple frequency to remove the selection of suitable composition inductor and capacitor filter circuit, the general can well reduce the ripple.

Sampling point selection in the LC filter before (Pa), the output voltage will decrease.Because any inductor has a DC resistance, when the current output, the inductancewill drop, causing the output voltage of the power supply to reduce. And this pressure drop is the change in output current.

Sampling points selected in the LC filter (Pb), so that the output voltage is what wehope to get the voltage. But in the power supply system is introduced into the interior of an inductor and a capacitor, may lead to system instability. On the system stability,a lot of information are introduced, there is no detailed written.

3, after the output switching power supply, the LDO filter

This is the most effective way to reduce the ripple and noise, constant output voltagefeedback system, does not need to change the original, but also the highest cost,high way power. Any one of the LDO have an index: noise suppression ratio. Afrequency of the -dB curve, the LT3024 curve is linear technology corporation.

To reduce the ripple. PCB routing switch power supply is also very important, this is a very he hand problem. The switching power supply PCB engineer specialized, thehigh frequency noise, due to high frequency large amplitude level after the filter,although there are certain effect, but the effect is not obvious. A special research on this aspect, simple way is in the diode and capacitor of C or RC, or a series inductance.

4, the diode and capacitor C or RC

Diode high speed on-off time, want to consider parasitic parameters. In the diode reverse recovery period, equivalent inductance and capacitance becomes a RCoscillator, high-frequency oscillation. In order to suppress the high frequency oscillation, the diode connected in parallel with a capacitor C or RC buffer network.General from 10 Ω -100 Ω resistor, capacitor 4.7pF-2.2n

In parallel with the diode capacitance on C or RC, its value after repeated

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