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1. Forewords:
Nowadays,
switching power supplies have replaced traditional linear power supply; become
the power source of most electronics equipment. It is being used in the desktop
computer, monitor, printer, notebook computer, fax machine, photocopy machine
and etc. The reason is because the advantage of smaller size, weight less and
high efficiency, however, the bad thing of it is the noisy output. This article
will discuss about the noise in switching power supply with DC output.
1.1. Definition:
Definition of output noise: the noise looks like a sine wave which overlaps
on the DC output, this sine wave contain PARD (Periodic and Random Deviation)
noise and it looks like the wave in Figure 1.
Figure 1 switching power supply's output noise
Since switching power supply use high switching frequency (>20kHz), accompany
by PWM (Pulse Width Modulation) and output filtering circuit, it can transform
the household electricity (AC) into DC voltage which IC circuit need, for example
+5V, +12V, -5V, -12V, +3.3V, and so on. The only problem with the switching
power supply is some sine wave in the DC output. If this sine wave (will call
noise from now on) is large enough, it will cause malfunction in the application
circuitry. We use 5V for example, if the noise is 1.0Vp-p, it will exceed the
4.75V-5.25V tolerance (a normal working voltage for logic IC), which can cause
malfunction or system shutdown, therefore output noise has a lot of influence.
Normally the output noise of switching power supply is control under 1% of the
output voltage, for example for +5V, +12V, its' output noise specification should
be 50mVp-p and 120mVp-p.
1.2. Controlling Output Noise:
When developing a switching power supply, the output noise has to be set
within a certain specification. During production, things like the parts (such
as transformer, diode, filtering capacitor and so on) with different material,
incorrect assemble, missing parts and so on that cause noise level over the
specification. To avoid these problems, checking output noise of each switching
power supply is a must during quality control.
2. Output
Noise Testing:
The
testing equipment can be either oscilloscope or ripple/noise meter, the differences
are describe as follow:
2.1. Oscilloscope:
This is the most popular equipment use for testing, but pay attention to
following condition otherwise the test result will not be accurate.
2.1.1. Avoid Ground Return: Differential input oscilloscope should be used,
because the BNC negative input in general oscilloscope is connected to the case
of the oscilloscope, and the case of the oscilloscope is connected to outside
line ground. If use a probe to measure the DC output, it can cause ground return
current which will affect the measurement. Because of that, ground return should
be avoided. Using a differential oscilloscope or oscilloscope with a
external differential amplifier is the correct method for testing connection,
see figure 2, INTEL suggested test connection method. Figure 3, HP suggested
test connection method. Both INTEL and HP suggestion use differential oscilloscope;
HP differential amplifier is shown in figure 4, as Tek differential probe is shown in figure 5.
Note: 7A13 plug-in is differential amplifier
Figure 2 PARD differential mode test connection diagram suggested by INTEL 
Figure 3 PARD differential mode test connection diagram suggested by HP
Figure 4 HP 1141A Differential Probe with HP 1142A
Power Supply
Figure 5 Tek P6046 Active Differential System
2.1.2. Test Condition:
When adding capacitor to the terminal during
test, the capacitor should be mentioned which include its material, capacitance
and etc., like figure 2 which INTEL add 10uF and 0.1uF capacitor in order to
simulate the capacitance on the main board. There is no rule set for this test
procedure; it is all depend on the condition. The capacitor should be mentioned
if it is used during test, like figure A and figure B recommended connecting
method, otherwise there will be big difference in the test results.
2.1.3. Frequency Bandwidth:
The
frequency bandwidth of the oscilloscope will affect the test result, usually
the standard is 20MHz, or 30MHz (INTEL test requirement), and the sampling frequency
for digital oscilloscope should be more than twice of the PARD frequency.
2.1.4. Input Resistance:
Usually we use 50 Ohm. For low input resistance like 50 (the oscilloscope can
be set to 1M) at the terminal for the purpose of eliminating the signal interference.
There is no specific requirement for this part, but the input resistance should
be mentioned, otherwise there will be big difference in the test results too.
2.2 Ripple and Noise Meter:
Due
to the reason that most of the oscilloscope have only two channel, it would
be difficult to monitor the power supply with many output, addition disadvantage
like it need human eye to observe and compare, and high cost. Therefore using
Prodigit 4030 (up to 4 channel) with ripple noise meter, it
can do the test in one time with upper and lower limit shown, plus the cost
is reasonable (much cheaper than oscilloscope), so this is a very good test
equipment, 4030 function block diagram is shown in figure 6. Its main specification
is described as below:
4030 Ripple/Noise (Peak to Peak) Meter Block Diagram
Figure 6 4030 Function Block Diagram ( Large picture
)
2.2.1. Input Structure:
Like
the oscilloscope description, the connection should avoid ground return, therefore
It is necessary to use differential mode, and the input
resistance normally is 50W (which can effectively eliminate the input noise).
2.2.2. Input Range:
Usually,
the specify noise for DC switching power supply output is 1% of its DC output,
for example 5 volts DC output, its output noise should be less than 50mVp-p,
for 12 volts DC output, the noise should be below 120mVp-p. Prodigit
4030 has 3 input range options to choose from when order, they are 3.0Vp-p,
1.5Vp-p, 0.75Vp-p respectively. Normally for 5V, 12V system voltage, choosing
option with 0.75Vp-p input range should be enough, for 24V, 48V system voltage,
1.5Vp-p or 3.0Vp-p option should be chosen.
2.2.3. Frequency Bandwidth:
Prodigit 4030 Ripple/Noise meter include 3 frequency bandwidth for
user to choose from, they are 20Hz~200KHz, 20Hz~2MHz and 20Hz~50MHz. 20Hz~50MHz
is mainly for checking output noise, 20Hz~200KHz or 20Hz~2MHz is for checking
output ripple or dynamic load over-shoot or under-shoot. Frequency bandwidth
range of 20Hz~50MHz means 4030 meter can measure the response of peak to peak
frequency range in input signal (noise). It usually means -3dB (0.707 X) frequency
range. For example if the input signal is 100mVp-p, at 50 MHz, 4030 will measure
70.7mV or more. 4030 Ripple/Noise meter classic frequency respond is shown in
figure 7.
Figure 7 4030 ripple noise meter typical frequency respond
2.2.4. Frequency Spectrum of the Power Supply output noise:
Power supply output noise includes many different frequency
(like 50Hz, 60Hz ripple on off frequency) and other noise. If output noise has
narrow and sharp FET with ON/OFF spike, it will contain many high frequency,
we can use Spectrum Analyzer to observe the frequency contain, figure 8, 9 are
the power supply output noise wave form and spectrum. When using 4030 Ripple/Noise
meter to measure noise, picking different frequency bandwidth will cause its
value to change. Normally, high frequency bandwidth (20Hz~50MHz) will result
in higher value than using low frequency bandwidth.
Figure 8 typical power supply output noise
Figure 9 Typical power supply output noise's spectrum
2.2.5. How to Define Pass/Fail with 4030
4030 has the ability to measure and compare between the upper and lower limit.
In every single PARD measure channel, when measure value is within upper and
lower limit, it will give a GO or Pass, if measure value is outside the upper
and lower limit, it will give a NG or Fail; if all four PARD measure channel
pass, green LED that shows PASS on the panel will light up, if there is one
fail the red LED that shows FAIL will light up on the panel, to show that it
doesn't pass the test condition. Every 4030 PARD measuring channel has its own
upper and lower limit value, operator can set its value individually, and its
PASS/FAIL comparison function block diagram is shown in figure 10.
Note: 4030 panel will light up green PASS LED when pass, red FAIL LED will light
up when fail.
If using logic symbol to represent, the result will be like as follow:
PASS = (PARD1L!OPARD1!OPARD1H) AND (PARD2L!OPARD2!OPARD2H) AND (PARD3L!OPARD3!OPARD3H)
AND (PARD4L!OPARD4!OPARD4H)
FAIL = (PARD1L!OPARD1) OR (PARD1!OPARD1H) OR (PARD2L!OPARD2) OR (PARD2!OPARD2H)
OR (PARD3L!OPARD3) OR (PARD3!OPARD3H) OR (PARD4L!OPARD4) OR (PARD4!OPARD4H)
Figure 10 4030 PASS/FAIL function comparison block
diagram ( Large picture )
2.2.6. Checking Connecting Cable:
4030 includes eight SMB high frequency cable and sixteen SMB connector,
providing connection between 4030 Ripple/Noise meter and test power supply,
its connecting method can be found in 4030 operating manual. Prodigit also prepare the optional fixture Model 9951 (P/N:
65233005), cost US$10.00 /pc, which is shown in figure 11 for PC power supply,
so that the user immediately connect the power supply to the 4030
Figure 11: 9951 PC Fixture with SMB connector
3. Conclusion:
3.1. Suitability:
4030
Ripple/Noise meter include four channel of noise measure circuit; it can measure
four output from power supply simultaneously. If the
output of the power supply is more than four set, two 4030 can provide up to
8 set of noise measurement simultaneously, therefore it will solve insufficient
input channel problem.
3.2. Input Structure:
4030/3600A equipment with differential input structure, which can avoid ground
return problem, addition advantage like the low cost and ease of use. This can
improve the problem of expensive differential probe and hard to use.
3.3. Definition:
4030/3600A equipment with PASS/FAIL define mechanism which improve the
difficulty in defining pass and fail using oscilloscope. This is very suitable
for quality control and checking on production line.
3.4. System Expansion:
4030 equipment with RS-232C & GPIB interface, and 3600A equipment with
RS-232C interface, which can use program to control. It is suitable for manually,
computer, or automatic control testing.
