Noticing the problem of creeping standby power, regulatory institutions all over the world have started to publish standards that specify limits for home appliances

BY VARUN MERCHANT
Mainstream Technical Marketing Manager
Tektronix
www.tek.com

Regulatory institutions
all over the world have started noticing the problem of creeping standby power
for electronic products that plug into a wall socket. Committees like the
International Electrotechnical Commission (IEC) have taken active steps to
publish standards, such as IEC62301, that specify standby power limits for home
appliances. Similar standards or derivations thereof are implemented in various
regions around the world.

Traditionally,
tests for compliance to such standards are performed at the end of the design
cycle, but the situation changes quickly if the design fails compliance testing
— board turns and re-testing can get expensive and time consuming. Avoiding such scenarios points to the need
for pre-compliance testing earlier in the design cycle.

Standby power measurement challenges

Looking at an
example of low standby power on the input side of a power supply recorded over
time (Fig. 1), it becomes immediately
apparent that the power is quite distorted, peaky, and irregular. It’s also very low. For
instance, if you were measuring 10 mW of standby power in Europe with 230-V
input, your current could be as low as 40 µA. For 5 mW, the current goes as low
as 20 µA.

farc_tek_oct2016_fig1_lres

Fig.
1: Low standby power measurements are often peaky and irregular.

Low current leads
to a number of problems. Waveforms are highly distorted because power supplies
operating at low loads often draw non-sinusoidal, very high-crest-factor
current. The power factor is low because the current may be predominantly capacitive
through the power supply’s EMC filter. The power draw may also be irregular if
the power supply is in a burst, or hiccup, mode to minimize input power.

The crest
factor, which is simply the peak value divided by the rms value, is often very
high when measuring stand- by power. To understand why, consider the front-end
stage of a typical ac-dc power converter (Fig.
2
). In most cases, the input rectifier is followed by a by-pass capacitor
designed to smooth out the input voltage ripple and provide a fairly stable dc
for the next conversion stage. The input current flows only when the bypass
capacitor voltage is lower than the input ac voltage peak. The amount and
timing of current flowing into the circuit is dependent upon the capacitor
value and total load current. This leads to narrow and peaky currents on the
input side. PFC circuits can be designed to mitigate this effect at full load,
but unfortunately, most PFC circuits are not active at no-load.

farc_tek_oct2016_fig2_lres

Fig.
2: In the front-end stage of a typical ac-dc power converter, the capacitor
plays a key role in determining crest factor.

When dealing with
such signals, another important issue to keep in mind is the power factor.
Traditionally, power factor is defined as: PF = cos Φ, where Φ is the angular
difference between the peak voltage and current. In cases like this, the non-sinusoidal
current is in line with the voltage, but VA — the  voltage-current product, or
apparent power — is much higher than the actual active watts, or real power.
This means that, for all practical
purposes, power factor should be defined as:

PF
= Real power (in W)       
        Apparent power
(in VA)

 

IEC 62301 testing

The latest IEC 62301 Edition 2 standard for standby power
recognizes the challenges involved with testing standby power, taking them into
consideration and recommending test methods.

The wall
socket voltage quality in most labs is likely not great and, in most cases,
could lead to failing the IEC stand- by power tests due to voltage harmonic
content or out-of-limit crest factors. It is, therefore, highly advisable to
use an ac source with defined
tolerances, even for pre-compliance testing. The voltage source should be
within 1% on voltage and frequency output and below 2% on total harmonic content up to 13th order. Voltage crest factor should be within
1.34 and 1.49.

Measurement
uncertainty is based on both the level of power to be measured and the
distortion and phase shift of the
waveform. To take into account both distortion and phase shift, maximum current
ratio, or MCR, is defined by IEC 62301 as:

MCR =   Crest factor  
               Power factor

The required
level of uncertainty is determined using a flow chart provided in the standard.
This defines the minimum accuracy and noise levels required for measurement equipment.
The required accuracy for a power analyzer is about 2% at 1 mW and 4% at 0.5 W.  Some standards, such as Energy Star, are more demanding, with a
2% accuracy requirement at 0.5 W. This means that your power analyzer should have
a minimum watt accuracy of 2% or better with a resolution of 10 mW or better.
It should be noted that, because there is so much variability, the uncertainty
needs to be calculated in real time while the test is running and then included
in reporting.

Both direct
meter reading and average reading test methods are effectively obsolete, with
the latest specification calling for a sampling test method. Stability of
measurements is determined from a least-squared linear regression through all
of the power measurements. Stability is established when the slope of the
straight-line regression    is either
less than 10 mW/h (input power ≤ 1 W) or less than 1% of the power if the power
is greater than 1 W.

Test setup is critical
for low power measurements. In a typical setup using a power analyzer (Fig. 3), a breakout box helps to make
connections safe and easy by providing two terminals to switch between source and
load side.

farc_tek_oct2016_fig3_lres

Fig.
3: In this IEC 62301 test setup using a Tektronix PA1000 or PA3000 power
analyzer, having a breakout box is very beneficial — and safe!

There are two ways
to connect the voltage and current channels for measuring power. One way is when
the voltage measurement across the load is more accurate than the current, and the
other way is when the current across the load is more accurate than the voltage.
The connection choice depends on whether the load current is very low or if the
power draw across the voltage channel impedance will be a significant part of the
total power measured. By moving the current measurement to the load side, the
current flowing through the voltage channel is ignored (Fig. 4, right).

farc_tek_oct2016_fig4_lres

Fig.
4: Proper connections are critical for low power measurements.

On the other
hand, if the load current is higher, the power dissipated across the physical
shunt can be high enough to generate an error in power readings. To prevent
this, power analyzer current shunts are set up so that a drop across it is
ignored by moving it toward the source side (Fig. 4, left). In the case of a test setup with a
Tektronix PA1000 or PA3000 power analyzer (Fig.
3
), the impedance across the voltage channel is 1 MΩ, and the impedance
across the 1-A current shunt is 600 mΩ. Although these values can vary for
different power measurement devices, most are very similar.

Taking the
impedance numbers into consideration, the drop across a voltmeter channel would
be 53 mW when supplied from 230 V. This
is not a significant number when measuring power in hundreds of watts, but when
the power is as low as 30 mW, this can lead to a very substantial error.

Similarly, the
power draw across a shunt with a 100-uA current is just 60 uW. But with a 1-A
current, the drop can be as high as 600 mW. Such drops across measurement
channels can significantly influence readings and generate wrong results.
Consequently, care should be taken in making connections, especially while
measuring very low power values. Again, a breakout  box helps by providing two different
terminals for switching between source and load sides.  ☐