Generic Skills for Microelectronic Engineers

Electrical Specifications and TestingAIM:

To understand how to read and write specifications for electronic equipment. The type of parameters commonly specified will be examined along with the common bounds that should be used for these parameters.

This session is available at http://maxwell.me.gu.edu.au/sok/gs

Specifications

All electronic equipment and components need to conform to given specifications. As an engineer you need to be able to both read supplied specifications and write specifications for new equipment.

This will be immediately relevant to you when you need to write the specification for your PSU.

Firstly we need to understand about the parameters and units that are common for devices and equipment. We then need to evaluate what parameters are important for inclusion in a specification sheet.

Specifications for devices and components

For simple components like resistors, capacitors, inductors etc we need to specify the characteristics for the device against any parameter that might cause that characteristic to vary.

For example:

What is the most critical parameter for a resistor?

Obviously the value of resistance.

Does anything make this value change?

Yes: Temperature, age, voltage, etc....

There are also many other parameters to examine that relate to lifetime etc.

Throughout your career you will constantly be referring to component data sheets.

Lets look at a simple resistor data sheet to observe how parameters vary due to external influence:

You will need to become used to refereing to data sheets as they are a vital tool for electronic design.

Once upon a time an electronics engineer would own a huge data book library. These days much of the information is available on the web or on CD.

Some useful data sites are

Hundreds of electronic manufacturershttp://www.crhc.uiuc.edu/~dburke/databookshelf.html

Integrated Circuit Datahttp://www.embeddedlinks.com/chipdir/

In particular you will need data sheets on integrated circuits which are often quite complex. Some chips are so complex that their data sheets are hundreds of pages long.

Look at a simple one.

Sometimes this data is tabulated or graphed. Other times it will simply be quoted as a figure. The units are most important and often unusual so you should pay special attention to them.

The base units should be familiar to you.

Basics

VIWPowerWatts

V/RICurrentAmperes

IRvVoltageVolts

N(dφ/di)HInductanceHenries

Q/VFCapacitanceFarads

V/IΩResistanceOhms

CalculationSymbolUseName

There are hundreds of other parameters used dependant on the application.

Levels

10log(P/1W)dBW

Comms levels....

20log(V/Vref)dBr

10log(P/1mW)dBm

20log(V/1µv)dBµv

20log(V/1v)dBv

Radio signals....

10log(P1/P2)

20log(V1/V2)

dBGeneral RatioDecibel

CalculationSymbolUseName

Misc

≤mv rmsMains noiseRipple

≤% + constVariation from fixedRegulation

%Total Harmonic DistortionTHD

ppm/°CChange with tempTemp coeff

SymbolUseName

A very important point to remember for many specifications is that they must include a qualifying parameter. In many cases this will be the tolerance or variation allowed for the measurement

For example the power rating for the resistors above must include the temperature that the rating refers to.

POWER RATING IBT1/4 IBT1/2Determined by load life test 100% load at 70°C ambient 1/4W 1/2W

You could measure the power handling of the IBT1/4 resistor by performing the test in a freezer at -25°C. The resistor may handle nearly 1W at this temperature. If you advertised that this resistor had a rating of 1W you would have a lot of unhappy customers with burnt resistors.

You must qualify how the value was obtained in your specification. Many standards organisations have defined standard testing conditions for many specifications. These should be adhered to where possible and advertised in the specification.

The resistors above show the standard:

PERFORMANCE CHARACTERISTICS (TESTED PER MIL-STD-202):

Specifications for equipment

For an example, have a dream about this!!!!!

Lets see what sort of things are specified...

NAD C370 Integrated Amplifier

105dB (178,000)A Weighted; ref rated power

±6 dB at 100 Hz; ±0.5dB

±5 dB at 10kHz; ±0.5dB

220 ohms

Source Z + 1k ohms

100 ohms

(IHF=Institute of High Fidelity standard)

Bass control

Treble control

Headphones output impedance

Tape output impedance

Preamp output impedance

94dB (50,000)A Weighted; ref. 1WS/N ratio

310mV ref. rated powerInput sensitivity

47k ohms /470pF R and CInput impedance Line Inputs

3dB

120WRMS (20.8dBW)

0.03%

140WRMS

-3dB 3Hz-70kHz

±0.3dB 20Hz-20kHz Frequency response

>150 Damping factor (ref. 8 ohm, 1KHz)

450W (26.5dBW) 2 ohms

340W (25.3dBW) 4 ohms

210W (23.2dBW) 8 ohmsIHF dynamic power (max short term power/channel)

IHF dynamic headroom at 8 ohms

Clipping power (maximum continuous power per channel)

Rated distortion (THD 20Hz-20kHz)

Continuous average power output into 8 ohms1

Physical Specifications

3dB

300WRMS (24.8dBW)

550W Power consumption

12kg26.45lbShipping Weight

12kg 26.45lbNet Weight

435 x 132 x 350mm 17 1/8 x 5 1/4 x 13 13/16"Dimensions (WxHxD)

Physical Specifications

700W (28.5dBW) 4 ohms

500W (27dBW) 8 ohmsIHF dynamic power

IHF dynamic headroom at 8 ohms

Continuous average power output into 8 ohms1

Bridged (Monophonic) Mode:

1 Min power per channel, 20Hz - 20 kHz both channels driven with no more than rated distortion

A classic example of where qualification of specifications is required is for frequency response and power output of amplifiers.

Frequency response 20Hz-20kHz ±0.3dB 3Hz-70kHz -3dB

Without the dB qualification the frequency response specification is meaningless

Similarly much deception occurs with power output specifications

Amplifiers are often specified with power or wattage ratings in

RMS (Root Mean Square) Music Power PMPO (Peak Music Power Output)

RMS is the true power that the amplifier can deliver

Music Power or dynamic power is the power that can be delivered for a short period (ie. 10 ms)

PMPO is a unit invented (or exploited) by marketing departments. It seems to be real power multiplied by some mystery factor that has been increasing over the years. It seems to now be up around 25!

You clearly need to be cautious as a consumer, and as an engineer, that you understand units and are not misled because the unit has not been clearly specified. In the case of consumer goods that is often deliberate.

Generally engineering equipment is specified correctly as the manufacturer will usually end up in court quite quickly for their deception.

Lets look at a piece of equipment that is close to your task of specifying your PSU.

3000 Series Lab PSU (3 Amp)

Load regulationThis is the variation in the supply output voltage when the load resistance is changed. Normally as the load resistance is reduced we see that the output voltage will decrease as more current is drawn.

We commonly graph this measurement as output voltage versus output current. This graph may then be used to ascertain the maximum current available from the supply. This will normally be calculated as the point where the output voltage has dropped by a certain percentage.

Line regulationThe output voltage will also vary if the 240v line input is changed. This measures how much the output varies for a given line variation. It is normally specified as a percentage change.

The line voltage is normally only reduced a small amount otherwise the equipment may not function correctly. This too can be specified.

Ripple and noise

A

NG

+V

0V

Regulator

If capacitornot present

0v

The type of power supply you are building firstly steps down the 240vAC line input with a transformer. The bridge rectifier acts as a valve and only lets current flow through each diode in the direction of the arrow. The result is that there are bursts of current coming out of the rectifier (100 per second). This is like spurts of water coming out of a pipe. If we let the water spurt into a bucket and draw water from the bucket then the flow of water from the bucket will be fairly constant instead of spurts. This is what the capacitor does. A regulator then tries to keep the voltage constant. This works fairly well but there will still be a small variation in the voltage due to the bursts of current. This is called the ripple.

You can imagine that if you try to draw a lot of current from the capacitor (water from the bucket) then the voltage (pressure) will drop between bursts as the level goes up and down, giving a ripply output voltage, ie the flow will vary.

If you try to draw lots of current (water) the capacitor (bucket) may even become empty between burst resulting in bursts from the output.

Making the capacitor (bucket) bigger will clearly reduce the ripple.

The ripple on the output will get worse as the load is increased (ie more current is drawn)

It is normally measured as the peak to peak variation or sometimes the RMS.

Water levelor

Voltage

Time

Medium current drawn by load

Water levelor

Voltage

Time

High current drawn by load

p-p ripple

Recovery time

100µs

100%

50%

When the load is varied it will take a finite time for the supply to settle to the new voltage. This is analogous to the voltage on the capacitor (level in the bucket) settling to a new level.

If a big capacitor is used to reduce the ripple then the recovery time may be slow.

The voltage regulator in your circuit can respond very quickly so the recovery time will be very quick.

How do we measure these parameters?

Load regulationTo measure the load variation we need to apply different levels of load to the supply. To do this we need a dummy load. For this level of load we often use light bulbs as loads. We need to measure the voltage of the supply and the current being drawn for as many different loads as possible in order to graph the performance.

PSU

A

V

ASIDE

How to calculate the current expected from a globe.

A 24 volt 21 Watt globe

Power = Voltage x Current Current = Power/Voltage

I = 21/24 = 0.87 Amps (870 mA)

But this is only at a supply voltage of 24 volts. What if we are supplying it with 12 volts

Calculate the resistance of the globe

P = V2/R R = V2/P = 242/21 = 27.4Ω

The current at 12 volts will therefore be I = V/R = 12/27.4 = 0.44Amps (440mA) half of the current at 24 volts. Note that the power is much less P = VI = 12 x 0.44 = 5.2W

This is only approximate as the resistance of the globe changes with temperature.

We normally calibrate any dummy load before use.

Enough points must be measured to allow the maximum current point to be accurately determined and to calculate the sag in the voltage over the operating region. The load regulation can often be best described by a constant plus a percentage.

Voltage

Current

Allowedvariation

Specified max current

Load regulation

Line regulation

Variac

PSU

A

V

To take this measurement we need to vary the line voltage to the supply. A variac can vary the 240 volt level. Normally the output voltage is measured for a given load and a range of line voltages. The line regulation is then calculated from this graph. It is also often specified as a constant plus a percentage. The line voltage is normally not reduced to zero but just low enough that the output of the supply falls below specification.

Ripple and noise

PSU

A

VCRO

A CRO is used to measure the AC ripple on top of the DC voltage. You learnt how to do this in TP2. Normally a range of loads are measured to produce a graph, or ripple voltage is specified for low and high current loads. The ripple is often measured in peak to peak voltage or RMS voltage.

Recovery time

PSU

A

VCRO

The load is changed with a switch from say 50% to 100% or reverse. The CRO is used in single shot mode to record the variation in voltage. The time to reach a specified percent of the correct voltage is then ascertained.

The Finished Specification

Your final document should include all your measured results, graphs etc. All test parameters must be included.

As is done in real life your results should be averaged over a number of samples of the product. (in your case 5)

Physical specifications should be included, dimensions etc.

All measurements that were taken must have their procedures explained. An appendix may be the most suitable for this.

This must all be compiled into a neat document. One copy must be handed in but you will all probably like to keep a copy.