Dr. Douglas C. Hopkins

1
Chapter 2 Electric Circuit Toplogies, Nomenclature
and Characteristics

• By
• Dr. Douglas C. Hopkins

2
Specifications

• Right is a power supply with inputs and outputs
• Electrically transfers power from the source to
  the load
• Mechanically has size and operates in a certain
  thermal environment
• So the power supply specd into two sections
• Electrical
• Mechanical
• The electrical specifications can be split up as
  follows

Iin
Iout
Power Supply
Vin
Vout
Source
Load
3
Electrical Specs
4
Input Specs

• The specifications of the input will depend on
  the source whether its
• DC
• AC
• Cover first specifying the input specs of a power
  supply running off a DC source

5
DC Input Spec

• Specifying the Vin for a PS (Power Supply) will
  depend on the DC source voltage range
• A DC voltage is described as a voltage that will
  very slowly over time or is a constant
• Here are some typical DC sources with their
  voltage range
• Car Battery ? typical 12V with 11 to 14 volts
  variation
• Solar Cell ? 0.5 to 1 volt per cell depending on
  sunlight
• Telecom Bus ? typical 12V with 36 to 72 volts
  variation
• PC Internal 5V Bus ? 5V /-10
• The range for Vin should be chosen such that it
  covers the DC source it intends to run off

6
DC Input Spec

• For a PS that runs off a Telecom bus would need
  to have a Vin operating range of 36 to 72 volts
• If the input voltage drops below 36V, typically a
  PS will shut down
• If the input voltage exceeds 72V, typically a PS
  will be damaged by the excessive high voltage
• A PS can be designed such it can handle short
  duration of high input voltage such as line
  transients due to lightning
• This is known as surge rating
• For example this PS can have a surge rating of
  100V for 100usec.

7
DC Input Spec

• The current drawn by the PS is known as Iin and
  measured in amps
• Iin is derived by this relationship
• Output power (Pout) of a PS is Vout x Iout
• Input power (Pin) of a PS is Vin x Iin
• ? is the efficiency of the PS
• Describes how much of the input power results to
  the output
• typically between 0.5 to 0.95
• This will be covered in more detail later
• ? Pout Pin
• Substituting and solving for Iin
• Iin (Vout x Iout) / (Vin x ?)

8
DC Input Spec

• Iin (Vout x Iout) / (Vin x ?)
• Note that if the output and ? is held constant,
  Iin will vary 1/Vin
• Worst case Iin occurs at lowest value of Vin
• For our example, the PS will draw the most
  current at Vin36 volts

9
DC Input Spec

• Ideally Iin should be a clean DC current
• But there will be the ripple current, Irip, from
  the switching stage within the PS itself
• Usually specd as a peak to peak current
• The frequency of this waveform is usually lt 10Mhz
• Typically this is lt 10 of maximum Iin
• In our example, If Iin max is 10A, Irip p-p
  should lt 1A

Irip
Iin
Time
10
DC Input Spec

• There will be noise that occurs at gt 10Mhz.
• Known as switching noise and its due to the
  internal parasitics
• Typically the peak to peak noise is less than 1
  of max Iin
• Finally there is surge current
• This is due to Vin rising quickly and charging int

11
DC Input Spec

• Finally there is surge current
• This is due to Vin rising quickly and charging
  internal capacitors in the PS
• Once the capacitors are charged, Iin will
  approach its steady state operation
• Usually Isurge is less than 5 times max Iin

12
AC Input Spec

• The other form of input for PS is AC
• The source for this is usually AC generators from
  the big utilities
• There are two types of transmission of AC utility
• Single Phase
• Three Phase
• Three phase is used for systems gt5KW
• This will not be covered in this course
• The main reason AC is used for transmission of
  power is because voltages can be easily stepped
  up and down with the use of transformers

13
AC Input Spec

• Some definitions of Vin for AC
• In discussing Vin, Vpk is rarely used
• Instead the DC equivalent value known as RMS is
  used
• RMS stands for Root Mean Square and the input
  voltage would be labeled as Vin-rms
• Vin-rms Vpk / 1.4142 or Vpk/(2)0.5
• Because the voltage is not constant, RMS some
  makes calculations easier
• For DC, Pin Vin x Iin
• For AC, Pin Vin-rms x Iin-rms
• Valid if Iin is a sine wave

Vpk
14
AC Input Spec

• Here are some typical AC utility voltage ranges
  for the home
• Doorbell, heating systems ? 24Vrms /-30
• Household wiring ? Typically 110Vrms with 90 to
  130 range
• Electric stoves ? Typically 220Vrms with 180 to
  260 range
• Actually 220Vrms with a center-tap is delivered
  to the home. 110Vrms is derived from the
  center-tap
• Here are some typical AC utility voltage ranges
  for the business with the AC single phase derived
  from three phase
• Office wiring ? Typically 120Vrms with 90 to 140
  range
• Inustrial/Computer ? Typically 208Vrms with 180
  to 260 range
• Smaller businesses will use the household AC
  utility

15
AC Input Spec

• Europe and some other countries do not use
  110Vrms or 120Vrms
• All homes and businesses are wired with either
  208Vrms or 220Vrms
• There are several reasons why this voltage will
  very
• Voltage drops due to the transmission of power
• IE a person living near a sub-station may have
  typically 130Vrms while someone 20 miles away may
  have only 110Vrms
• The more current drawn on the wiring more voltage
  drop
• IE turn on a vacuum cleaner in a house and the
  lights will dim slightly
• Voltage can increase temporarily because the load
  is suddenly removed and the generators have not
  responded to the change

16
AC Input Spec

• Here are some specs for Vin for typical products
• Desktop PC sold in the US, 90Vrms 140Vrms
• Desktop PC sold in the Worldwide, 180Vrms
  260Vrms
• High end server sold worldwide, 180Vrms 260Vrms
• Desktop PC with universal PS sold worldwide,
  90Vrms 260Vrms
• Why not use a universal PS in all desktop PCs
  ?
• Universal PS are more expensive and difficult
  to design

17
AC Input Spec

• Another spec for Vin is the frequency
• Frequency is defined as one over the time it
  takes to complete one whole cycle
• In the use we use 60Hz
• In Europe and some other countries, they use 50Hz
• The range is only /-3Hz
• A universal PS operates from 47Hz to 63Hz
• This is not a cost or a design problem

Frequency1/time For a complete cycle
18
AC Input Spec

• Like DC, AC utility will have voltage surges
• These typically be quite large
• 2000V spikes are no uncommon
• Florida is the worst US state
• They are due to lightning, some industrial
  equipment and solar flares
• The front-end circuitry must be able to shunt
  this energy
• Since the AC utility voltages are much higher
  than DC and there is nothing to limit the
  current, the PS have no direct connection between
  input and output
• This is known as isolation
• This is a safety requirement
• More will be discussed later

19
AC Input Spec

• Another problem with AC utility is brown outs,
  sags or drop outs in the power
• This is when voltage drops below its specified
  Vin range
• This occurs if there are problems with the
  utility grid
• Transformer in a sub-station goes bad
• Too much load on the grid like air-conditioners
  in the summer
• Solar flares can induce too much voltage and
  pop the breakers
• These occur quite often
• How many times have the lights flickered ?
• That was a drop out
• More than 99 of the drop outs are less than 20ms
  in length

20
AC Input Spec

• PS in most products will just shut down
  temporarily during these events
• Televisions, radios, VCRs
• Non mission critical devices
• In mission critical devices such as high-end
  servers, the PS shall maintain operation for a
  loss of AC line up to 20ms
• After 20ms it can shut down
• This is known as hold-up time
• This is accomplished by a large energy storage
  device like a capacitor
• Typical specifications for hold-up is 20ms

21
AC Input Spec

• Ideally Iin should follow Vin as shown on the
  right
• This is known as emulating a resistor

22
AC Input Spec

• Usually for a PS, the input is a bridge diode
  rectifier with a large capacitance
• Iin with respect to Vin will be distorted
• In both cases the input power is the same
• However, Iin-rms is now significantly higher
• PinVin-rms x Iin-rms is the apparent power
• The real power is the average of (Vin x Iin)

23
AC Input Spec

• The term to describe this Power Factor, PF
• PF Real Power / Apparent Power
• The lower the PF, the higher the Iin-rms for the
  given power
• The problems with lower PF are
• Wire sizes must be increased to handle the higher
  Iin-rms current because Power Loss resistance
  of the copper x (Iin-rms)2
• This is extra power utilities must size for and
  supply
• Iin is rich in harmonics which adds noise and
  circulating currents in 3 phase systems

24
AC Input Spec

• Another criteria is the Total Harmonic
  Distortion, THD
• Any waveform can be broken down into a sum of
  sine waves with different amplitudes
• A pure 60Hz signal should only be made up of a
  60Hz signal
• If there is any distortion, then
• II1sin(2?ft)I2sin(4?ft)I2sin(6?ft).
• I1 is the root-mean-square (rms) of the
  fundamental current waveform
• I2 is the rms of the second-order harmonic
  current waveform
• I3 is the rms of the third-order harmonic current
  waveform, etc.
• THD square root ((I2)2 (I3)2 (I4)2
  ...)/ square root (I1)2 x 100 (to convert to
  percentage)

25
AC Input Spec

• Any distortion will show up with THD gt 0
• A good value for THD lt 5
• Will discuss these regulations later

Reference http//www.lrc.rpi.edu/NLPIP/Online/Answ
ers/LA-Power/Sidebars.htm
26
AC Input Spec

• Most PS with a simple diode bridge and capacitor
  has a PF?0.67 and THD?20
• The above PS with a large inductor in series can
  achieve a PF?0.9 but still THD?20
• A Pwer Factor Correction circuit used in a
  front-end PS can achieve PFgt0.98 and THDlt5
• However a PFC adds cost complexity to the PS

27
AC Input Spec

• Noise on Iin is another issue
• Its not measured the same way as DC inputs
• Since these PS connect directly on the grid, this
  noise can propagate
• If this noise if high enough in level it can
  effect equipment on the grid significantly
• If the frequency is high enough, it can use the
  external wiring as an antennae

28
AC Input Spec

• If the frequency is less than 30Mhz, this is
  known as conducted
• This is measured with a box called a LISN which
  filters out the 50/60hz
• This is connected to the line cord
• A frequency spectrum analyzer will then display
  the noise spectrum
• If the frequency is greater than 30Mhz, this is
  known as radiated
• This is measured with an antennae usually 10
  meters away
• At these frequencies, line cords and cables
  become very effective antennae
• There are federal specifications that must be met
• This will be discussed later

29
AC Input Spec

• Like DC, Iin has inrush issues with AC
  applications
• Usually peak Iin is specd to be less than 5Xof
  the steady state of Iin-rms
• Another factor to take account is fusing and
  circuit breakers
• If the inrush current is too high or can occur
  throughout the day, fuses and circuit breakers
  can be weakened, damaged or open up