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Good power quality is necessary for providing stable, sustainable power for the world's increasing and varying load demands. Power quality  management  is an essential way to maximize the efficiency of the power grid, lessen the burden for resources and reduce operating costs. Power Factor (PF) is a measure of power quality.  Power Factor Correction (PFC) is the implementation of methods to increase power factor, thereby increasing power quality.  This article is a quick review of Power Factor (PF) and Power Factor Correction (PFC). PFC circuits are beyond the scope of the article. Power Factor Definition 

Power Factor in it's simplest terms is the ratio of true  power to apparent power. True power is the working or useful power measured in watts (W), while the apparent power is the  total voltage and current consumed by the load, measured in volts x amps (VA). In cases where the true power and apparent power are equal the PF is considered ideal and expressed as the number 1. All the power consumed by the load has been turned into useful power. This  occurs when the  voltage and current are  in phase and the wave shape of the current is sinusoidal.   See Figure 1 Linear loads that are purely resistive have a power factor of 1.
  
Figure 1.
PF < 1

Power quality is decreased whenever the power factor is less than 1. This happens for 2 reasons. See Figure 2   
Figure 2.
1. Distortion factor: 

Non-linear loads which draw current in short bursts or spikes distort the current waveform causing it to be non-sinusoidal. These distorted waveforms appear at multiples of the fundamental frequency, also known as harmonics.  Distortion adds a fourth type of power called distortion reactive power (QDIST).  Switching power supplies are non-linear loads that cause waveform distortion. 

2. Displacement factor:

Linear loads that are inductive or capacitive result in an additional power called  reactive power (Q). The impedance of the reactive power causes a displaced phase angle. The cosine of the angle θ = PF.  This is considered the traditional or displacement power factor (DPF).  Electric motors and pumps are linear loads that cause phase displacement or shifting.  Switching power supplies are non-linear loads that cause waveform distortion. See Figure 3 Vector representation of linear and non- linear loads.   
Figure 3.
The Math 

Displacement or traditional power factor = True power (P)/Apparent power (S) or the cosine of the displacement phase angle θ. In the case of ideal power factor P=S, therefore the phase angle is 0 and the power factor = 1 

Distortion Factor = 1/ (√1 + THD^2) where THD equals Total Harmonic Distortion
Total PF = Displacement Power Factor x Distortion Power Factor or cos θ ? (1/ (√1 + THD^2) 
Standard 

While there isn't  a formal PFC regulation, there is an important  EN standard to limit harmonic content  which  is often  accomplished by raising power factor  EN 61000-3-2:

This is a standard for line power utilization. PFC is often used to meet limits on harmonic currents. The standard is divided into 4 classes and covers electrical equipment  greater  than 75W, except for class C.  Note: the Nominal voltage is 230Vac for this standard. 

  • Class A  Balanced three phase equipment 
  • Class B  Portable Tools 
  • Class C  Lighting equipment 
  • Class D  Information Technology Equipment  (ITE)
 
Program 
 
Energy Star / 80 plus is a voluntary certification program. It certifies products to have more than 80% energy efficiency at 20%, 50% and 100% of rated load, and a power factor of 0.9 or greater at 100% load. 
 
Power Factor Correction 
There are two types of Power Factor Correction, Passive and Active.
 
  • Passive PFC: Passive PFC yields a  PFC number of approximately  0.6 to 0.9 , much less than the ideal power factor of 1.  The elements used  to implement passive PFC  are components like  inductors, capacitors and ferrite cores. 
  • Active PFC: Active PFC uses semiconductors and switching elements like transistors to reach PFC numbers greater than 0.9 
 
There are advantages and disadvantages  such as cost and complexity to each method and you will need to determine what's necessary for your application 
 
PSUI have a full range of power factored products for all your design needs.
 
By John Benatti 
 
POWER FACTOR CORRECTION Links
DC-DC converters are used in applications where voltage levels need to shift or where an isolation barrier is necessary. Voltage conversion can be achieved locally or at the point of load (POL).   All converters fall into two types, isolated and non-isolated. Let's look at the differences.   

Isolated Converters
   
As the name implies, isolated converters have an isolation barrier. The isolation barrier is typically provided by a transformer which separates the input and output terminals.
It can withstand hundreds to thousands of volts. Most isolation voltages are in 1500 to 4000V range, depending on the application.

Isolation falls into three categories:
  • Operational:  An isolated output that offers fault protection
  • Basic: A transformer isolation with single fault protection
  • Reinforced: Two isolation barriers that may offer physical separation
There are several reasons to use an isolated DC-DC converter.

  • Safety: Isolation barriers are required to protect humans from dangerous voltages. Whether it be operator safety or as applied in the medical world, the critical safety of the patient.
  • Galvanic Isolation: Impeding an electrical charge through isolation reduces galvanic corrosion.
  • Noise/transients: Isolation results in better noise filtering and the prevention of unwanted ground loops.
  • Floating Ground Requirements: Floating grounds are useful for providing negative or positive grounds. Examples: negative grounds for datacom, positive grounds for telecom.  (See Figure 1)
  • Floating Outputs: Positive  or negative voltages can be obtained  by changing the placement of the output reference ground, regardless of the input polarity. (See Figure 1)
Figure 1.
View Isolated DC-DC's Quick Links

Non-Isolated Converters
  
Non-Isolated Converters are used in applications not requiring an isolation barrier. There are several reasons to use a non-isolated converter.

  • Cost: Non-isolated converters are less expensive than their isolated counterparts.
  • Size: Without a transformer and associated circuitry, compact sizes are available including DIP(dual in- line),SIP(single in- line pin) and SMT(surface mount module)
  • POL: Smaller package sizes enable Point of Load placement which reduces I²R board losses, while increasing transient response and load regulation.
  • Output Voltages: Additional board level voltages obtainable using existing converters as their source of input.
  • Efficiency: Efficiencies are quite good given the lack of the copper and switching losses of a transformer.


Figure 2.

View Non-Isolated DC-DC's Quick Links

A power supply's main job is to deliver power, but sometimes delivery of that power is delayed, interrupted or otherwise compromised. These situations can be deleterious to sensitive systems like computers and other applications that rely upon a stable, reliable source of power. In these applications, it's incumbent on the user to monitor the status of the power supply at all times. To that end, some power supplies are equipped with status signal lines to alert the user to changes that threaten the stability of the power source. The careful monitoring and manipulation of status signals affords the user a method to identify and avoid adverse situations and promote a safe operating system. Two such status signals are Power Good and Power Fail.

Power Good - Power Good (PG) is also called DC OK or Power OK.

When input power is turned on or enabled  it will generate output power, but until the Power Good signal is high, the output power might  be unstable and potentially  harmful for it's  intended operation. It's not until all the power supply output voltages are stable and all internal testing has been done that the output  power is considered "good" and ready for use.
The Power Good signal is often a +5 TTL compatible voltage* generated internally by the power supply. The signal is triggered at a voltage threshold (Vth), where the DC output voltage is in tolerance as specified by the manufacturer. The Power Good signal typically takes 10-500mS to become enabled after reaching Vth.

A few applications that benefit from Power Good:
  • Central Processors: Computers can't arbitrarily turn on at any voltage without risking damage to the motherboard and other related devices. If the computer were to try to turn-on at a less than required voltage a premature boot-up could occur which in turn might cause  unreliable operations and results.  While the consequences in either case can be significant, they can also be avoided.  If the computer processor monitors the Power Good  signal and sees  that it's in a  low state, it will stay in reset mode and the computer will never start the boot-up sequence. Once the Power Good signal is high, the processor stops the reset signal to the computer and a safe  boot-up  is launched with all the required DC voltages for proper operation.
  • Sequencing: When there's a need to sequence multiple power rails, a Power Good signal can be used by a PLC or other controlling device. Sequencing is often used in automotive assembly and industrial automation.
  • Parallel Redundacy: A Power Good signal can be used to easily identify a failure in a parallel redundant system. 
  • Voltage Regulation/Reset: Supervisory ICs, sometimes called voltage regulators, monitor Power Good signals and will force a system reset if any of the signals fall below the power good threshold (Vth). These signals help the supervisory IC  monitor the health of systems such as automotive, industrial and telecom.
  • Status alert: The Power Good signal does not always have to take an action other than providing a simple alert through an LED or alarm.
  • Input Problems: Any time there is a deteriorating input, it will eventually result in a failing output. The Power Good signal will note the failed output once it is out of tolerance.
* Common  topologies for Power Good signals are  open collector, open drain, TTL compatible, isolated circuits or relays.
Figure 1. Simplified Power Good Timing Diagram


Power Fail - Power Fail (PF) is also called AC Fail or AC OK.

Power Fail is distinguished from Power Good because Power Fail is concerned with the condition of the  AC mains  input , not the  DC output. Power Fail is used to alert the user that the input AC is  no longer available or is dropping  to a point where the regulated output cannot be sustained or could be lost entirely. A typical Power Fail scheme uses an opto-coupler circuit. The comparison of the primary side housekeeping voltage to a reference voltage provides the drive to the opto-coupler. When the AC input falls below a set value, the drive to the opto-isolator is removed and the Power Fail signal changes to a low state. When this happens, energy in the large input capacitors allow the DC output to be held up long enough (usually 5 - 10mS) for memory and other data to be saved during a controlled shutdown.  Back-up power may also be enabled during this time.  See Fig 2.

Power supplies with power factor are treated differently. The PFC boost section does not react to  normal changes in the AC input, so the time from Power Fail signal low to the loss of output power should be measured.

An industry that is particularly sensitive to AC failures is industrial automation control which needs to know the exact status of all the operations before the loss of input power.  Saving data and initiating back-up power is critical. In the case of industrial control, relays are used as the interface for the AC fail signal since they easily can tie into loop process controllers.

Watts News Tip: The causes of AC failures are well known and some preventative steps can be taken to reduce Power Fails.
  • Keep power supplies and UPS systems on maintenance and calibration schedule.
  • Make sure all mains wiring is done by a professional electrician
  • Don't spare expense on power cabling and accessories
  • Size fuses and breakers properly
* Common topologies for Power Fail signals are open collector, open drain, TTL compatible, isolated circuits or relays.

Figure 2. Simplified Power Fail Timing Diagram


Summary
  • Power Good is essentially the signal the tells you that  DC output  is ready to use.  It can also tell you that DC output has fallen out of regulation and is failing.
  • The Power Fail signal is an important safeguard against system failures, since unlike Power Good it can warn of impending failures.
  • Power Fail is better suited to critical applications.
  • Power Fail and Power Good signals are sometimes used simultaneously for a more fail safe system.

Written by John Benatti