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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

  • 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