Tuesday, August 26, 2014

Ground Continuity & Ground Bonding Tests on Power Supplies

I heard some discussion on this subject in our facility recently, and thought it would make a good blog article.
The safety bodies (UL, CSA, IEC etc.) require that electrical and electronic products are suitably protected and tested; to ensure the user does not get an electrical shock that could injure or even kill.

One of the areas of concern is the grounding (earth) of the product, and the following tests are conducted; not just during product safety certification testing, but also in production.  This is mandated on all products with a pluggable power cord.
Ground Continuity
The ground continuity test verifies the connection between the ground pin on the power cord and any exposed metal parts on the equipment.  An AC or DC voltage can be used, and the current is typically quite low, less than 1A.  A simple handheld device can be used for testing

Ground Bonding
Unlike the continuity test, the bonding checks the integrity of the grounding.  This is typically measured using a 25 or 30A current (depending upon the rating of product’s internal AC fuse or branch circuit) simulating an actual internal fault.  The applied voltage is less than 12V and the maximum resistance between the earth and exposed metal surfaces is 0.1 ohm. The resistance can be determined by measuring the voltage drop.  Depending upon the safety agency requirements, this test is performed for 60 to 120 seconds.
Using a higher current than the continuity test ensures that any hardware in the ground path is fully tightened, any wire joints are properly crimped, and any printed wiring board traces are truly capable of handling the current.  The fuse or breaker should open before a loss of the ground connection.
There are a number of commercially available testers on the market than can be programmed for production use.
If you design your own tester there are two things you should note:
  1. Make sure that you do not include the cable drops when measuring the voltage (have the meter read at the connection points)
  2. Apply the test probes when there is no power applied; otherwise the resulting spark can mark the metal parts and damage the plating.
As a note ground bonding may be also be referred to as earth bonding.
Power Guy

Thursday, July 31, 2014

UL 60601-1 and ANSI/AAMI ES 60601-1


On datasheets for new medical power supplies, you might notice that there is no mention of UL 60601-1, but a new safety standard called ANSI/AAMI ES 60601-1:2005 is being called out.

So what has happened?  ES 60601-1 is in fact identical to IEC 60601-1 but with U.S. deviations to comply with U.S. National Electric Code.  UL is now using that standard to write their reports and is the standard used in the US to comply to the 3rd edition.

The FDA now officially recognizes ANSI/AAMI ES 60601-1:2005 in the Federal Register

Older power supplies are calling up this new standard, and will also reference the older UL 60601-1 standards to keep continuity for existing customers with UL’s “grandfather” clause.

ANSI is the American National Standards Institute.  AAMI is the Association for the Advancement of Medical Instrumentation.
Power Guy

Tuesday, May 20, 2014

Cathodic Protection Using Active Corrosion Control


To avoid corrosion in large metallic structures, passive cathodic protection is widely employed.  Such structures include steel used as reinforcement in concrete buildings, bridges, piers, pipelines, offshore platforms and ships.

Basically the steel in the structure is made the “cathode” and a more easily corrodible “sacrificial” metal is connected to it, acting as the “anode”.  The chemical reaction between the two metals generates an electrical current.  The sacrificial metal then corrodes, protecting the original structure.  Eventually that metal part has to be replaced, like the rod in most domestic water heaters. 

Below is an example of a passive system.



For both environmental and operating cost reasons, the traditional passive protection is being replaced by active corrosion control.  In the ‘active’ method, a sophisticated electronic current control system is used to inject a reverse current to that generated from corrosion to protect the structure. Since current flow is closely related to the flow of charge over time (I = dQ/dt), having constant current control allows the user to accurately control the process.  This is also known as impressed current cathodic protection (ICCP)



Active corrosion control was initially discovered in the early 1800s, but was unsuccessful due to the lack of suitable materials and current source.

In larger systems like pipelines, the passive anodes cannot deliver enough current to provide protection, and sophisticated monitoring and control is often needed.

The initial cost of an active system is higher, but in the long term, the environmental & maintenance benefits outweigh this.

TDK-Lambda’s new Z+ series of 200 to 800W programmable power supplies offer a wide range of models and options suitable for active corrosion systems. The series can operate in constant current mode with currents ranging from 2A to 72A.  In addition, the units can be remotely programmed and monitored using a variety of isolated analog and digital interfaces, including RS232/485, IEEE488 and LAN.  Up to six units can be paralleled to supply additional current.

 

Monday, March 3, 2014

When should external diodes be used with a power supply?


I have written a few of articles over the years regarding the use of external diodes with power supplies (or FETS), one was back in 2007 concerning fault tolerance, another was when driving DC motors and a third on operating power supplies in series.

Recently two other applications came up, just when I thought I had the subject covered!

Redundant Operation

Connecting two power supplies together for redundancy is widely used.  (Not to be confused with brute mode current sharing)



If PSU #1 fails, theoretically PSU #2 takes over, right?  Not quite….


Looking inside the power supply, the output voltage is usually monitored by an op amp and is then compared to an internal reference.  If the output voltage is too high, then the comparator will make the control circuit lower the output voltage by reducing the switching converter pulse width.  Likewise, if the output voltage is too low, the switching pulse width will increase to make the output voltage rise.
Let’s say PSU #1 is at 24.0V and PSU #2 is set at a slightly higher voltage, say 24.1V.  PSU #1’s control circuit now “sees” 24.1V as an output voltage and will turn the switching converter off believing that its output voltage is too high. 
In the event of PSU #2 failing, the load demand will fall on PSU #1, which will have to turn the switching converter back on and may cause a temporary loss of voltage provided to the load.
 
Adding a diode in series with each power supply output will stop the power supplies from “seeing” the other’s output voltage, and although PSU #2 may provide the entire load, if it fails PSU#1 will be active, ready to provide power, and be able to keep a voltage available to the load.




Battery Back-Up
On many low cost, low wattage power supplies, overvoltage protection is provided by a Zener diode connected across the output terminals of the power supply.  In the event of a control circuit malfunction causing the output to rise, the Zener will fail short circuit, forcing the power supply into overcurrent protection (“hiccup” type current limit mode must be used by the power supply designer).
If battery back-up is being used (or another power supply), then current will flow into the faulty power supply and cause overheating of the Zener and surrounding circuitry.
Again, a diode in series with the power supply will prevent this.

Tuesday, January 21, 2014

Power Supply Operation on a 400Hz Source

This article is intended to provide a general overview on using industrial power supplies with an aircraft 400Hz electrical source.

Most large aircraft are fitted with an Auxiliary Power Unit (APU) supplying a phase to neutral 115VAC 400Hz source.  The APU is used primarily to start the aircraft engines, but is also used to run accessories on the plane while passengers are on board and for preflight checks by the crew when the aircraft has left the gate.
The reason 400Hz was chosen over the traditional 50/60Hz is because of weight.  A 400Hz generator is much lighter, thus saving fuel, and the need to support a heavier unit making the airframe lighter.
MIL-STD-704F is the specification that covers Aircraft Electrical Power Characteristics for US military aircraft, and covers in detail all aspects of the requirements.
If an aircraft is being serviced on the ground, it is more convenient and safer not to have either the main engines or the APU running.  In this case an external 400Hz generator or Ground Power Unit is usually available.  Often diagnostic equipment is not required to meet the full flight specifications and for cost purposes a regular industrial power supply can be chosen.
TDK-Lambda is often asked if one of our AC-DC industrial power supplies will work off 400Hz input, usually the answer is “yes”; the following explains why:
For low wattage power supplies (50W or less), the input circuit is a simple full wave bridge rectifier.
Simple full wave bridge rectifier circuit
The AC voltage is filtered and then full wave rectified into high voltage DC.  With a 60Hz input, the ripple voltage on the bulk cap is 120Hz. With a 400Hz input, the ripple voltage is 800Hz (hence smaller), having no impact on the power supply performance.

For power supplies greater than 50W, most AC-DC power supplies have active Power Factor Correction circuitry.  Simply put, a boost converter is used to reduce the input harmonic currents by changing their shape to appear more sinusoidal - as if the load were resistive.
Boost Converter
 
The Boost FET in the above circuit is driven by a control IC.  The IC regulates the converter receiving feedback from three sources: the 100-120Hz rectified input voltage (Point “A”), the inductor current and the DC voltage across the bulk capacitor.
Although most PFC circuits will operate from a 400Hz input, the wave shape of the current harmonics is slightly degraded due to distortion at Point “A” (now an 800Hz waveform).  This, however, is usually acceptable for ground based equipment.
Simplified Diagram Showing “Y” cap locations
To reduce high frequency radiated and conducted noise, power supply input filters use special “Y” decoupling capacitors connected from the Line & Neutral to Chassis (Earth) ground.  In addition to high frequency current, these Y capacitors also provide a path for 50/60Hz current.  The maximum value of this “earth leakage” current is dictated by the safety agencies like UL, particularly for equipment that is plugged into a regular wall socket found in an office (for example).  For large pieces of equipment that is hard wired to an AC source, the limits are much higher.
With a 400Hz input, the earth leakage current is significantly increased through the “Y” (input to ground) capacitors as that current is directly proportional to the input frequency, I = V x 2πfC.
However, ground based equipment running off 400Hz generators falls into the hard wired category, so this is not normally a problem.

One last note; although commercial power supplies are safety certified to the Information Technology Equipment standard IEC 60950-1, testing for the report is usually conducted with a 50-60Hz input.  Most ground based aircraft systems do not fall under IEC 60950-1 but using a power supply certified to that standard means the product has been rigorously tested.

Power Guy

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