Thursday, April 16, 2015

Reducing noise on open frame power supplies

We get a lot of questions on how to reduce noise, both output & EMI (ElectroMagnetic Interference), on open frame power supplies.  Usually it is a result of a failure to ground the product correctly.  With an enclosed power supply, encased in a metal box, it is simple as all the connections are made for the user by the chassis.  Connect up the input and output wiring and everything works fine.  With an open frame (pcb type) it is a little different.

TDK-Lambda’s ZPSA open frame power supply

First a look at what we are aiming to do.  Below is a simplified diagram of the noise decoupling capacitors in a typical power supply.  The Y capacitors on the left provide a low impedance path for high frequency noise to ground.  This avoids electrical noise (EMI) exiting the power supply and interfering with other devices on the AC input.   The capacitors on the right have the same function, but in this case stops electrical noise from appearing on the output of the power supply and interfering with the load.  In some cases, just one capacitor is sufficient.

You can see two blue Y capacitors on the ZPSA photograph, close to one of the mounting holes.

Looking at the underside of the ZPSA pcb, we can see the locations of those capacitors.

The red arrow shows the Y capacitors CY2 and CY3 are connected to a common trace that leads to the bottom left mounting hole.  This hole is in fact a plated through hole and the mounting screw and standoff will make a connection with that trace.
The blue arrow shows the output to ground capacitor CY1, again connected to a copper trace leading to the bottom right mounting hole.

Note that in the case of the ZPSA, there is no pcb trace between those two holes.  (The top two mounting holes do not have any traces going to them, so we can ignore them electrically.)

Looking at our schematic again, we need a connection from chassis ground to both the input and output capacitor traces to reduce the electrical noise.  This we do by mounting the power supply on a grounded metal plate, with metal standoffs and screws.

Follow these guidelines and the open frame power supply will meet the EMI and output noise specifications.

Power Guy

Monday, March 9, 2015

Should power supply manufacturers adopt IEC 62368-1:2014 early to replace IEC 60950-1 and IEC 60065?

In 2014 the IEC introduced edition 2 of the Hazard-Based Safety Engineering (HBSE) IEC 62368-1 standard.  Intended to replace new submittals for IEC 60950-1 and IEC 60065 in June 2019, it covers hazards and hazard prevention for ITE (Information Technology Equipment) and audio and visual equipment.
UL & CSA published the 2nd edition of the IEC 62368-1 standard on December 1st, 2014, following Europe’s publication in September 2014.

The current standards are known as “prescriptive” and closely control product design.  The new standard retains some of those design rules, but offers performance options that might allow some design flexibility.  It must be stressed that unlike IEC 60601-1, a formal risk analysis is not required.  It is also not a simpler merger of 60950-1 and 60065.
No date has been set yet for when “grandfathering” of the 60950-1 and 60065 standards ends.  It has been recommended by some test authorities that if a product is going to be removed from the market by 2018, then do not proceed with certifying to IEC 62368.  As there may be some regions that have not accepted the new standard, products that will be on the market after 2018 should look at obtaining certification to IEC 62368, but keep the existing 60950-1 and 60065 certifications.

Does this mean that power supply manufacturers should start transitioning over now?  No, the IEC committee recognizes this would impose a huge burden on everyone’s resources, including the certification bodies.  Clause 4.1.1 states that “Components and subassemblies that comply with IEC 60950-1 or IEC 60065 are acceptable as part of equipment covered by this standard without further evaluation other than to give consideration to the appropriate use of the component or sub-assembly in the end product.”
One snippet of information I found this week; ever wonder where the numbering convention of standards comes from?  If a standard is IEC 50xxx it is based on a standard of CENELEC (European Committee for Electrotechnical Standardization) origin.  If it is IEC 55xxx, it is based on CISPR (Special international committee on radio interference) origins.  For standards IEC 6xxxx, they originate from the IEC (International Electrotechnical Commission).
Power Guy

Monday, January 5, 2015

Pollution Degree Ratings for Power Supplies

A less common question that TDK-Lambda’s Technical Support team gets asked is “what is the pollution degree of your products?”  It is very important for the safety of the end equipment and can be found listed in the safety certification reports.

Our products go into a wide range of industrial applications, from semiconductor fabrication facilities to off-shore drilling platforms.  The environment that they operate in varies dramatically, and a walk through the service department will show which customers haven’t paid attention to pollution degree!  By “pollution” we mean contaminants that could be condensation, water and a variety of dusts.


The three main safety standards for power supplies (IEC 60950-1, IEC 60601-1 and IEC 61010-1) all call up pollution degree classifications, and in general the wording is similar.

Pollution Degree 1 is the least stringent.  It applies where there is no pollution or only dry, non-conductive pollution.  This not only applies to applications like clean rooms, but also where the power supply is placed in a sealed cabinet or enclosure.

Pollution Degree 2 is a little tougher, applying to non-conductive pollution that with occasional condensation could become temporarily conductive.  Applicable for products used in office environments, laboratories and test equipment.

Pollution Degree 3 you would find in harsh industrial and farming, particularly with unheated rooms.  Conductive pollution is to be expected, with or without condensation.

Pollution Degree 4 is outdoor equipment.  Persistent conductivity, rain or even snow is the norm.
Could a pollution degree 2 power supply be used in an outdoor application?  Yes, providing it is mounted in a suitable enclosure.

When the power supply is submitted to the safety test houses for certification, careful attention is paid to distance between components, pcb traces and the product housing.  The voltage measured say between two traces on a pcb will determine the insulation thickness or creepage/clearance distance.  Creepage is the shortest distance measured on the surface of an insulator; clearance is the shortest distance through the air.  With pollution, this distance could become reduced, leading to the risk of electrical shock or failure.  The manufacturer will advise upon submittal what pollution degree they want the product evaluated to.  For TDK-Lambda’s ZMS100 series of AC-DC power supplies, pollution degree 2 was chosen and because of the product’s 5,000m altitude specification, those spacings were multiplied by 1.48 according to IEC 60664-1.

Power Guy

Friday, November 28, 2014

What pin material should I use for my power supply connector pins?

Open frame power supply manufacturers typically use a supplier like Molex or JST for their input and output connectors. These connectors are low cost, readily available, reliable and easy to use.  In addition, it makes it easier for the customer to second source a power supply, if required, when some standardization exists.
Many power supply manufacturers will specify the mating connector series name in their product documentation, but will often leave it up to the user to determine the actual part numbers.  This usually provokes a call to TDK-Lambda’s Technical Support for a recommendation.

Why do we do this?  Let’s take the industry standard low power 2x4” single output power supply.  The Molex KK® 09-50-3041 housing is widely specified as the output mating connector. Made of nylon, it has a friction lock and 4 circuits; two for the + output & two for the – output.
When looking for the mating pin, one has to be a little more careful.  The suggested pin for the connector is available in 2 materials; brass and phosphor bronze.

Brass is a common material for contacts and pins.  It is low cost, has good conductivity and generally dependable in a benign, low temperature environment like an office.
Phosphor bronze should be considered for more challenging environments.  At higher temperatures, brass contacts can lose their spring properties unlike phosphor bronze.  If there is some vibration, this can cause reliability problems. Brass does have better conductivity, so check current rating capability.

Phosphor bronze is more expensive, 13c compared to 5c for brass (1000 piece pricing from a distributor).  For a 2x4” power supply that could add $0.56 to the bill of material cost.  The user will have to consider the environment and desired field life.
As a note, on higher power 2x4" open frame power supplies (~100W), there are alternatives to the single point of contact KK style pins like those used with Molex's 09-50-1041 housing (SPOX™ series).  These have multiple points of contact for lower resistance.
As Molex advised “Different terminals have different performance and different characteristics”.

Power guy

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

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