Thursday, October 29, 2015

Power Supply Safety Reports and Certifications

One can find a great deal of information on a power supply by studying the manufacturer’s datasheet and other technical articles, but sometimes more information is required for the actual installation.  Where is this information?  It is in the power supply’s safety reports and certifications.  Failure to review and follow these can cause delays when system certification is sought.

To keep this article simple, we will just review a product that is certified to IEC 60950-1.

Usually there are three main documents; the CB certificate, an IEC 60950-1 CB report and / or EN 60950-1 test certificate and of course for North America, the UL or CSA 60950-1 test report.  Due to confidential information like schematics, full test reports are often restricted and may only be released with a non-disclosure agreement (NDA).  Fortunately reproduction is allowed by the test houses for the relevant pages of the report.

Usually the CB test certificate, which should always accompany the CB test report, is just two or three pages.  This is often public information and details the part numbers that have been certified, their input and output ratings along with the safety standard (including revisions and amendments).  Its function is to give a quick snapshot of the product and to show if all the certifications are current.  A product that has out of date certifications may only be suitable where the safety bodies have allowed the use of “grandfathering” for older systems, and will not suitable for new designs or major system upgrades.

To reduce cost, many power supply manufacturers are using the CE Mark to indicate compliance with EN 60950-1 rather than pay for and maintain a separate EN 60950-1 test report and certificate.  In this case the CB test certificate (and CB test report) will indicate that the product was “additionally evaluated to EN 60950-1”.  This is perfectly acceptable.

Even an abridged CB or UL 60950-1 test report (the full report may extend to over 300 pages) has useful information.  The section “Engineering Conditions of Acceptability” has the all-important details for how the product should be used.

For example:

Are the outputs SELV?  Those outputs that are not should be insulated or have their access restricted to ensure that an operator or service technician cannot receive an electric shock.

Do any outputs have hazardous energy levels?  240VA is considered potentially dangerous if a screwdriver or metallic item accidentally shorts them, and a cover should be installed to protect them.  Metal watch straps have caused serious burns to car mechanics when they have shorted the positive battery terminal to the automobile body.

Is “field wiring” allowed?  If not, any cabling has to be attached by trained personnel.  Products like DIN rail power supplies do allow field wiring and do not have crimped wire terminations.

The maximum investigated branch circuit rating is given.  This reflects the size of the circuit breaker that was used during the safety testing, particularly when abnormal tests were performed.

The investigated Pollution Degree rating is stated.  A rating of 2 is normal for office or laboratory equipment.  That product should not be used where a pollution degree of 4 is required for an outside application where it may be subject to rainfall.

Proper bonding to the end-product main protective earthing termination is listed as required or not required.  Failure to correctly earth the product can result in electric shock.

The temperature class of any magnetic component components is given.  Usually this is Class A (105oC) and system testing should check to make sure that is not exceeded under worse case conditions.

“The following end-product enclosures are required:” Here the types of enclosures are indicated for mounting the power supply in.  If an open frame power supply is being used, the report will state that it has to be housed in an enclosure.

Other notes may be listed, like product orientation.
Many power supply companies are now posting this information on their website, along with the CE D of C (Declaration of Conformity); even some distributors are doing this too.  The recent surge of amendments to the standards though is keeping many webmasters busy!

Monday, August 31, 2015

IEC/UL/EN 60950-1 Amendment 2

CENELEC (Comité Européen de Normalisation Électrotechnique) published what is believed to be the last change to the EN 60950-1 standard in August 2013.  60950-1 covers general safety requirements for Information Technology Equipment (ITE).  Commonly known as “Amendment 2”, the safety files will now refer to either EN 60950-1:2006 + A11:2009 + A1:2010 + A12:2011 + A2:2013 or EN 60950-1:2006/A2:2013.  The date of withdrawal is July 2nd, 2016 and power supply manufacturers are updating their files to the new version of the standard.

UL & CSA announced their update to 60950-1 on October 14th, 2014, but unlike Europe, the Amendment 2 revision is not mandatory to existing files.  New products though are being certified to UL 60950-1, 2nd Edition, 2014-10-14 and CAN/CSA C22.2 No. 60950-1-07, 2nd Edition, 2014-10.

This should be the last change as June 2019 will see the move to IEC 62368-1:2014 for both IEC 60950-1 and IEC 60065.  The changes are considered as mainly clarifications to smooth that transition.  The safety bodies have commented on their websites that there would be minimal impact to power supply manufacturers for Amendment 2.

The changes for most power supplies for Amendment 2 are:

Any graphics used are to comply with ISO standards and must be explained in the installation manual.

Humidity testing has been introduced for equipment “designated for tropical regions”.  CCC & CQC certifications for China are already referring to this.

Similarly, to reflect a common Chinese requirement for 5,000m operation, the addition of a symbol will indicate if the equipment is to be operated at a maximum of 2,000m.  This is not an issue for UL, EN or CB.

Non lead acid batteries (with the exception of button cells) must comply with IEC 62133, and will only affect a very limited number of power supplies.

The use of VDRs - voltage dependant resistors – (a major change for Edition 1), have had a flammability requirement added.

As a note, all the self-certified Declarations of Conformity for CE marking will have to be re-written and posted.

Wednesday, July 29, 2015

Using Droop Mode Current Share Power Supplies

Connecting power supplies in parallel is commonly used to increase the available output power or to provide system redundancy in the event of a power supply failure.  The correct and reliable way to connect two or more power supplies in parallel is to have them equally share the load current.

If “brute force” current share is used (connecting power supplies in parallel without regard to load sharing), it can lead to one or more of the units operating in overload and a reduced field life due to overheating.  Here is an earlier article I wrote:

There are two main (analog) techniques for getting power supplies to share the load - “active” and “droop” mode. 

Active current share uses a signal wire interlinking two or more power supplies.  Simply put, the voltage on the wire is proportional to the total current supplied to the load.  That voltage is used to “inform” a unit that it is not contributing enough current, thus raising its output voltage to produce more current.  In telecom and datacom systems, the power supplies are usually rack mounted and plugged in to a pcb backplane.  The current share connection is therefore identical from installation to installation.

On the other hand, industrial applications tend to use hard wired DIN rail mount power supplies.  The cable routing can change dramatically between installations.  Cable lengths can be quite long causing the current share signal connection to be vulnerable to EMC interference from motors and relay switching.  One solution that does not need any current share connection is droop mode.

Droop mode is a very simple way of paralleling power supplies.  The output voltage drops (droops) in proportion to the current drawn from the power supply.  If one power supply is supplying more current than the others, the output voltage will fall and load balancing will occur.  Certain electronic loads can be sensitive to variations in the supplied voltage (3.3V or 5V for logic ICs for example), but typically 12V, 24V or 48V outputs drive relays, DC-DC converters or motors and are more tolerant to droop mode current share.

Very often this feature is standard on DIN rail power supplies 100W and above and is enabled by either a switch, or like on TDK-Lambda’s DRF series (shown below), the removal of a wire link or jumper on a connector.
TDK-Lambda DRF DIN rail power supply

When the DIN rail power supply is not operating in parallel, the switch is closed (or wire link kept in place).  The internal control circuit will compare the output voltage to a reference (Figure 1).  Any change in the output voltage due to load will be compensated for, and the output regulation will be minimal – in the order of 10mV.
Figure 1

When two or more power supplies are connected in parallel, the switches (or links) are opened (Figure 2).

Figure 2

As the output current increases, the voltage across the shunt will add to the voltage across the output, causing the control circuit to compensate and lower the power supply’s output voltage.

As an example, for the 24V 10A TDK-Lambda DRF240-24-1 power supply, the droop characteristic is at 64mV / A.  Table 1 shows the output change against load when droop mode current share is enabled.

Output Voltage
Output Current

Table 1

When using power supplies in droop mode current share, care must be taken to:

1. Set the output voltages of the power supplies to the same voltage.  The output voltage can be adjusted slightly higher if needed, to offset for the droop voltage*

2. Use the same length and same gauge of wire from the output to the load for each unit

3. Note any additional de-rating stated by the manufacturer.  This avoids tolerances from overloading a unit

4. Do not exceed the manufacturer’s recommendation for the number of power supplies that can be paralleled.

5. Make sure the parallel switch is in the right position, or the wire link is removed!
* If the power supplies are being used in redundant mode with ORing diodes, set the output voltages higher to compensate for the forward voltage drop (Vf) of the diode.  The function of these diodes is to prevent the bus voltage from being pulled down due to an internal short in a faulty power supply.  The diodes should be rated to carry the full output current of the power supply.

Redundant mode connection with ORing diodes

Power Guy

Tuesday, June 30, 2015

Comparing DC-DC Converter’s Usable Power

Power supply manufacturers rarely use the term “usable power” in their AC-DC product literature, but it is used frequently when referring to a DC-DC converter’s performance against temperature. Is it just a fancy reference to a de-rating curve? As usual, before we answer that question, we need to look a little deeper.

An AC-DC power supply, like the TDK-Lambda’s LS50 series, has a de-rating “curve” as shown below. It can deliver full power at 50C ambient and it de-rates linearly to 70% load at 70C. (The knee points of the chart vary from product to product but not normally dramatically between competitors of like products.)

The chart is very simple because the LS50 does not require any forced air, and has a metal case that is used as a heat sink and to provide a level of physical protection.

Looking at TDK-Lambda’s iQG ¼ brick DC-DC converter, we can see a much more complex set of de-rating curves.

To be fair, the industry standard ¼ brick has migrated from a product where 50W output power was leading edge, to products that are fast approaching 1000W. The emphasis for DC-DC converters has been put on package size. Even when fitted with an integral baseplate, like TDK-Lambda’s iQG ¼ brick (shown below), the iQG’s volume is 1.6 cubic inches, compared to the LS50’s 21 cubic inches. That is 10 times the output power in less than a 1/10th of the volume.

The “brick” style DC-DC converters are designed to be either conduction cooled (to a cold plate), or forced air cooled, often without an external heat sink. The rate of airflow available will depend on the user’s application, and so a number of performance curves are provided. It can be noticed that in some cases for low airflow conditions, de-rating has already occurred already at 30C ambient.

Usable power really refers to the slope and start point of the de-rating curve. Often Engineers will focus on the output current of the converter, and choose a higher power, more expensive product, expecting to significantly better performance. This is where “usable power” comes into play.

Below is a simplified pair of curves for 2m/s airflow. The blue line is for a 12V 67A (800W) DC-DC converter, and the green line for the TDK-Lambda 12V 42A (500W) converter. The 800W model is 1.6 times more powerful at low ambient temperatures, but in the yellow area at higher ambient the ratio drops to 1.35 times at 70C and 1.24 times at 75C. (Typically customers operate DC-DC converters in the 65 to 80C range.)

Although the 800W converter has more available power, the 500W unit has more usable power, demonstrated by a much less steep de-rating curve. It can be seen that at higher ambient temperatures, it would be more cost effective to use the 500W converter.

Wednesday, May 27, 2015

Understanding convection cooled power supplies

There are a number of commonly used terms to describe cooling methods in the power supply industry:
  • Fan cooled - Unit has an internal fan
  • Convection cooled - Unit requires no fan cooling
  • Forced air cooled - Unit requires external airflow
  • Conduction cooled - Unit relies on a cold plate to remove the waste heat
The most misunderstood and hence most misapplied is probably convection cooled. Many Engineers assume that a convection cooled power supply is one that does not need any airflow to operate.

One definition of convection is “The transfer of heat by the circulation or movement of the heated parts of a liquid or gas”. In our case – the circulation or movement of hot air.

Open frame power supplies, for example, are typically mounted on a flat surface upon standoffs, and below, we can see how the air behaves.

As the hot air rises, cooler air is drawn in from the sides. Although the airspeed is quite low, just 0.3m/s, it is sufficient to reduce internal temperatures. During the safety certification process for the power supply, this is taken into account during thermal testing.

It is very important to ensure that there is adequate space for the air to be drawn in from the sides and allowed to exit above the power supply. A distance of 50mm is considered adequate.

Orientation of the product is also very important. Most manufacturers will state a recommended mounting orientation and any de-rating associated if that is not followed. Mounting the product upside down for example can severely reduce field life unless heavy de-rating is applied, and is often forbidden.

The ramifications of mounting the power supply vertically should also be studied. Ideally the input (bulk) and output electrolytic capacitors should be located at the bottom, where the temperature will be the coolest.

If in doubt, consult the manufacturer’s installation manual. For high density products, recommended maximum component temperatures will be advised for critical parts.

Power Guy

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