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.

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



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

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

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

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

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 3^rd 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

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.

 

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.

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