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

Friday, December 20, 2013

The advantages of using a power supply incorporating digital control to power non-linear loads


I was recently tasked with doing a presentation on the advantages of power supplies incorporating Digital Control to power non-linear loads, so I thought I would share the content with you.

A non-linear load is one that does not behave like an ideal resistor, in that the current drawn from the power supply is not proportional to voltage and/or the initial currents are often much higher than the rating of the power supply.

These loads can cause problems for power supplies, but are actually present in many applications:

Large switched capacitor banks

Point of Load DC-DC converters

Thermal printers

DC motors

The main issue from a power supply’s point of view is that the load can activate the internal over-current protection.  Over-current protection (OCP) is an essential feature for a power supply, but the power supply is usually expected to recover automatically with no manual intervention.

So to start with, let’s look at the types of OCP. There are several basic methods used:

Constant Current

Fold Back

Fold Forward

Hiccup

Constant Current

When an overload condition occurs, the output voltage falls but the output current remains at a fixed level.  This type of protection is not well suited for delivering peak loads as it can lead to the power supply latching.

 
Fold back

When the current drawn reaches the OCP limit, the voltage falls, but this time the current decreases as the overload gets heavier.  Again this type of protection is not well suited for delivering peak loads as it can result in the power supply latching.
 
 
Fold forward

When the current drawn reaches the OCP limit, the voltage falls.  This time the output current increases to a set maximum at short circuit.

Fold forward is well suited for powering up motors, but requires heavier system load cabling to handle the additional overload current.

 
Hiccup

At the OCP limit, the power supply turns off for a short interval and then automatically tries to restart.  Hiccup mode reduces the need for heavy cabling or pcb traces, and this type of protection can be modified to deliver a peak load. 

With traditional Analog Control though, the OCP points and recovery timing are fixed.

With Digital Control we can use software settings to adjust the limits and timing; for example we can set:

10s for an initial overload condition

60ms for heavy overloads

5ms for a short circuit condition with recovery times or 1 to 2 seconds

Let’s take an application example of a discharged capacitor bank being switched onto an operating power supply with Analog control.

The lower (blue) trace is the power supply current; restarting twice with the power supply current limit set at around 60A.

The top (gold) trace shows the output eventually recovering, but tolerances with the hiccup mode timing could have prevented a full recovery. 



This time the same discharged capacitor bank is switched into a TDK-Lambda CFE400M supply incorporating digital control.

The blue trace is the current, gold trace is the output voltage

Using Digital Control, we can set the thresholds and timing accurately.

50A for 1.5ms (The short circuit condition)

30A for 50ms (The over current condition)
 



Notice that there are no multiple attempts to recover after the capacitor bank is applied to the power supply.

To summarize:

Digital control can allow for precise and repeatable current limiting using load dependant timing.  We are not restricted to the value of a timing capacitor which can change due to:

a) Batch tolerances

b) Aging of the capacitor

c) Capacitor values changing with temperature

Digital control allows for easy tailoring for different applications with no physical modification of the power supply is needed.  All changes are handled with software programming!

Power Guy

 

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