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

 

Wednesday, October 30, 2013

Reducing Switching Power Supply Radiated & Conducted EMI


One application issue that comes across my desk on a regular basis is where a customer has gone to an outside lab to certify their equipment for EMC, and they have failed conducted or radiated noise.

Usually the power supply in question is an open frame type, which does not have the shielding that a metal enclosed power supply has.  There are two areas that are worth checking; grounding points and wire harnesses.

1. Grounding Points

Power supplies utilize decoupling capacitors; two are typically connected from input to earth ground (see below). Likewise, two are connected from the output to earth ground. This keeps the noise currents circulating close to the power supply, rather than allowing them to radiate around the end user’s system.
 
In an enclosed power supply these capacitors will be grounded through the metal case, but with an open frame type, it is up to the user to connect these points to ground.  With the power density of products today, there often is more than one point on the power supply printed circuit board that needs to be connected.  A common mistake is to only connect one, which can cause excessive radiated and conducted noise.

The installation manual will show which mounting holes / points need to be grounded.  In the product below, three mounts should be connected (A, B & C).


 
 
The photo below shows the same power supply undergoing EMC testing, and it can be noted that the unit is connected to a metal plate with metal standoffs.


 
 
A quick glance of the underside of the printed circuit board will show which mounting holes have traces that need to be grounded. This smaller model has only one grounding point at the bottom right hand side of the board.

2. Wiring harnesses

In the test photo, it can be seen that the cable harnesses are neatly dressed and are kept away from the power supply.  Wiring that is positioned above or below the unit will pick up radiated noise, thus defeating the purpose of having the EMI filter components.

If I am assisting customers on site, I always pack some tie-wraps in my tool kit to re-route any offending harnesses.


 

Monday, September 23, 2013

“Brute Force” Parallel of Power Supplies


You will see in many of our instruction manuals a warning about not connecting power supplies in parallel that do not have current share capabilities.

At first it would seem a nice easy way to get extra current.  Take two like power supplies, connect them together and they will deliver twice the current?
 
Unfortunately there is a good chance that the two power supplies will not current share due to their output voltage set points.  The power supply with the highest output voltage setting will deliver as much current as it can until it reaches its current limit threshold and then the output voltage starts to drop.  The second power supply will then take over and provide the balance.  The output voltage might glitch during the transition, affecting system operation.

For example, take two 24V 10A power supplies with an over current set point of 120% powering a 15A load:

Power supply A might deliver 12A (now at its current limit point)

Power supply B would then deliver 3A.

One could argue that the power supply is being protected by the current limit.  There are two issues with this though:

1.  A power supply is not designed to operate in current limit indefinitely. Internal temperatures will rise, reducing the life of the product

2.   The safety certifications for UL, CSA are based on 100% load, not 120%

My recommendation is to use a power supply with a higher current rating, or choose one with a current share feature.

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