Wednesday, July 31, 2019

When are dual fused power supplies not permitted?


For many years AC-DC power supplies for the medical market are not only safety certified to the IEC 60601-1 medical standard, but also to the industrial IEC 60950-1 / IEC 62368-1 standards.  Dual certification reduces the number of individual model numbers that have to be produced and stocked; this improves production efficiency and reduces the amount of inventory held in distribution and the supply chain.  One small cost adder is that two AC input fuses are needed in many medical applications, one in the Line side and one in the Neutral side.

Dual fusing is necessary to guarantee full protection if the polarity of the Line and Neutral wiring to the power supply should be reversed.  This might be due to a wiring error at the building’s AC socket or in the wiring feeding the power supply.

I had to say I was mildly surprised when TDK-Lambda started offering a single (Line) fuse option on their new medical/industrial power supplies.  Two protective devices surely must be better than one?  Some investigation was required, because someone else was bound to question this too!

If an industrial system is consuming more current than a regular AC socket can support, it would have to be permanently connected to a distribution panel or building wiring.  This task is normally carried out by a professional electrician, who would be aware of the potential dangers of a polarity reversal.  Figure 1 shows a power supply connected to the AC input, with the Neutral connected to the earth ground at the panel.

Figure 1: AC-DC power supply permanently installed to a distribution panel

In the event of an over-current fault condition in the power supply or fuse aging, there is a 50% chance that either F1 or F2 would open.  If F2 was to open, a service technician may believe that there is no AC power being applied to the power supply.  Inadvertent contact with the Line while touching the earth ground would result in an electrical shock.  This is even more likely if the power supply in question is of an open frame type construction.

To avoid this, the NEC, CEC, IEE Wiring Regulations and IEC 364, specifically prohibit fusing in the Neutral in this type of equipment.  To overcome this a single fuse power supply must be selected.  This can be achieved by using an industrial (single fuse) or a medical/industrial power supply that has a single fuse option.

TDK-Lambda currently offers a single fuse option on the medical and industrial certified QM modular series and CUS-M models.

Power Guy

Thursday, May 30, 2019

Does it matter how an EMC/EMI filter is wired to a power supply?


An email was recently received by TDK-Lambda’s Technical Support from a technician who was fault-finding a video system.  His questions about how an EMC filter should be connected were relevant and warranted a blog article on the subject!

As indicated in Figure 1, the EMC filter is situated between the AC input to the system and the power supply providing DC voltage(s) to the system load.  The filter’s function is to reduce incoming noise from the AC input and/or outgoing noise to the AC input from the power supply.




Figure 1: System block diagram






Regarding connection to the filter, let us now look at a typical EMC filter - the 250Vac 10A rated RSEN-2010 for example (see Figure 2 and 3).


Figure 2: RSEN-2010 filter

Figure 3: RSEN schematic

Reviewing Figure 2, the nomenclature “LINE/LOAD” on the left hand and right hand side of the label indicates that either set of the terminals can be used for the input or load connection.  It can be also noted that there is no indication of where the Line or Neutral should be wired to, just numbers 1, 2, 3 & 4.  Which connections are made to these terminals is very important.

On the right hand side of the label and schematic (Figure 3), two additional capacitors, known as “Y” capacitors can be seen.  These provide a low impedance path to earth ground to reduce high frequency common-mode noise.   When the filter is used to reduce system noise from the power supply reaching the AC source, terminals 3 and 4 should be connected to the power supply.  If the filter is being used to reduce externally generated high frequency noise from entering the system then terminals 3 and 4 should be connected to the AC source input.

It is essential that if the AC line is connected to terminal “1”, then terminal “4” should be connected to the power supply Line terminal.  Likewise with terminals “2” and “3” for the neutral wiring as shown in Figure 4.



Figure 4: Connection of the filter to the AC and the power supply

If the wiring to “3” and “4” is crossed, there could be safety issues for a power supply with a single input fuse situated internally in series with the AC Line terminal.  

If the AC is correctly wired (Figure 5a), in the event the internal power supply fuse was to open due to a fault, the internal circuitry is isolated.  It is important to note that at the breaker panel, where the AC first comes into the building, the neutral input is grounded to earth.




Figure 5a: Correct AC connection to a single fuse power supply

If the Line and Neutral is reversed to the power supply due to incorrect wiring (Figure 5b), in the event of the fuse opening the internal circuit is still live.  Two issues can arise with this scenario.  A service technician fault-finding the system could receive an electrical shock, particularly with an open frame power supply without a cover.  Secondly, if an internal short was to occur in the power supply between Line and the earthed chassis, the fuse would be out of circuit and not open.  Even the simple use of a mounting screw that is too long could cause this!




Figure 5b: Incorrect AC connection to a single fuse power supply

Always ensure that adequate inspection and testing techniques are in place with AC wiring.
Power Guy


Monday, April 29, 2019

How do I balance the current between two DIN rail power supplies used in parallel?


Before economically priced DIN rail power supplies gained popularity, many 100W or higher power rated units provided users with a parallel operation function in the form of switch fitted on the front panel.  This switch allowed the user to select between “droop mode” current share between two or more supplies and single unit operation. Droop mode allowed for balancing of the output currents between the units.

If power supplies without a parallel operation function are being used and additional current is required for the system load, a second power supply could be connected in parallel, although many manufacturers do not recommend this.  Unless the output voltages of both power supplies are set to the same voltage, the unit with the greatest output voltage may deliver the majority of the current and operate in an over current condition. This will reduce the unit’s field life due to excessive internal heating.  Ideally the output current of each power supply should be routinely measured during preventative maintenance to ensure they are balanced, which is both time consuming and cumbersome.

One alternative is to use a “redundancy” module with a load balance option, like TDK-Lambda’s DRM40.  Figure 1 shows a DRM40 used to connect two 20A power supplies to deliver up to 40A and Figure 2 the DRM40 with its current balancing LED.


Figure 1: Two units connected in parallel with the DRM40


Figure 2: DRM current balance LED

If the LED is not illuminated, measure the output voltage on both power supplies. Adjust the voltage of the power supply with the lowest voltage higher until the LED turn on.  Alternatively, one can adjust the voltage of the power supply with the highest voltage lower.  The current balance LED will be illuminated when the difference between the voltages is less than 50mV and the output currents are balanced.

The DRM40 has internal MOSFETs, used to block reverse currents in the event of one power supply failing short circuit.  They also allow the measurement of each power supply’s output voltage without disconnecting any load cables.

The DRM40 can also be used in redundant power systems, where two power supplies are used in a 1+1 configuration as shown in Figure 3.  If one power supply fails the other will continue to provide current to the load.  The load balancing function can be used in same manner.




Figure 3: DRM40 used in a 1+1 redundant configuration

Power Guy

Friday, January 25, 2019

What is the difference between efficiency and average efficiency?


Efficiency

Power supply datasheets include product efficiency in a percentage format for each voltage and output power model, as a guidance to how much power is lost in wasted heat when the product is running.   As the actual operating efficiency varies with input voltage, output load, ambient temperature and component tolerance, usually there is a test condition noted. 
Phrases like “up to 95%” or “typically 93% at 230Vac input, 100% load and 25oC ambient” are widely used.

If the selection of the power supply is being made purely on efficiency, then the manufacturer’s evaluation data has to be studied in order to determine the measured efficiency at the user’s load condition.  Figure 1 shows the efficiency vs. output current plot for TDK-Lambda’s 600W rated 24V output GXE600-24 for different input voltages.  At 60% load, 230Vac input one could expect the efficiency to be 94%.


Figure 1: GXE600-24 Efficiency vs Output Current

Average efficiency

External power supplies complying with the DoE (Department of Energy) and EU efficiency regulations will sometimes only state the standard (and its revision) they comply with.  TDK-Lambda’s DTM110PW240C8 datasheet for example, states compliance with the latest DoE Level VI & EU Tier 2 Efficiency standards and also includes that the average efficiency is >89%. The average efficiency for an external power supply rated between 49-250W has to be at least 89% to comply with the current and proposed standards.

Is “Average Efficiency” the same as “Efficiency”?  No.

Average efficiency is calculated by measuring the efficiency at 25%, 50%, 75% and 100% loads.  These four values are added together and the total is divided by four to obtain the average. Measurements are taken at 115Vac and 230Vac inputs.

Using the measurements from Figure 2 for the DTM110PW240C8, the calculated average efficiency at 115Vac is 90% and 90.5% at 230Vac.

--
Input
Output
--
Load (%)
Power
(W)
Voltage
(Vdc)
Current
(A)
Power
(W)
Efficiency
(%)
Vin: 115V/50Hz
100
121.53
24.16
4.54
109.69
90.0
75
91.67
24.20
3.43
83.11
91.0
50
61.27
24.26
2.28
55.40
90.0
25
31.73
24.27
1.17
28.35
89.0
10
12.44
24.27
0.46
11.07
89.0
0
0.08
24.25
--
--
--
Vin: 230V/50Hz
100
119.33
24.16
4.52
109.07
91.0
75
92.19
24.21
3.47
83.87
91.0
50
62.11
24.27
2.30
55.83
90.0
25
30.74
24.34
1.14
27.63
90.0
10
13.26
24.35
0.49
11.82
89.0
0
0.11
24.25
--
--
--

Figure 2: DTM110PW240C8 Efficiency Measurements

Efficiency readings are also taken at 10% to check compliance to the EU Tier 2 Efficiency standard.  For a power supply rated at 49-250W it must have a minimum efficiency of 79%.  At 10% load the DTM250-D has an efficiency of 89%.

Power Guy


Popular Posts

TDK Corporation - Americas | Press Release