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


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