How to Select a Right DC Power Supply Part 2: Key Considerations

Welcome back to Part 2 of this series! Part 1 is all about the functions and characteristics of DC power supplies. In Part 2 here, we will take a look at the considerations when selecting a right power supply.
1. Key Considerations
There are too many considerations when selecting a DC power supply. This makes it hard to select a right power supply without any understanding, which is the topic we are going to explore here.
In Part 2, we will look at two main considerations: basic considerations and specific requirements for a DC power supply. First, start with the basic considerations which are most important factors in the power supply selection. The next is the specific requirements, which are mainly related to:
1) performance requirements 2) function requirements 3) system expansion requirements 4) requirements for flexibility in your system 5) safety requirements and 6) maintenance requirements.
In choosing a power supply, it is very important to be clear about what you want to achieve. So, Part 2 guides you to determine exactly what you are looking for in a DC power supply. Each of the sections below is considered in more details with particular examples or technical advice. Furthermore, we will not cover here in details, but you may need to think about your priorities, point of compromise, how to use or combination with other components in your system to find a best DC power supply.
1-1. Basic Considerations
The following are very important considerations that you should take into account.
1) Voltage and Current
Determine how much voltage and current that you need.
2) Wattage
Calculate the maximum power wattage to be achieved according to the voltage and current. Consider using a multiple-range power supply depending on your application.
3) Load Type and Current
Check your load type, load current (e.g. pulse current) and load current waveforms.
One last recommendation: You can use the five Ws and one H approach to find out what you need from a DC power supply: who, what, when, where, why and how.

1-2. Specific Requirements
This section uses the table below again:

1-2-1. Performance Requirements
1) Low Ripple and Noise
To achieve a low ripple and noise, select the B-type or C-type power supplies. The B-type power supplies, series regulator DC power supplies, have a low ripple and noise. The C-type power supplies, linear DC power supplies, can offer high speed and low noise. Read data sheets or specifications for the details.
2) Pulse-waveform
If your project requires sharp pulses such as sharp rising and falling in 5 µs, choose the linear
power supplies (C-type). If your project requires 30 µs, you can also use the inverter DC or AC
power supplies (D and F-type) or linear AC power supplies (E-type). The E-type and F-type
power supplies are high-voltage power supplies, but if you need higher voltage and high-speed pulses, add a pulse generator in your system. With a pulse generator, the switching DC power supply (A-type) or series regulator DC power supply (B-type) can be also used.
3) Absorb reverse current from load
The bipolar linear DC or AC power supplies (C or E-type) can absorb a current source such as a motor reverse current. Some bipolar inverter AC power supplies can return the absorbed current to a power line. Other than them, place a resistor or electronic load in parallel with your power supply to absorb it.
4) Fine Voltage Adjustment
A voltage setting resolution is stated in data sheets or specifications; e.g. 0.012 % of full-scale (max. voltage). The more voltage the power supply has, the more rough adjustment is achieved in constant voltage mode. The voltage setting resolution may differ depending on the setting method: set by panel or communication command.
5) Inrush Current
You need to consider the voltage rise time before selecting a power supply.
Pulsed inrush current is drawn by DUT when first turned on. The voltage rise time for typical DC power supplies is 50 ms or more, so the inrush current cannot be easily obtained even if your DUT has capacitive properties. To correctly measure the inrush current, the input voltage should have the rapid rise time. Placing a switch at the power supply output is effective to control the voltage rise time within 1 µs or less.
Capacitive current (Ic) = C x dV/dt; Large capacitors may cause a large current spike.
The switching DC power supply (A-type) has a large capacitor at its output side to be able to flow more inrush currents. There are power supplies with specific inrush current capability designed for use of large motors.
6) Power Efficiency
The power loss is the power difference between electricity input and output as a result of an energy transfer from AC voltage to DC voltage. The power conversion efficiency is usually 70 – 90 % and the high efficient power supply is the switching type power supply such as A, D or F-type on Table 1.
The most efficient way is to use the rated power from your power supply. Do not select a power supply with much higher power ratings than you actually need.
7) Low Current Consumption
To reduce a current consumption, use a high efficient power supply (read 6. Power Efficiency) or get a higher input voltage as much as possible.
The power supplies with a power factor correction circuit use less input current. A high power factor (close to 1) indicates good use of the incoming supply.
8) Takt Time
Takt time is the rate at which a product needs to be completed and is used to describe how fast or slow production takes place based on customer demand. Manufacturers are expected to reduce the takt time while increasing the productivity per unit time.
The DC power supplies are typically used in production test systems to apply various test voltages on a DUT. Their high speed response is important to reduce the takt time. Also, the signal lags between transmitting an output-on command and actual voltage rising needs to be minimized.
C, D, E or F type power supplies, the high speed power supplies, can quickly switch the output voltage, but the time taken for the signal lag was not much different between all types of power supplies. You can find a multi-output power supply with high set speed, but consult with the vendor or maker to confirm the exact information before making any decisions.
For the analogue control power supplies, the takt time can be reduced depending on how they are used.
9) Pulse Current
In pulse current, a duty ratio is the ratio of the on-time to total time of the current waveform, where total time is an on-and-off cycle (pulse cycle). If your load accepts the pulse current to flow with duty ratio 50 % and pulse cycle 5 ms or more, use any DC power supplies. If a pulse cycle is 1 ms or less, use either of B, C, E or F type power supply.

1-2-2. Function Requirements
1) Various Applications
When testing your DUT with different voltages or currents within the same operating area, the multiple-range DC power supplies are recommended. If you use a high-power power supply for small-power applications, its efficiency is decreased.
2) Bipolar Output
Using two units of DC power supplies can make a bipolar output. Place a switch to control the output on/off synchronization, if needed.
With a master-slave parallel operation function, the entire system can be controlled by the master unit. This function is similar to the dual tracking function (check Part 1 – Figure 9) of the multi-output power supplies. The multi-output power supplies often feature the output on/off synchronization function.
3) Remotely control power supplies
The modern DC power supplies often feature a LAN port. Multiple power supplies can be controlled with a LAN hub connected. Output on/off synchronization may be also available via LAN interface. The LAN interface will increase in popularity for DC power supplies.
4) Use as DC and AC power supply
C, D, E and F type power supplies, bipolar power supplies, can provide both DC and AC outputs. Especially, E and F type power supplies, bipolar AC power supplies, are more appropriate to provide high voltage output.
5) Voltage Waveform Generation
C, D, E and F type power supplies tend to have the voltage waveform generation function. These are the high speed power supplies and the voltage rising and falling time is from 3 µs to 30 µs, which can generate desired waveforms.
Some power supplies allow users to customize waveforms with their internal function generator. You can use this sequence feature to customize certain times or levels of waveforms and save the sequences into the power supply itself.

1-2-3. System Expansion Requirements
1) Increase power supply capacity
There is the master-slave parallel function which is performed by designating one master unit and connecting it to one or more of the same models being the slave units. The entire system can be controlled by operating the master unit. Output current and power can be greatly amplified under this operation.
To double the output voltage, connect two units in series. The maximum number of series connection units differs by power supply models.
2) Increase control units
The number of control units can be adjusted via communication network.

1-2-4. Requirements for Flexibility in Your System
1) Low Noise
DC power supplies were originally designed for use in laboratories and factories, so it is quite normal that their cooling fans get larger and louder. In some power supplies, the cooling fan can make less speed and noise during low-power outputs.
2) AC Input
Depending on the model, but mostly in E and F type power supplies, the input voltage range is specified from 85 VAC considering the input voltage drop. See the data sheets or specifications to check the input voltage range.
3) Under 15 A Circuit Breaker
Keep the power supply operation within the 15 A circuit breaker rating. Check the output power rating of a power supply before use.
4) Harmonic Current Reduction
A power factor correction circuit is necessary in high-power power supplies to improve the power factor. Please be noted that some power-saving power supplies do not have it.
5) Inrush Current Protection
Inrush current protection circuit is installed in almost all power supplies to prevent it. However, small-sized power supplies installed a commercial transformer may not have it.
6) Backup or Redundant Power Supply
It is the best way to prepare a back-up power supply to ensure the operation for your critical application during power failure or breakdown, or you can have redundant power supplies.
A redundant power supply is when a DUT operates using two power supplies in parallel with diodes as below. Each of the power supplies has the capacity to run this DUT on its own, which allows it to operate even if one goes down. During normal operation, each of the power supplies provides half of the power that is needed.

7) High and Low Temperature
The typical safe temperature range for DC power supplies is 0 – 40 C°, but the range can be extended to 50 C° depending on the models.
8) Mount in 19-inch rack
Check the availability of 19-inch rack mount accessories.

1-2-5. Safety Requirements
1) Safety
Most DC power supplies comply with IEC61010.

1-2-6. Maintenance Requirements
1) Warranty Period
The warranty period has been recently extended. Ask your vendor or maker.
2) Lifetime and Mean Time Between Failure (MTBF)
DC power supplies can last a very long time by making a repair. There might be no specific expected lifetime but the mean time between failure (MTBF) is defined.

Products Mentioned In This Article:

• DC Power Supplies please see HERE