Notes For Power Supply Selection

- Jul 18, 2016-

Notes For Power Supply Selection

When design circuit for electric device,it is very important to consider the supporting power supply.Below give some notes for power supply selection.

One.The basic foundation selection of power supply

Voltage and current range, these two indictators are the most easy to determine, as long as can be calculated according to power consumption of circuit.Also consider to test high and low voltage extremum.Most fixed power allow output voltage to change + / - 10% range,if the changing range cannot meet the requirements of the circuit,choose the output adjustable or allow a greater changing range of power supply.

If use power supply to feed a modular device, then 75% to 90% current of the biggest current is powered by one power supply,and connect two or more power supplies for insufficient part.

Two.The expansion And Safety of Power Supply

1.Work in parallel or in series

When a power supply can not meet the required voltage or current range, two or more power supplies use (or different output of one same power supply) in parallel or in series. Under this working mode,the connection of various power supply voltage regulator and control circuit is still remaining,just a power supply act as master and control another one

2.Overload Protection

Because a power supply feed different circuits, and perhaps the current flow of these circuits is unknown,in order to avoid damage the power supply, it is important to set protection scope of circuit.

Almost all power supply has following characteristics: when it falls outside the range of output, it hold at the maximum output value, or turn off power by itself. Some programmable power supply is not only available to set output scope, but also automatically set the type of power supply's stable output. That is to say, when the voltage or current exceed setting limit value of external circuit,power supply automatically turn constant voltage source to constant current voltage or change value current source to constant voltage source.Adding a protection diodes for power supply can prevent damage from misconnection of external power supply polarity. Thermal sensors can also be used to prevent to burn power supply under continous work in an overload condition or invalid cooling.

Three.The potential root cause exist in power supply

1.Ripple and noise

Ideal DC power supply should provide pure DC, but there are always some interference is present, such as power supply output port superimposed ripple current and high-frequency oscillations. Both interference plus noise spikes generated by the power supply itself,make it appears intermittent and random drift.


When the line voltage or load current change, the output voltage of the DC power supply will be ups and downs. The extent of Regulators is determined by the voltage regulator circuit parameters, parameter refers to the capacity of filter capacitor and rate of energy release.If use a relatively constant supply for power, simply need basic load to make voltage stable. The degree of stability is generally defined as the percentage of no-load or full load output voltage or the changing value of voltage.

3.Internal impedance

Relatively large resistance has two disadvantages for load, the first is not conducive to the work of load regulator circuit, more unfavorable is to cause the output of the DC power supply fluctuations at any change of load current, these fluctuations affect test results exactly the same as impulse and noise caused.

4.Power Supply transient response or recovery

The costing time of power supply transient response and recovery indicates the capacity of power regulator circuit to recover normal when output load changes suddenly. There are two parameters to calibrate the power transient response and recovery: one is the output offset value when load suddenly changes; the second is the costing time to restore to output its original. For consistency, generally when load change 10%, mark the output deviation vector by output the milli figure of deviating from the peak voltage, and recovery time by millivolt output recovered to normal. Some other manufacturers, test recovery time by a larger load current changing. For example, take the recovery time when output current changes 50% to 100%.

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