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Tech Tips

Input Voltage

The range of figures for input voltage shows the range of voltages input into the power supply that will enable the power supply to perform at the rated levels of output and specifications.

E (rms) = rms formula
E (ave) = average formulas

AC inputs come in two types, single phase input and three phase input (for larger output powers)

a waveform of period Time, Em, E (ave) and E (rms)
Fig. The sinusoidal AC voltage waveform
Input Current

The input current is the amount of current input to a power supply expressed in RMS. Both series regulators and switching power supplies have capacitive rectifier circuits. This causes the input current waveform to not be sinusoidal but in a waveform as expressed below. The AC waveform of units with PFC (Power Factor Correction) will be sinusoidal.

Input current (rms) = Output power divided by Input voltage (rms) x Efficiency
a waveform of positive and negative peak
Fig. Input current waveform
Input Power (Input Capacitance)

Because power supplies have capacitive rectification circuits and smoothing circuits, the waveform of the input current will be distorted. The product of the input voltage (RMS) and the input current (RMS) is apparent power. The integrated value of the product of input voltage and input current is active power. Apparent power is the power that flows in the power supply, active power is the power consumed by the power supply. The gap between the apparent power and active power is the power not utilized, and return into a commercial power.

Apparent power = E (rms) x 1 (rms)
Active power =
Active power formulas

Efficiency is the ratio of output power to input active power. This value in the specification is at the rated output power. The smaller the input current gets, the worse the efficiency value becomes.

Efficiency (%) =
Output power divided by Input active power
x 100
Power Factor

Power factor is the ratio of apparent power to active power. In the case of a power supply with capacitive input, the power factor is normally approximately 0.6. A power supply with PFC (Power Factor Correction) is approximately 0.9.

Power factor =
Active power divided by Apparent power
Rush Current

As switching power supplies apply capacitive input circuits, the rush current can be from multiples of ten to multiples of hundreds of times that of normal operation. Rush current is the crest value of current that flows when applying an input voltage to a power supply. In the case of series regulators, there is current that flows to the capacitor input rectifying circuit of secondary and which flows when the transformer is saturated when input at 0 phase 5 to 10 times of rated current of rush current flow.

Leak Current

Leak current is the current specified by the safety regulation of each country or area for safety purposes to avoid electrical shock or other hazardous incidents.

Stray capacity between the primary and secondary transformers, or input to primary components, or current to the ground capacitor of the noise filter are considered as leak current. The diagram below shows how to measure leak current.

Leak Current circuit
Fig. Leak Current Measurement Circuit
Input Regulation
Input regulation is the fluctuation of the output voltage when the input voltage is changed within a specified range.
Load Regulation
Static Load Regulation

Static load regulation is the fluctuation of the output voltage when the load is changed from no load to the rated load.

Dynamic Load Regulation

Also called transient response time, the dynamic load regulation is the output response time when the load is changed from no load to rated load quickly.

represent Emax and Emin of output voltage drops
Fig. Dynamic load regulation

Ripple is a wave synchronized with the switching frequency and/or input frequency added to the output voltage. Ripple is usually expressed in peak-to-peak terms. When a capacitor is added to the output of a switching power supply, the composition of the ripple synchronized with the switching frequency becomes smaller, but the composition synchronized with the input frequency shall be the same. For series regulators, the ripple is only the noise synchronized with the input frequency.

Ripple Noise

Output noises consist of noise other than ripple added to the output voltage and is expressed in peak-to-peak terms. Most switching power supplies use rectangular waveform inverters, so noise from switching transistors or output rectifier diodes is added to the output. Below shows the relation of ripple, output noise, and ripple noise. For series regulators, ripple is only the noise synchronized with the input frequency.

a relationship between ripple and noise in the standards
Design of Heat Radiation

As power supplies are a power apparatus, all the differences in conversion efficiency translate into heat radiation. One major component in designing a power supply is the dissipation of heat.

a. Natural Air-Cooling

Natural air-cooled power supplies dissipate the heat via radiation and convection with the use of forced air-cooling (ie: fan). Most heat dissipation is done via convection and thus sufficient space for airflow is required. When placing a natural air-cooled power supply inside an enclosure, be sure there is proper ventilation to prevent the accumulation of hot air. Placing a larger vent near the exhaust is more effective than intake larger. Always install the power supply in a specified manner that in which it was designed.

b. Forced Air-Cooling

Forced air-cooled power supplies to use a fan to force air flow to cool the power supply components and therefore control of the ambient temperature is enough to cool the fan. Always be sure that the air intakes or exhaust vents on the power supply are not blocked.

Remote Control

The remote control function is used to turn the power supply on or off, or to toggle the settings of voltage rise or fall times for multiple power supplies. This is a function to turn the power supply output on or off with an external signal. Relay contact transistors or integrated circuits can also be used to power on or off. Depending on the model, LOW = ON and HIGH = OFF or vice versa. Always check the power supply's specification before use.

Isolation/Dielectric Strength
Isolation Resistance

Isolation resistance is the resistance when applying a specified DC voltage between specified terminals.

Dielectric voltage strength

Dielectric voltage is a voltage that does not break the power supply or does not cause a spark discharge when a specified AC voltage is applied between specified terminals. To test the dielectric voltage strength, increase the voltage gradually from 0 V so that rapid voltage change does not cause impulse voltage, or increase the voltage with a zero crossing switch.

Operating Temperature

Operating temperature is the ambient temperature without wind and when placed horizontally, at which the power supply can be operated within its specifications. Avoid placing the power supply vertically or at extreme angles. For forced air-cooled power supplies, the operating temperature is the temperature of airflow into the power supply. The lifetime of the power supply is reduced by 50% for every 10°C rise in temperature. Operating the power supply at as low a temperature as possible (in the specified operating range) will prolong the life of the power supply.

Storage Temperature

Storage temperature is the ambient temperature at which the power supply can be safely stored without deterioration or performance of the power supply. Avoid storing power supplies in hot areas so as not to deteriorate the aluminum electrolytic capacitors within the power supply.