Friday, June 18, 2010

What is the Load Transient Recovery Specification Telling Me?

Whenever a load change occurs on the output of a power supply, that power supplies output voltage will momentarily change from its programmed value. The figure to the right is a screen shot from an scope showing the change of a supplies voltage level after its load went from 1 mA to 500 mA in about 30 us.

The specification for this power supply characteristic is called the Load Transient Recovery Time or Transient Response Time. It represents how long it takes a power supply to return to its set voltage level after a sudden change in load current.
The specification typically has three parts to it:
•Magnitude of the load change, such as from 50% of full load to 100% of full load. So if we had a power supply that was rated for a max current of 10 A the spec would be referring to a load change from 5 A to 10As
•Voltage settling band is how close the voltage level will settle to its original level before the load change. Note that after a load change the power supply’s never recovers to its original level. How close it gets to returning to its original level is dependent on the magnitude of the load change.
•The time it takes the power supplies voltage level to settle within the voltage settling band

Below is an example of a transient response specification for two of Agilent’s N6700B modular power system’s supplies.

N6751A & N6761A Transient Response
Magnitude of load change: 60% to 100% and from 100% to 60% of full load for models N6751A & N6761A
Voltage settling band: ± 75 mV
Time: < 100 μs

The example spec can be interpreted as the output of the N6751A will return to no more than 75 mV within its original value in less than 100 us when a load transient that is 60% to 100% or 100% to 60% of its full scale load current occurs.

The transient response time is highly dependent on the speed of the supplies internal voltage output monitoring loop. Speeding up this loop provides better transient response time, but the output becomes more susceptible to instability and oscillations. That is why supplies with short transient response times typically cost more because of the extra investment in engineering it to ensure good output stability.

If the load is changing too fast for the supplies transient response time to keep up the power supply will never be at its programmed voltage level. With that in mind, the transient response time of a power supply is strongly related to the output bandwidth of the supply. The reason you typically do not see an output bandwidth spec is because the bandwidth of the supply is dependent on its load and power supply manufacturers cannot predict what type of load their customers will connect to the output of their supplies.

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