4
Load Transient Recovery Time
A constant voltage DC power supply is designed
with a feedback loop which continuously acts to
maintain the output voltage at a steady-state level.
The feedback loop has a finite bandwidth, which
limits the ability of the power supply to respond
to a change in the load current. If the time delay
between the power supply feedback loop input and
output approaches a critical value at its unity gain
crossover, the power supply will become unstable
and oscillate. Typically, this time delay is measured
as an angular difference and is expressed as a degree
of phase shift. The critical value is 180 degrees of
phase shift between the loop input and output.
Power Supply Tests
Table 2
Load Transient
Current Limit
Efficiency and
Recovery Time
Load Effect
Characterization
PARD
Power Factor
Start-Up
Electronic Load
t
rise
≤
15 µs
1% programming
1% programming
1% programming
1% programming
1% programming
accuracy
accuracy
accuracy
accuracy
accuracy
Trigger output to
CC or CR mode
CR or CC mode
CC or CR mode
CC or CV mode
CR mode
the oscilloscope
Low PARD
Digital
t
sample
≤
100 ns
N/A
N/A
t
sample
≤
25 ns
N/A
t
sample
≤
1 µs
Oscilloscope
DC to 20 MHz
Record length 1 K
minimum bandwidth
samples minimum
Record length
≥
2 K samples
100 µ/Div (linears)
1 mV/Div (switchers)
Digital
N/A
5
1
/
2
Digits
5
1
/
2
Digits
N/A
N/A
N/A
Multimeter
±0.005% accuracy
±0.005% accuracy
Wattmeter
N/A
N/A
N/A
N/A
1% accuracy with
N/A
crest factors to
10:1 in current
waveforms
Regulated
>1% regulation
>1% regulation
N/A
>1% regulation
>1% regulation
>1% regulation
AC Source
Adjustable peak
Adjustable peak
Adjustable peak
Adjustable peak
Adjustable peak
and frequency
and frequency
and frequency
and frequency
and frequency
Power factor
Phase control
measurement
capability
RF rms
N/A
N/A
N/A
100 µV Full scale
N/A
N/A
Voltmeter
DC to 20 MHz
minimum bandwidth
Figure 1. Load Transient Recovery Time
Load transient recovery time measurements require an electronic
load with a risetime and falltime at least five times faster than the
power supply under test.
