Today we're going to talk about power supply transient response.
What is it
How is it measured
and why is it important.
Power supply transient response what is it
it's a characteristic which describes a power supply response to a large change
in output current
it's typically specified for 50% change in full-scale output current
It is specified in constant voltage mode
with a resistive load.
Transient response measurements of a real battery
let's first consider response to a load change for battery
on the left here we have an actual battery shown with its internal voltage,
and internal resistance
on the right
we have a load that can be switched from 1.2 Ohms to 2.4 Ohms in the
graph on the right here we show that the switch starts in the open position
and moves secondly to the closed position
and then returns back to the open position
the current in the circuit
When the switch is open is going To be V(L)/2*R
and when the switches closed is going to be the V(L)/R
what you can see in the third graph
is what happens when you switch
from open to closed
the output voltages at the load V(L)
starts out here
then when you change to the higher current
that internal resistance causes an additional voltage drop so the voltage
at the load drops down to this level
which can be defined as V(battery)-I(L)* R(internal)
and then when the switch is open it returns to its original voltage over here
now in the second case let's consider power supply
where the power supply on the left here
which has very low output impedance
and program it to V(out)
we have the same setup on the right where you're switching in
a resistor and then
you’re shorting it out.
So we start out with a switch open
we go to the closed position and back open.
So it's the same conditions is for the battery
you have the current in load
is defined as the V(out/2R) when the switch is open
And V(out/R) when the switch is close
What you will notice in the third graph
is that the output impedance of power supply
causes the V(out) to vary far less than for the battery.
If you zoom in on that condition there are some overshoots and undershoots
which we'll discuss next.
Let's talk about transient response specifications first. Power Supplies
have some overshoot and undershoot
associated with them.
We specify power supply
both for the time it takes to recover and the amplitude it has to recover to
within over here.
So the typical spec for a power supply would be
less than 100 microseconds to recover to within 50 mV for a load
change of 50%.
So we have the two boundaries here: the 50 mV and the 100 microsecond.
so as long as the power supplies is back within that limit before the end of the
time is up. It’s meeting it's spec
There are other parts of the waveform that are not specified
the V(peak) undershoot and overshoot are not specified.
The reason these aren't specified is there are too many variables that can
cause these voltages to go up and down
the load lead length
dressing of the leads whether they are twisted or not
and the load itself whether it's reactive can all affect V(peak)
overshoot and undershoot,
So you must be careful when attaching load leads that you are making them
as short as possible
and you're twisting them together to minimize the amount of inductance in the
actual load leads.
Transient response issues that can come up with a power supply that is poorly
regulated or has poor transient response is that you can actually have
an undershoot that actually goes below the low voltage
shut down of the circuit
A good example of this will be a
digital cell phone when it goes in to talk about it draws a large amount of
current and has a
battery detect circuit which once it goes below will shut off the phone in
that condition. So this
power supply that has bad transient performance
would be unacceptable for cellphone testing
The other problem you can run into a power supply transient responsive
that’s it’s to slow
the condition here shows that the power supply is still actually settling
by the time you go to the next transient This causes a problem when you're
actually trying to make fast measurements in a test system when you
want to go from one condition to the next. If the
Power supply hasn't settled out then you can’t make an accurate measurement.
So slow transient response time can actually affect how long it takes you to
run through your test in a test system which can cost you money.
I hope this has outlined some of the issues that come up and [demonstrates]
how to measure transient response in a power supply