A lot of various products that run off of DC power, often
destined to be used in automobiles and other types of vehicles, but even quite
a number in stationary applications as well, require validation testing for
impact of having AC disturbances riding on top the DC powering them.
Conducting this
type of testing is often a big challenge for the test engineer in finding a
solution that adequately addresses the disturbance test requirements. It
usually requires multiple pieces of hardware:
- A DC power supply is used to provide the DC bias voltage and power.
- A power amplifier is used to generate the AC disturbance.
- A separate ARB /function generator is needed to produce the reference signal for the disturbance
Coupling the DC power supply and power amplifier together
is extremely problematic. While it would be great to just directly connect the
two in series, this rarely can be done in practice as the power amplifier
usually cannot handle the DC current of the power supply. A variety of custom
approaches are then typically taken, all with their associated drawbacks.
An article about this very topic was published last year,
written by a colleague I work with, Paul Young in our R&D group. As he
noted it’s great when the power source can provide both the DC power as well as
the AC disturbance as this is a big savings over trying to incorporate multiple
pieces of equipment. Paul’s article “Extending the Usable Bandwidth of a
Programmable Power Supply for Generating Sinusoidal Waveforms” (click here to review) is an excellent reference on this and the inspiration for my blog
posting this week.
Our N6705B DC Power Analyzer in Figure 1 and recently introduced
N7900A series Advanced Power System (APS) 1KW and 2KW power supplies in Figure
2 have proven to be very useful for doing a variety of testing where transients
and audio disturbances are needing to be introduced on top of the DC that is powering
the DUT.
Figure 1: Agilent N6705B DC Power Analyzer and N6700
series DC power modules
Figure 2: Agilent N7900A series 1KW and 2KW Advanced
Power System and N7909A Power Dissipator
The reasons for these products being useful for
disturbance testing are due to their built in ARB generation capability in
conjunction with having a respectable AC bandwidth, on top of being able to
source the DC power. Everything can be done within one piece of equipment.
A very common test need is to superimpose a sinusoidal
disturbance in the audio range. One example of this is in automobiles. The
alternator “whine” AC ripple induced on top of the DC output falls within this category.
Our 1KW and 2KW N7900A series APS are good for applications needing higher DC
power. However, at first glance the specified AC bandwidth of 2 kHz on does not
look like it would work well for higher audio frequencies. The AC response of
an N7951A from 1 kHz to 10 kHz is shown in Figure 3. This was captured using
the 14585A companion software to set up its ARB. There is noticeable roll off for higher
frequency, as expected.
Figure 3: N7951A APS AC response characteristics captured
using companion 14585A software
However, it’s worth noting that the roll off is gradual
and very predictable. In the case of superimposing a relatively small AC signal
on top of the DC output it is easy to compensate by measuring the attenuation
at the given frequency and applying a gain factor to correct for it, as I did
as shown in Figure 4. As one example, for 5 kHz, I programmed 2.38 volts peak
to get the desired 1 volt peak.
Figure 4: N7951A APS AC response characteristics after
gain correction
As can be seen it was simple to now get a flat response
over the entire range. A limiting factor here is sum of the programmed DC value
plus programmed AC peak value needs to be within the voltage programming range
of the power supply being used. In practice, when the AC disturbance is
reasonably small it is easy to cover a wide range of frequency.
Another factor to consider is capacitive loading. Some DC
powered products sometimes have a fairly substantial filter capacitor built in
across the DC power input. This will increase the peak current drain from the
power supply when AC is applied on top of the DC. As an example a 100
microfarad capacitor will draw a peak current of 6.28 amps when a 10 kHz, 1
volt peak AC signal is applied. There may also be series impedance limiting the
peak current, but whatever this AC peak current is it needs to be included when
determining the size of the power supply needed.
With these basic considerations you will be able to
perform AC disturbance testing over a much greater bandwidth as well!
.
