| Brick |
This is a standardized size for PCB mounted power supplies. I suppose that
the term brick comes from the first engineer that dropped a power supply on his toe. It
could have been worse! The brick power supply dimensions are as follows:
Full brick 4.6 x
2.4 x 0.5 (116.8 x 61.0 x 12.7) mm
Half brick 2.3 x 2.4 x 0.35
Quarter brick 2.3 x 1.45
x 0.35 |
| Centering |
In a multiple output power supply it is sometimes awkward to have every
output voltage spot-on. Power supply centering refers to this. This is often due to
things like needing a full integer wind on a transformer. So, although the voltage might be
well regulated, the nominal voltage can be slightly off. |
| Compliance Current |
The range of current over which the load regulation is within specified
limits is called compliance current. There can be compliance current problems at both
high or low currents. |
| Compliance Voltage |
The range of output voltage (in a constant current power supply) over
which the load regulation is within certain limits is called the compliance
voltage. |
| Crowbar protection |
A sometimes used fail-safe mechanism which shorts the outputs of the power
supply under failure conditions. Crowbar protection can also refer to a circuit which
has its sole purpose to cause a fuse to blow by subjecting it to high current. |
| Current limit techniques. |
One of the advantages of switching power supplies is that they can sense
if they are overloaded and do something about it (other than catching on fire or blowing a
fuse). When an over current situation is detected there are two possibilities. Some power
supplies reduce the voltage so the current stays within safe limits. This is essential for some
applications such as battery chargers, heating or electrochemical processing where you don't
want to stop the process when the current limit is reached. However, it might be dangerous for
cases where you are normally operating below the current limit and over current is always due
to a fault. So for the second case the power supply is designed to pulse when it detects an
over current situation. Thus it is self-resetting but won't supply a lot of current to a dead
short. For obvious reasons this is called "hiccup mode" current limiting. |
| DC Transformer |
A simple DC/DC converter which typically uses a self-resonant square wave
generator without a voltage or current feedback loop is a DC Transformer. |
| Direct off line |
A switching power supply that is isolated by the high frequency
transformer and has no 60 Hz transformer at the front end is called direct off
line. |
| Derating |
The fundamental limit of a power supply is how hot they can get. A 100
watt power supply can't run at 1000 watts just because something in it would get too hot.
Therefore if you are running a power supply in a hot environment you will have to cut back its
rated power to compensate for the fact that it can't get rid of its heat as efficiently. A
derating curve or specification tells you what this tradeoff is. |
| Drift |
In a power supply,Drift is a failure of the feedback loop to
control the voltage or current because the gain of the feedback loop is changing for some
reason, for example the sense resistor is changing resistance due to heating. |
| Flyback |
When the current flowing through an inductor is cut off the energy stored
in the magnetic field is released by a sudden reversal of the terminal voltage. If a diode is
in place to conduct the stored energy somewhere useful, the diode is called a flyback
diode. This only requires one winding on the inductor, so the inductor would be called a
flyback transformer. This arrangement has the interesting property of transferring
energy to the secondary side of the power supply only when the primary switch is off. Such
switchmode power supplies are called flyback converters. |
| Forward Converter |
A forward converter is a switching power supply circuit that
transfers the energy from the primary to the secondary while the switching element is "on,"
which is the opposite of a flyback converter. |
| Inrush current |
When a power supply is first turned on there are a lot of capacitors and
inductors that are empty. Inrush current is the transient amount of current that it
takes to charge these capacitors and inductors and thus start up the power supply. |
| Isolation |
Isolation is the electrical separation between the input and the
output. Some power supplies, such as many DC/DC converters have common grounds, and therefore
are not isolated. Most AC/DC and AC/AC converters are isolated for safety reasons. The measure
of isolation is resistance, and is measured with an ohm meter. In some instances you might also
want to know the isolation capacitance between the input and output to make sure the RF
isolation is low. |
| Isolation Voltage |
Say you have one part of your circuit that you need to float above ground.
Or say your equipment has a possibility to feed high voltage back into the power supply. You
want to know what the maximum isolation voltage is. This is usually defined as the voltage
between the input and the output (or chassis) of a power supply that would cause it to either
fail, or to conduct a certain number of microamps of DC current, and is often in the
kilovolts. |
| Leakage Current. |
An isolated power supply will still "leak" a tiny amount of current,
either AC or DC to the chassis, or to ground, from the input or the output. This is critical in
cases where a human can be part of the circuit, such as in an operating room, so leakage
current is limited to a low value in medical grade power supplies. |
| Line Regulation |
Line reguation means that the power supply will provide a constant voltage
as the input voltage varies. Inexpensive transformer based wall mount power supplies, for
example, do not have good line regulation, and will change the output proportional to the input
voltage. Switching power supplies, by their very nature, have very good line regulation. This
has a plumbing analogy. When you are taking a shower your spousal unit usually takes the
opportunity to flush the toilet. This causes the cold water pressure to drop, and thus the flow
of cold water in your carefully designed mix drops and you are scalded. Then, as if to
compensate, the sink faucet is turned on full hot and an analogous phenomenon causes you to
freeze. We power supply designers would say that the line regulation of your shower head is not
adequate. |
| Load Regulation |
Load regulation means that the power supply will provide a constant voltage
as the current drawn from the load goes up and down. This has a plumbing analogy. During the
Super Bowl, when a lot of beverages are being consumed in front of the TV and no one wants to
miss the action, city water pressure is at a constant. During the commercials, when the
football fans are at liberty to engage in plumbing activities, the city water pressure drops
precipitously, sometimes triggering water-failure alarms in industry. We power supply designers
would say that the city water pressure does not have good load regulation. Again, inexpensive
transformer-based wall mount power supplies are designed to give a certain voltage at a certain
current draw. When the current changes voltage changes, how much depends on the design of the
transformer. Switching supplies are fully regulated, so have good load regulation, as do linear
regulated supplies. |
| MTBF |
Mean time before failure. This is a statistical calculation, not a
measurement. All the parts in a device have mean time between failure ratings. These are
mathematically combined to get an MTBF number, often dependent on the stress put on them. Even
the via holes in a circuit board have MTBF ratings. As you might think, as a rule of thumb, the
more parts the less the MTBF. However, sometimes you can replace one part with a few more
reliable parts, or spread the stress of one part over several parts and increase the MTBF. The
human body has a trillion cells and each cell has an MTBF of 6 years. No wonder I don't feel so
good. By the way, PowerStream engineers are well equipped to calculate MTBF using
MIL-HDBK-217E. |
| Parallel Forced Sharing |
Two switchmode power supplies with feedback loops will do strange and
sometimes unexpected things when connected in parallel. Often one supply will carry the entire
load until its current limit is reached. If it starts hiccuping before the other one comes to
its rescue the two supplies may start a chaotic oscillation. The answer to this problem is to
use power supplies that are made to communicate with each other so they can collaborate, a
switchmode power supply feature know as parallel forced sharing. Sometimes this takes
the form of a master-slave operation, and sometimes they just force each other to put out the
same current. |
| PFC |
Power factor correction. This originally referred to the fact that an
inductive load, where the current and the voltage are not totally in phase, draws instantaneous
power of the instantaneous current times the instantaneous voltage. However, it has been
extended to mean any distortion or phase shift in the power drawn from the mains. An AC power
supply must use some method to control the power coming from the mains. Usually this is done
with some kind of pulsing. There are also components in the power supply that cannot draw
current until the voltage gets high enough, or draw current only at the peak of the sine wave.
Drawing power from different parts of the sine wave can distort what is left for everybody else
to use and the utilities have to compensate for it. In Europe, where the utilities are mostly
owned by the government the governments are tempted to use the power of the law to benefit
their industries, and have required an increasingly onerous set of PFC values over the years,
now requiring PFC for power supplies as low as 75 watts. In the USA the utilities are at the
mercy of their customers and PFC requirements have lagged behind Europe. |
| Remote sensing |
When current is flowing the voltage at the end of a cable is different
from the voltage at the beginning because of the resistance of the wire. The higher the current
the higher this loss. Since switching power supplies contain feedback loops to keep the voltage
constant it is feasible to sense the voltage at the other end of the cable with wires that do
not conduct current, thus regulating the voltage were it is used, rather than where it is
generated. Clever! |
| RFI |
Radio frequency interference, see EMI. |
| Ripple. |
Ripple is the amount of fluctuation left over from the rectification of AC
to DC. It is measured in volts peak to peak or volts RMS. Switching power supplies have high
frequency ripple, linear power supplies have low frequency ripple (some multiple of 60
Hz). |
| RMS |
This is defined as root mean squared. RMS is a clever way to measure
average voltage when the voltage is AC. After all, the time average of a 60 Hz sine wave is
zero, since half of the time it is positive and half the time it is negative. So what they do
is square the signal, which makes everything positive, average this, and then take the square
root to get it back to volts instead of volts squared. |
| Soft start |
Sometimes the equipment that you are powering from the power supply has so
much inrush current that it will shut down the power supply, or blow a fuse, or cause other
damage. In this case you want to use a power supply that has a soft start to gradually increase
the output voltage to keep the "out rush" current within a reasonable range. |
| Synchronous Rectification |
One of the many efficiency problems with power supplies is that diodes have
a forward voltage drop. This means that when they are conducting there is about 0.6 volts
difference between one side of the diode to the other. Multiply this voltage drop by the
current and you have the power that is lost forever to heat in the diode. This is a
particularly troublesome thorn-in-the-flesh when low voltage, high current power supplies are
designed, for example 2.5 or 3.3 volt supplies for computers. One way to eliminate this is to
use a back-to-back set of MOSFETs that are turned on and off by an active circuit that detects
the zero crossings and turns on the FET at just the right time. Since the on-resistance of
MOSFETs can be very low, you can thus have very high efficiency rectification. This is known as
synchronous rectification. Impressive, no? There is an engineering solution to
everything except cash flow! |
| Thermal protection. |
Thermal protection means the power supply studies its own temperature, and
when it feels that is has had enough, shuts down in time to prevent permanent damage. |
