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Switching Transformer Glossary

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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


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