Pulsed DC (PDC) or pulsating direct current is a periodic current which changes in value but never changes direction. Some authors use the term pulsed DC to describe a signal consisting of one or more rectangular ("flat-topped"), rather than sinusoidal, pulses.[1]
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Pulsed DC is commonly produced from AC (alternating current) by a half-wave rectifier or a full-wave rectifier. Full wave rectified ac is more commonly known as Rectified AC. PDC has some characteristics of both alternating current (AC) and direct current (DC) waveforms. The voltage of a DC wave is roughly constant, whereas the voltage of an AC waveform continually varies between positive and negative values. Like an AC wave, the voltage of a PDC wave continually varies, but like a DC wave, the sign of the voltage is constant.
Pulsating direct current is used on PWM controllers.
Most modern electronic items function using a DC voltage, so the PDC waveform must usually be smoothed before use. A reservoir capacitor converts the PDC wave into a DC waveform with some superimposed ripple. When the PDC voltage is initially applied, it charges the capacitor, which acts as a short term storage device to keep the output at an acceptable level while the PDC waveform is at a low voltage. Voltage regulation is often also applied using either linear or switching regulation.
Pulsating direct current has an average value equal to a constant (DC) along with a time-dependent pulsating component added to it, while the average value of alternating current is zero in steady state (or a constant if it has a DC offset, value of which will then be equal to that offset). Devices and circuits may respond differently to pulsating DC than they would to non-pulsating DC, such as a battery or regulated power supply and should be evaluated.
Pulsed DC may also be generated for purposes other than rectification. It is often used to reduce electric arcs when generating thin carbon films,[2] and for increasing yield in semiconductor fabrication by reducing electrostatic build-up.[3] It is also generated by the voltage regulators in some automobiles, e.g., the classic air-cooled Volkswagen Beetle.
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Pulsed DC is also commonly used in driving light-emitting diodes (LEDs) to lower the intensity. Since light-emitting diodes cannot be reliably dimmed through the simple reduction of voltage as in an incandescent bulb, pulsed DC is used to produce many rapid flashes of light that to the human eye are indiscernible as individual flashes, but are seen as a lower brightness. Since an LED has no traditional filament to stress, this also has the added effect of prolonging the lifespan of the LED by reducing its on-time. This can sometimes be seen on videos where an LED or lamp assembly composed of LEDs is filmed at a frame rate very close to - but not exactly - the same as the pulsed DC frequency, which causes the lamp to slowly and occasionally fade in and out as the LEDs fall out of synchronization with the video frame.
While smoothed DC is less dangerous than the same current level of AC, pulsed DC can be more dangerous than AC. Short DC pulses below 100 μs are more efficient at evoking action potentials and disturbing the conduction systems in the body than AC which usually has a much smoother and longer wave form. Therefore lower levels of current will be enough to cause the same effects equal to those of AC.[4][failed verification] On the other hand at shorter waveform AC will also have a higher frequency which is less effective at stimulating nerves and muscles due to its biphasic nature which doesn't allow a build up of enough charge, in the short time between phases, to depolarize cell membranes in order to evoke an action potential.
I get this question a lot: 'How do I know when to use DC and when to use RF for a sputtering application?'
Of course, the first thing to consider is film requirements. Typically, RF makes a better thin film than DC, pulsed DC, or AC. The RF-sputtered film will be smoother and have better packing density. RF also deposits the film at about 20% of the DC rate. If you want to sputter using DC, pulsed DC, or AC, you must have a conductive (or semi-conductive) target. I always check the conductivity of a target by placing my ohm meter probes anywhere on the target surface. I need to see less than 650 kΩ.
We used to only have analog ohm meters with a needle. A 'wiggle' on the meter confirmed that we could use DC, pulsed DC, or AC. Now that we have digital meters, it's necessary to put a value on the 'wiggle.' On the digital readout, 650 kΩ seems to coincide with an analog meter 'wiggle.' This is why we need to use a dopant such as Al on the Si targets for DC-sputtered SiO2 films. The dopant makes the target surface semi-conductive enough (about 400 kΩ and less) to successfully sputter with DC, pulsed DC, or AC. We can also sputter this same doped target with RF. We would need RF to sputter SiO2 from a quartz target, as its resistance reading on the target surface is infinite. RF hasn't commonly been used with rotatable magnetrons yet, as the end blocks tend to bind up and leak water with the 13.56 MHz RF signals. However, we are safe with DC, pulsed DC, and AC on rotatables.
Good luck and be safe.
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