DC is not always DC. When systems are developed to operate on DC power, the DC electrical environment will have noise and may experience other disturbances. This article will explore considerations for generating low frequency disturbances (glitches, sags, surges, spikes) that occur in the 100 microsecond and longer region with bandwidths of 10 kHz or less. In contrast, noise, which is outside of the scope of this article, can be low frequency (like 50/60 Hz hum) but becomes a challenge to generate in range from kHz, to MHz or up to the GHz range.
Disturbances can be caused by a variety of factors. Source side disturbances can be due to events that cause a variation of the source, such as changing source output power due to environmental conditions (like a cloud passing over a solar panel or a car alternator spinning at a different speed). Load side disturbances can be due to changing loads being placed on the DC power, such as a DC rail being pulled down by new loads (devices) plugged onto it, or the powering up/down of subsystems that are turned on/off to conserve power.
When designing systems that run on DC, engineers need to consider the effects of DC disturbances on the overall operation of the system. Certainly, normal DC disturbance events, such as plugging in a new device, should not cause the DC rail to collapse and cause other devices on the same common DC power bus to lock up, reset, lose data or fail. Thus, design validation of DC powered systems and devices will require simulation of DC disturbances for testing to ensure proper operation of the system.
DC Systems and Possible Disturbances
Many systems and devices operate from a DC bias or rail. The system will operate properly if the DC rail stays at the desired DC operating voltage with some operating margin. Here are some examples of DC systems and how DC disturbances can occur, causing undesirable results.