Powering up can be one of the most dangerous events an electrical system endures: current rushes in as the circuit passes through voltages that are outside of the normal operating range and as different sub-circuits begin functioning sooner than others. Those without much electronics experience might imagine an instantaneous change in voltage or at least a gradual, monotonic rise at the power node:
|Ideal power-up voltage transitions.|
This might not be the case, however, especially as the circuits get more complex and the connection to power is initially intermittent, as in the case of a bouncing switch. In this article, we explore the potentially destructive voltage spikes that result from the seemingly innocuous act of applying power to a circuit with a low equivalent series resistance (ESR) capacitor across its power input.
As with many unexpected problems that we encounter in electronics, the causes of the destructive spikes we are exploring here are the non-negligible parasitic elements of the components we try to idealize. The power supply and power leads can have a relatively large inductance (L) that causes a large electromagnetic energy buildup when charging a capacitor (C). That energy is there in the magnetic field around the wires even after the capacitor voltage has reached the power supply voltage, and the collapsing magnetic field continues delivering current into the capacitor, causing its voltage to continue rising. That voltage can rise to several times the power supply voltage before the magnetic field is exhausted, which then causes the current to flow in the other direction as the capacitor discharges back into the power supply. This charging and discharging of the capacitor can happen a few times before the system stabilizes, and the frequency of the oscillations is determined by the inductance and capacitance (L and C).
|Oscilloscope capture showing LC spike voltage (yellow) and current (green).|
Capacitors (and wires) have a prominent parasitic feature of their own, known as the equivalent series resistance (ESR). On older capacitors, that resistance tended to be high enough to dampen the LC oscillation to the point that any spikes were negligible or even non-existent. However, newer, increasingly common ceramic capacitors have extremely low ESRs that are generally a benefit but do very little to dampen LC oscillations.
These LC-induced voltage spikes and oscillations might last for only a few milliseconds, so they’re not something you would even notice if you weren’t looking for them. However, the spikes can last long enough to destroy your electronics, so it’s definitely worth being aware of. The problem can be exacerbated by mechanical switch bouncing, which can introduce a new set of spikes with each bounce of the contacts. We hope that the oscilloscope screen shots we provide here will help illustrate this phenomenon to make it easier to understand and remember, and we suggest a few techniques for limiting the problem.