Column: Circuit drill
The harmonic sensitivity of an overvoltage protection circuit
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By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions Te charging process starts with a
attery-charging circuits are used to charge portable batteries from an external power source, usually AC, which, as we know, are very venerable to harmonics.
A simple definition of harmonics is “additional noise frequency signals that affect the regular power source”. If the main power supply has a
frequency of, say, 60Hz, then it’s possible that a small 60Hz noise signal leaks out to the output circuit, in this case our battery charger. Te noise signal may be 60Hz or a multiple thereof, referred to as “nth
specific DC voltage from a power source, shown as V1. At this stage there are no harmonics. Te circuit starts to charge the load battery V2 at 3.7V, shown in Figure 2, which shows the relation between V1 and the charging current passing through the load battery, V2. As V2 charges above V1, the charging
current that passes through it goes higher, too. Tis threshold voltage, or “breaking voltage”, represents the minimum voltage required for V1 to start charging the load battery V2 – in our case, a Li-ion battery.
-order
harmonic”. For example, a 2nd-order harmonic is a 120Hz signal, whereas a 3rd-order – 180Hz; and so on. Overcoming harmonics is crucial when designing any power system.
Leaked noise Here we will explore the impact of any leaked noise frequencies (harmonics) on a typical Li-ion battery-charging circuit; see Figure 1.
Experiment and results In this experiment we assume that: • Te harmonics signal is 1V with zero phase shiſt.
• Te experiment was conducted using a 60Hz harmonics signal and its multiples, to 660Hz.
• We assumed the ambient temperature to be fixed. By observing the breaking voltage at various harmonic levels, there’s a
corresponding shiſt, starting at a harmonic frequency of 480Hz, equivalent to the 8th harmonic order. From 480Hz upward, the breaking
voltage goes higher than the nominal 3.7V. Tis means that the voltage source (V1) needs more voltage to start the charging process when the harmonic signal is at 480Hz or above. In other words, charging circuit performance will decline linearly at 480Hz and higher-degree harmonics; see Figure 3. Tis leads to the conclusion that
the sample Li-ion battery-charging circuit shows declining charging performance at 480Hz or higher. Te minimum voltage required as a power source to the circuit V1 must be higher than 3.7V to allow the circuit to start charging battery V2. Tis finding emphasises the need for harmonic filters in power supplies connected to the mains, since battery chargers show lower performance at higher harmonic frequencies.
Figure 1: A typical Li-ion battery-charging circuit 14 April 2022
www.electronicsworld.co.uk
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