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The common mode current of a non-isolated power supply can become an electromagnetic interference (EMI) source. Have you ever eliminated it? In some high-voltage power supplies, such as the ones used in LED bulbs, you may find that you cannot eliminate them. After careful inspection, it was found that the non-isolated power supply is no different from the isolated power supply. The switching node grounds the parasitic capacitance to produce a common mode current.
Figure 1 is a circuit diagram of an LED power supply showing the main causes of common mode current generation in the buck regulator. The reason is the grounding capacitance of the switch node. Surprisingly, such a small capacitance still causes problems. CISPR Class B (for residential equipment) radiation regulations allow 50 supply impedances for 46 dBuV (200 uV) signals at 1 MHz. This means that only 4 uA of current is allowed. If the converter switches the 200 Vpk-pk square wave of the Q2 drain at 100 kHz, the reference voltage is approximately 120 volts peak. Since the harmonics decrease proportionally with the frequency reduction, there will be about 9 Vrms at 1 MHz. We can use it to calculate the allowable capacitance, which is about 0.1pF, which is 100 fF (equivalent to 2 megaohm impedance at 1 MHz), which is the full possible capacitance of this node.
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