Stabilized high frequency switching power supply with suppressed EMI noise

摘要

An improved switching power unit is of the type which uses a capacitor C1 of large capacitance value as a primary DC power supply, inputs primary DC power from the capacitor C1 to a series circuit formed of the primary winding N1 of a high-frequency transformer 6 and a switching element Q, induces AC power in the secondary winding N2 of the transformer 6 by switching the switching element Q, and rectifies and smooths the AC power to produce and output DC power. In the improved switching power unit, an inductor L1 and/or inductor L2 of small inductance value are/is connected in series with the capacitor C1 of large capacitance value for smoothing the primary DC power, a bypass capacitor Cp is connected in parallel with a series circuit formed of the primary winding N1 of the high-frequency transformer 6 and the switching element Q, thereby bypassing high-frequency noise caused by switching and preventing it from passing through the capacitor C1, and a series circuit formed of a grounding resistor Rg and a grounding capacitor Cg is connected between frame ground G and a hot line 4p or a common line 4n. As a result, the operation of a switching circuit 11 of the switching power unit is stabilized, while electromagnetic interference noise is markedly suppressed since high-frequency noise leaking to frame ground G is converted to heat by the resistor Rg.