The following SMPS design assumes that the components are ideal – transistor and diode voltage drops are zero, inductor and capacitor are ideal (no parasitics). In a continuous mode and a steady-state operation, the inductor current and output voltage always stay positive and never go to zero. When it reaches the desired value, V OUT < V IN, the switch opens again. When the switch subsequently opens, the output voltage rises. This is the preferred mode in EMC – it results in a smaller output ripple, smaller load-current variations, and lower EMC emissions. If the switch closes before the inductor current (and output voltage) goes to zero in the switching cycle, the SMPS will operate in the so-called continuous conduction mode. If the switch remains open long enough the inductor current decays to zero and subsequently the output voltage goes to zero. Note that in both cases, when the switch is closed and open, the inductor current is positive and flows in the same direction. Figure 6: Step-down SMPS circuit: a) transistor ON b) transistor OFF
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