Effectively, placing capacitor as a divider or as a series element (blocker) is basically the same thing. If you take the AC equivalent of the circuit the net capacitance of the bridge configuration will be in series with the primary winding:
simulate this circuit – Schematic created using CircuitLab
In both configurations the capacitor voltage will swing around a DC value which is set by the duty cycle and the bus voltage, or in other words, the average voltage of the switching node.
There are a few advantages of using the capacitor as a divider (i.e. split caps):-
- During start up the switches are off (or maybe only the low-side is kept on by the controller) therefore the bridge node (a.k.a. switching node) is floating. So these caps will be charged up through the DC bus (PFC or rectifier) instantly (assuming 1) the output impedance of the DC rail is really low, and 2) The output capacitance of the MOSFETs is negligibly lower than the split caps). So when the switching starts, since the caps are already charged up, the initial winding current will be lower (see the images below).
- Two capacitors share the current so the current per cap becomes lower. This also allows the designers to pick lower-rated-voltage capacitors instead of a single, chunky one.
- These caps (the series equivalent) will be across the boost rail, so they bring extra holdup and filtering.
Test circuit and waveforms:
Both circuits are running with the same conditions: Same duty cycle (350 n / 1 us), same bus voltage (8V), identical switches and identical transformers. The pink waveform is the primary current of the circuit to the right. And also I started switching after 100 us.