Supply of the primary side of the transformer in a forward, half-bridge converter

Question

Why is it that in most forward, half-bridge converters, one of the transformer windings is connected with two capacitors (to VCC and GND):

when my simulations shows that one capacitor (that block DC form half bridge) is enough and it provides the same effect?

and with 10% duty cycle:

Answer 1

There might be even more precise terminology, but the two capacitors used to split the rail to half voltage is usually called a symmetrical half-bridge and the single capacitor in series is usually called asymmetrical half-bridge. Try varying the duty cycle to something else (lower) than 50 % and you will observe the difference between them.

The dual capacitor serves dual purpose, it both splits your rail to half and acts as hold-up/energy storage after a PFC or bridge rectifier.

The series capacitor passively prevents staircase saturation of your transformer as it won't pass DC but will also "steal" voltage if you run anything but 50 % duty cycle.

Answer 2

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.