Why Are Half-Bridge Capacitor Voltages Not Vs/2 in IR2153 LTspice Simulation?

The discussion addresses why the voltages across the two capacitors in an IR2153-based half-bridge converter simulation in LTspice do not equal half the supply voltage (Vs/2) as theory predicts. The key issue is that for the midpoint voltage to remain stable at Vs/2, the charge transferred through each switch must be identical over time. Any imbalance in switch ON times causes one capacitor to charge while the other discharges, resulting in the midpoint voltage drifting toward one rail. This phenomenon is exacerbated by resonant effects and the presence of the transformer, which can alter the expected voltage distribution. Adjusting the switching frequency via the resistor on the Rt pin affects the capacitor voltages, and the circuit's resonant frequency and quality factor (Q) influence operation stability. Some designs include circuitry to compensate for ON-time differences to maintain the midpoint voltage. It is also suggested to test the circuit without the capacitive midpoint or transformer to isolate the cause. Driving the IR2153 against circuit common rather than the capacitive divider may also impact the voltage balance.
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