Parasitic Capacitance Effects in Step-Up Transformer Design

One of the most critical components in a step-up design like Figure 1 is the transformer. Transformers have parasitic components that can cause them to deviate from their ideal characteristics, and the parasitic capacitance associated with the secondary can cause large resonating current spikes on the leading edge of the switch current waveform. These spikes can cause the regulator to exhibit erratic operating conditions that manifests itself as duty cycle instability. This effect is exacerbated in very high voltage designs. Attention to transformer design will cure this problem. Figure 2 shows the high frequency current paths of the parasitic capacitors. In the analysis of operation assume the input and output voltages are at AC ground. Thus, the parasitic capacitors are all in parallel. The transformer’s secondary provides the AC current path for these capacitors. The current flowing through the secondary produces N times the current in the primary. As the parasitic capacitance and turns ratio increase, the primary current becomes progressively larger.