Effects of Winding Capacitance & More

Now consider the equivalent pulse transformer circuit of Figure 5


The circuit has all the components of the circuit in Figure 2B, but also has primary winding capacitance, secondary winding capacitance, core loss, and secondary leakage inductance. Start with both switches open and no capacitive energy and no inductive energy. All currents are initially zero. Close the secondary switch then close the primary switch. The primary leakage inductance, Lkp, restricts the flow of primary current by opposing the source voltage. The opposing voltage is generated by Lkp x d(I)/dt action. Current flow ( from the source ) finds the uncharged winding capacitance, Cp to be a much easier path, hence a relatively large amount of current flows into the winding capacitance. This large amount of current could be called a surge current because it will diminish over time as the capacitance is charged. The surge causes a relatively large voltage drop across the primary winding resistance, Rp, thereby initially lowering the voltage available to Lkp and Lm. Over time, as the surge current diminishes, the voltage drop across Rp diminishes, and the voltage across Lkp and Lm reaches full ( peak ) value. The surge effectively delays the peak voltage across Lm. This in turn delays peak secondary voltage. The delay contributes to rise time, hence Cp contributes to rise time. As discussed earlier, Lpk restricts flow of the reflected load current and consequently also contributes to rise time.A similar consequence occurs with the secondary winding capacitance, Cs. Any current supplied by induced secondary voltage must charge Cs as the secondary voltage tries to rise to peak value. This delays the secondary in reaching peak voltage, hence Cs also contributes to rise time.

TSecondary leakage inductance, Lks, restricts secondary current flow just like Lkp restricted primary current flow. Lks also delays the secondary peak output voltage, hence it also contributes to rise time.

TCore loss resistance, Rc, provides a relatively small current shunt path across Lm just like the reflected secondary load current does. It has the same effect but the effect is much smaller.

To summarize, Winding capacitances and leakage inductances act to increase rise time. (They also generate trailing edges which is discussed later.) They may also contribute to spurious oscillations. In a typical pulse transformer design, core loss does not have much effect.