Blocking Oscillator - Circuit Operation

Circuit Operation

The circuit works due to positive feedback through the transformer and involves two times—the time T_closed when the switch is closed, and the time T_open when the switch is open. The following abbreviations are used in the analysis:

• t, time, a variable
• T_closed: instant at the end of the closed cycle, beginning of open cycle. Also a measure of the time duration when the switch is closed.
• T_open: instant at the end of the open cycle, beginning of closed cycle. Same as T=0. Also a measure of the time duration when the switch is open.
• V_b, source voltage e.g. V_battery
• V_p, voltage across the primary winding. An ideal switch will present supply voltage V_b across the primary, so in the ideal case V_p = V_b.
• V_s, voltage across the secondary winding
• V_z, fixed load voltage caused by e.g. by the reverse voltage of a Zener diode or the forward voltage of a light-emitting diode (LED).
• I_m, magnetizing current in the primary
• I_peak,m, maximum or "peak" magnetizing current in the primary. Occurs immediately before T_open.
• N_p, number of primary turns
• N_s, number of secondary turns
• N, the turns ratio defined as N_s/N_p, . For an ideal transformer operating under ideal conditions, I_s = I_p/N, V_s = N*V_p.
• L_p, primary (self-)inductance, a value determined by the number of primary turns N_p squared, and an "inductance factor" A_L. Self-inductance is often written as L_p = A_L*N_p2*10−9 henries.
• R, combined switch and primary resistance
• U_p, energy stored in the flux of the magnetic field in the windings, as represented by the magnetizing current I_m.

A more-detailed analysis would require the following:

• M = mutual inductance, its value determined by degree to which the magnetic field created by the primary couples to (is shared by) the secondary, and vice versa. coupling. Coupling is never perfect; there is always so-called primary and secondary "leakage flux". Usually calculated from short-circuit secondary and short-circuited primary measurements.

• L_p,leak = self-inductance that represents the magnetic field created by, and coupled to the primary windings only
• L_s,leak = self-inductance that represents the magnetic field created by, and coupled to the secondary windings only

• C_windings = interwinding capacitance. Values exist for the primary turns only, the secondary turns only, and the primary-to-secondary windings. Usually combined into a single value.