Issue link: https://resources.pcb.cadence.com/i/1180526
PSpice User Guide Convergence and "time step too small errors" October 2019 850 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. (such as a Schmidt trigger built out of switches) then such loops should include capacitances. Another way of saying all this is that during transient analysis the circuit equations must be continuous over time (just as during the bias point calculation the equations must be continuous with the power supply level). ■ Inductors and transformers While the impedance of capacitors gets lower at high frequencies (and small time steps) the impedance of inductors gets higher. Note: The inductors in PSpice have an infinite bandwidth. Real inductors have a finite bandwidth due to eddy current losses and/or skin effect. At high frequencies the effective inductance drops. Another way to say this is that physical inductors have a frequency at which their Q begins to roll off. The inductors in PSpice have no such limit. This can lead to very fast spikes as transistors (and diodes) connected to inductors turn on and off. The fast spikes, in turn, can force PSpice to take unrealistically small time steps. ■ It is recommended that all inductors have a parallel resistor (series resistance is good for modeling DC effects but does not limit the inductor's bandwidth). The parallel resistor gives a good model for eddy current loss and limits the bandwidth of the inductor. The size of resistor should be set to be equal to the inductor's impedance at the frequency at which its Q begins to roll off. The value of this resistor can be calculated using the following formula: where f is the roll-off frequency. Adding parallel resistors limits the inductor impedance at high frequencies. Example: A common one milli-henry iron core inductor begins to roll off at no less than 100KHz. A good resistor value to use in parallel is then R = 2*p*100e3*.001 = 628 ohms. Below the roll-off R 2 Π × f × L × =