FET-Jet Calculator

There are tradeoffs between accuracy, ease of use, and speed. FET-Jet Calculator balances these by minimizing sources of error wherever possible and making simplifications that allow quick results with little loss of accuracy for typical applications. Accuracy is quite sufficient for the intended purpose of FET-Jet Calculator, which is a power device selection and topology comparison tool for use early in a design cycle.

Many applications require electrical isolation between the conductive backside of the package and the heatsink. A value of 0.6 °C/W is a conservative value for modern phase-change isolators. If using a non-phase-change isolator pad, then a value of 1.8 °C/W would be typical, but please refer data from the isolator material manufacturer. For surface mount on insulated metal substrate (IMS), please refer manufacturer’s data.

There are many variations of SVM, but typically the time spent in each zero-state is balanced to minimize switching transitions and loss. In this case, the conduction and switching losses of SPWM versus SVM are almost identical, and it is practical to simply use SPWM for the loss calculations for both SPWM and SVM. Overmodulation (pulse skipping) is not supported in Revision 1 of FET-Jet Calculator. Both SPWM and SVM yield mathematically meaningful results with a modulation index up to . However, only space vector can physically realize the 1.15x higher AC voltage with a given DC link voltage compared with SPWM. Furthermore, SVM applies almost exclusively to 3-phase systems. With SPWM, each phase is independent of the others, and therefore any number of phases is supported.

Sine-triangle pulse width modulation (SPWM) is used to calculate power loss in AC/DC topologies. Although most systems are 3-phase, each phase is independent of the others with SPWM, and entering line-to-neutral voltage and then calculating line-to-line voltage allows support for any number of phases. Losses calculated with SPWM and alternating-nulls space vector modulation (SVM) are almost identical, but unlike SPWM, the switching of each phase with SVM is interdependent on the other phases. For this reason, the vast majority systems using SVM are 3-phase.

FET-Jet Calculator makes no distinction between input and output power. Power semiconductor efficiency in most systems is high enough to make the small error in power loss calculations from ignoring input versus output power negligible. The power rating can therefore be thought of as output power.

When a FET turns off and voltage across it increases from near zero to its steady off-state voltage, energy is stored in the output capacitance. In circuits with zero drain-source voltage turn-on (ZVS), this output capacitance energy is recycled to the load and/or input capacitance. This is different from hard switching where output capacitance energy becomes lost (as heat) in the FET as it switches on. During turn-off, some of the drain current flows through the FET channel and creates heat, while the remainder of the current charges the output capacitance. It is impossible to measure these currents separately during turn-off; only the total is measured and used in turn-off energy calculations of the datasheet. It is however possible to measure the output capacitance and therefore to calculate its energy stored after turn-off. Again, because the output capacitance energy is recycled, the turn-off energy in circuits with ZVS turn-on is calculated as the difference between the datasheet (hard switched) turn-off energy E_off minus the output capacitance energy corresponding to the switched voltage. Sometimes the resulting turn-off switching loss is so low that the result is displayed as zero.

Inductor ripple current as an input parameter is used to fine-tune switching loss estimates. It has negligible effect on conduction loss in the corresponding topologies.

Automatic transient analysis is not supported in this revision of FET-Jet Calculator.

Some topologies have the option of multiple legs, meaning all the FETs, diodes, and inductors are replicated, so the converter circuit is “soft paralleled”. Power rating is the sum of the power in all legs (not per leg). Multiple legs would typically have interleaved switching, although this has no effect on the loss calculations. Number of parallel FETs or diodes means the number of directly paralleled FETs or diodes within each circuit leg. Power loss is calculated per FET and per diode, and then summed according to the total number of FETs and diodes in all legs to yield the total converter loss. Consider a boost converter as an example having two parallel FETs and one diode per leg, and there are three legs. Each leg would process one-third the power rating, and there would be 3 legs * 2 FETs per leg = 6 FETs total, and 3 legs * 1 diode per leg = 3 diodes total; total loss = 6 FETs * loss per FET + 3 diodes * loss per diode.