0.2 us 0.2 (IS 0.1 (is 5 [is 0.2(is

10 (IS

Fig. A.IS Example: an approximate transistor eurrem waveform, including estimated current spike due to diode stoied charge.

Example

A transistor current waveform contains a current spike due to the .stored cliarge of a freewlieel-ing diode. Tlic observed waveform can be approximated as shown in Fig. A1.18. Estimate the rms current.

The waveform can be divided into six approximately linear segments, as shown. The Dj. and for each segment are

l.Ttiangularsegmeut:

2. Constant segment:

3. Trapezoidal segment:

4. Constant segment:

5. Triangular segment:

fl, = (0.2(is)/(10fcs) = 0.02 u, = /f/3 =f20A)/3 = 133A

Dj = (0.2 fi£y(10(is) = 0.02 j= ==(20А) = 4даА

Лз = (:0.1 м)/(10 д.ч> = П.а1 Hj= (/, + /; + ;}/3 = 148 А-

D = (5ii,sy(10s) = 0.5 =(2А) = 4А

Dj = (0.2(is)/(10 ;i.s) = 0.a2 и= ll/2 =[2А)Ьг= 1.3 А

6. Zero segment:

The rms value is

Even though its duration is very short, the current spike has a significant impact on the rras value of the current-withont the current spike, the rms current is approximately 2.0 A.

Appendix В

Simulation of Converters

Computer simulation can be a powerful tool in the engineering design process. Starting from design specifications, an initial design typically includes seJection of system and circuit configurations, as well as component types and values. In this process, component and system models are constructed based on vendor-supplied data, and by applications of analysis and modeling techniques. These models, validated by experimental data whenever possible, are the basis upon which the designer can choose parameter values and verify the achieved performance against the design specifications. One must take into account die fact that actual parameter values will not match their nominal values because of inevitable production tolerances, changes in environmental conditions (such as temperature), and aging. In the design verification step, worst-case analysts (or other reliability and production yield analysis) is performed tojudge whether the spccification.4 aic met under all conditions, i.e., for expected ranges of component parameter values. Computer simulation is very well suited for thi.s ta.sk: using reliable models and appropriate simulation setups, the system performance can tie tested for various sets of component paiatiictcr values. One can then perform design iterations until the worst-case behavior meets specifications, or until the system reliability and production yield are acceptably high.

In the design verification of power electronic systems by simulation, it is often necessary to use component and system models of various levels of complexity:

]. Detailed, complex models that аиетрг io accurately represent physical behavior of devices. Such inodels are necessary for tasks that involve finding switching times, details of switching transitions and switching loss mechanisms, or instantaneous voltage and current sbesses. Component vendors often provide libraries of such device models. To complete a detailed circuit model, one must also carefully examine effects of packaging and board interconnects. With fast-switching power semiconductors, simulation time steps of a few nanoseconds ot less may be required, especially during on/off switching transitions, Because of the complexity of detailed device models, and the fine time resolution, the simulation tasks can be very time consuming, hi practice, time-domain simulations using detailed device intMlcIs are usually performed only