Estimating Power IC contributions to your embedded design power budget

Among the greatest challenges in designing today's power-consuming products is managing the system's thermal budget. Since most electronic equipment include some form of power conversion, it is necessary to understand the design's thermal constraints, which form the context for many design decisions.

In most power-conversion circuits, the hottest elements are the power ICs - diodes, MOSFETs and IGBTs. For a given circuit topology, these components heat up as functions of applied voltage, load current, switching frequency, gate-drive circuit, package type and mounting.

Of these, the first four dissipate power and model as thermal sources, while the last two models as thermal sinks because they remove heat from the system.

A good first-order estimate of power dissipation in switchmode circuits is P = DVI, where I is the average conduction-cycle current through the power IC, V is the average conduction-cycle voltage across the device, and D is the duty cycle.

Figure 1: The power IC's datasheet provides thermal response curves, from which you can calculate the device's temperature rise above the case temperature when operating in switched-mode.

In physical circuits, current is a function of circuit operation. Voltage is a function of current, the device type, junction temperature and IC control method. For example, the forward voltage across a diode is simply a function of current and temperature.

The voltage across a MOSFET in the on state is IDRDS(on) - the product of drain current and channel resistance. RDS(on), in turn, is a function of ID, gate drive and temperature. The voltage across an IGBT in the on state, V=VCE(sat), is a function of current, gate drive and temperature.

To determine the IC's temperature rise, multiply the power dissipation by the thermal impedance. The limitation with this analysis is that it oversimplifies the power calculation and does not account for transient conditions.

The power device's data sheet provides thermal response curves, however, with which you can overcome that limitation (Figure 1 above).

The curves assume a rectangular power pulse of amplitude P for duration t with duty cycle D. Follow the curve appropriate to your circuit's duty cycle to the point along the horizontal axis corresponding to the pulse duration. Read the corresponding thermal response from the vertical axis and multiply that value by the power dissipation to arrive at the temperature rise from case to junction.

Figure 2: The power IC's thermal stack includes the junction, substrate, case, thermal paste or other thermal interface material, heat sink, and ambient.

The thermal response curves only address the case-to-junction temperature rise. They cannot account for the case's mounting method, which contributes to its rise above ambient as a complete thermal-stack model indicates (Figure 2 above).

Rather than approach the problem piece-by-piece, using different tools and data sources to solve each part of the problem, a circuit simulator can calculate the total thermal response. The simulator also allows you to observe the effect of the thermal system on the circuit's parametric performance, which is difficult to deduce from pen-and-paper or spreadsheet analyses.

Circuit simulation uses component models and network analysis, which closely approximates the operating conditions for each device in the circuit. The simulator automatically calculates the power dissipation of power devices, taking into account a full range of circuit and device behaviors that include gate drive, switching transitions and diode reverse-recovery.

Figure 3: A quasidynamic thermal wrapper model accounts for the power device's parametric dependence on temperature.

However, traditional circuit simulators calculate power based on a static thermal model. In other words, they fix device behavior with respect to temperature. This is adequate for low-power IC simulation because devices in such circuits exhibit little self-heating.

Power ICs do self-heat, however, and an accurate simulation must account for the device behavior's temperature dependence. Adding a quasidynamic thermal wrapper model to the static 25 degrees C device model overcomes this limitation (Figure 3 above).