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.
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| 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.
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| 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.
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| 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).
