| Hypothetical calculation | Result | Interpretation |
|---|---|---|
| 2kW /12V;2kW /48V | 166.7A;41.7A | Ideal terminal currents |
| Current² ×0.1mΩ | 2.78W;0.174W | Connection loss illustration |
| 60°C +100W ×0.12K/W | 72°C base | Shared cooling term |
| 72°C +12W ×2K/W | 96°C device | Local branch term |
| Additional20W ×0.12K/W | 2.4K rise | Shared heat from another source |
Define which converter architecture is being budgeted
An EV can contain DC-DC functions with different voltage levels, isolation requirements and power-flow directions. A low-voltage bidirectional converter is not thermally or electrically identical to an isolated high-voltage auxiliary converter. State the architecture, voltage ranges and permitted operating modes before selecting a ceramic power substrate. TI's TIDA-01168 and Toshiba's RD210 provide examples of four-phase bidirectional low-voltage automotive converters; neither reference is presented here as a ceramic-board design. Their relevance is that the operating direction and phase structure are explicit. The thermal budget must be equally explicit about which devices, magnetic components and interconnections produce heat in each mode.
Use operating cases rather than a single rated-power point
Create a matrix covering the relevant input voltage, output voltage, load, direction, switching strategy and cooling condition. Include cases in which fewer phases operate if the controller allows phase shedding. The case with the highest total loss need not be the case with the highest temperature in one component. Different devices can carry different conduction or switching burdens as the operating condition changes. Keep transient overloads separate from continuous ratings and identify their permitted duration. A ceramic selection based on only one nominal efficiency figure cannot capture that range. The thermal management guide provides a framework for translating each case into a source map and temperature check.
Recognize the thermal significance of the low-voltage current path
For an ideal power comparison, delivering two kilowatts at twelve volts requires approximately 166.7 amperes, while the same power at forty-eight volts requires approximately 41.7 amperes. These are terminal-current examples and do not describe the waveform in an individual switch or inductor. Through a hypothetical resistance of one tenth of a milliohm, those currents would dissipate approximately 2.78 watts and 0.174 watts respectively. The sixteenfold ratio follows from current squared. This illustrates why a low-voltage connector, bus path or shunt can deserve attention even when the semiconductor substrate receives most of the thermal-design discussion.
Separate local heat sources from shared heat exits
Draw the thermal network so that each semiconductor has its own local path to a shared base or cooler where appropriate. Add magnetic components, interconnect losses and control electronics at their actual coupling locations. Do not send every watt through the ceramic merely because the converter uses a ceramic power stage. Some heat may enter the same base through another route, while some may leave through the enclosure or air. The distinction matters because remote heat can raise the common reference temperature seen by the semiconductor without passing through its local ceramic layer. A useful budget preserves both the local temperature rise and the shared cooling burden.
Worked network: local and shared contributions add differently
Consider an invented steady model with a coolant reference at sixty degrees Celsius. Assume one hundred watts enters a common base whose effective resistance to that reference is 0.12 kelvin per watt. The modeled base temperature is then seventy-two degrees Celsius. A particular device dissipating twelve watts through a local junction-to-base path of two kelvin per watt would reach ninety-six degrees Celsius. A second device dissipating eight watts through an otherwise identical local path would reach eighty-eight degrees Celsius. This is a simplified network calculation, not a temperature prediction for a named converter. It assumes an isothermal base and defined effective resistances under the stated condition.
Use the budget to compare the right improvement
In that invented network, reducing the first device's local resistance from two to 1.5 kelvin per watt lowers its calculated temperature by six kelvin at twelve watts. Reducing the common base-to-coolant resistance from 0.12 to 0.08 kelvin per watt lowers the base temperature by four kelvin at one hundred watts and benefits both devices in the model. These changes are not equally easy or costly to implement, and the example does not recommend either one. It shows how a ceramic or attachment improvement and a shared-cooler improvement act on different terms. Compare their demonstrated effects on the limiting components instead of comparing only nominal material conductivities.
Include heat from components outside the ceramic tile
If an additional twenty watts from another component enters the same common base in the example, the base rises by a further 2.4 kelvin with the assumed 0.12-kelvin-per-watt path. The semiconductor temperatures rise with it even though their individual losses and local ceramic paths remain unchanged. This is why a substrate prototype tested alone can appear better than the same substrate inside a populated converter. Identify the intended thermal coupling of inductors, transformers, connectors and any neighboring power stage. For a nonuniform cooler, replace the single common-base approximation with a representation that captures the important spatial relationships and verify it with measurements.
Assign loss estimates to the actual electrical waveforms
A terminal-current calculation is useful for a bus connection but should not be copied directly into a switch or inductor loss model. Use the relevant current waveform, duty cycle, switching conditions and temperature-dependent device information. Keep conduction, switching and other loss terms separate enough to understand how they change between buck and boost operation. For magnetic components, use a suitable model or measurements for the actual flux and current conditions. Label each source estimate as measured, calculated or assumed. When the thermal model predicts a different operating temperature, revisit loss terms whose values depend materially on that temperature rather than treating the first estimate as fixed evidence.
Budget the ceramic path within its complete attachment stack
The local path can include a package or bare die, attachment material, upper metal, ceramic, lower metal and the interface to the base. Identify the temperature nodes used in the calculation and avoid counting a layer twice when a supplied package parameter already includes it. If a proposed ceramic change also changes the copper thickness, attachment area or insulation arrangement, keep those changes visible in the comparison. The power substrate guide can organize the construction. A lower calculated ceramic-layer resistance may be useful, but it does not establish the complete junction-to-coolant resistance without the other layers and spreading effects.
Treat current sharing as a thermal validation question
A multiphase architecture distributes work across phases, but the thermal design should verify the actual operating behavior rather than assume perfect equality from the schematic. Measure or estimate phase losses under the controller's supported conditions and examine the consequences of permitted imbalance. Differences in component temperature can in turn affect electrical behavior. Include the phase arrangement in the source map, particularly if cooling or mounting is asymmetric. TI's reference design explicitly describes its current-control architecture; that is a reminder to use the control behavior of the selected implementation. A generic equal-power split is an assumption to validate, not a property of every multiphase converter.
Keep isolation requirements independent of the heat budget
A ceramic layer may form part of an electrical isolation system, but the thermal calculation alone cannot establish the required insulation performance. Identify whether the converter topology requires galvanic isolation and what electrical boundaries the substrate is expected to provide. Do not infer those requirements from the presence of ceramic or from a reference design with a different topology. Changes intended to improve heat transfer, such as reducing thickness or altering conductor spacing, must be reviewed against the actual electrical and manufacturing requirements. Keep the approved insulation design and its verification separate from the thermal-resistance arithmetic so that a thermal optimization does not silently change another essential function.
Match the cooling condition to the vehicle installation
Specify the coolant or ambient reference, available flow where relevant, mounting orientation and the permitted operating range of the cooling system. A laboratory cold plate held at a uniform temperature may not reproduce the local temperature distribution of the production cooler. Record where the reference temperature is measured and how it relates to the component locations. Include the required degraded or extreme conditions defined by the system team. Avoid using an inlet temperature as though it were the temperature at every point on the base without checking the consequence. The temperature budget should remain traceable to the actual vehicle-level boundary supplied to the converter design.
Verify the budget with measurements that distinguish its terms
Measure enough locations and operating cases to test the important parts of the network. A single external case temperature may not distinguish an underestimated local attachment resistance from an underestimated shared cooling resistance. Compare the measured response when local device loss changes and when shared heat load changes, where the test setup permits that distinction. Use the selected semiconductor's appropriate temperature-estimation or measurement method and document its uncertainty. Do not compare an internal modeled maximum with an unrelated surface measurement as if they were the same quantity. Preserve the electrical loss measurements and cooling conditions alongside the temperature data so that the budget can be revised coherently.
Review thermal cycling separately from the peak-temperature limit
Meeting a steady peak-temperature target does not fully describe the mechanical demands created by repeated operating changes. Record relevant temperature swings, dwell periods and the locations at which they occur. A material or attachment change can alter both the peak and the distribution of temperature, which may change the loads experienced by different interfaces. The thermal cycling guide provides related planning context. Use qualification evidence for the selected assembly and operating conditions rather than assuming that a lower steady temperature proves every reliability requirement. Keep the temperature budget and the exposure-validation plan linked, while recognizing that they answer different engineering questions.
Release a budget with ownership and revision triggers
The final thermal budget should identify each source, its basis, the relevant temperature nodes, the local and shared paths, and the operating cases that set the limits. Assign responsibility for updating it when devices, phase control, magnetic components, ceramic construction or cooling conditions change. Preserve measured evidence and unresolved assumptions separately. This makes the budget a usable design record rather than a one-time spreadsheet created to justify a material selection. Ceramic substrates can contribute to an EV DC-DC converter's thermal performance, but their value is clearest when the entire network shows which temperature term improves and which components remain limited by another part of the system.
Engineering example
The electrical and thermal network numbers are invented. The TI and Toshiba references demonstrate converter architectures, not ceramic implementations or the performance values used here.
Before you release the design
- Identify topology, isolation boundary and both power-flow directions.
- Map loss by phase and operating mode.
- Separate local substrate paths from shared cooling loads.
- Check low-voltage interconnect losses using the appropriate current.
- Validate model nodes and revision triggers against the production installation.
Sources and further technical reading
Manufacturer references support the material and process context. Worked examples and checklists are engineering guidance; they are not test results or supplier guarantees.