Read a Power Device’s Thermal Response Over Time

Why the thermal response has a time axis

During a finite pulse, some dissipated energy warms the device and its surrounding thermal mass while heat is transferred away. This energy storage gives the system thermal inertia: its temperature takes time to change. Nexperia’s RC Thermal Models, AN11261 Rev. 5.0, dated March 18, 2021, explains that inertia and represents dynamic response using thermal resistance and capacitance.

A steady resistance describes the eventual temperature difference per watt for its stated boundary and conditions. It leaves the speed of that response unresolved. Two devices can therefore share the eventual value without sharing the response along the way. The buying question is whether the supplied data describes the time interval that matters.

References: [TT3]

Read duration, duty cycle and reference together

Nexperia’s official thermal-characteristics explanation presents Zth(j-x) as a function of pulse duration and duty cycle. Here j denotes the junction and x the specified thermal reference. For a sufficiently long pulse, the response approaches the corresponding steady Rth; that statement supplies no universal settling time.

In that explanation, duty cycle zero denotes an isolated pulse and duty cycle one denotes continuous operation. Keep the curve’s pulse conditions and ordinate units attached to its value; the fictional comparison below uses absolute K/W values.

The related thermal-metrics guide covers temperature endpoints, heat paths and power denominators in detail. Establish that basis before comparing time response. A junction-to-mounting-base curve cannot silently become a junction-to-ambient curve.

References: [TT1][TT3]

TR-01: the same 10 K/W gives different rises at 100 ms

TR-01 is an original fictional comparison of two conceptual component offers. Both use the same junction-to-mounting-base boundary and have a stipulated steady resistance of 10 K/W. Assume the same simplified linear response convention, a fixed mounting-base temperature, no other heat sources and no preceding dissipation. Each independent case begins in thermal equilibrium with that reference.

Stipulate that offer A’s single-pulse Zth at exactly 100 ms is 2 K/W and offer B’s is 6 K/W. These are invented absolute ordinates, not normalized ratios or values read from a real manufacturer’s curve. Apply one rectangular pulse of 4 W of device dissipation for 100 ms to each case.

AN11156, Using Power MOSFET Zth Curves, Rev. 1 dated September 28, 2012, relates a single rectangular dissipation pulse to the junction-to-mounting-base temperature difference using power multiplied by the applicable Zth. It is a historical NXP note now hosted by Nexperia. Its named device and numerical example supply none of TR-01’s inputs.

At the end of the stipulated pulse, A gives 4 W × 2 K/W = 8 K. B gives 4 W × 6 K/W = 24 K. Watts cancel the per-watt denominator, leaving a temperature difference in kelvins. A difference of 1 K has the same size as a difference of 1 °C.

Those rises are above the fixed mounting-base reference. An absolute junction temperature would additionally require that reference temperature; no actual junction reading is supplied here. The example contains no measurement or simulation.

If 4 W were instead maintained to steady state under the same assumptions, both offers would give 4 W × 10 K/W = 40 K. That eventual equality leaves the 100 ms results different by 16 K. It is a direct counterexample to selecting on steady resistance alone, without declaring either conceptual offer suitable for an actual product.

References: [TT2]

A recurring pulse needs its thermal history

Now change TR-01’s requirement to recurring operation: 4 W for 100 ms, then zero dissipation for 100 ms. The period is 200 ms, so the duty cycle is 100/200 = 0.5 and the arithmetic average power is 4 W × 0.5 = 2 W. A constant 2 W input has the same average.

That equality describes energy delivered per period. It does not describe the temperature variation within the period. An off interval allows cooling, but its duration alone does not establish a return to the starting temperature. A later pulse can begin with heat remaining from previous pulses, changing the rise above the reference.

The supplied 2 and 6 K/W ordinates answer only the independent first-pulse question. They provide no complete cooling response or periodic peak. Equally, replacing the recurring waveform with its 2 W average erases the timing that the peak question depends on.

AN11156 treats composite waveforms and pulse bursts separately from its single-shot example. For this changed requirement, request applicable recurring-pulse data or a manufacturer-supported transient assessment accounting for the actual waveform, initial history and thermal-reference behaviour. Identify whether the evidence describes startup cycles or an established periodic condition. The given inputs do not determine either offer’s recurring peak.

References: [TT2][TT3]

Compare what each data package actually establishes

For a supplier comparison, apply these distinctions to the proposed requirement. A package containing only the matched 10 K/W steady figure supports the stipulated continuous comparison in TR-01. It cannot distinguish A’s and B’s 100 ms excursions. A package that adds the applicable isolated-pulse response resolves that finite-pulse comparison while leaving the recurring requirement open.

For a real offer, request the exact part and package, source revision, thermal reference and characterization conditions alongside the response data. Retain whether the supplied values are typical or guaranteed limits; a typical curve alone does not establish a guaranteed worst-case peak. Ask the supplier to identify which waveform and operating history the evidence addresses.

For recurring operation, the useful response must connect the actual on and off dissipation, timing and reference behaviour to the relevant temperature response. A number labelled pulse thermal performance without those conditions cannot close the comparison. This is a review of supplied evidence, rather than an instruction to operate or test the device.

A lower Zth at one duration establishes only the smaller rise in a comparison whose other assumptions match. Package temperature limits, safe operating area and other product constraints remain separate requirements. The appointed product and thermal specialists must establish suitability for the intended assembly.

References: [TT1][TT2]

Carry the selected system’s requirements into physical planning

An AI server workload adds another boundary. A rack’s electrical-input trace is not the dissipation waveform of an individual power device. A component assessment needs that device’s dissipated power and applicable thermal path; the assembled server also needs evidence for its own configuration and thermal environment. The related room-cooling article addresses a separate capacity comparison at room-equipment level.

Once equipment requirements are established, Leviathan’s Physical Infrastructure Planning & CAD service can discuss a separately agreed package of rack layouts, cable pathways, point-to-point schedules, labels and installation documents. Authoring capability, reviewers and outputs are confirmed for each engagement. Component thermal design and thermal-performance approval remain with the appointed specialists.

Use the related planning service and project inquiry to share the selected equipment, available rack and connection plans, requested documentation and unresolved interfaces. Those inputs support a discussion of physical planning scope, requirements and availability.