Inverter and VFD Cooling: Heat Sink Sizing for Drives
Short answer: a variable-frequency drive wastes 2–5% of its rated power as heat, so a 75 kW VFD at 97% efficiency dissipates roughly 2 kW that must leave through the power module's heat sink. Sizing starts from the IGBT module datasheet: junction limit typically 150 °C with 125 °C as the practical operating ceiling, minus case-to-sink and sink-to-air drops, each typically 0.05–0.3 K/W, solved backwards for the maximum sink thermal resistance. Natural-convection sinks handle roughly 0.05–0.15 W/cm² of base footprint; anything denser needs a fan or a liquid path.
Drive manufacturers do not advertise their losses, but every watt of loss has to go somewhere, and in a VFD that somewhere is the heat sink bolted to the IGBT module. Sizing that heat sink wrong means either a drive that derates in summer or a cooling fan that fails in year two. The method is the same for a 2 kW micro drive and a 200 kW cabinet: account the losses, budget the temperature rises, and choose the cooling mode that fits the installation.
Where the Losses Come From
Drive losses split into three parts. Conduction and switching losses in the IGBTs and diodes dominate, scaling with output current and switching frequency. Losses in the DC bus, gate drivers, control board and cooling fans add a smaller fixed load. The efficiency story is roughly flat across load: a drive at full load runs 95–98% efficient depending on topology and switching frequency, and because most of the loss is in the semiconductors, the heat appears on the power module baseplate where the heat sink mounts.
A practical estimating rule: heat to the sink is roughly 2–4% of rated drive power at full load, rising at light load because the fixed losses stay. Example: a 22 kW drive at 97% efficiency dissipates about 660 W; a 75 kW drive at 97% dissipates about 2.2 kW; a 250 kW drive at 98% still dumps about 5 kW. Those watts must cross the module's junction-to-case resistance, the interface, and the sink-to-air resistance without exceeding the junction limit at the worst-case ambient, usually 40–50 °C inside the cabinet.
| Drive rating | Typical efficiency | Heat to heat sink | Cooling normally used |
|---|---|---|---|
| 2–7.5 kW | 95–97% | 80–250 W | Natural convection or small fan |
| 11–45 kW | 96–97.5% | 300–1,200 W | Fan-assisted extruded sink |
| 55–160 kW | 97–98% | 1.5–4 kW | Ducted fan or liquid cold plate |
| 200 kW+ | 97–98.5% | 4–7 kW | Liquid cooling or air-conditioned cabinet |
Indicative numbers at full load; check the specific drive datasheet because switching frequency changes losses by tens of percent. A drive run at 8 kHz switching instead of 4 kHz can dissipate 30–50% more switching loss in the modules.
The Junction Budget: Working Backwards From 150 °C
IGBT modules specify a maximum junction temperature of 150 °C and a recommended operating maximum near 125 °C. The datasheet also gives Rth j-c per switch and for the module case, usually 0.05–0.4 K/W depending on module size. The design budget, per power device, is:
Tj = Ta + P·(Rth j-c + Rth c-s + Rth s-a)
Work backwards. A 22 kW drive dissipating 660 W total across six IGBTs sees roughly 110 W per switch. With Rth j-c of 0.2 K/W, a 0.05 K/W interface, and a target junction of 120 °C at 45 °C internal ambient, the sink may have at most (120 − 45)/110 − 0.25 ≈ 0.43 K/W case-to-ambient per switch — a number that dictates a specific extruded profile and airflow.
The margin question decides reliability. Sizing to the datasheet maximum leaves zero margin for TIM aging, dust loading, fan degradation, or a hot day. Competent designs target junction temperatures 10–25 °C below the module limit at worst-case conditions, and that margin is exactly where heat sink quality shows. An anodized sink, for example, radiates better than bare aluminum, which matters more at the high temperature rises and natural convection typical of small drives; see why in our black anodized heat sink article.
Natural Convection, Fans, or Liquid
The cooling mode sets the whole mechanical design. Natural convection is silent, fan-free and failure-free, but weak: a vertical extruded sink in free air rejects roughly 0.05–0.15 W/cm² of base footprint at a 40–60 °C rise, which is why small drives hang a chunky finned backplate on the enclosure. Fan cooling multiplies that by roughly 3–8× but adds a wear part, dust accumulation, and acoustic noise, plus the fan itself dissipates power inside the cabinet. Liquid cooling, once reserved for the largest drives, now appears on mid-size regenerative and high-switching-frequency drives where air ducts cannot keep up.
| Cooling mode | Heat density on sink base | Fan power | Typical drive range |
|---|---|---|---|
| Natural convection | 0.05–0.15 W/cm² | None | Up to ~7.5 kW |
| Fan-assisted, shrouded | 0.3–1.0 W/cm² | 5–50 W | 11–160 kW |
| Ducted external airflow | 0.5–1.5 W/cm² | System fan | Cabinet builds |
| Liquid cold plate | 2–10+ W/cm² | Pump in loop | 100 kW+ or dense cabinets |
The enclosure matters as much as the sink. A VFD heat sink mounted to the rear wall of an IP54 cabinet dumps heat outside the sealed volume, which is why that construction dominates industrial drives; mounting the same sink inside a sealed cabinet just heats the air the electronics breathe. If you are sourcing a custom drive enclosure, the stamped and machined enclosure decision, sealing class, and sink mounting are one design, not three.
Specifying the Heat Sink for a Drive
A useful drive heat sink inquiry carries seven numbers: drive power and efficiency or measured heat load, switching frequency, worst-case ambient inside the enclosure, target or maximum junction temperature, module baseplate size and mounting pattern, available space, and whether airflow is allowed. Without the ambient and the junction target, every factory will guess, and guesses go two ways: undersized sinks that derate the drive, or oversized sinks that lose your cabinet bid on cost and weight.
Extrusion is the default for drive heat sinks because a long profile cut to module width is cheap at volume, and the fin direction aligns with the fan duct. Where the profile is complex, or quantities are small and the module layout changes, CNC-machined heat sinks take over, with machined pockets for the module, tapped mounting holes, and a base flatness that keeps the TIM thin across a large baseplate. At BQUQ we hold ±0.005 mm on critical features, black-anodize for radiation and corrosion, and machine from 6063-T5 or 6061 stock with hardness and surface finish matched to the module's clamping force.
Sizing Mistakes That Show Up in the Field
The most expensive drive cooling mistakes share one shape: they size to the datasheet maximum instead of the field worst case. Designing the sink so the junction reaches 148 °C when the module limit is 150 °C leaves no budget for the TIM drying out, the fan bearing wearing, the filter loading with dust, or the cabinet sitting next to a furnace in July. Field returns of drives are full of perfectly calculated designs that had no margin, so competent practice targets 110–125 °C junction at the worst case and spends the extra metal willingly.
The second class of mistake is the ambient guess. The air around the sink is not the air in the room: inside a sealed IP54 cabinet the internal ambient can run 10–20 °C above the room from the drive's own losses and its neighbors, and a sink mounted inside that volume sees only that heated air. This is why back-wall mounting dominates, why cabinet ventilation is part of the thermal design, and why quoting an ambient of 40 °C when the installation will see 55 °C converts a pass into a derate.
The third mistake is treating the fan as a free resource. A fan-assist sink that needs its fan to meet the rating will silently lose performance as the fan slows with bearing wear or the filter loads, so the thermal rating should be achievable at, say, 80–90% of nominal airflow, not only at the fresh-fan operating point. Where derating is unacceptable, the honest options are a bigger sink, a more efficient fin field, or liquid, and the cost comparison should include the fan replacement cycle, because a fan that dies in year three takes the drive's thermal design down with it.
These are exactly the trade-offs a source factory should surface in the quote, not bury in the fine print. When BQUQ quotes a drive heat sink, we state the assumed internal ambient, the junction target and whether the number depends on a fan, and we machine the module pocket, mounting holes and flatness to the module's clamping spec so the interface resistance assumed in the budget is real. Send the drive rating, losses, ambient and module drawing to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours.
Frequently Asked Questions
Q: How much heat does a VFD need to dissipate?
A: Roughly 2–4% of rated drive power ends up as heat at full load, most of it in the IGBT module. A 22 kW drive typically dissipates 500–800 W and a 75 kW drive around 1.5–2.5 kW, depending on efficiency and switching frequency.
Q: What junction temperature should an IGBT heat sink design target?
A: IGBT modules rate 150 °C absolute maximum, but competent designs target 110–125 °C at worst case to leave margin for TIM aging, dust and fan wear. Sizing exactly to the 150 °C limit guarantees derating or early failure in the field.
Q: Can a VFD run without a cooling fan?
A: Yes, up to a point. Small drives up to roughly 7.5 kW commonly use natural convection with a large finned backplate, rejecting about 0.05–0.15 W/cm² of base area. Above that power density, fan-assisted or ducted cooling is required to keep the junction budget.
Q: Why do VFD heat sinks get mounted on the enclosure back wall?
A: Because that puts the fins outside the sealed cabinet, dumping the heat to the surrounding air instead of into the electronics compartment. This is the standard construction for IP54 and higher drives, and it is why the sink, the enclosure wall and the gasket are designed together.
Q: What should I send for a drive heat sink quote?
A: Drive rating, heat load or efficiency, switching frequency, internal ambient, junction or case target temperature, module baseplate drawing, space envelope, and cooling mode. Send those to sc@bquq.com or WhatsApp +86 13713157787 and BQUQ will quote the extrusion or machined sink within 12 working hours.
Related Resources
- IGBT Power Module Cooling Guide — module-level thermal design for power electronics.
- Heat Sink Sizing Calculation — step-by-step from watts to profile.
- CNC-machined heat sinks — machined module pockets, flat bases and anodizing for drive hardware.
- Contact us — send your drawing for a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
