Dual MOSFETs have skyrocketed in popularity over the past two years. Two die chips are integrated inside one single package, cutting PCB footprint by half — an incredibly appealing selling point at first glance.

Yet I’ve seen countless engineers fall into pitfalls. During component selection, they only fixate on RDS(on) and package dimensions while ignoring thermal coupling effects. Once prototypes are powered on, temperature rise far exceeds design expectations, accompanied by deteriorated energy efficiency.

Today we’ll thoroughly unpack the heat dissipation challenges of dual MOSFETs and analyze the actual performance of three Hotek part numbers: HKTG35C06, HKTQ20C03 and HKTQ20C04.

The Bittersweet Trap of Dual-Die Packages

Common package formats for dual MOSFETs include TSSOP-8, DFN5x6 and PDFN3x3. Two MOSFET dies are encapsulated within one housing and share common source pins.

Its direct advantages are clear:

• 50% reduction in PCB footprint
• More streamlined circuit routing
• Lower parasitic inductance from traces
• Ideal for space-constrained portable devices

However, a hidden drawback is often overlooked: thermal coupling effect.

With two dies housed together, heat dissipation no longer functions independently. Heat generated by one die conducts to the other, pushing the junction temperature of both dies higher than that of discrete single-die MOSFET packages.

This is thermal coupling: heat inside the package does not dissipate separately, and the two chips thermally interfere with each other. When designing with single-die MOSFETs, you only need to monitor one die’s junction temperature; for dual-die variants, you must account for the combined thermal load of both chips.

How to Interpret Key Parameters: RDS(on), PD & Package Thermal Resistance

When selecting dual MOSFETs, these critical parameters must be evaluated collectively.

RDS(on) – On-Resistance

The two dies inside a dual MOSFET can operate independently or be paralleled. Paralleling reduces overall RDS(on), yet uneven current sharing between the two dies becomes a major risk. Uneven current distribution leads to one die overheating while the other operates underutilized.

PD – Maximum Power Dissipation

The PD value listed on datasheets refers to the maximum power rating of a single die under specified test conditions. For dual-die packages, the actual allowable power dissipation must be derated to account for thermal coupling; the single-die PD ratings cannot simply be added together.

Package Thermal Resistance

This is the decisive parameter for dual MOSFET selection:

• RθJC (Junction-to-Case Thermal Resistance): Thermal resistance between the die junction and package exterior. Lower values mean heat transfers from the chip to the package shell more efficiently.
• RθJA (Junction-to-Ambient Thermal Resistance): Thermal resistance between the die junction and surrounding air. Lower values equate to superior heat dissipation.

Thermal resistance calculation for dual-die packages is far more complex than single-die counterparts, as both dies share the same thermal conduction paths. Datasheets typically only specify thermal resistance figures for one die; engineers must factor in cumulative heat buildup when both chips conduct simultaneously in real designs.

Performance Comparison of Three Hotek Dual MOSFET Models

Below is a comparative analysis of heat dissipation performance across Hotek’s three dual MOSFET products.

Let’s break down the differences clearly: HKTQ20C03 boasts the lowest RDS(on) at just 3.2 mΩ, a 3 W power dissipation rating, and an ultra-low RθJC of 15 °C/W, delivering the best thermal performance. Its DFN5x6 package, however, occupies more PCB space than TSSOP-8 and PDFN3x3 packages.

HKTG35C06 adopts the compact TSSOP-8 package with the smallest footprint, but its RθJC reaches 25 °C/W, resulting in relatively weaker heat dissipation. It fits applications with moderate power demands and extreme space limitations.

HKTQ20C04 features a PDFN3x3 package as a balanced middle-ground option. It offers lower RDS(on) than HKTG35C06 and higher RθJC than HKTQ20C03, with a medium-sized package footprint.

Core Judgement Criterion

Power density (PD divided by package footprint area) is the definitive metric. Component selection cannot rely solely on RDS(on) or package size alone.

Thermal Layout Design Guidelines

Picking the right dual MOSFET is only half the battle; thermal layout design poses the real challenge. Below are actionable engineering recommendations:

1. Adequately Sized PCB Thermal Pads

Most dual MOSFETs feature exposed thermal pads on their bottom side, whose area directly determines thermal conduction efficiency. Datasheets provide recommended pad dimensions — in practical design, oversize the pad rather than cutting corners to save space.

2. Sufficient Copper Weight for PCB Traces

Heat from the dies dissipates outward through copper pours. Both copper pour area and copper thickness require careful consideration. 1 oz copper is the baseline standard; high-power applications should adopt 2 oz copper cladding.

3. Optimize Current Balancing Between Two Dies

When operating the two dies in parallel, current flow may distribute unevenly. Refine PCB routing to match the loop impedance of both channels as closely as possible and mitigate current skew.

4. Reserve Ample Temperature Rise Margin

Datasheet PD ratings are maximum values measured under ideal lab environments. Real-world operation introduces elevated ambient temperature, heat from adjacent components, and variable cooling performance. A conservative design practice is to derate the datasheet PD value to 70% and retain sufficient thermal headroom.

Component Selection Principle: Smaller Packages Do Not Equal Better Performance

A core rule for dual MOSFET sourcing: power density carries greater weight than RDS(on).

• Low-power portable devices (TWS earbuds, smartwatches): Choose HKTG35C06. The TSSOP-8 package minimizes PCB space and meets modest cooling requirements, fully leveraging its footprint-saving advantage.
• Medium-power equipment (power tools, POS terminals): Choose HKTQ20C04. The PDFN3x3 package balances compactness and thermal performance, with mΩ-level RDS(on) sufficient for most use cases.
• High-power equipment (electric two-wheelers, garden power tools): Choose HKTQ20C03. The DFN5x6 package delivers top-tier heat dissipation, and its 3.2 mΩ RDS(on) creates distinct advantages under high-current loads.

Another frequently overlooked factor: larger packages provide superior cooling but come with higher material costs. Engineers must strike a balance between performance, cost and form factor — there is no universal perfect solution, and selections need to align with individual project specifications.

Summary

Dual MOSFETs are excellent components that shrink PCB space and simplify routing, yet thermal performance cannot be overlooked during development.

1. Evaluate thermal coupling impact upfront: Dual-die packages deliver inferior effective cooling compared to discrete single-die parts. Always cross-reference RθJC and PD during selection.
2. Prioritize power density as the core metric — do not fixate exclusively on RDS(on). Power density is calculated as PD divided by package footprint area.
3. Rigorous thermal layout design is mandatory: Do not compromise on thermal pad sizing, copper pours, current balancing or temperature safety margins.

Next time a colleague claims dual MOSFETs cut PCB area in half, remember to follow up with one critical question: Have you factored thermal performance into your design?