A DC-DC converter power module, a self-contained unit that takes a direct current input and delivers direct current at a different or regulated voltage, is classified in HS subheading 8504.40 (static converters) under HS 2022. Heading 85.04 names static converters, and the WCO Explanatory Notes to that heading cover DC converters, so the heading is settled under GRI 1 and the subheading under GRI 6. Headings 8537, 8542, 8473 and 9032 are sometimes proposed instead; each fails for the reasons set out below.
These modules are used in telecom equipment, industrial automation, automotive electronics and medical devices, and they are sometimes misclassified, which leads to duty errors, customs disputes and post-clearance audits. The analysis follows the order the law sets (see the legal structure of HS classification): product facts first, then the heading text and notes, then the subheading.
What the product is
A DC-DC converter power module typically:
- accepts a direct current (DC) input;
- converts the voltage level (step-up, step-down or regulated output);
- delivers a DC output;
- contains semiconductor devices, transformers, capacitors and controllers;
- is housed as a self-contained functional unit; and
- is designed to be built into larger electronic systems, not sold as a bare PCB component.
These facts have to be pinned down before any tariff rule is applied, because every step that follows depends on them.
The rules that apply
Classification starts with GRI 1: the terms of the headings and the relevant Section and Chapter Notes. GRI 6 then applies the same method at subheading level. For this product the relevant legal notes are those to Section XVI and Chapter 85, and the main interpretive guide is the WCO Explanatory Notes to heading 85.04. For the rules themselves, see this practical guide to the GRIs; for the legal weight of the Explanatory Notes, see the role of Explanatory Notes in classification disputes.
Why heading 85.04 applies
Heading 85.04 (HS 2022)
“Electrical transformers, static converters (for example, rectifiers) and inductors.”
Static converters in the Explanatory Notes
The WCO Explanatory Notes to heading 85.04 list, among static converters:
- AC/DC converters
- DC/DC converters
- power supply modules
- voltage regulators
- switch-mode power supplies
Because DC-DC converters are recognised as static converters in the Explanatory Notes, heading 85.04 is decided under GRI 1. The module’s principal function is converting electrical power. It does not control, switch or distribute circuits.
Subheading 8504.40
At six-digit level (HS 2022) the module falls in 8504.40, “Static converters”:
- it is not an electrical transformer (subheadings 8504.2x and 8504.3x);
- it is not an inductor; and
- it performs active electronic conversion of DC power.
For a United States example, CBP ruling NY N328742 (21 October 2022) classified a 12 V vehicle DC-DC converter with two USB outputs in HTSUS subheading 8504.40.9580. That product was a consumer charger, not an industrial module, but the ruling applies the same subheading to DC-DC conversion.
Headings that are wrongly proposed
| Heading proposed | Why it fails |
| 8537 (boards and panels for electric control or distribution) | The module does not switch or control circuits. |
| 8542 (electronic integrated circuits) | A converter module built from semiconductors, transformers and capacitors is not an integrated circuit within Note 12(b) to Chapter 85. Very small encapsulated modules need a closer look: one that combines an IC with inductors or capacitors formed indivisibly into a single body, like an IC, may be a multi-component integrated circuit under Note 12(b)(iv) and fall in 8542 (compare the Samsung IC-Codec case). |
| 8473 (parts of machines of headings 84.70 to 84.72) | A static converter is a good of its own heading, so Section XVI Note 2(a) keeps it in 85.04 even when it is built into a computer or office machine. |
| 9032 (automatic regulating or controlling instruments) | There is no measuring and control loop. |
These are the errors that tend to appear in import documentation and that trigger reassessment during audits.
Section XVI Note 2(a): goods of their own heading are not “parts”
Note 2(a) to Section XVI provides that parts which are goods included in any heading of Chapter 84 or 85 (other than the parts headings the note lists) are in all cases classified in their own headings. A DC-DC converter module is a static converter of heading 85.04. It therefore stays in 8504.40 even when it is designed for one particular machine, and the parts provisions of the machine it goes into are never reached.
National tariff lines and duty
8504.40 is harmonised only at six digits. National tariff lines at 8 or 10 digits, and the rates attached to them, vary by importing country, by free trade agreement and by rules of origin (see how country-specific tariff variations affect supply chains). Once the HS subheading is settled, the importer still has to:
- find the national tariff line and rate in the importing country;
- check whether a preferential rate applies and whether the origin documents support it;
- value the goods under the WTO Customs Valuation Agreement; and
- check local excise or regulatory surcharges.
Worked example
| Fact or step | Result |
| Product | Encapsulated DC-DC converter, 48V input, 12V output |
| Function | Voltage conversion |
| Mounting | Mounted on a PCB but electrically independent |
| Heading | 8504 |
| Subheading | 8504.40 |
| Basis | Static converter, as described in the WCO Explanatory Notes to heading 85.04 |
Practice points
- Start with what the product does, documented from the datasheet.
- Apply GRI 1 and the Section XVI notes before looking at any parts provision.
- Check the Explanatory Notes to heading 85.04 and keep the reference in the file.
- Record why each rejected heading fails.
- Where the duty at stake is significant, apply for an advance ruling.