WCM420 Series: How Do You Choose a High-Power Switch-Mode Transformer for Up to 5 kW?
Published by West Coast Magnetics, July 2026, based on our 2024 WCM420 product sheet (available for download below); content reviewed and confirmed current as of publication.
The WCM420 series of switch-mode transformer core-and-bobbin sets delivers up to 5 kW in a compact, PCB-mount package, using FEM-optimized core geometry to equalize flux distribution, two purpose-matched core materials to cover 25 kHz through 1 MHz, and standoff creepage/clearance distances that meet the most stringent medical-device standards.
The WCM420-10, -20, -30, and -40 are core-and-bobbin sets, not wound transformers: you wind them to your turns ratio and topology. What WCM provides is a precisely engineered geometry and material combination so that the core performs as specified across the frequency and power range you need. This page covers the series engineering, the power-vs.-frequency lineup, how to choose between WCM-F200 and WCM-FM400, the medical creepage question, and how to select a part. For the full dimensional drawings, material-loss graphs, and ordering, see the WCM420 product page.
What Is the WCM420 Series, and What Problem Does It Solve?
High-power SMPS transformers in the 500 W to 5 kW range present two competing design pressures: board space is shrinking while power density requirements are rising, and creepage/clearance requirements in medical and industrial equipment demand generous physical spacing that a generic core geometry may not deliver. The WCM420 series addresses both.
The series is a PQ-style core-and-bobbin set with four size grades. All four share the same design philosophy: the core geometry is finite-element-optimized to equalize flux distribution throughout the core cross-section. That means flux does not concentrate in corners or in limited cross-sectional areas of the core, which is the common failure mode in a conventionally shaped core under high-power, high-frequency conditions. By spreading the flux uniformly, the design achieves higher power density than you can reach with a non-optimized geometry of the same external dimensions.
The result is a part that fits a compact PCB footprint while handling power levels that would normally require a larger form factor. The WCM420-40, the largest in the series, fits in a 55.0 × 58.0 × 54.5 mm envelope (maximum), yet handles up to 5,600 W at 1 MHz.
How Does FEM-Optimized Geometry Equalize Flux Distribution?
Finite-element optimization of a magnetic core geometry works by iterating the cross-sectional shape, corner radii, and leg dimensions until the flux density is as uniform as possible across the entire core cross-section. In a non-optimized core, corners and narrow sections concentrate flux and reach saturation before the rest of the core. That limits the usable power density of the material: you must derate to keep the hot spots below Bsat, which means the bulk of the core is operating well below its capability.
By equalizing flux distribution, the WCM420 geometry uses the core material more uniformly. The same volume of core material can handle more volt-seconds before any point in the cross-section approaches saturation. That is the direct engineering mechanism behind the “outstanding power density” claim in the product sheet. It is also why the WCM420 is more space-efficient than standard PQ cores of similar external size.
The 10-pin PCB bobbin design completes the space-efficiency picture. All four variants use a 10-pin layout with standardized pin spans, so the footprint maps cleanly to a PCB layout without the awkward placements that come from adapting a chassis-mount core to a PCB application.
What Power Can Each WCM420 Variant Handle?
Output power scales with both the core size and the switching frequency. Higher frequency means more volt-seconds per second, so the same core handles more power as frequency increases. The table below gives output power in watts across the standard frequency points for all four variants.
WCM420 Output Power vs. Switching Frequency (Watts)
| Part | 100 kHz | 200 kHz | 400 kHz | 600 kHz | 1 MHz |
|---|---|---|---|---|---|
| WCM420-10 | 500 | 660 | 850 | 1,100 | 1,500 |
| WCM420-20 | 1,200 | 1,800 | 2,200 | 2,600 | 3,000 |
| WCM420-30 | 1,800 | 2,400 | 2,800 | 3,300 | 4,000 |
| WCM420-40 | 2,700 | 3,400 | 4,000 | 4,700 | 5,600 |
Source: WCM420 product sheet, 2024-06.
A few design notes on reading this table:
- The power figures are for the core-and-bobbin set operated with the appropriate core material (WCM-F200 for 25 kHz to 500 kHz, WCM-FM400 for 500 kHz to 1 MHz). Using WCM-F200 above 500 kHz or WCM-FM400 below 500 kHz will not deliver these numbers.
- The WCM420-40 at 1 MHz shows 5,600 W, which exceeds the “up to 5 kW” series headline. The series headline is conservative and reflects the mid-range operating conditions most SMPS designs use. At higher frequencies with WCM-FM400, the core handles more power.
- If your design runs at an intermediate frequency, interpolate from the nearest two columns. The relationship is not linear, but the table gives good bounding values.
WCM also offers SMPS transformer design kits: the WCM420-10/-20 kit covers up to 2.5 kW, and the WCM420-30/-40 kit covers up to 5 kW.
For design guidance on choosing topology, turns count, and winding strategy for SMPS applications, see our switch-mode power supply transformer design guide.
How Do You Choose Between WCM-F200 and WCM-FM400?
The WCM420 series is available with two core materials, and the selection rule is straightforward: use WCM-F200 for switching frequencies up to 500 kHz; use WCM-FM400 for switching frequencies above 500 kHz.
The reason comes down to loss behavior. WCM-F200 is a high-permeability MnZn material (µi 3000 ±25%) optimized for the traditional SMPS frequency range. Its core loss at 100 kHz / 200 mT is 430 kW/m³ at 25°C, dropping to 360 kW/m³ at 100°C, which is favorable: the core runs cooler at elevated temperature. Above 500 kHz, however, eddy-current losses in a high-permeability MnZn material rise steeply enough that the part thermally underperforms relative to its flux-density capability.
WCM-FM400 is a low-permeability MnZn material (µi 900 ±25%) designed specifically to minimize loss at frequencies above 500 kHz. Its resistivity is twice that of WCM-F200 (10.00 Ω·m vs. 5.00 Ω·m), which reduces eddy-current loss at high frequency. At 1 MHz / 50 mT, WCM-FM400 loss is 80 kW/m³ at both 25°C and 100°C, a very low figure at that frequency. The Steinmetz constants for FM400 (K1 = 1.72×10⁻⁸, kf = 1.98, kb = 2.71) give you what you need to use the generalized Steinmetz equation or iGSE in your core-loss model.
Core Material Comparison
| Property | WCM-F200 | WCM-FM400 |
|---|---|---|
| Frequency range | 25 kHz to 500 kHz | 500 kHz to 1 MHz |
| Initial permeability (µi) | 3000 ±25% | 900 ±25% |
| Saturation flux density (Bs) at 25°C | 520 mT | 540 mT |
| Saturation flux density (Bs) at 100°C | 420 mT | 450 mT |
| Curie temperature | >220°C | ≥280°C |
| Resistivity | 5.00 Ω·m | 10.00 Ω·m |
| Density | 4.90 g/cm³ | 4.80 g/cm³ |
| Core loss at 100 kHz / 200 mT (25°C) | 430 kW/m³ | — |
| Core loss at 1 MHz / 50 mT (25°C) | — | 80 kW/m³ |
Source: WCM420 product sheet, 2024-06. Loss figures at the relevant frequency for each material; see the product sheet graphs for full loss-vs.-frequency and loss-vs.-flux-density curves.
One additional distinction: WCM-FM400 has a higher Bs at both temperatures (540 mT vs. 520 mT at 25°C; 450 mT vs. 420 mT at 100°C) and a higher Curie temperature (≥280°C vs. >220°C). At high-frequency / high-flux-density operating points, WCM-FM400 gives you more thermal headroom before saturation becomes the limiting constraint.
The lower permeability of WCM-FM400 (900 vs. 3000) means you will need more turns to reach a given magnetizing inductance. That is expected and accounted for in the AL values in the engineering data table. Work from the AL value for the specific part you select.
For more on winding and conductor selection for high-frequency SMPS transformers, see our switch-mode transformer application notes.
What Does “Meets Medical Creepage/Clearance Standards” Mean for a Core-and-Bobbin Set?
In a switch-mode transformer, creepage and clearance are the distances that determine whether the part can pass isolation testing and meet safety certification requirements. Creepage is the distance along a surface between two conductive parts; clearance is the shortest through-air distance between them. For medical devices, IEC 60601-1 sets the minimum values based on working voltage, pollution degree, and applied-part classification (Type B, BF, or CF). These distances can be demanding, and depending on the working voltage and pollution degree the standard may call for several millimeters or more of creepage; consult the standard for your specific classification.
A generic ferrite core and bobbin will often fail to meet those distances when the designer specifies a winding arrangement, because the standoff between the core and the bobbin (the plastic shoulder that physically spaces the core from the winding) was not designed with medical clearances in mind.
In the WCM420 series, the standoff geometry between core and bobbin is designed to meet the most stringent medical-device creepage and clearance standards. That means the physical geometry of the assembly, as supplied, provides the surface path and through-air spacing that medical standards demand. The winding engineer works within a pre-cleared physical envelope, rather than having to add custom insulation spacers or creepage extenders to bring a non-compliant geometry into compliance.
This is practically valuable in two ways: it simplifies the winding design for medical applications, and it removes a common point of failure in safety certification when using off-the-shelf cores.
What Are the Core Parameters (AL, Ae, Le, Ve) for Each WCM420 Part?
The table below provides the core parameters you need for design calculations, including inductance factor (AL), effective cross-sectional area (Ae), effective magnetic path length (Le), and effective core volume (Ve). These are the standard inputs to the transformer design equations in Faraday’s law and the Steinmetz core-loss model.
WCM420 Engineering Data
| Part | AL (nH/N²) | Ae (mm²) | Le (mm) | Ve (mm³) |
|---|---|---|---|---|
| WCM420-10 | 6,414 | 107.7 | 63.3 | 10,625 |
| WCM420-20 | 8,218 | 173.9 | 79.7 | 21,285 |
| WCM420-30 | 9,329 | 223.2 | 90.2 | 30,958 |
| WCM420-40 | 10,384 | 276.0 | 100.2 | 42,335 |
Source: WCM420 product sheet, 2024-06.
Use Ae to calculate the peak flux density at your operating conditions. For a bipolar push-pull or full-bridge topology, Bpk = V × ton / (N × Ae × 2), where ton is the switch on-time (SI units); half-bridge, forward, and flyback topologies use a different form, so see the design guide for the topology-specific equation. Verify that Bpk stays below the material’s Bsat at your worst-case operating temperature, using the 100°C Bs values for thermal derate: 420 mT for WCM-F200, 450 mT for WCM-FM400. Use Ve and the Steinmetz parameters (or the loss curves in the product sheet) to estimate core loss and confirm thermal performance before committing to a turns count.
The product sheet also provides window area (Wa) and area product (WaAc), which you need for window utilization calculations when specifying the wire gauge and fill factor. Download the WCM420 product sheet for the full engineering data table.
How Do You Select the Right WCM420 Part?
Part selection follows a straightforward sequence:
1. Establish your switching frequency. This determines the core material: WCM-F200 for 25 kHz to 500 kHz, WCM-FM400 for 500 kHz to 1 MHz.
2. Establish your output power requirement. Locate the power column in the output-power table that matches your switching frequency. Select the smallest part whose power rating meets or exceeds your requirement. For example: if you need 2 kW at 200 kHz, the WCM420-30 (2,400 W at 200 kHz) is the fit; the WCM420-20 (1,800 W at 200 kHz) is below the requirement.
3. Verify flux density. Use the Ae from the engineering data table and your primary turns count to confirm Bpk at full load stays below the material’s 100°C Bsat.
4. Verify window utilization. Use the Wa (window area) from the full engineering data table to confirm your winding fits with adequate insulation.
5. Verify footprint. Check the dimensional envelope against your PCB layout. The four sizes range from 36.0 × 37.0 × 35.5 mm (WCM420-10) to 55.0 × 58.0 × 54.5 mm (WCM420-40), all with standard 10-pin PCB layouts.
If your power, frequency, or form-factor requirement falls outside this series, WCM also offers sibling switch-mode families (the PC-mount WCM410, up to roughly 5 kW, and the lower-power, high-density WCM430) and designs fully custom switch-mode transformers wound to your specification, not just the core set. Contact us with your topology, frequency, power level, and isolation requirement and we will scope the design.
FAQ
The WCM420 series is a family of four switch-mode transformer core-and-bobbin sets (WCM420-10, -20, -30, and -40) from West Coast Magnetics. The series covers output power from 500 W (WCM420-10 at 100 kHz) up to 5,600 W (WCM420-40 at 1 MHz). The series headline is “up to 5 kW,” which reflects mid-range operating conditions; the -40 variant exceeds that at 1 MHz with WCM-FM400. All four use FEM-optimized core geometry and meet medical-device creepage/clearance standards.
Both are MnZn ferrite materials, but they are optimized for different frequency ranges. WCM-F200 has higher permeability (µi 3000) and lower loss at traditional SMPS frequencies up to 500 kHz; use it when your switching frequency is 25 kHz to 500 kHz. WCM-FM400 has lower permeability (µi 900) and much lower high-frequency loss, achieved through higher resistivity (10.00 Ω·m) that reduces eddy currents; use it for frequencies above 500 kHz up to 1 MHz. The power-vs.-frequency table in the WCM420 product sheet applies when you match the correct material to your frequency range.
Finite-element optimization iterates the core cross-sectional shape until flux density is as uniform as possible throughout the core volume. In a conventional core, flux concentrates in corners and narrow cross-sections, which reach saturation before the rest of the core. That forces a design derate to protect the hot spots, leaving much of the core underutilized. An equalized flux distribution means the entire core approaches its flux-density limit simultaneously, so more usable power fits in the same physical volume.
The WCM420 standoff geometry between core and bobbin is designed to meet the most stringent medical-device creepage and clearance standards. The physical spacing provided by the standoff is built into the part, so the winding engineer is working within a geometry that already meets these distances, rather than adding external insulation or creepage extenders. Whether a specific assembly meets a specific IEC 60601-1 working-voltage and pollution-degree combination depends on the complete winding design, but the core-and-bobbin geometry provides the required clearance envelope. Contact WCM to discuss your specific isolation and safety requirements.
For a bipolar push-pull or full-bridge topology, use Faraday’s law: N = V × ton / (Ae × Bpk × 2), where V is the primary voltage, ton is the switch on-time, Ae is the effective area from the engineering data table (in m²), and Bpk is the peak flux density, kept below the material’s Bsat at 100°C (420 mT for WCM-F200, 450 mT for WCM-FM400). Half-bridge, forward, and flyback topologies use a different form. For a detailed worked example and topology-specific design guidance, see our SMPS transformer design guide.
Both options are available. The WCM420 is sold as a core-and-bobbin set for engineers who wind their own transformers. WCM also designs and manufactures fully wound custom switch-mode transformers, including designs built on WCM420 geometry where that fits the requirement, or on entirely custom core geometries where it does not. If your design requires a specific turns ratio, winding configuration, wire type, or insulation system, contact us or visit our custom switch-mode transformer page.
WCM420 Series: High-Power Switch-Mode Transformer Core-and-Bobbin Sets Up to 5 kW 
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