EPS: Efficient Power Rectification for Modern Power Electronics

An EPS power chain loses watts, not percentages. How to model rectification loss on a 12 V or 48 V steering path and pick a device that stays cool.

Request a Quote

EPS: Efficient Power Rectification for Modern Power Electronics

In an electric power steering system the number that decides your device selection is not the rail voltage — it is the 15 W that a 30 A motor current dumps into a series element with a 0.5 V drop. Power steering is a high-current, low-duty application, and the loss budget of the steering path, not the part list, is what separates a design that stays inside its thermal limits from one that throttles itself on a hot day.

Last updated: October 11, 2026

Key Takeaways

An EPS power chain is a 12 V or 48 V system carrying motor-class currents, so rectification and protection devices are chosen by loss and surge behaviour rather than by convenience.
The loss anatomy is simple to state and easy to underestimate: series drop multiplied by current, converted into junction temperature by the package’s thermal resistance.
Motor-drive paths and supply paths demand different things. The drive side wants low conduction loss in switches; the supply side wants robust rectification and transient protection.
Qualification follows the device family: AEC-Q101 for discrete semiconductors, with load-dump behaviour examined as part of the protection plan rather than as an afterthought.
Read the reverse-voltage rating against the transient, not the nominal rail. A family rated at 40 V is a plausible 12 V part and a poor 48 V choice.

The Loss Anatomy of an EPS Power Path

Every loss in the steering path has the same shape: a voltage dropped across a device, multiplied by the current through it, converted into a temperature rise by the package. Only the first factor changes from device to device.

That structure is what makes the design decision tractable. If a path carries 30 A, a device that drops 0.5 V dissipates 15 W; a device with 2 mΩ of on-resistance dissipates 1.8 W at the same current. Both are “conducting”. One needs a heatsinked package and the other may not, and the difference is the first factor, not the circuit topology.

The EPS environment sharpens this because the current is high, the duty cycle is genuine — a driver holds steering load for seconds at a time — and the assembly sits where the cabin and the engine bay meet. Low forward drop on the supply side and low conduction loss on the motor side are not marketing preferences; they are what keeps the junction temperature inside the limit on a long steering hold.

Path in the EPS system	What the device does	What dominates the choice
Motor drive (inverter legs and freewheel paths)	Switches and rectifies motor-phase current	Conduction loss, switching behaviour, and AEC-qualified switches
Supply and bias rectification	Provides rectified rails for control and gate drive	Forward drop, reverse-voltage rating, and thermal path
Transient protection at the input	Clamps load-dump and switching transients	Energy handling and clamp behaviour, not forward drop
Reverse battery and polarity duties	Blocks a reversed or mis-wired supply	Reverse blocking plus the loss the blocking device introduces

A Worked Loss Model for a 30 A Path

Take a 12 V-class path carrying 30 A and compare two ways of passing that current. The assumptions are stated so you can substitute the real numbers from your own device datasheets.

Series element with a 0.5 V forward drop: P = 0.5 V × 30 A = 15 W.
Switch with 2 mΩ on-resistance: P = 30 A × 30 A × 0.002 Ω = 1.8 W.
At a 5 °C/W junction-to-case thermal path, 15 W means a 75 °C rise on the die before the case even starts conducting into the housing; 1.8 W means a 9 °C rise.

This is a model, not a measurement, and the 0.5 V and 2 mΩ figures are illustrative assumptions: the real values come from the datasheets of the specific devices you intend to use, at the current and junction temperature the design will actually see. What the model establishes is the ratio. On a high-current path, a series diode and a low-resistance switch are not variations on the same idea; they are an order of magnitude apart in self-heating, which is why the motor-drive side of an EPS system uses qualified switches rather than rectifiers carrying full motor current.

The result also explains why the answer changes with the rail. On a 12 V bus a 0.5 V drop costs about 4 % of the rail and 15 W of heat; on a 48 V bus the same drop costs about 1 % of the rail but exactly the same 15 W, because the current is what generates the heat. The voltage class changes the efficiency argument; it does not change the thermal one.

Reverse Voltage: Read the Transient, Not the Rail

A device’s reverse-voltage rating has to clear the event, not the label on the system. This is the boundary condition that turns a comfortable-looking selection into a field failure.

Consider the small-signal Schottky family documented in the ST BAT54 datasheet: its repetitive peak reverse voltage is 40 V. That is a perfectly sensible figure for 12 V logic-level duties and a poor one for a 48 V rail, where the nominal voltage already sits above a safe margin and load-dump events add a large, slow overvoltage on top. The selection question is therefore not “is 40 V more than 12 V” but “what is the highest voltage this node can see, including the transients the programme requires, and how much margin does the derating policy leave”.

The same reasoning applies at the input protection stage. Load-dump behaviour belongs in the protection plan as a specification the TVS device is chosen against, not as a property one hopes the rectifier has. And because automotive programmes define the transients in the standards they follow, the honest engineering sequence is: state the events, state the levels, then choose discrete devices whose ratings and qualifications cover them.

What to Qualify and What to Ask For

Qualification is a document set, and it is requested by name. For an EPS supply or drive path that means:

The AEC-Q101 qualification report for the exact discrete part, including the package variant, since AEC-Q100 covers integrated circuits and AEC-Q102 covers optoelectronic devices rather than diodes and MOSFETs.
Datasheet derating curves with the thermal resistance that applies to the copper area you will actually use — the same device in the same package changes junction temperature with the pad area under it.
Load-dump and surge data for the protection devices, and the clamp behaviour the module designer is relying on.
Package and outline drawings, so the mechanical fit and clearance are checked before layout freeze rather than after.
Lot traceability and a material declaration, so a change of source becomes visible to the programme.

Where the data does not exist for a given part, the correct answer is that it is available on request or not applicable, not a synthesised figure. A supplier that can hand over the qualification report for the exact part has answered the question that matters; one that can only describe the brand has not.

Where the Same Logic Applies Elsewhere

The loss anatomy and the transient rule travel to every high-current DC system. The same reasoning appears in server power at 48 V, where rectification efficiency is the margin being competed for; in PV inverter stages, where the rectifiers sit in a genuinely hot environment; and in automotive lighting drivers, where the module is sealed and the air path that would carry heat away does not exist. The device families change across those applications; the two questions — what does this drop cost me in watts, and what is the highest voltage this node will ever see — do not.

FAQ

Why does a 0.5 V drop matter so much in an EPS system?

Because motor currents are large. At 30 A a 0.5 V drop is 15 W of heat in whatever carries the current, and the package’s thermal resistance converts that into a junction rise that a high-duty steering manoeuvre can hold for seconds.

Does moving from 12 V to 48 V reduce the loss problem?

It reduces the loss as a fraction of the power delivered, but not the watts. The heat generated by a series drop is the drop multiplied by the current, and the current is unchanged by the rail voltage for the same motor power.

Which AEC standard applies to rectifiers and MOSFETs in an EPS design?

AEC-Q101, which covers discrete semiconductors. AEC-Q100 applies to integrated circuits such as gate drivers and controllers, and AEC-Q102 to optoelectronic devices.

Can a small-signal Schottky diode be used on the 48 V side?

Rarely. Its reverse-voltage rating has to clear the highest transient the node sees, not the nominal rail. A family rated at 40 V repetitive peak reverse voltage illustrates the problem: it is comfortable at 12 V and inadequate above a 48 V nominal rail once load-dump margin is applied.

What should be requested before a device is approved for production?

The AEC-Q101 qualification report for the exact part and package, derating curves with the thermal resistance for the intended copper area, transient and load-dump data for protection devices, package drawings, and traceability for the production lot.

Official Resources

References

STMicroelectronics, BAT54 — Small signal Schottky diodes, DocID5505 Rev 12 — source of the 40 V repetitive peak reverse voltage, forward voltage and thermal resistance figures. st.com/resource/en/datasheet/bat54.pdf
Diodes Incorporated, 1N4001–1N4007 1.0 A Rectifier, DS28002 Rev 11–3 — source of the automotive qualification statement (AEC-Q100/101/104/200, PPAP capable, IATF 16949 certified facilities). diodes.com/assets/Datasheets/ds28002.pdf
AEC-Q101 — stress test qualification for discrete semiconductors (standard designation; documents are distributed by the Automotive Electronics Council).
IEC 60747-15 — semiconductor devices, isolated power semiconductor devices, used as the reference for device-level isolation and rating terminology. webstore.iec.ch/en/publication/102658

Continue through this engineering topic

For the complete selection, design, verification, and troubleshooting path, explore the Automotive Power Electronics Hub.