Electric power steering (EPS) is the rare automotive load that goes from gentle assist to full stall within seconds. One moment the motor is drawing a few amps to help a light turn; the next it is holding a parking manuvre at full lock, the motor stalled against its mechanical stop and pulling its peak current. Every diode in that drive — the freewheel, the bridge rectifier, the clamp — is rated against the pulses of that profile, not against a constant load. This article builds the EPS load profile, maps the diode roles in the drive, adds the clamping and TVS protection around the motor bridge, works the thermal duty of assist bursts and parking cycles, and closes with a worksheet that sizes the diodes from the profile itself.
From Gentle Assist to Full Stall: The EPS Load Profile
The EPS load profile is the foundation of every diode decision in the drive, and it is not a single number but a schedule. The motor’s demand varies across the driving day in a pattern that repeats: light assist for small corrections, stronger assist for maneuvers, and full-stall current when the driver holds the wheel at lock or when the system fights a parking resistance.
The numbers frame the schedule. A light assist may draw a few amps; a strong assist burst draws tens; and a stall event holds the peak current for seconds at a time while the steering holds position. The stall is the brutal corner: the motor cannot turn, so it sits at locked-rotor current, dissipating heat while the driver holds. The profile is the product of these phases — the duty cycle of assist bursts, the frequency of parking cycles, and the duration of full-stall events. The EPS rectifier design article documents the electrical demand of the system, and the motor bridge sizing article sets the bridge context this article builds on.
The design rule that follows is to size against the profile, not against a single rated load. A diode chosen for the average assist current will overheat on the stall corner; one chosen for the stall peak will be oversized and costly for the rest of its life. The profile-derived sizing worksheet at the end of this article turns the schedule into a parts decision, and the sections between map the roles and the protection that the profile demands.
Diode Roles in the EPS Drive: Freewheel, Clamp, and Sense
The EPS motor drive uses diodes in three distinct roles, and each role must survive a different part of the load profile.
The freewheel role is the most stressed. The drive’s motor controller — a brushless or brushed stage — switches the motor current, and when the switching element turns off, the freewheel diode carries the motor winding’s inductive current. In an EPS, every assist burst writes a train of switching edges into the freewheel, and the stall corner writes the most current through it. The freewheel must carry the peak stall current and dissipate the switching loss of the burst without exceeding its junction limit. The clamp role is the protection element around the motor bridge: it absorbs the spikes and the back-EMF that the switching and the mechanical events generate, holding the drive’s electronics to survivable voltages. The sense role is the diode in the current-sense or position-sense path of the steering controller, which carries small currents and must read accurately through vibration and temperature. The EPS rectifier design article maps these roles, and the rectifier selection for SMPS guidance uses the same role language for the switching stage.
The three roles have different rating priorities. The freewheel prioritizes current, surge, and switching loss; the clamp prioritizes pulse energy and clamping voltage; the sense role prioritizes accuracy and stability. Reading the profile against each priority is the sizing method, and the worksheet pulls the three roles together.
Clamping and TVS Around the Motor Bridge
The motor bridge in an EPS drive is a source of spikes as much as a source of useful torque. Every switch edge, every commutation, and every stall generates transient energy that must go somewhere, and the clamping and TVS protection around the bridge is what controls it.
The clamping story starts with the motor’s inductance. When the controller switches the winding current off, the winding attempts to sustain the current and drives a voltage spike across the switching element; without a clamp, the spike stresses the controller and the bridge diodes. The freewheel diode is the first clamp, holding the winding current through the off-time; the TVS or reverse-polarity protection is the second line, absorbing the larger spikes and the rail events that reach the drive. The EPS rectifier design article and the TVS selection guide cover the sizing of both lines.
The protection order matters because the two clamps absorb different energies. The freewheel carries the repetitive switching current — thermal duty spread over the drive’s life; the TVS absorbs the discrete events — the spike, the load-dump-fed surge, the occasional hard fault. Mixing the roles fails both: a TVS asked to carry the continuous freewheel current overheats, and a freewheel asked to clamp a large spike lacks the energy rating. The motor drive protection article documents the field signatures when the protection order is wrong.
Thermal Duty in Assist Bursts and Parking Cycles
The thermal duty of an EPS drive is the schedule of the load profile written as heat. Each assist burst heats the freewheel and bridge diodes; each parking cycle adds a stall corner; and the pauses between events let the parts cool. The junction temperature swings with the schedule, and the life of the parts is set by the hottest corners and the delta-T of the cycles.
The stall corner is where the thermal design is decided. During a full-stall hold, the motor current and the diode current sit at the peak for seconds, and the dissipation accumulates in the sealed steering-column environment where airflow is minimal. The junction temperature during the stall — ambient plus the peak dissipation times the thermal resistance — must stay below the derated limit, and the number of stall events per day sets the cycling duty that drives the life. The rectifier thermal design guide and the thermal management worked case provide the junction-temperature and thermal-resistance method, and the thermal cycling guide covers the delta-T life trade.
The thermal worksheet follows the profile. Convert the assist bursts and stalls into an average and a worst-case dissipation; run the junction-temperature equation at the worst case; and check the life curve against the number of parking cycles per day. A drive whose stall corner keeps the junction inside the margin with the derating applied survives the profile; one that runs the stall at the absolute limit is a warranty event waiting to happen.
Selecting Diodes for an EPS Bridge: Worked Example
The worksheet closes the article with a worked example that sizes the bridge diodes from the load profile. The method is the point — the same three steps work for any EPS stroke and any vehicle.
The example: an EPS drive with a 60 A stall current, a 15 A average assist current, and a duty of ten stall events per day, each holding two seconds. First, size the freewheel and bridge diodes for the stall current with the standard surge margin: a part rated for the peak with margin, not for the average. Second, run the thermal check: the stall dissipation at the diode’s VF times the 60 A, folded through the Rth at the steering-column ambient, must keep the junction under the derated limit. Third, check the switching duty: the assist bursts at the motor’s switching frequency need a diode with recovery fast enough that the switching loss stays small. The motor bridge sizing method and the EPS rectifier design article supply the rating checks the worksheet uses.
The example’s verdict is typical: a diode that carries the stall with margin and runs its junction under the limit at the sealed ambient passes; a part sized only for the average assist fails the stall corner. The worksheet turns the load profile into a repeatable selection, and it is the practical tool this article is built to deliver. The general rectifier category supplies the parts the worksheet lands on, and the EPS design article is the field reference that validates the result.
EPS is a load-profile application, and the diodes survive it the way the profile is treated: as a schedule of assists, stalls, and cycles, each with a rating that must hold. The freewheel carries the peak and the switching loss, the clamp absorbs the spikes, and the thermal duty decides whether the drive lives or fails at the parking corner. Size the diodes from the profile with the worksheet — stall current first, thermal second, switching third — and the EPS drive stops being the part that overheats in the column. The general rectifier category and the motor drive failure guide close the loop with the parts and the field signatures.