The AMBR40250S is a 40 A, 250 V Schottky barrier rectifier in a TO-220AB package from Good-Ark, with a maximum forward drop of 0.90 V at 20 A, a 180 A surge rating, and 2.0 °C/W junction-to-case resistance. It belongs in soft-switched power stages where the actual reverse peak stays inside its 250 V class with margin—not in every “high-voltage” node. This article covers its ratings, application boundaries, derating, mounting, and the surrounding portfolio.
AMBR40250S at a Glance
All values below come from the official Good-Ark datasheet (Rev. B), read at TA = 25 °C unless noted:
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Maximum repetitive peak reverse voltage | VRRM | 250 | V |
| Maximum RMS / DC blocking voltage | VRMS / VDC | 175 / 250 | V |
| Maximum average forward current | IF(AV) | 40 | A |
| Peak forward surge current (8.3 ms half-sine) | IFSM | 180 | A |
| Forward voltage, IF = 20 A, TJ = 25 °C | VF | 0.83 typ / 0.90 max | V |
| Forward voltage, IF = 20 A, TJ = 125 °C | VF | 0.69 typ / 0.77 max | V |
| Reverse leakage, TJ = 25 °C / 100 °C | IR | 100 µA max / 10 mA max | — |
| Reverse recovery time | trr | 35 max | ns |
| Junction temperature range | TJ / TSTG | −55 to +150 | °C |
| Thermal resistance | RθJC / RθJA | 2.0 / 62.5 | °C/W |
The part ships in TO-220AB, 50 units per tube, with AEC-Q101 qualification available on request. Read the VF columns at both junction temperatures before comparing parts: the difference between 0.83 V typical at 25 °C and 0.77 V maximum at 125 °C changes the loss budget more than most suppliers’ headline tables suggest.
Where the 250 V Class Fits—and Where It Does Not
Silicon Schottky diodes are common below 100 V and become physically harder to build as the blocking class climbs: forward drop and leakage both rise with the voltage rating, which is why most suppliers stop around 150–250 V. The AMBR40250S occupies that narrow window, and the window has a hard precondition: the repetitive peak reverse voltage across the diode, in the actual topology, must stay inside the 250 V rating with 20–30% margin.
The class fits soft-switched DC-DC stages—resonant, LLC, and phase-shifted topologies where di/dt and dv/dt are controlled. There, recovery loss matters less and the low forward drop pays directly in conduction efficiency. The same part is not a general-purpose answer for every node near 250 V. A PFC boost output, for example, sits at roughly 380–400 V bus voltage, which exceeds the rating; that role belongs to a fast-recovery or SiC device, not this part. Before any application claim, write down the diode’s maximum reverse peak from the real waveform, add the margin, and only then compare parts.
Derating and the Honest Thermal Calculation
The 40 A rating is valid at a stated case temperature with a defined heat-flow route; it is not a blanket continuous-load number inside a sealed module. Use the datasheet derating curve, which shows how IF(AV) shrinks as case temperature rises toward 150 °C.
Do the loss math at the hot working condition, not at 25 °C typical values. Forward loss is IF × VF, where VF is the maximum value at the operating junction temperature, and the temperature itself depends on the loss—so a single pass is only an estimate. Walk the full chain from junction to ambient:
TJ = TC + (PD × RθJC), where TC is set by the interface, heatsink, and ambient.
Worked check at 20 A: with the maximum VF of 0.77 V (20 A, 125 °C), forward loss is about 15.4 W, and 2.0 °C/W junction-to-case resistance gives roughly 31 °C of rise from case to junction. If the measured case sits at 90 °C, the junction lands near 121 °C—inside the 150 °C ceiling, but with leakage and interface resistance still to add. The same calculation using 25 °C typical values (0.83 V, about 17 W, about 33 °C) paints a misleadingly optimistic picture because VF, IR, and the operating temperature interact. Iterate the estimate or confirm it with a case-temperature measurement on the first prototype before freezing the thermal design.
TO-220AB Mounting and Heatsinking
The TO-220AB tab is the thermal lifeline, and mounting quality decides how much of the 2.0 °C/W RθJC you actually keep:
- Use a thermal interface material. Bare metal-to-metal contact traps air and can double or triple the interface resistance; choose a pad sized to the tab.
- Control the mounting pressure. A common TO-220 screw torque range is roughly 0.4–0.6 N·m—enough to seat the TIM without cracking the plastic body. Follow the mechanical drawing for the exact hardware.
- Insulate when the tab is live. If the tab sits at a circuit potential, use an electrically insulating pad and bushing; if the system can share a common tab potential, direct contact with TIM gives the lowest thermal resistance.
Alternatives in the Portfolio
When the design needs the same current class in a board-mount footprint, the AMBRB30200CT offers 30 A / 200 V in TO-263 (D2PAK). When the reverse peak genuinely exceeds 250 V with margin, the honest answer is a fast-recovery or SiC device rather than overstressing this part. The full Schottky rectifier diodes category groups the family by voltage, current, and package so the comparison starts from the real working condition.
Engineering note. The selection discipline above follows the rating structure of the AMBR40250S datasheet: VRRM 250 V, IF(AV) 40 A, IFSM 180 A, VF max 0.90 V at 20 A / 25 °C, and RθJC 2.0 °C/W. A 250 V device must be validated against the actual repetitive peak reverse voltage in the specific topology with margin; a PFC bus near 400 V is beyond its rating. Thermal validation should use maximum VF at the operating temperature and the complete junction-to-ambient chain, confirmed by measurement rather than a single-pass estimate at 25 °C typical values.
Frequently Asked Questions
What is the maximum reverse voltage of the AMBR40250S?
The maximum repetitive peak reverse voltage is 250 V, with 175 V maximum RMS and 250 V maximum DC blocking. Apply 20–30% margin over the maximum reverse peak in the actual circuit before selecting the part.
Can the AMBR40250S be used in the PFC output of an OBC?
Only if the reverse peak across the diode stays inside 250 V with margin. A typical PFC boost bus of 380–400 V exceeds the rating, so that role belongs to a fast-recovery or SiC device. Confirm the topology’s maximum reverse waveform first.
What is the correct thermal calculation for this part?
Use the maximum VF at the operating junction temperature, then walk the full chain: TJ = TC + (PD × RθJC), with the interface, heatsink, and ambient included. At 20 A and max VF 0.77 V, forward loss is about 15.4 W and the junction-to-case rise about 31 °C; iterate or measure because VF and IR depend on temperature.
Is the AMBR40250S AEC-Q101 qualified?
The datasheet states AEC-Q101 qualified available, meaning the part can be supported for automotive qualification with the appropriate documentation. Confirm the exact qualification status and documents with Good-Ark before project approval.
What should I choose if my design needs more than 250 V blocking?
Move to a fast-recovery or SiC rectifier. In the same portfolio, the AMBRB30200CT covers 200 V in a board-mount D2PAK, and the Schottky category page lets you compare the rest of the family by voltage class.
Conclusion
The AMBR40250S answers a narrow, real question: what conducts 20–40 A in a soft-switched stage where the reverse peak stays within a 250 V class? Verify the reverse waveform and margin first, derate from the case temperature, run the loss math with maximum VF at the hot junction, and mount the tab properly. If any of those checks fails, the part is not the bottleneck—the application boundary is.
Review the AMBR40250S product page for the full datasheet and packaging details, and contact Good-Ark with your topology, reverse peak voltage, load current, and thermal conditions to confirm the part, samples, and qualification documentation.