An on-board charger contains two rectifier problems with two different mindsets: the PFC stage blocks a high-voltage bus at high frequency, and the isolated DC-DC secondary carries high current at low voltage with conduction loss as the enemy. This guide allocates the 6.6 kW loss budget across the two stages and maps each to its part family.
OBC Architecture: PFC Plus Isolated DC-DC
The OBC converts mains AC to a DC bus and then to the battery voltage through an isolated DC-DC converter, typically a PFC boost followed by an LLC or phase-shifted full bridge. The two stages live under different stress: the PFC diode blocks a bus near 380–400 V and switches at high frequency, while the DC-DC secondary rectifies tens of amps at the battery voltage with the transformer reflected voltage on top.
The design mindset follows the stress: the PFC side is a voltage-and-frequency problem, the secondary side is a conduction-and-thermal problem. Selecting one “rectifier” for both is the same mistake as in any SMPS, amplified by the automotive power level.
The two stages also have different switching styles: the PFC boost is hard-switched at the line-modulated envelope, while the LLC secondary turns off softly, so the recovery requirement is different on the two sides even at the same power level. The mindset difference is therefore electrical, not just organizational.
The architecture also sets the isolation boundary: the PFC side sits on the mains side of the isolation barrier, and the DC-DC secondary sits on the battery side, so the two rectifiers live in different electromagnetic environments and different safety domains. The selection for each side is made with its own barrier in mind.
The PFC Side: 250 V-Class Schottky and SiC
The PFC boost diode sees the 380–400 V bus, which rules out the silicon Schottky class on voltage alone. The realistic candidates are a fast recovery diode or a SiC Schottky, and the choice follows the gates—voltage, recovery, temperature, cost—rather than a topology slogan. The 250 V silicon Schottky class, represented by the AMBR40250S, belongs to the DC-DC side or to soft-switched stages where its reverse-peak margin closes, not to the PFC boost; the boundary decision is covered in the 250 V article.
The temperature gate is part of the same decision: an OBC sits in a vehicle and can see high ambient under charging, so the PFC diode’s leakage and recovery behavior at the hot junction enter the budget. A SiC Schottky holds its low-recovery behavior at temperature but costs more; an FRD is proven and cheaper but its recovery loss grows with the heat. The comparison runs the loss math at the hot condition and the system cost at the working conditions.
The DC-DC Secondary: 200 V-Class D2PAK
The isolated DC-DC secondary rectifies the transformer output at the battery voltage, with the reflected voltage setting the reverse stress. For a 200 V-class secondary, a D2PAK Schottky such as the AMBRB30200CT (30 A, 200 V) fits: the 200 V class covers the reflected peaks with margin, and the D2PAK turns the board into the heatsink for the tens of amps of load current. In a soft-switched LLC secondary the recovery term is largely removed, so the low forward drop decides the efficiency story.
The secondary current is where the wattage concentrates: at 20 A of battery current with a 0.84 V typical drop, the forward loss is about 17 W per conduction path, and the D2PAK board design—pad, vias, copper—must move that heat into the enclosure. The thermal design guide and the D2PAK design guide own the details; the OBC point is that the secondary is the thermal center of the product.
The secondary’s reflected voltage is set by the transformer turns ratio, and the class selection reads the worst reflected peak across the battery’s full range—charging at the top voltage and discharging at the bottom—before the 200 V class is confirmed.
Loss and Thermal Budget for a 6.6 kW OBC
| Stage | Typical loss share (illustrative) | Dominant term |
|---|---|---|
| PFC diode | 15–25 W | Recovery / conduction at high frequency and voltage |
| DC-DC secondary | 25–40 W | Conduction loss at tens of amps |
| Total rectifier budget | 40–65 W | Must leave through the enclosure |
The figures are illustrative design-budget ranges, not measurements—the real numbers come from the loss model at the working conditions and the case-temperature measurement on the prototype. The message is the allocation: the secondary dominates because it carries the load current, and the PFC side is smaller but harder to cool because it sits at high voltage and frequency.
The budget also defines the cooling architecture: a 6.6 kW OBC with a 40–65 W rectifier budget needs an enclosure that can move that heat without a fan or with a carefully placed one, and the mechanical design is set by the same numbers the rectifier selection uses. The thermal design and the electrical design are one document.
The allocation is also where the efficiency target lands: if the program targets a specific peak efficiency, the rectifier budget is the allowance the two stages must share, and the secondary’s share is spent first because it is the easier win at high current. The design conversation starts from the allowance, not from the part.
AEC-Q101 and Automotive Reliability Requirements
An OBC is automotive hardware, so the rectifiers carry the automotive qualification question. The wording on the datasheet matters: qualified means the evidence exists, qualified available means it can be supported, and unstated means no claim. For the OBC’s production program, confirm the qualification status and documentation for the selected parts before the BOM—the AEC-Q101 guide explains the wording and the reading rule.
The automotive environment adds two more checks beyond qualification: the thermal cycling of the module stresses the solder joints of the D2PAK and the PFC parts, and the transients of the vehicle network stress the input side. The reliability checklist folds these into the design review, and the qualification wording gates which parts can enter the program at all.
The qualification documentation is requested in the same order as the electrical data: before the BOM, with the revision date checked, because the automotive program’s own approval depends on the evidence being current.
The selection closes with a walkthrough: for a 6.6 kW OBC, the PFC side lands on a fast recovery or SiC diode sized for the 380–400 V bus and the switching frequency, the secondary lands on the 200 V-class D2PAK Schottky sized for the battery current, and both are checked against the qualification wording and the thermal budget before the BOM is approved. The two mindsets meet in one enclosure, each part is selected for its own stage, and the design review reads both sides against the same efficiency and reliability targets.
Design note. The 6.6 kW allocation above is an illustrative engineering budget, not a measured result; the dominant-term logic—secondary conduction at load current versus PFC recovery at high voltage and frequency—follows the two-stage architecture, and the final numbers come from the loss model and prototype measurement. The 250 V Schottky’s role is limited to stages where its reverse-peak margin closes, never the 380–400 V PFC bus.
Frequently Asked Questions
Why are the PFC and DC-DC rectifiers different?
Because the two stages face different enemies: the PFC diode blocks 380–400 V at high frequency, the DC-DC secondary carries tens of amps at low voltage. Voltage and frequency drive one, conduction loss drives the other.
Can a 250 V Schottky be used in the PFC boost?
No—the bus sits near 380–400 V, beyond the 250 V class. The PFC diode is a fast recovery or SiC part; the 250 V Schottky belongs on the DC-DC side where the reflected peak fits with margin.
What rectifier fits a 200 V DC-DC secondary?
A 200 V-class D2PAK Schottky like the AMBRB30200CT, sized for the output current with the reflected voltage and margin in the class selection.
Which stage dominates the rectifier loss?
The DC-DC secondary, because it carries the load current and conduction loss is current times drop. The PFC side is smaller but harder to cool at high voltage and frequency.
What automotive qualification do OBC rectifiers need?
The AEC-Q101 status on the datasheet, read literally—qualified, qualified available, or unstated—with the documentation confirmed before the BOM. The wording guide explains the difference.
Conclusion
The OBC is two rectifier designs in one enclosure: the PFC stage buys voltage and frequency capability, the DC-DC secondary buys conduction loss at current. Allocate the thermal budget by stage, match each part to its dominant term, and close the automotive qualification question before the BOM.
Review the AMBR40250S and AMBRB30200CT product pages on the Good-Ark site, and contact Good-Ark with your power level, bus voltage, and qualification requirements for a stage-by-stage recommendation.