Power electronics module for electric vehicles: the liquid-cooled on-board charger unit, alone or combined with the 12–14 V DC-DC converter
Sealed, liquid-cooled charger units on electric vehicles · battery voltage window, coolant flow and inlet temperature, isolation and sealing, and the 14 V DC-DC stage some units add
In an electric vehicle, a power electronics module of this kind is the cast-aluminium box that sits on the vehicle's coolant loop and converts AC from a charging point into DC for the traction battery; some units also carry the DC-DC converter that feeds the 12–14 V system. Inside are the power switches, magnetics, capacitors and a control board, but the buyer specifies the box from the outside: which battery voltages it can charge, how much coolant at what temperature it needs, how well its high-voltage side is isolated and sealed, and whether it can also send power back.
Vehicle-mounted, liquid-cooled on-board charger (OBC) units for battery-electric and hybrid vehicles, including bidirectional units and integrated units that combine the OBC with a high-voltage to low-voltage DC-DC converter, specified at the enclosure, coolant, connector and battery-window level. IGBT power modules are covered on the IGBT module page and silicon carbide modules on the Silicon carbide (SiC) MOSFET power module page; the power-stage board inside an inverter on the Power electronics board page; packaged DC-DC converter modules on the DC-DC converter page; stand-alone DC-to-AC inverters on the Power inverter page; and off-board chargers on the EV charging pile page. TO-220 device mounting and rectifier cooling are covered on their own pages. Traction inverters and wireless charging receivers are outside this page.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Battery voltage window and AC input: units for 400 V and 800 V packs
The battery voltage range on the data sheet decides which vehicles a charger unit can serve. The three units reviewed span two different windows.
The Bel data sheet gives two different lower limits for the battery voltage; ask Bel which applies to the unit you are buying. [3] ASTRI's output current tops out at 25 A on three-phase and 17 A on single-phase. [1]
Editorial calculation: 11 kW into a 450 V battery is about 24.4 A, and into 850 V about 12.9 A; at ASTRI's 96.8% the unit dissipates about 0.36 kW at 11 kW output (11 ÷ 0.968 − 11), and at Bel's 94% about 1.3 kW of a 22 kW input (22 × 0.06), all of which the coolant has to carry away.
State the pack's minimum, nominal and maximum voltage, the grid supply (single- or three-phase, voltage and current per phase) and the charging power you need on each.
02
Coolant flow, pressure drop and inlet temperature before the unit derates
Bel's BCL25-700-8 is the only one of the three that publishes a full coolant specification, and it shows what a vehicle's thermal loop has to supply. [3]
ASTRI's unit is water-cooled through double self-sealing connectors, runs from −40 °C to +55 °C with derating at 45 °C and above, and measures 254 × 262 × 101 mm at 11 kg with a stated power density of 1.6 kW/L. Bel's unit measures 705 × 106 × 359 mm and weighs 19 kg. [1][3]
Editorial calculation: ASTRI's box is about 6.7 litres (254 × 262 × 101 mm), and 11 kW ÷ 6.7 L gives the 1.6 kW/L it states; Bel's is about 26.8 litres, or about 0.82 kW/L at 22 kW.
M18×1.5 to DIN 9974-1, aluminium alloy fittings [3]
Give the coolant mixture, the flow and inlet temperature available at the charger's position in the loop, and the pressure drop the loop can spare, so the supplier can confirm full power or state the derating.
03
Isolation between grid and battery, leakage current and sealing class
Bel states that its power transformers and safety insulation are designed for 4,242 VDC (3,000 VAC) between AC input and HV output, and 2,500 VDC from the HV output to chassis. Leakage current is given as 3.5 mA at 528 VAC three-phase and 5 mA at 264 VAC single-phase, the power factor as 0.99, and the protection class as IP67 and IP6K9K when all mating connectors are installed. Environmental tests follow SAE J1455, and immunity follows IEC 61851-21-1. [3]
ASTRI uses an isolated dual-active-bridge (DAB) topology and lists AC and DC over-current, over-voltage and under-voltage protections, with CAN bus communication. [1] Editorial note: the IP rating holds only with every connector mated; ask for the rating with the service caps fitted too, if the vehicle ships before the harness is connected.
Ask for the isolation test voltages and where they apply (input to output, output to chassis), the leakage current at your highest grid voltage, and the IP class with and without mating connectors.
04
Adding the 14 V DC-DC stage and reverse power flow in one housing
TI's TIDM-02021 reference design puts an 11 kW OBC and a 2.5 kW low-voltage DC-DC converter (nominal 14 V output) under one F29x microcontroller. The OBC uses a three-phase single-stage series resonant dual-active bridge with a 150 kHz resonant frequency inside an 80–250 kHz operating range, and supports grid-to-vehicle, vehicle-to-load and vehicle-to-grid power flow. [2]
Bel's unit runs in reverse as a three-phase export source, 3 × 400 or 3 × 480 VAC at 50 or 60 Hz selected over CAN, with 93% efficiency at a 350 VDC battery; it can be paralleled up to four units in charge mode. ASTRI's 11 kW unit is also bidirectional and names vehicle-to-grid as a use. [3][1]
Editorial note: a combined OBC and DC-DC unit shares one coolant connection and one housing, but the DC-DC side must keep running whenever the vehicle is on, not only while charging; confirm its rating and cooling with the charger idle.
05
Data for an on-board charger unit
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Battery: [minimum, nominal and maximum voltage; maximum charge current]
Grid: [single or three phase; voltage; current per phase; frequency]
Power: [charging power; export power if bidirectional]
Low voltage: [DC-DC needed or not; output voltage and power]
Cooling: [coolant mix; flow; inlet temperature; allowable pressure drop; fittings]
Isolation and sealing: [test voltages; leakage current; IP class]
Interfaces: [CAN protocol; HV and AC connectors; mounting]
Standards: [SAE J1772 or IEC 61851; environmental test plan]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
Can one on-board charger serve both 400 V and 800 V vehicles?
It depends on its battery window: ASTRI's unit covers 240–500 V, TI's reference design 450–850 V, and Bel's BCL25-700-8 states 240 or 250 to 800 V in different parts of its data sheet. [1][2][3]
How much coolant does a liquid-cooled charger need?
Bel specifies at least 10 L/min of 50/50 ethylene glycol and water at 20 °C, with a 0.05 bar pressure drop, full power up to 60 °C coolant and 50% power up to 75 °C. [3]
Can the charger and the 12 V DC-DC converter be one unit?
Yes. TI's TIDM-02021 combines an 11 kW OBC and a 2.5 kW, 14 V DC-DC converter under one microcontroller. [2]
Sources and what still needs a direct check
Sources checked 2026-10-13. A product data sheet from the Hong Kong Applied Science and Technology Research Institute (ASTRI), Texas Instruments' TIDM-02021 design guide (March 2026), and Bel Power Solutions' BCL25-700-8 preliminary data sheet.
