Surge protection in DC fast chargers is a certification-critical subsystem that is often over-engineered with legacy component stacks inherited from earlier industrial power products. The IsoMOV hybrid device from Bourns simplifies the SPD circuit while meeting UL 1449 4th edition and IEC 61643-11 requirements more cleanly than the conventional MOV+GDT combination.
The Conventional Approach
Traditional surge protection for DCFC AC inputs uses a combination of:
- Metal Oxide Varistors (MOVs) for initial clamping of surge transients
- Gas Discharge Tubes (GDTs) for follow-current interruption and arc quenching
- Thermal fuses or disconnect mechanisms for end-of-life MOV failure
This three-component stack performs adequately but has failure modes that complicate UL 1449 testing: MOVs degrade over repeated surge events, GDTs have slow response times (hundreds of nanoseconds), and the thermal disconnect adds a latency before the circuit disconnects a failed MOV from the mains.
The IsoMOV Advantage
The IsoMOV integrates a MOV element with a series-connected positive temperature coefficient (PTC) thermistor in a single package. The PTC element provides thermally activated disconnection: as the MOV degrades and begins to conduct at lower voltages, the PTC heats up and increases resistance, limiting follow current and safely disconnecting the failing MOV without a separate thermal fuse.
Key benefits:
- Reduced component count: One IsoMOV replaces MOV + thermal disconnect
- Faster response: The integrated MOV still clamps within nanoseconds
- Improved end-of-life behaviour: No risk of MOV thermal runaway causing fire
- UL 1449 4th edition compliance path: IsoMOVs are specifically addressed in the updated standard
Circuit Design Considerations
IsoMOVs are available in varistor voltage ratings from 130V to 625V RMS, covering single-phase 120V/240V and three-phase 208V/480V applications. For a 240kW DCFC with a 480V three-phase AC input, use IsoMOVs rated for 625V RMS in a line-to-line and line-to-neutral topology.
PCB layout: IsoMOVs have the same lead spacing as standard disc MOVs. However, because the PTC element heats up during a surge event, ensure 10mm minimum clearance from adjacent PCB components or electrolytic capacitors that are sensitive to elevated temperatures.
Enclosure requirement: UL 1449 4th edition requires SPD components to be within a listed enclosure or tested as part of the complete assembly. If your charger enclosure is plastic, verify the flammability rating of the enclosure material in the vicinity of the SPD circuit.
Testing and Certification
Certification testing for the IsoMOV-based SPD circuit under UL 1449 4th edition includes:
- 20 surge events at the rated surge current (typically 20kA for Class 1 SPDs)
- Operating duty cycle tests simulating end-of-life degradation
- Temperature rise measurement at rated surge current
The IsoMOV's integrated PTC behaviour simplifies the temperature rise test, as the device self-limits its dissipation through PTC action rather than relying on an external thermal fuse whose trip time must be characterised separately.
Conclusion
For new DCFC designs, the IsoMOV-based SPD circuit is the right choice. It reduces component count, simplifies PCB layout, provides better end-of-life safety behaviour, and has a clear certification path under current UL and IEC standards. The cost premium over a discrete MOV is offset by eliminating the thermal fuse and simplifying assembly.