Nexperia — Application-Specific MOSFETs for 36V Battery Systems

The most common requirements for such a MOSFET are:

• Adequate drain-source voltage rating
• Low on-resistance, which reduces conduction losses
• A high dc current rating, so that the MOSFET can handle peak overload and fault currents in high-power applications
• Low drain-source leakage current, reducing the rate of battery discharge during long periods of inactivity
• A robust package and good board-level reliability. This is important for applications exposed to harsh environments, such as extreme temperatures or vibration.

Other factors which designers consider when evaluating options for a high-side discharge MOSFET include:

• High avalanche rating. When a high-current load is isolated quickly, an event typically triggered when the battery is disconnected by a failure protection mechanism, high avalanche energy may be dissipated through the MOSFET.
• Large safe operating area (SOA). In fault conditions, or when trying to turn off the motor during an overload, the discharge transistor will often be pushed briefly into its linear mode, when its resistance rises substantially. In these conditions, the battery voltage can decay under a high load current, and the MOSFET gate voltage might not be sufficient to ensure the transistor is fully turned on. A MOSFET with a large SOA can withstand these conditions.
• Low gate leakage, to avoid unwanted turn-off events in linear mode.

In a battery pack which contains 10 cells in series, the voltage is typically 36V; the maximum charging voltage is 42 V. A MOSFET which has a breakdown voltage rating between 45 V and 52.5 V provides at least 80% derating.

A 60 V MOSFET is typically used in these applications, however, since it provides even higher derating. But this comes at the expense of other factors such as avalanche rating, SOA, gate charge, and leakage performance. In many MOSFETs, the silicon architecture deliberately thins the cell pitch to achieve low on-resistance, but this has the effect of shrinking the SOA and reducing avalanche energy capability.

Design engineers, then, have become used to accepting a compromise between efficiency and isolation capability. The latest application-specific 50 V and 55 V MOSFETs from Nexperia, however, eliminate the need for compromise. They benefit from a superior superjunction technology which produces lower on-resistance without impairing performance in the other parameters.

For example, the Nexperia PSMNR90-50SLH in an LFPAK88 clip-bonded package features maximum on-resistance of 0.9 mΩ at a gate-source voltage of 10 V. It has a maximum drain current rating of 410 A. Despite its low on-resistance, this MOSFET also offers best-in-class SOA current capability of 8.1 A at 40 V for a 1 ms pulse, shown in Figure 2.