Nexperia — Application-Specific MOSFETs for 36V Battery Systems
Nexperia

Nexperia Application-Specific MOSFETs for 36V Battery Systems

How to select the ideal MOSFET for 36V battery-powered equipment

A 36V lithium-ion battery is today a common power source for tools and outdoor power equipment. Products powered by a 36V battery benefit from high power output and long battery life, while also being relatively light and easy to use. The high energy density of the 36V battery, however, also entails the implementation of efficient and safe battery isolation.

 

The need for isolation arises from the safety risks inherent in the technology. Lithium-ion batteries contain a flammable electrolyte: if damaged or incorrectly charged or discharged, this can cause a fire or explosion. To safely charge and discharge the cells and isolate them from the load, battery system designers generally use a MOSFET, as illustrated in Figure 1.

Nexperia — Application-Specific MOSFETs for 36V Battery Systems

Fig. 1: A simple high-side discharge MOSFET in a typical application circuit

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.

Nexperia — Application-Specific MOSFETs for 36V Battery Systems

Fig. 2: 40 V/1 ms SOA plot for the PSMNR90-50SLH MOSFET

For more space-constrained applications, 50 V and 55 V parts with a 200 A current rating are also available in a 5 mm x 6 mm LFPAK56E package.

In addition to optimizing the key parameters for use in 36V battery systems, Nexperia has also hardened the operation of these MOSFETs in high temperatures. It has performed high-temperature reverse bias life testing (HTRB) for 1,000 hours at 175°C and 100% of the device’s drain-source voltage rating.

The industry standard for 60 V parts is 1,000 hours at 175°C and only 80% of the drain-source voltage rating, 48 V, according to MIL-STD-750-1 M1038 Method A.

 

Wide range of MOSFET product options

Nexperia discharge MOSFETs are supplied in a 100% clip-bonded LFPAK package. This package is robust, offers high board-level reliability, and provides excellent thermal performance. The LFPAK package is suitable for automotive as well as industrial and consumer applications.

Part NumberMaximum Drain-source VoltagePackageOn-resistance @ 10 VMaximum Drain CurrentAvalanche Energy @ 25 A SOA Current @ 40 V for 1 ms
PSMN1R5-50YLH50 VLFPAK56E1.7 mΩ200 A2000 mJ5 A
PSMN2R0-55YLH55 VLFPAK56E2.0 mΩ200 A1770 mJ4.5 A
PSMNR90-50SLH50 VLFPAK880.9 mΩ410 A7400 mJ8.1 A
PSMN1R2-55SLH55 VLFPAK881.2 mΩ330 A6400 mJ8.1 A

Nexperia MOSFETs with a large SOA for use in 36V li-ion battery packs

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