How to Select a No-Compromise Discharge MOSFET for a Battery-Powered Motor Drive
The use of lithium-ion battery packs in high-powered motor-drive applications such as power tools, cordless appliances and unmanned aerial vehicles, or drones, has grown markedly in recent years. The benefits of li-ion battery technology include high energy density, the absence of a memory effect, and low self-discharge: they make li-ion batteries ideal for use in products which require a high power capability, long battery life and fast charging.
Design engineers, however, harbour concerns about the safety of the technology, since li-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, system designers generally use a MOSFET, as the typical application circuit in Figure 1 shows.
Fig. 1: a simple high-side discharge MOSFET
The most common requirements for such a MOSFET are:
- Low on-resistance for reduced conduction losses
- A high 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 reasonable drain-source voltage rating. This is usually determined by the number of cells in the battery pack, and their tolerance of transient conditions at exposed connectors.
- 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 that designers consider when evaluating options for a high-side discharge MOSFET include:
- High pulse-current rating. When a rotor becomes locked or the motor stalls, a dead short occurs across the motor - this calls for a high current pulse to get the motor started.
- 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 four or five cells in series, the voltage ranges from 14.4V to 21V. A MOSFET which has a breakdown voltage rating between 25V and 30V provides at least 80% derating.
In order to achieve on-resistance and a high current rating, MOSFET manufacturers normally have to settle for a compromise in the parameters of SOA, gate charge and leakage performance, building their devices with a silicon architecture that has a narrow cell pitch (which gives low on-resistance).
Nexperia, however, avoids the need for compromise, since its MOSFETs benefit from a superior superjunction technology which produces lower on-resistance without impairing performance in the other parameters.
For example, the Nexperia PSMNR51-25YLH in a PowerSO8 package features maximum on-resistance of 0.57mΩ at a gate-source voltage of 10V. It has a maximum drain current rating of 380A. Despite its low on-resistance, this MOSFET also offers best-in-class SOA current capability of 38A at 10V for a 10ms pulse (see Figure 2).
Fig. 2: 10V/10ms SOA plot for the PSMNR51-25YLH MOSFET
In a battery pack which contains between eight and ten cells in series, the voltage ranges from 28.8V to 42V: this calls for a discharge MOSFET which has a breakdown voltage rating of 50V-55V.
This means that the industry-standard 40V rating is too low. A 60V MOSFET provides headroom, but could be expected to impose a trade-off in parameters such as SOA and drain-current rating. Because of this, a 50V-55V MOSFET would be better suited to batteries with a voltage up to 42V. To meet this requirement, Nexperia is to introduce in July 2020 the PSMN1R5-50YLH, a MOSFET which has a voltage rating of 50V, and the 55V PSMN2R0-55YLH.
Wide Range of MOSFET Product Options
The Nexperia discharge MOSFETs are supplied in a 100% clip-bonded LFPAK package suitable for automotive applications. This package is robust, offers high board-level reliability and provides excellent thermal performance.
Table 1: Large-SOA MOSFETs for use in li-ion battery packs