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Intelligent Power Switches for 48V battery applications

Editor’s note: I particularly like this tutorial blog about Power switches because not enough good technical content is written about this device. I am pleased to introduce our guest authors for this article: Nicholas Aupetit and Alberto Villegas from ST Microelectronics

Introduction

In the not-too-distant future, high-end diesel and gasoline cars may have two batteries on board: the main one is defined as “48V,” and the secondary will be a “12V.” The main interest of the 48V is to support the trend of fuel economy and reduction of pollutant emissions, especially CO2 reduction. The use of a 48V battery should help to increase the cars’ efficiency: for example, by using thinner wires in the cars. Actually, to drive the same power, if the battery voltage is higher, the current is lower. A 48V battery is a good trade-off between safety and functionality.

Several new electronic systems powered by the 48V battery are already available. More and more often it will be necessary to have Intelligent Power Switches (IPS) to drive some of the loads. The IPS offer useful types of diagnostics such as short-circuit, overload, and thermal protections, and they can supply an actual image of the current flowing into the load. The key “switching” element, in this case, is an N-MOSFET.

Therefore, because of the maximum possible voltage (60 to 70V) on the 48V batteries, and the spikes generated inside the car, the MOSFET switch has to withstand voltages of an order of magnitude of 80V at 25o C.

Today the number of available “Smart Power MOSFETs” or “IPS” that can comply with these requirements is extremely low, and they are expensive.

In this application note we demonstrate a way to design circuits able to drive all kinds of loads with components already in mass production, AEC-Q100 or AEC-Q101 qualified, and that offer functionalities similar to those of the 12V or 24V IPS available in the market today.

General Overview

Figure 1

General system overview

General system overview

This “intelligent power switch” for the 48V battery application is intended for driving resistive or inductive loads with one side connected to ground.

This application performs an analog current measurement through a shunt resistance, which delivers a voltage proportional to the load current. The TSC1031 is a high side current sensing device used to amplify the voltage across the shunt. This information is then analyzed by a microcontroller. As this measurement is used to control the current into the load, it must be done accurately. The maximum guaranteed error is described in Figure 2.

Figure 2

Maximum output voltage error of the TSC1031

Maximum output voltage error of the TSC1031

The current measurement combined with a high speed comparator (TS3021) and a D flip-flop (HCF4013) perform an overcurrent protection function, which can switch-off the MOS and latch-off the application in case of overload. After an overcurrent event, the system takes less than 6.5µs to switch-off the power NMOS.

A thermal sensor is used to control the MOS temperature to protect the PCB when it gets beyond 125o C. The temperature information analyzed by the microcontroller can also be used to switch-off the MOS in case of overheating.

The 80V NMOS (STH275N8F7-6AG) is driven thanks to the gate driver L6491, which is PWM controlled by the microcontroller STM8L. The PWM frequency can be configured in the range 250 Hz to 20 kHz. Nevertheless by working at higher frequencies it increases the switching losses leading to an increase of the junction temperature of the NMOS.

Figures 3 and 4 summarize the MOSFET switching losses and junction temperature increase as a function of the PWM frequency.

Figure 3

Switching losses vs frequency

Switching losses vs frequency

Figure 4

Junction temperature vs frequency

Junction temperature vs frequency

This application note describes in details all the schematics block defined in Figure 1 and explain how the function of “Intelligent Power Switch” for a 48V application is realized. It also provides results based on application tests employing the following dedicated application board.

Figure 5

Application board

Application board

Please do share your comments and experience with our readers on this topic.

About the authors

Nicolas Aupetit, Analog product definition, ST Microelectronics

Nicolas Aupetit started his professional career at STMicroelectronics in 2004, after he achieved his master’s degree in Electronic Engineering from Polytech’ Grenoble, France.

He began as an application engineer and for four years he was responsible for display drivers for Oled and Epaper technologies.

Then he continued in a more analog range activity, by taking charge of the product definition and characterization of audio amplifiers, Op Amps, current sensing and DAC products, in a wide range of applications (automotive, industrial, consumer and space). Additionally, Nicolas insures worldwide customer support for these product families.

Alberto Villegas, EMEA Sales and Marketing team, ST Microelectronics

Alberto Villegas was born in Manizales, Colombia (south America) and obtained his Engineer diploma from ENSEA – Paris (a Graduate School in Electrical & Computer Engineering), in 1985. After 6 years of design, manufacturing and characterization of GaAs MMICs in the Philipps research laboratory in Paris, he joined the Sales & Marketing team of Philipps Semiconductors in Paris. For five years he was involved mainly in small signal MOSFETs, Infrared Sensors, and Power RF&Microwave transistors. When he joined ST Microelectronics in Paris in 1990, Alberto was in charge of the Marketing of RF & Microwave power products (for applications in Radars, Avionics, TV & Radio emitters, PMR, CB) for the Mediterranean countries. Since 2000, Alberto has been contributing to boost the sales in the Automotive field in Europe. He works in the EMEA Sales and Marketing team, promoting in particular AEC-Q Power Solutions, including Smart MOSFETs, MOSFETs (Silicon and Silicon Carbide), Gate Drivers, Rectifiers (Silicon and Silicon Carbide), and Protection.

For a more detailed technical treatment of this subject see my article on EDN: Intelligent-Power-Switches-for-48V-battery-applications

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