Automatic Voltage Switch Design

Portable electronic devices or cordless devices are becoming ubiquitous in recent years thanks to the advantages they bring to the table in terms of size mobility and flexibility in their deployment. For starters, the numerous problems arise with the need to manage different voltage rating switched power supplies to maintain charge all devices. Practically it’s possible to connect a wrong power supply and eventually damage the device, sometimes beyond repair. There is a solution to solve this kind of problems which can enable multiple devices operating at different voltage levels to share the same power supply without a problem as long the voltage range is supported and if not, it will prevent the wrong voltage from passing through. To achieve this, a High Voltage GreenFET technology device is required to perform within the selected voltage range, over voltage and under voltage protection, and MCU to automate the process.

Automatic Voltage Switches are devices that provide protection to a connected load. AVS respond to input voltage to activate or deactivate a load when the supply voltage exceeds or underlines the programmed limits. They are typically used in applications like UPS and Monophasic smart switches. It may also be used in power management unit of electronic devices.

In general, AVS device have a single undervoltage and overvoltage reference voltages that serve as the system operation limits. A simplified block diagram of AVS is illustrated in Figure 1.

From technical standpoint, the implementation of a high voltage DC AVS require a MOSFETs, because of its low ON resistance and high drain current.

For flexibility purpose, the system requires a GreenFET with two selectable voltage limits.

From controlling standpoint, the voltage limits can only be controlled by MCU that can decide under what conditions to set the configurations for the specified voltage limits and maintain the protection features of the system.

The SLG59H1008V is a load switch designed to control 12 V or 24 V power rails up to a current of 4 A. It contains all the necessary logic blocks for protection against Short-circuit, Overcurrent, Undervoltage and Overvoltage lockout windows, overheating protection and current monitoring.

The following list provides more details on the functions of each pin:

· ON pin: Initiates the operation of the SLG59H1008V’s state machine when asserted HIGH. To disable the IC, the pin must be connected to GND because there is no internal pull-down resistor.

· GND pin: Is a Ground connection. Connect this pin to system analog or power ground plane.

· VIN pin: Supply the power for the operation of the SLG59H1008V, its internal control circuitry, and the drain terminal of the nFET load switch.

· VOUT pin: Source terminal pin of the nFET load switch.

· SEL pin: · Selects one of the two undervoltage/overvoltage lockout window. When SEL = LOW, the VIN undervoltage/overvoltage lockout window is set for 12 V ± 10% applications (mode 0). When SEL = HIGH, the VIN undervoltage/overvoltage lockout window is set for 24 V ± 5% applications (mode1).

· FAULT pin: Open drain output. FAULT is asserted within TFAULTLOW when a VIN undervoltage, VIN overvoltage, a current-limit, or an over-temperature condition is detected.

· CAP pin: Sets the VOUT slew rate and overall turn-on time.

· IOUT pin: Is the SLG59H1008V’s power MOSFET load current monitor output.

· RSET pin: Sets the active current limit threshold. A 91 kΩ resistor sets the SLG59H1008V’s active current limit to 1 A and a 20 kΩ resistor sets the active current limit to 4.5 A.

The SLG59H1008V is designed from the ground up to be suitable for voltage switching applications. The part comes with a 4 A rated load switch that is well suited for a variety of systems that can handle two voltage extremes. The part is based on a N-Channel MOSFET, suitable for low and high-power loads, what makes the part shines it’s the presence of voltage select, analog current monitoring and fault detection modules allowing an external MCU to control the part and read operation parameters.

When using SLG59H1008V in AVS applications, an external grounding resistor RSET = 20 kΩ is connected to set the load switch current limit to 4.5 A. A grounding capacitor CSLEW = 10 nF is connected to set VOUT slew rate and overall turn-on time while a grounding resistor RIOUT = 84 kΩ and capacitor CIOUT = 180 pF are connected to generate VIOUT with optimal linearity.

The system is composed of four main blocks with different functions:

· The SLG59H1008V block is the heart of the system. This block handles the power input and has all the features required to perform load switch, current sensing, fault detection and pin selectable 12 V/24 V input overvoltage and undervoltage lockout.

· Voltage Regulator block is the final check point of the system. Since the system is designed to allow 12 V/24 V input, this stage is responsible for regulating the 24 V input down to 12 V.

· Bypass control block is responsible for selecting the source of output voltage. Bypass control is implemented on two SLG55021 MOSFET drivers and a pair of N-channel-MOSFETs. The bypass control selects one of the two voltage sources for the load — 12 V directly from the High Voltage GreenFET or 12 V from the regulator. Only one of the MOSFET should be active at a time. The cases are VIN = 12V − VMVLine = HIGH and VVreg = LOW and if VIN = 24 V − VVreg = HIGH and VMVLine = LOW.

· Microcontroller block is the brains of the system. It controls all the operations of the systems from current monitoring input voltage to input voltage range selection. The MCU works as a voltmeter and ammeter to verify if voltage and current parameters are within nominal values. In case of overcurrent or overvoltage events, the MCU shutdowns the whole system and prevents any damage.

In voltmeter mode the MCU reads VIN to decide what VSEL logic to apply. In case VIN = 12 V — VSEL = LOW and if VIN = 24 V — VSEL = HIGH.

In ammeter mode the MCU reads and calculates the MOSFET load current to determine the power drawn. The output current signal is proportional to the current thought the n-channel MOSFET, please see Figure 4 for reference.

Since the IOUT pin is a current output there is a need for grounding resistor and capacitor to convert the current into voltage and then the MCU can measure it. The resultant voltage is proportional to the IOUT output current. The voltage is a product of: V_IOUT = R_IOUT x I_OUT.

The internal circuit has seven integral parts. U1 is the Atmega328 system MCU. The IRF7403 is a generic n-MOS transistor. The SLG55021 is a high-side MOSFET driver. U3 is the L78L05_5V voltage regulator. J1 is the system I2C slave communication port. J2 is the input voltage terminal. J3 is the circuit output voltage terminal.

For testing the circuit in the application, the host system was simulated and debugged using Proteus, which was used for the circuit design, programming of the MCU and performance analysis of the circuit. The circuit is not designed with tight timings because of the low voltage and current sample rate at 10 samples/second.

The test software algorithm is show in Figure 7. Operation starts with applying voltage to the circuit. By default, the MCU sets the GreenFET and MOSFET drivers in OFF state and waits for 250 ms before passing to the next step. After power up completion of the first step, the MCU begins to read the VIN (at rate of 10Hz) to verify if VIN meets operating ranges. In case VIN = 12 V, the MCU sets system at mode 0 and in case of VIN = 24 V, the MCU sets the system to enter mode 1. In case VIN > 25.5 V or VIN < 10.7 V, the MCU will return to step 1 and repeat the process. The High Voltage GreenFET current verification is done to ensure the load current draw doesn’t exceed the operating limit of the SLG59H1008V and prevents it from an overcurrent/short-circuit event. The current verification is implemented by reading the voltage at IOUT (VIOUT) pin, see Figure 5 for reference. If VIOUT < 3379 mV the system will go to the previous step (current sensor verification) to retry the process. If VIOUT > 3380 mV the system will disable all outputs and power OFF the High Voltage GreenFET and wait for 2000 ms before retrying the whole process.

To ensure functionality of the circuit below procedures were performed:

· Power ON Procedure

· Voltage Verification

· Current Sensor Verification

Turn ON Procedure

It’s possible to verify that control pins VON, VSEL, VVREG and VMVLine are LOW for the first 250ms after power is established.

After power ON, MCU begins to read the input voltage VIN to determinate correct SLG59H1008V configurations. The first pin to be toggled is the SEL (VSEL) to set the voltage lockout at 12 V or 24 V. For 24 V operation VSEL must be set HIGH and for 12 V operation VSEL must be set LOW. Then ON (VON) pin set VON = HIGH to turn on the SLG59H1008V and VON = LOW to turn it OFF. At last, the voltage bypass control MVLine (VMVLine) will be set HIGH if input voltage is 12 V or VReg (VVReg) will be set HIGH if input voltage is 24 V.

Current sensor verification is implemented by reading the current coming from the SLG59H1008V’s current sense output (IOUT) and comparing it to a pre-programmed max value in the MCU. Proper calibration of the MCU analog reference supply voltage is important to prevent erratic behavior at high current. The overcurrent state is maintained for about 100 ms since some loads have high start-up current.

The load current can be obtained by the following formula I = VIOUT / (RIOUT × 0.01) where VIOUT is the voltage generated at the SLG59H1008V IOUT pin and RIOUT is the grounding resistor at IOUT pin.

Figure 11 shows an example of a current curve. This current curve shows the various current zones. When the load current is between 0 A and 3.99 A the system is at normal operation. Once the load current is at 4 A the system is at a transition state, but operation is allowed since it’s at the 4 A maximum continuous operating current level. When the load current is greater than 4 A the system is at overcurrent state. This state is only allowed for a pre-programed 100 ms duration and allows current to reach 4.5 A, see Figure 10 for reference. If overcurrent state is maintained for more than 100 ms the system will power OFF the High Voltage GreenFET.

In this article, the SLG59H1008V GreenFET load switch is implemented in AVS application intended for use in systems that can work with different input voltage ranges. This GreenFET device helps to reduce application’s cost as all required functions for the system are already integrated into a single small IC, especially operating voltage range switch, current sensing and fault detection. When designing a circuit to implement this solution, attention is required to maximize PCB passive cooling efficiency.