Hot-Swap Circuits with Inrush Current Limiter

 

To provide continuous current sensing and limiting, this hot-swap circuit uses a PNP transistor and a current sense resistor of 0.333R. When input power is applied, current flows through R1 and Q1 and into the load. 

Current flowing through R1 generates the VBE bias voltage for Q2. If the current is sufficient to bias Q2 ON, Q2 reduces Q1's gate-source voltage, limiting the current to the load. In this circuit, the maximum current limit is at 2.1A, as Vbe=0.7V and R1=0.333. A Zener diode is used to prevent overvoltage in Q1 and VCE breakdown in Q2.

Alternatively, a chipset solution from Maxim could be used. The MAX4370 is a circuit-breaker integrated circuit designed to provide protection in hot-swap applications using Maxim's DualSpeed/ BiLevel detection. 

The chip offers many features in a small package, and it requires few external components. The circuit offers inrush-current limiting and dual overcurrent fault protection, which consists of a fast response from the MAX4370 for high-amplitude circuit faults and a slow response for low-amplitude disruptive overcurrent conditions.

Protecting Microcontroller Input TINA (TI) Simulation

Over Current Protection at Output Stage/ Pins

 

The purpose of this circuit is to restrict the output current to less than 20mA, and this limit can be adjusted using resistors R1 and R8. Furthermore, the circuit also can disconnect the input/output when the output short circuits to a high positive or negative voltage by limiting the current. 

Additionally, the diode D1, located at the output and the 5V0 pin, serves to suppress the kick-back voltage from an inductive load. This occurs when the drivers are deactivated (stop sinking) and the stored energy in the coils causes a reverse current to flow into the coil supply through the kick-back diode.

Overvoltage Protection

 

This circuit monitors the input voltage (VIN) and safeguards the output voltage using a series-connected transistor (Q14). In the event of over-voltage conditions, Q14B is activated, which in turn causes Q14A to disconnect the output from VIN. The desired overvoltage limit is established by the combination of D10 and the voltage across the base emitter (VBE) of Q14. For instance, if the required cutout voltage is 12V, the Zener voltage of D10 should be 11.4V (12V - 0.6V). 

You can view a simulation of the above circuit in the video below.

 

CAN-BUS Termination and Protection

 A properly terminated CAN bus is terminated at each end with the characteristic impedance of the cable, this is typically 120Ω on each end for a 60Ω load on the CAN driver. The 60Ω–60Ω split termination is preferred to reduce high-frequency noise and common-mode drift. Usually, a 60R resistor is cost effective and a choice of 60.4R with 1% tolerance is better.

If CAN-BUS communication is chosen for long distances of more than 50m, it is advised to reduce the bitrate to minimize errors and signal distortions.

The maximum possible CAN Bitrate depends on the total CAN bus cable length: 

- 1 Mbit/s: max. 40 m

- 500 kBit/s: max. 100 m

- 125 kBit/s: max. 500 m  

A varistor should be added near the connectors to enhance CAN Bus reliability.

If the device is intended to be used in a harsh environment and requires isolation due to ground potential differences between subsystems, a Galvanic isolation barrier can be placed between the CAN-BUS transceiver and the MCU. Using an isolated DC-DC power supply for the transceiver is important.

Basics of eFuses

 eFuses serve as integrated power path protection devices designed to limit circuit currents and voltages to safe levels during fault conditions. These components offer numerous advantages to the system, including protection features that can be challenging to implement using discrete components. In contrast, discrete solutions tend to be bulky, inefficient, and lack thermal protection, necessitating more components and occupying additional board space. These factors can raise concerns about the robustness and reliability of protection circuits.

While various types of eFuses are available in the market, I particularly favour TPS259271 from TI. 

 This eFuse provides a robust defence against overloads, short circuits, voltage surges, excessive inrush current, and reverse current. Additionally, it offers Controlled Power Down, Over-Temperature Protection, and Health Monitoring and Reporting features.

Soft-Start and Protection Circuit

When the input is below 18 V, Q1 is OFF state allowing C3 to be charged though R3. Thus turning on Q105. Once the input voltage exceed 18 plus two diode drop Volts, Q1 will turn on that discharge C3 and Q105 is OFF.

Once the input goes back below 18 V, Q1 is turn off again. This allows C3 to be charged slowly resulting soft-start at the output.

Micro-controller Output Protection by Current Limiting

Q1B is the pass or output transistor. R2 sense the output current. Q1A  is the protection transistor which turns on as soon as the voltage across R2 becomes about 0.65 V.
Maximum current = VBE,Q1A / R2 = 0.65 / 33 = 19.7 mA

Reverse Polarity Protection

VGS (th), Gate Source threshold voltage = -2.0 V
Zener Voltage = 10 V, Q1-VGS max = +/- 20V 
Zener diode, D3 will protect the gate from excessive voltages/ unwanted spikes

By referring the Gate signal to the ground line, the device is fully turned on when the battery is applied in the right polarity. For the first start-up, the intrinsic body diode of the MOSFET will conduct, until the channel is switched on in parallel. The Zener diode will clamp the Gate of the MOSFET to its Zener voltage and protect it against overvoltage. By reverse polarity, the MOSFET will be switched off, because the Gate Source voltage for this case will be positive (voltage drop over the Zener diode). 

Protecting microcontroller Inputs and outputs

1) Filtering

This circuit can be used to protect the I/Os of microcontrollers/ processors.  D1A is used for Transient Voltage Suppression or ESD protection. R1 acts as a current limiting resistor as well as a low pass filter together with C1. The value of the resistor and the capacitor must be sized so that the microcontroller does not miss any signals.

The rise time of the fastest income edge = 2.2 RC

2) Limiting Current

This circuit limit the input current to 23.2 mA (VZ/R1 = 5.1 / 220).

3) Limiting Voltage

Use Schottky diodes with 0.2 Vf for better performance. Once the voltage at the I/O pin is greater than VCC by about 0.2V, the top diode will start to conduct. The bottom diode will conduct for a voltage less than -0.2V.

Short Circuit Monitoring

Comparator Circuit for Open/Short Detection

The Comparator Circuit for Open/Short Detection works as follows:

1. Open Circuit Detection: When there is an open circuit across the input connector, the voltage at V- will be approximately 5.1V, which is higher than the V+ voltage of 2.5V. As a result, the output to the microcontroller will be Low (0V).

2. Short Circuit Detection: In the event of a short circuit across the input connector, the voltage at V- will drop below 2.5V, causing the output to go High (5V).

In addition to open and short circuit detection, this circuit can also be utilized as a simple continuity test by connecting an LED and Buzzer at the output.