Design and simulation for ov/uv/overcurrent/reverse polarity

The project focuses the design and simulation an undervoltage, overvoltage, overcurrent, and reverse polarity protection with LTC4368. This small 3x3mm chip acts as a controller for 2 N-Channel MOSFETs, through which the power is controlled. When the LTC chip senses any abnormaities in the incoming power, it opens the MOSFETs, cutting off power to the rest of the circuit, protecting everthing downstream. The LTC chip is particularly useful in circuits powered by battery, namely to protect against: 

• Reverse polarity (plugging the battery in the wrong way) 

• Overvoltage (Plugging in a battery of a higher voltage than designed or a discharged battery) 

• Undervoltage (Plugging in a battery of a lower voltage than designed) 

• Overcurrent (Shorts or sudden power draw excedding the capacity of downstream components)

LTC4368

The LTC4368 is a 100V UV/OV and Reverse Protection N-channel MOSFET Controller with Bidirectional Circuit Breaker that protects applications from overvoltage, undervoltage and overcurrent faults in both forward and reverse directions, as well as reverse polarity. The LTC4368 controls the gate voltage of a pair of external N-channel MOSFETs to ensure that the load is connected to the input supply only when there are no voltage or current faults. 

Two comparator inputs allow configuration of the overvoltage (OV) and undervoltage (UV) set points using an external resistive divider. A current sense resistor sets the forward and reverse circuit breaker current thresholds. After a forward current fault, the LTC4368 will either latchoff power, or retry after a user adjustable delay. After a reverse current fault, the LTC4368 waits for the output to fall 100mV below the input to reconnect power to the load. 

It has a wide range of operation with 100V overvoltage protection and -40V reverse supply protection. The bidirectional electronic circuit breaker has a +50mV forward sense threshold and a -50mV reverse for LTC4368-1, –3mV Reverse for LTC4368-2.

Schematic

The overview of schematic is shown below. The board features overvoltage, undervoltage, overcurrent and reverse polarity protection. With 12V input, the overvoltage (OV) and unvervoltage (UV) threshold is set to 18V and 6V respectively. The SI7942DP-T1-E3 MOSFET dual N-channel was selected for reverse polarity protection. Since most of ICs on the board don’t exceed 1A, the current limit is set to 5A. The retry time is set to 121ms. The fault pin is connected to a red LED for fault indicator.

Voltages at VIN outside of the 6V to 18V range are prevented from getting to the load and can be as high as 100V and as negative as –40V. Load currents above 5A (forward from VIN to VOUT) and below –300mA (reverse from VOUT to VIN) will cause the load to be disconnected from VIN. The circuit below protects against negative voltages at VIN as shown. A more detail of the component selection and calculation will be discussed in next section.

Overcurrent Protection

A current sense resistor sets the forward and reverse circuit breaker current thresholds. After a forward current fault, the LTC4368 will retry after a set time. After a reverse current fault, the LTC4368 waits for the output to fall 100mV below the input to reconnect power to the load. 2 For overcurrent, the current sense resistor of 10mΩ was selected to set current limits at +5A in forward direction and -300mA. This limit was selected because most of IC on flight computer didn’t exceed 1A but a higher limit can be set using a smaller value for the sense resistor. Forward overcurrent protection prevents large currents from flowing from VIN to VOUT. This threshold current is determined by the external sense resistor (RSENSE) and an internal comparator with a 50mV threshold.

If 5A flows to the output across the 10mΩ sense resistor, the external MOSFETs are immediately turned off. This disconnects the load from the input supply. Reverse overcurrent protection prevents large currents from flowing from VOUT to VIN. While the LTC4368-1 (–50mV) bidirectional circuit breaker allows load current to flow in either direction: from VIN to VOUT or from VOUT to VIN, the LTC4368-2 provides diode-like behavior by making the reverse overcurrent threshold (–3mV) significantly smaller than the forward overcurrent threshold (+50mV). The reverse overcurrent fault threshold is determined by the external sense resistor (RSENSE) and an internal comparator. 

If –300mAh flows from the output across the 10mΩ sense resistor, the external MOSFETs are immediately turned off. To turn the MOSFETs back on, an internal comparator detects when VOUT drops 100mV below VIN: VOUT < VIN – 100mV. Once this condition is met, the gates of the external MOSFETs are turned on again to reconnect the input supply to the load. The overcurrent trip occurs when the SENSE to VOUT voltage exceeds 50 mV for the forward direction (VIN to VOUT) and -3mV for negative direction (VOUT to VIN).

Gate Resistor and Output Capacitor

Since SI7942 has a lower maximum total gate charge Qg (24nC), meaning it requires less additional capacitance for stability, a 2.2nF capacitor was chosen for gate capacitor. Meanwhile, other MOSFET usually has higher Qg around 80 to 100nC and require a higher gate capacitor (3.3 to 4.7nF). A smaller gate capacitor means faster reverse protection. For output capacitors, multiple capacitors (distributed capacitance) was used instead of a single one to reduce ESR, ESL, and spread startup current. Disributed capacitance also makes inrush easier to regulate, improves transient response, lowers voltage overshoot, offers greater stability of the LTC4368 control loop. In addition, 10Ω resistor at R7 and R8 are added to the MOSFET gates to prevent circuit oscillations The gate resistor R1 prevents gate capacitor from slowing down the reverse polarity protection circuits. It also stabilizes the fast pull-down circuits and prevents chatter during fault conditions. The resistor value of 22kΩ is recommended for most applications.

Overvoltage (OV) and Undervoltage (UV)

The LTC4368 provides two accurate comparators to monitor for overvoltage (OV) and undervoltage (UV) conditions at VIN using an external resistive divide. If the input supply rises above the OV threshold, the gates of the external MOSFETs are quickly turned off, thus disconnecting the load from the input. Similarly, if the input supply falls below the UV threshold, the gates of the external MOSFETs are quickly turned off. The external resistive divider allows the user to select an input supply range that is compatible with the load at VOUT. This configuration comprises of three resistors (R1, R2, R3) with R3 connected to VIN and UV, R2 connected to UV and OV and R1 connected to OV and GND. The datasheet provided list of equations to help select the resistor values for the resistive divider shown in schematic. This minimizes UV and OV offset errors caused by leakage currents at the respective pins

Reverse Polarity

The LTC4368’s VIN helps protect circuits at the output load. If the input supply is plugged in backwards, or a negative supply is connected, the LTC4368 prevents this negative voltage from passing to the output load. As shown in schematic, external back-to-back N-channel MOSFETs are required for reverse supply protection. When VIN goes negative, the reverse VIN comparator closes the internal switch, which in turn connects the gates of the external MOSFETs to the negative VIN voltage. The body diode of MOSFET Q1A turns on, but the body diode of Q1B remains in reverse blocking mode, meaning that the common source connection of Q1A and Q1B remains about a diode drop higher than VIN. Since the gate voltage of Q2B is shorted to VIN, Q2B will be turned off and no current can flow from VOUT to VIN. To avoid large currents when the reverse voltage happens, the gate resistor is set to 22kΩ

RETRY

Retry Input is connected to ground to latch off the MOSFETs after a forward overcurrent fault. The RETRY is connected to an external capacitor of 22nF to configure a 5.5ms/nF delay before the MOSFETs automatically turn on again. This gives us 121ms for a retry. Some applications use 220nF (1200 ms) to protect MOSFET from heating during repeated faults, but a 1-second delay is too slow for systems that need fast recovery. Others choose very small capacitors, such as 0.1uF for 5.5 ms, allowing rapid restart after transient events and minimizing downtime but risking MOSFET overheating if the short persists. With that, the 121ms of retry time is the most sufficient choice as it balances between protection and responsiveness, as well as prevents chattering.

LTSpice SIMULATION

To simulate the behavior of the circuit, a program called LTSpice is used to perform a DC Sweep Analysis, which involves setting a range of voltages and sweeping through them at regular time intervals. Then the voltage and current can be plotted at different locations around the circuit

In this simulation, the input voltage VIN starts at -40V at t = 0s, is held at that voltage for 200ms, and then the voltage is raised over time, reaching 70V at t = 1.7s. After reaching peak voltage, it is then reduced over time, reaching 24V at t = 2.0s, where it is then held for the rest of the simulation. This tests 3 of the 4 protections the LTC4368 provides, that being under and overvoltage protection, as well as reverse polarity protection.

There is also a simulated load, represented by V2 in the schematic. This waits until t = 2.2s to apply a near-instantaneous load of 1V, causing a large current draw, to test to overcurrent protection of the chip

The green line represents VIN, which is the voltage being input into the simulation, while the blue line is the output voltage VOUT. The red line represents the current output IOUT 

From t = 0s to t = 0.75s, the voltage is negative, so both VOUT and IOUT are zero, as the polarity is reversed, and thus the LTC4368 has cut off the output. From t = 0.75s to t = 1.25s, VIN = VOUT, as the MOSFETs are passing the power through. At t = 1.25s, VIN is greater than 36V, and this is the overvoltage protection that is tripped, causing VOUT and thus IOUT to drop to zero. As VIN dips back below 36V at t = 1.9s, VOUT once again equals VIN. At t = 2.2s, the simulated load is applied, causing the current to spike to 24A nearly instantly, which sets off the overcurrent protection, causing VOUT to drop to zero. VOUT remains zero for a further 1.2 seconds, per the data sheet, and finally at t = 3.4s, VOUT is reconnected to VIN, and the circuit remains steady for the rest of the simulation.