【Introduction】Traditionally, fuses are used for overcurrent protection. However, fuses are bulky, slow to respond, have large trip current tolerances, and need to be replaced after one or several trips. This article presents a compact, thin, and fast-response 10 A Electronic fuse that does not have the disadvantages of passive fuses described above. electronic fuses provide overcurrent protection on DC rails up to 48 V.
To minimize system downtime caused by electrical faults, high-utilization power supplies or systems that operate 24/7 require added overload and short-circuit protection on the power strip. When a power supply is powering multiple subsystems or boards, such as RF power amplifier arrays or backplane-based servers and routers, overcurrent protection must be provided for the power supply. Quickly disconnect the failed subsystem from the shared power bus, allowing the remaining subsystems to continue operating normally without rebooting or taking them offline.
Traditional overcurrent protection (OCP) is based on fuses, but they are bulky, slow to respond, have large tolerances, and require replacement after one or more trips. Integrated circuit OCP solutions for DC power supplies, also known as electronic circuit breakers or electronic fuses, overcome these fuse shortcomings. To save board space and have the simplicity of passive fuses, electronic fuses contain power MOSFET switches and control circuitry is integrated in the same package.
Surge suppressor with internal power MOSFET
A surge suppressor is an integrated circuit device that controls an N-channel power MOSFET in a power supply line placed on a DC power supply (eg 12 V, 24 V or 48 V) and needs to withstand input voltage and load current surges between the system electronics. Built-in output current and output voltage limits enable surge suppressors to protect load electronics from high voltage input surges and protect the power supply from downstream overloads and short circuits. Adjustable timers activate during voltage or current surge limiting events, keeping the system uninterrupted and running continuously for brief failures.
If the duration of the fault exceeds the timer time, the system is powered off.
The LTC4381 is the first surge suppressor with an internal power MOSFET. It can operate from supply voltages up to 72 V, but consumes only 6 µA of quiescent current. Internal power MOSFETs provide 100 V drain-to-source breakdown voltage (BVDSS) and 9 mΩ on-resistance (RDS(ON)) to support input surges up to 100 V and 10 A applications. The LTC4381 offers four options to select fault restart behavior and fixed or adjustable output clamp voltage.
Figure 1. 48 V, 10 A Electronic Fuse and LTC4381
The surge suppressor function of the LTC4381 is easily expandable and can be used as an electronic fuse. Figure 1 shows the LTC4381-4 in a 48 V, 10 A electronic fuse application that protects the power supply from overload or short circuit on the output. During normal operation, the output VOUT is connected to the power supply input VIN through the internal power MOSFET and external sense resistor RSNS. During output overload or short circuit, when the RSNS voltage drop exceeds the 50 mV current limit threshold, the TMR pin capacitor voltage begins to rise from 0 V and the internal MOSFET turns off when the TMR voltage reaches 1.215 V (more on this later). The 4 mΩ RSNS sets the typical overcurrent threshold to 12.5 A (50 mV/4 mΩ) and the minimum threshold to 11.25 A (45 mV/4 mΩ), providing enough headroom for a 10 A load current.
Figure 2. LTC4381 10 A Fuse Circuit Using (a) 48 V (Left) and (b) 60 V (Right) Supplies to Start 220 µF Load Capacitors
Due to the parasitic inductance of the circuit or cable returning to the circuit, when the internal MOSFET switch is turned off during current flow, the input voltage can skyrocket above the nominal operating voltage. Zener D1 protects the 80 V absolute maximum rating of the LTC4381 VCC pin, while D2 protects the internal 100 V MOSFET from avalanche. D1 also sets the output clamp voltage to 66.5 V (56 V + 10.5 V) in case D2 is not used. R1 and C1 filter VIN rising and falling. If there is a capacitor close to the LTC4381 to limit the voltage spike, below 80 V, the VCC pin can be connected directly to VIN. In this case, the use of D1, D2, R1 and C1 can be eliminated.
During normal operation, with 10 A flowing through the internal MOSFET, the LTC4381 initially drops 90 mV and dissipates 900 mW. However, at room temperature, this power dissipation increases the temperature of the LTC4381 package on the DC2713A-D evaluation board to about 100°C, twice the RDS(ON), and increases the voltage drop to 180 mV . The 4 mΩ sense resistor drops another 40 mV at 10 A. It may be necessary to consume more copper, especially at the SNS node, to reduce the temperature rise of the LTC4381. The DC2713A-D SNS node uses 2.5 cm2 2 oz copper, which is evenly distributed on the two outer layers of the board, the information above is for reference.
Figure 3. Safe operating area for the LTC4381 MOSFET.
When the ON pin is not connected to ground, the circuit in Figure 1 enables a 220 µF load capacitor, as shown in Figure 2, for both 48 V and 60 V supplies. Assume that 60 V is the upper end of the operating range of the 48 V supply. Assuming no additional load current during startup, 220 µF is the maximum load capacitance that this 10 A current can safely charge. When the 220 µF capacitor is charged to 60 V with the 12.5 A current limit, the inrush time is 220 µF × 60 V/12.5 A = 1.06 ms. The safe operating area (SOA) diagram of the LTC4381 MOSFET, shown in Figure 3, shows that it can operate normally for 1 ms at 12.5 A and 30 V. 30 V is used because it is the average input-output differential voltage, starting at 60 V and dropping to 0 V after that.
With no GATE pin capacitor to slow its ramp rate, the output charges within 2 ms and the inrush current reaches 17 A peak, exceeding the current limit threshold (see Figure 2), before being controlled. The LTC4381 has a 50 mV current limit detection threshold, or 12.5 A when the voltage at the OUT pin is >3 V with a 4 mΩ sense resistor, but when the voltage at the OUT pin is <1.5 V (as shown in Figure 4), It will rise to 62 mV or 15.5 A. The graph also shows that during startup, if the electronic load current in the sense resistor drops by more than 20 mV (5 A at 4 mΩ), the output remains at 2 V (and the TMR times out).
The waveforms in Figure 2 show that the inrush current pulses are instead regulated due to the lack of the 47 nF gate capacitance required to keep the loop stable. In fact, during a 60 V inrush, the current is switched off for about 0.5 ms. The LTC4381 TMR pull-up current is proportional to the power dissipation in the internal MOSFET. Therefore, even if the current is below the current limit threshold, during the startup inrush, the TMR will rise. We deliberately removed the gate capacitors to use small TMR capacitors so that the 220 µF load capacitor still started successfully. The small TMR capacitor protects the MOSFET during short-circuit faults, which we will discuss in detail in the next section.
The smallest TMR capacitor is 68 nF and keeps the TMR voltage rising to around 0.7 V during 60 V startup. For example, choosing a TMR capacitor of 47 nF allows the TMR to reach 1.15 V during 60 V startup, which is very close to the 1.215 V gate turn-off threshold. The 0.7 V peak TMR target voltage was chosen to provide sufficient headroom from the 1.215 V gate turn-off threshold, while using these tolerances: TMR pull-up current ±50% (ITMR(UP) specification in the LTC4381 data sheet), TMR capacitance ±10%, ±3% of 1.215 V TMR gate turn-off threshold (VTMR(F) specification).
Figure 4. LTC4381 Current Limit vs Output Voltage
Table 1 lists the TMR capacitors recommended for the maximum load capacitance to limit the TMR voltage rise to around 0.7 V during 60 V startup.
Table 1. Recommended CTMR for CLOAD(MAX).
Output Short-Circuit Behavior
The circuit in Figure 1 is primarily used to protect upstream power supplies from downstream faults such as overloads and short circuits, either during startup or during normal operation. Figure 5 shows the LTC4381 turning on its MOSFET when there is a short at the output. The gate voltage (blue curve) increases. When the 3 V threshold voltage is exceeded, the MOSFET turns on and current (green curve) begins to flow. With the output shorted and with no gate capacitance, the MOSFET circuit ramps up quickly, exceeds the current limit threshold of 15.5 A at 0 V output, and reaches a peak of 21 A before the LTC4381 reacts, pulling down the MOSFET gate and turning off current flow . The current exceeds 15.5 A for less than 50 µs. Due to the brief power dissipation in the MOSFET, the TMR voltage (red curve) rises by about 200 mV. Since the TMR is well below the 1.215 V gate turn-off threshold, the gate turns on again, causing another current spike. At each current spike, the TMR voltage rises to nearly 1.215 V.
Figure 5. LTC4381 with 48 V power on into output short
After several such current spikes, the TMR voltage reaches the gate turn-off threshold of 1.215 V and the MOSFET remains off. The TMR now enters a cooling cycle and the LTC4381-4 does not allow the MOSFET to turn on again until the cooling cycle is complete. According to Equation 8 in the LTC4381 data sheet, the cooling cycle duration for a 68 nF TMR capacitor is 33.3 × 0.068 = 2.3 s. Because of the LTC4381-4 auto-retry, this pattern of current spikes and cool-down cycles will repeat indefinitely until the output short is cleared. This pattern will repeat if an output short circuit occurs during normal operation (ie, the output is enabled). Note that the LTspice® simulation does not show the behavior shown in Figure 5 unless a 4 µH input rail inductance is added.
The LTC4381’s internal power MOSFETs provide a compact circuit for electronic fuses or circuit breakers in 48 V, 10 A systems. In this way, there is no need to spend time selecting power MOSFETs during the design phase. The SOA of the LTC4381 MOSFET is production tested and each device can be guaranteed quality, which is not provided with discrete MOSFETs. This helps build a reliable solution to protect expensive electronics in servers and network equipment.
The 10 A circuit discussed in this article has some peculiar behavior that should be noted due to the lack of gate capacitance of the stabilization loop. Specifically, during a short circuit, there are no inrush and pulse currents that are controlled by conventional dV/dt. However, these are brief, instantaneous events lasting less than a few milliseconds. Input bypass capacitors can help prevent any interference with the 48 V supply, especially if it is shared with other boards, such as the backplane. In the latter case, the load capacitance of the adjacent board also plays the same role as the input bypass capacitance.
About the Author
Pinkesh Sachdev is a senior applications engineer on the Cloud Power team at Analog Devices. He holds a bachelor’s degree in electrical engineering from the Indian Institute of Technology (Mumbai, India) and a master’s degree in electrical engineering from Stanford University. Contact: [email protected]