COVER STORY
operational while protecting the MOSFET from damage by longer-lasting overvoltage events. Some devices feature a retry function,
Figure 8: The LTC7860, a switching surge stopper
would require the surge stopper to ride through the surge event. In this case, a linear or a switching surge stopper can achieve this functionality (provided the power levels were reasonable for the topology and FET selected).
Fault timer
Ride-through operation requires some protection for the MOSFET against persistent surges. To remain within the safe operating area (SOA) of the FET, a timer can be implemented. The timer is essentially a capacitor to ground. When an overvoltage condition occurs, an internal current source starts to charge this external capacitor. Once
enabling the device to turn on the output again after a cool down period.
Overcurrent protection Many surge stoppers have the ability to monitor current and protect against overcurrent events. This is achieved by monitoring the voltage drop across a series sense resistor
and responding appropriately. Inrush current can also be monitored and controlled to protect the MOSFET. The response can be similar to an overvoltage condition, as it ei- ther disconnects by latching off or riding through the event if the circuitry can handle the power levels.
Figure 9: The LTC4368, a protection controller
the capacitor reaches a certain threshold voltage, a digital fault pin pulls low to indicate the pass transistor will soon turn off due to the extended overvoltage condition. If the timer pin voltage continues to rise to a secondary threshold, the GATE pin pulls low to turn off the MOSFET.
The rate of change of the timer voltage varies with the voltage across the MOSFET - that is, a shorter timer for larger voltages and a longer timer for smaller voltages. This useful feature enables the device to ride through short overvoltage events, allowing downstream components to remain
Reverse input protection Reverse input protection is possible due to the wide operating capabilities of the surge stopper devices (capable of withstanding up to 60V below ground potential on some devices). Figure 10 shows a back-to-back MOSFET implementation of reverse current protection. During normal operation, Q2 and Q1 are turned on by the GATE pin, and Q3 doesn’t have any impact. However, when a reverse voltage condition exists, Q3 turns on, pulling Q2’s gate down to the negative input and isolating Q1, protecting the output. Reverse output voltage protection is also achieved with robust device pin protection, with up to 20V below ground potential possible, depending on the device selected. For applications that require wide input voltage ranges, a floating topology surge stopper can be used. When a surge event occurs, the full surge voltage is seen by the surge stopper IC so the internal transistor technology limits the voltage range of the IC. With a floating surge stopper such as the LTC4366, the IC floats just below the output
Figure 11: LTC4366 high voltage floating topology
Figure 10: LT4363 reverse input protection circuit
voltage, giving it a much wider operating voltage range. A resistor is placed in the return line (VSS), which allows the IC to float up with the supply voltage. The result is an input voltage limitation set by the voltage capability of the external components and MOSFETs. Figure 11 shows an application circuit capable of operating from a very high dc supply while protecting the downstream load.
Choosing the right device for my application
In many ways, because of their inherently robust design, using a surge stopper simplifies protection circuit design. Data sheets can help greatly with sizing components, with many possible applications already shown.
As with all product selection, it is important to understand your system requirements before looking for the correct device. Some important considerations are the expected supply voltage and the voltage tolerance of downstream electronics (important for deciding the clamp voltage), as well as any particular features that are important for the design.
Regardless of the surge stopper type implemented, active, IC-based surge stopper designs eliminate the need for bulky TVS diodes or large profile inductors and capacitors for filtering. This results in an overall smaller area and a lower profile solution. The output voltage clamp is more accurate than a TVS with 1% to 2% accuracy possible. This prevents overdesign and allows for downstream
devices with tighter tolerances to be selected.
Analog Devices
www.analog.com
MAY 2021 | ELECTRONICS TODAY 17
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