COVER STORY
frequency before the filter can be correctly sized. The drawbacks of this path are the cost of BOM and real estate requirements - the board area and cost of the components required to achieve the level of filtering - as well as the need for overdesign - rating the component tolerances to compensate for changes over time and temperature.
Active protection using a surge stopper
One way to overcome the challenges and disadvantages of the passive protection solutions described is to instead utilise a surge stopper IC. A surge stopper eliminates the need for bulky shunt circuitry (TVS devices, fuses, inductors, and capacitors) with an easy to use controller IC and a series N-channel MOSFET. Surge stopper controllers can greatly simplify system design since there are few components to size and qualify. A surge stopper continuously monitors the input voltage and current. Under nominal operating conditions, the controller drives the gate of an N-channel MOSFET pass device fully on, providing a low resistance path from the input to the output. When an overvoltage or surge condition occurs - with a threshold dictated by a feedback network at the output - the IC regulates the gate of the N- channel MOSFET to clamp the output voltage of the MOSFET at the level set by the resistor divider.
Figure 5 shows a simplified schematic of a surge stopper implementation, along with the results of a 100V input surge on a nominal 12V rail. The output of the surge stopper circuit is clamped to 27V for the duration of the surge event. Some surge stoppers also monitor for overcurrent conditions using a series sense resistor (the circuit breaker in Figure 5), and adjust the gate of the N-channel MOSFET to limit the current presented to the output load. There are four broad types of surge stopper, classified by their response to an overvoltage event: • Linear surge stopper • Gate clamp • Switching surge stopper • Output disconnect protection controller
The choice of surge stopper depends on the application, so let’s compare their operation and advantages.
Surge stopper type: Linear A linear surge stopper drives the series MOSFET much like a linear regulator would, limiting the output voltage to the pre-
16 MAY 2021 | ELECTRONICS TODAY
31.5V gate clamp and a MOSFET threshold voltage of 5V, the output voltage is limited to 26.5V. Alternately, an external gate clamp allows a much wider range of voltages to be selected. An example of a gate clamp surge stopper is shown in Figure 7.
Figure 5: A high level diagram of a surge stopper implementation
Surge stopper type: Switching For higher power applications, a switching surge stopper is a good choice. Like linear and gate clamp surge stoppers, a switching surge stopper fully enhances the pass FET under normal operation to provide a low resistance path between the input and output (minimising power dissipation). The main difference between a switching surge stopper and a linear or gate clamp surge stopper appears when a surge event is detected. In the event of a surge, the output of a switching surge stopper is regulated to the clamp voltage by switching the external MOSFET much like a switching dc-to-dc converter.
Protection controller: Output disconnect
Figure 6: The LT4363, a linear surge stopper
A protection controller is not officially a surge stopper, but it does stop surges. Like a surge stopper, a protection controller monitors for overvoltage and overcurrent conditions, but instead of clamping or regulating the output, the protection controller disconnects the output immediately to protect downstream electronics. This simple protection circuit can have a very compact footprint, suitable for battery-operated, portable applications. The LTC4368 protection controller is shown in a simplified schematic in Figure 9, along with its response to an overvoltage event. Protection controllers are available in a number of variants.
Figure 7: The LTC4380, a gate clamp surge stopper
programmed safe value, dissipating excess energy in the MOSFET. To help protect the MOSFET, the device limits the time spent in the high dissipation region by implementing a capacitive fault timer.
Surge stopper type: Gate clamp The gate clamp surge stopper operates by utilising either an internal or external clamp (31.5V or 50V internal, for example, or an adjustable external clamp) to limit the gate pin to this voltage. The threshold voltage of the MOSFET then determines the output voltage limit. For example, with an internal
A protection controller operates by monitoring the input voltage to ensure that it remains within a voltage window configured by the resistor divider on the OV/UV pins, disconnecting the output via back-to-back MOSFETs when the input is outside this window, as shown in Figure 9. The back-to- back MOSFETs can also protect against a reversed input. The sense resistor at the output enables overcurrent protection capability by continually monitoring the forward current, but without a timer-based ride-through operation.
Surge stopper features In order to select the most suitable surge stopper for your application you will need to know what features are available and what challenge they are helping to solve.
Disconnect vs. Ride-through Some applications require disconnecting the output from the input when a surge event is detected. In this instance, overvoltage disconnect would be required. If you needed the output to remain operational in the face of surge events, thus minimising the downtime of downstream electronics, you
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