Longer Lasting Protection from Surges

A fairly recent innovation that can increase the usefulness of surge stoppers is the switching surge stopper, which is able to keep the circuit operating during longer surges.

Electrical surges can be devastating to electronics equipment. They can happen when equipment is plugged in, unplugged, or even from external factors, such as lightning. There are four different types of surges that can affect electronic equipment - forward voltage, reverse voltage, forward current and reverse current. The protection that each circuit will need is dependent on the application and how it has been designed. To counter this threat, there are different types of components that can be used, for example, e-fuses, load switches, diodes, surge stoppers and high-side switches. These measures can be effective at stopping one, some or all of the surge types. However, most applications will use several types of protection to cover all eventualities. For example, a single circuit could use a transient voltage suppressor (TVS) for overvoltage protection, an in-line fuse for overcurrent protection, capacitors and inductors for filtering low energy spikes and a series diode for reverse battery protection. Although this type of protection is effective, it adds to the size, weight and cost of the design. The measures also bring other problems, such as the diode dissipating power making the circuit less efficient, and when the TVS or fuse does its job, it may stop cirvcuit operation until it can be replaced or repaired manually.

Another way of tackling protection is by using a surge stopper, which in practice is an IC that drives an external MOSFET. The surge stopper is capable of dealing with most types of surge. If there is a surge, the surge stopper forces the MOSFET into a state where it is half open, limiting its output voltage to a higher level than normal, but to a level that the circuit can handle. Surge stoppers have the capability of holding the output at this level for a period of time, as opposed to a fuse, which will just blow or trip. When the IC detects the surge is over, it will then switch the MOSFET to operate as normal. The amount of time that the circuit can be held half open depends on the MOSFET selection. As this process dissipates a lot of heat, the MOSFET should be overspecified to stop heat from destroying it. The surge protection limits of the surge stopper can be set to a single value, or allowed to float. The silicon is protected by resistors on all pins and it floats according to the gate voltage. If the resistors are selected properly, then the floating surge stopper can be used against surges of 1000s of volts.

A practical example of a typical surge stopper is Analog Devices’ LTC4366, which has been designed to protect loads from high voltage transients. Using an external N-channel MOSFET, the LTC4366 regulates the output during an overvoltage transient. The circuit continues to operate as normal, while the overvoltage is dropped across the MOSFET. Placing a resistor in the return line isolates the LTC4366 and allows it to float up with the supply meaning that the upper limit on the output voltage depends only on the availability of high valued resistors and the MOSFET ratings. The device features an adjustable overvoltage timer to prevent MOSFET damage during the surge while an additional 9-second timer provides for MOSFET cool down. A shutdown pin reduces the quiescent current to less than 14µA during shutdown. After a fault the LTC4366-1 latches off while the LTC4366-2 will auto-retry.

A fairly recent innovation that can increase the capabilities of surge stoppers is the switching surge stopper, which is able to keep the circuit operating normally during longer surges. It can do this using a smaller, cheaper MOSFET that does not need to be overspecified like a regular linear surge stopper. The switched surge stopper achieves this by taking the high surge input voltage and operating like a buck regulator, stepping the voltage down efficiently and fixing the output voltage closer to the expected output. The switching surge stopper differs from a regular buck regulator in that it can perform at a 100% duty cycle. As most of the time, input voltage equals output voltage, the switching action only starts happening if there is a surge. A buck regulator is typically built with a maximum duty cycle of around 90% and is not intended for 100% duty cycle operation constantly. Another feature that distinguishes a switching surge stopper from a buck regulator is it will typically have additional pins to offer relevant features, such as looking for surges or indicating that power is good.

In real terms, a typical switching surge stopper would be Analog Devices’ LTC7860, which allows operation with higher currents and permits smaller solution sizes. During an input overvoltage event, the LTC7860 controls the gate of an external MOSFET to act as a switching DC/DC regulator to bring the output voltage to a safe level. During normal operation, the LTC7860 turns on the external MOSFET continuously, passing the input voltage through to the output. An internal comparator limits the voltage across the current sense resistor and regulates the maximum output current to protect against overcurrent faults.

An adjustable timer limits the time that the LTC7860 can spend in overvoltage or overcurrent regulation. When the timer expires, the external MOSFET is turned off until the LTC7860 restarts after a cool down period. By strictly limiting the time in PROTECTIVE PWM Mode when the power loss is high, the components and thermal design can be optimized for normal operation and safely operate through high voltage input surges and/or overcurrent faults. An additional PMOS can be added for reverse battery protection.

The LTC7860 uses a p-channel MOSFET, and its sister device, the LTC7862 works similarly but uses an n-channel device. Both types of MOSFET are effective in the role, and both have pros and cons. Frederik Dostal, Subject Matter Expert, Power Management, at Analog Devices explains, “p-channel MOSFETs are more expensive than N channel MOSFETs and have a higher Rds(on), which causes a higher voltage drop. However, they are much easier to drive. N-channel MOSFETs are more difficult to drive, but cheaper due to not requiring a well during the manufacturing process. There are more n-channel MOSFETs to choose from, and they are also more efficient due to their lower Rds(on). Driving an external n-channel MOSFET requires a built in charge pump to make sure the MOSFET is always turned on and running.”

www.analog.com