In automotive, industrial, and avionic applications, high voltage power supply spikes with durations ranging from a few microseconds to hundreds of milliseconds are commonly encountered. The electronics within these systems must not only survive transient voltage spikes, but in many cases also operate reliably throughout the event.
In systems where power is distributed over long wires, severe transients are generated by load steps (abrupt changes in load current). Negative load steps happen when load current drops from a high value to a low value. Negative changes in current (dI/dt) cause the wire's parasitic inductance to generate a positive-going high voltage spike which can cause damage to neighboring devices powered from the same wire.
High values of dI/dt are produced by fast load switching, such as caused by relays, switch contacts, and solid state load switching. Corroded connections between a power source and load can lead to an abrupt interruption of current flow, and a high value of dI/dt. The best example of this condition is the automotive load dump, where there is a sudden break in the connection to the battery caused by vibration and corroded terminals.
A worst case
Load dump causes a voltage surge that stays elevated for hundreds of milliseconds (see below). The amplitude of the transient, according to the Society of Automotive Engineers (SAE), may be as high as 125V. A typical load dump profile has a rise time of 5 milliseconds and decays exponentially with a time constant of 200 ms. In industrial systems similar events can be caused by regeneration in solenoids and motors.
Electronic circuits have become more complex in automobiles, and they must be reliable. In addition, sophisticated consumer electronics such as smart phones, laptops, MP3 players, GPS, and data entry devices that charge through automobile power points (cigarette lighters) must also protect their products from both repetitive transients and unexpected voltage spikes. Inadequate protection from high voltage transients leads to degraded performance or failure and costly replacement.
These transients pose a difficult challenge for engineers focused on protecting sensitive electronics. Historically, this protection was achieved using bulky capacitors, TVS (transient voltage suppression) diodes
, and fuses, but this discrete solution consumes a lot of real estate, and may be impractical.
But modern developments can address these challenges. Take for instance the Linear Technology LT4356
surge stopper, which operates from 4 to 80V and provides -60V of reverse protection on the input pins. During an overvoltage transient, the output clamps to a user-defined voltage, defined by the resistor divider network on the output. The LT4356 is capable of suppressing surges >100V as long as a resistor and TVS diode is used at the input to avoid exceeding the absolute maximum operating voltage (see below). Because the current sensing circuitry is upstream of the MOSFET, overcurrent protection must be disabled if the device is used to protect from transients above 100V.
The LT4356 withstands 150V at input.
Two new devices have recently been added to the company’s surge stopper family, the LTC4366
high-voltage floating surge stopper and the LT4363 high -voltage surge stopper with overcurrent protection. The former is designed for systems that continuously operate at voltages above 100V, or where protection from extremely high voltage transients (>200V) is required (see below).
is a second generation version of the LT4356, moving the overcurrent sensing downstream of the pass FET so that it provides overcurrent protection while withstanding voltage transients greater than 100V (see below).
The absolute maximum rating for the LT4363 is 100V, so the input must be protected from high voltage transients >100V using a resistor and TVS diode as shown in the figure prior to the one immediately above. In contrast, the LTC4366 uses a floating topology—external voltage dropping resistors allow it to float up with the supply, isolating it from the high voltage surge. The upper limit on the operating voltage is only limited by the availability of the high valued resistors and sizing the MOSFET to handle the power dissipated during voltage regulation.