Back to Basics: Why do we need active surge suppression?

December 7, 2013
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It’s well understood that unexpected electrical disturbances can cause data loss or even permanent damage to any unprotected electronic equipment in their path. Active as well as passive suppression is essential to provide comprehensive protection from all such events. This post shows why this is so, and how active suppression works to fulfil its role.

Electrical events threaten electronic equipment by placing excessive electrical energy onto its input power rails in the form of a transient, a surge, or possibly both. A transient is typified as a fast-rising voltage spike of high magnitude – up to, say, 1800 V but of limited duration, typically around 50 μs. As this short duration limits their energy, they can be successfully managed with a passive Transient Voltage Suppressor (TVS) such as a Zener diode, MOV or varistor. With a clamping voltage chosen to be below the high line voltage or transient limits of the input power converter, this shunts the higher voltage energy away from the protected device. A TVS able to withstand 1.5 J is suitable for many applications.

Transients are caused by events such as lightning strikes, and start-up or shut down of inductive loads close to the protected equipment. Surges are created by train braking, or by fuse clearing. Often found in telecoms applications, this occurs after a fuse has short-circuited, allowing buildup of a large inductive field. When the fuse is cleared (opened), the field collapses, inducing a surge in the adjacent cabling. Similar load dumping can occur in vehicles, if a battery becomes disconnected during high engine speeds. 100 ms or more can elapse before the alternator regains control.

Voltage surges are very different from transients. Although measured in tens rather than hundreds or thousands of volts, they can last for 20 ms or more – and originate from a source impedance as low as 0.2 Ω. This is too much energy for a TVS to handle, so an active suppression circuit must be provided. A simple example is shown in Fig. 1.

active-transient-and-surge suppression-circuit

Fig 1: An active circuit to prevent fast transients and surges

Although called an active suppression circuit, its first stage, D1, actually provides a passive suppression function by clamping fast, high voltage spikes. The active circuit is formed around Q1, D2 and D3. MOSFET Q1, which is in series with the critical load, is controlled to behave like a linear regulator during a voltage surge. Protection is provided without need for excessive power dissipation while a known current is passed to the load. Planning and sizing become manageable.

Q1 is kept in full conduction during normal operation by the charge pump circuit comprising the 1N4148 diodes and 470 pF capacitor. The Gate Out signal from the equipment’s converter drives the charge pump. When overvoltage occurs, D2 conducts, with current limited by R1; Q2 is turned on. With Q2 in conduction, the gate voltage of Q1 is held to the clamping voltage of D3. The output voltage equals this level minus the gate source voltage of Q1. When overvoltage ceases, the system resumes normal operation.

The FIAM family devices are available as completely integrated packages, including the power MOSFET. FIAMs can be used in MIL, Rail, Telecom and other 48 V power environments. Vicor can also provide advice and reference designs for DEFSTAN and other highly stringent standards where discrete component solutions become essential.

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