Bodo´s Power Systems

May 2016 www.bodospower.com70

CONTENT

Transient and surge suppressors are com-

monly used to protect systems from lightning

strikes, ESDs and short-circuit scenarios.

They fall roughly into two categories, the

rst of which is a ‘fuse and crowbar circuit’;

examples of which are shown in Figure 1.

Such circuits are used to prevent an input

over-voltage (surge) condition from damag-

ing downstream circuitry. They all operate by

creating a very low resistance path - using

a thyristor, TRIAC, trisil or thyratron - to

eectively create a temporary short-circuit.

Current is then limited on the source side by

a circuit breaker tripping or a fuse blowing.

Or, to put it another way, circuit breakers and

fuses rely on overcurrent to protect against

overvoltage. Both leave downstream circuitry

without power once tripped/blown.

The second type of transient/surge suppres-

sor is a clamping device. Whilst not short-

circuiting, it does decrease its resistance to

limit the voltage (to a rated clamping volt-

age). Common devices used are the Zener

diode and the Metal Oxide Varistor (MOV),

which have both strengths and weaknesses.

For instance, in the ideal world, the clamped

voltage would be constant, irrespective of

current. In reality, MOVs and Zener diodes

are not linear devices and above a certain

current the voltage will start to rise. Of the

two device types, an MOV is able to sustain

a high voltage and dissipate a large amount

of energy, but above a certain current the

voltage across its terminals will once again

increase. Conversely, a Zener diode can

maintain a relatively constant voltage, but is

unable to dissipate a lot of energy while trig-

gered (typically just a few mJ).

Clearly, in order to eciently protect down-

stream circuitry from the eects of a lightning

strike or ESD, voltage clamping alone will not

suce. An optimal Surge Protection Device

(SPD) must: clamp the over-voltage at the

load; limit the over-current; dissipate the

power surge energy and remain functional

after the surge has passed. It must also have

a fast response time, as a limiting factor with

traditional Silicon-based suppressors is that

they take about 10μs to short-circuit.

Best of Both

CALY Technologies, a spin-o from the

Ampere-Lab at INSA de Lyon, is currently

developing a hybrid Surge Current and

Voltage Limiting Device (SCVLD), which will

ultimately take the form of a three-pin mod-

ule. It comprises an MOV, a Zener diode and

a SiC-based Current Limiting Diode (CLD).

The architecture (see gure 2) is such that,

with suitably-sized components, it is possible

to get the clamping benets of both the MOV

and the Zener while avoiding their shortcom-

ings; i.e. not being able to limit voltage over

certain currents and not being able to dis-

sipate high energies, respectively.

When subjected to an ESD with a pulse

shape of 8/20 or 4/440, the SPD has four

distinct phases of operation:

• Phase 1 [0 < t < 0:8μs]: The induced over-

current is limited to the saturation current

of the SiC-based CLD;

• Phase 2 [0:8μs < t < 2 μs]: The increased

(but saturated) current produces a voltage

across (and is clamped by) the Zener di-

ode (avalanche voltage Vz), which clamps

in less than 10ns;

• Phase 3 [2μs < t < 2.5 μs]: With the cur-

rent limited to the value of the CLD satu-

ration current and voltage across the load

limited to Vz, the voltage across the MOV

will increase to its clamping voltage.

• Phase 4 [t > 2.5 μs]: Once triggered, the

voltage clamping devices are dissipating

the remaining energy of the surge, until the

voltages fall below their triggering values

CLAMP

and V

). At the same time, the

voltage drop across the CLD remains high,

as long as the current is at saturation.

The principle of operation depends on the

MOV clamping voltage being higher than the

Zener avalanche voltage (Vz). Also, the CLD

must be able to sustain short circuit opera-

tion and have a very fast response time. In

this respect, Silicon Carbide’s low electron

mobility comes in to play; 1,000cm

/V.s com-

pared to Silicon’s 1,500cm

/V.s (and GaN’s

1,250cm

/V.s, by the way). The CLD must be

able to dissipate the bulk of the surge energy,

which it does by producing heat. Here, Sili-

con Carbide’s high Thermal Conductivity is

essential; 4.9W/cm.K, compared to Silicon’s

1.5W/cm.K (and GaN’s 1.3W/cm.K). In ad-

dition, when two CLDs are placed in series,

back to back, the circuit is able to protect AC

as well as DC power lines.

PROTECTION

To Protect and to Serve

This article is about a novel Silicon Carbide based hybrid surge suppressor module for

safeguarding AC and DC power circuitry.

By Dr. Dominique Tournier (CTO) and Dr. Pierre Brosselard (CEO),

CALY Technologies and

Phil Burnside, Business Development Manager of

Raytheon UK’s Semiconductors Business Unit

Figure 1: A variety of fuse and crowbar surge protection circuits. The shorting components are (left to right) a MOSFET, a TRIAC and Thyristor

(SCR). In all cases the shorting elements producing an over-current which blows the fuse.