CHARGED Electric Vehicles Magazine – July-August 2019

CURRENT

SENSORS

JUL/AUG 2019 29

There is an even more diverse array of current sen-

sors which output a signal in proportion to a mag-

netic field, from the mundane current transformer to

the truly exotic fiber-optic interferometer (sadly, not

used in any EVs I’m aware of, but see ABB’s catalog if

you need to measure 600 kA). Which type of current

sensor to choose depends on several factors, such as

whether DC must be measured, AC frequency range,

current range, overload capability (or tolerance, at

least), linearity, accuracy and, of course, budget. Note

that all magnetic-type current sensors automatically

isolate the circuit being monitored from the circuit do-

ing the monitoring, and this benefit alone is often the

deciding factor in choosing one of them over a shunt.

As hinted at above, the simple shunt resistor can

hide some unpleasant surprises, but some of the most

vexing are the spikes produced by their usually tiny,

yet not insignificant, stray inductance. A very common

application for a shunt is to monitor the current of a

switch or bridge leg by placing it between the switch

and the negative rail, where it will be hammered with

rectangular pulses of current. Unless (quasi-) resonant

switching is used, these pulses will have extremely fast

rise and fall times (that is, a high dI / dt), which can

provoke nasty little thin spikes from any stray induc-

LTSpice

simulation

of a shunt

with parasitic

inductance

included

99.6μs 99.7μs 99.8μs 99.9μs 100.0μs 100.1μs 100.2μs 100.3μs 100.4μs 100.5μs 100.6μs 100.7μs 100.8μs 100.9μs

V(vshunt) V(vo)

Compensated Shunt Amplifier

R1 * C1 = L2 / R 2

R3 Vshunt R1

1 10k C1

10m 10p 10k

L2

1n

R4

10k

R6

V2

15v 15v

LT1007

U2

R5

10k

Vo

R8

1Meg

V3

R2

V1

PULSE(0 20 0.1m 20n 20n 0.1m)

.tran 0 0.3m 0 50n

Current can be sensed or

measured via the voltage

drop across a resistor

or via the intensity of

the magnetic field which

radiates from any current-

carrying conductor

purpose, or it could be a creative use of circuit/compo-

nent parasitics, such as the on-resistance of a MOSFET,

the winding resistance of an inductor, or even a trace

on a printed-circuit board. Despite the differences in

form, all shunts perform the same function, which is

to drop a predictable amount of voltage vs current, but

don’t let that apparent simplicity fool you into thinking

shunts are foolproof.