Introduction to the ISO test pulses 7637-2

The sources of ISO 7637-2 test Pulses are explained. Pulse 1, 2a, 2b, 3a, 3b and 5

1. Introduction to ISO Test
Pulses (7637-2)
Vinoth Kumar K
iNARTE-certified Associate EMC Engineer

2. Pulses means…?
•Transients…!
•Before switching –
•After switching -
•But during switching – That’s the Only problem…! Called test
pulses

3. Introduction to the culprit…
•Can you believe that Inductor is the whole
reason for ISO 7637-2 pulses…!
•Yes… Inductor only creates Pulse 1, 2a, 3a,
and 3b
V
L
= L * di/dt
How much is the
transient
Amplitude?
Reason no 1 : Inductor (L)
Reason no 2 : Current (I)
Reason no3 : Switching (d/dt)
It can convert 12V into 30kV
Ignition coil can
generate 30kV..!

4. Cont…
•The Inductor :
•Window lift motor, Door lockers, Wipers, Seat adjustments, Fans, AC,
Alternators and Generators
•Wiring harness…
•1 meter of a normal wire has 1uH
•Inside a vehicle we have at least 1 km lengthy wires
•Current :
•Maximum current in alternators is 50-70A…! (passenger cars)
•Normal motors handle at least 1A

5. Cont…
•Switches :
•Relay, Ignition, fuses and multipurpose ON/OFF switch
•For an example,
•L=10uH, i=10A, d/dt=1usec
•
VL= 100V….!
V
L
= L * di/dt

6. How an inductor works…?
•Inductor – Cycle wheel
•Even after stopping the pedal, wheel still rotates in the same direction
(Inductor drives the current in the same direction even after removing the
supply)
•It doesn’t allow the faster variations
•Magnetic field can’t response as faster as higher freq hence it gives high
resistance.
Let’s go…..!

7. Pulse 1
•Don’t switch off the ignition when an inductive load is connected
across the DUT.
When the switch is closed inductor stores energy by magnetic field
Step 1: Before opening the switch
i=∞

8. Cont…
But in reality, The load has a small resistance, that only helps powering on the DUT
i= V/RL i.e. the huge current…!
RL
L
Step 2 : As soon as opening the switch
Now inductor will act as a source to the DUT
but it will maintain the same current
direction
i=v/RL
But in opposite to the conventional current direction
Hence negative peak voltage.
The inductor will push the same current
For any DUT

9. Cont…

10. Pulse 2a
•When an inductive load switch is made off DUT gets affected by the
wiring harness inductance.
Step 1 : Before opening the switch
When the switch is closed inductor stores energy by magnetic field
i = i_DUT + i_Inductor
Across the harness inductor more current passes. Hence harness
inductor stores the huge magnetic fields
5uH
5uH

11. Cont…
Step 2: After opening the switch
Load inductor doesn’t affect the DUT since open circuit to the
DUT
But the harness magnetic fields suddenly collapse. Hence it will
push the huge current across the DUT but in the same direction
to the conventional current (Positive peak)
i = i_DUT + i_Inductor
Don’t forget ignition is always connected
5uH
5uH

12. Cont…

13. Pulse 3a/3b
•Don’t use mechanical switches to the sensitive devices. Stray
capacitance and stray inductance can give the burst of transients
Ls
Cs
RL
Can you believe, while the switch is made off we had seen only one
transient but the improper mechanical contact creates the burst
Step 1 : After opening the switch
The collapsed magnetic field pushes the current to the capacitor.
Capacitor gets charged up fully
Please visualize in a slow motion of switch
How small the capacitor is the big the impact is…!

14. Cont…
•Switch contact position as soon as the switch is made off.
The capacitor voltage is enough to break air dielectric medium (10^9 V/m)
between the minimum spaced contacts. It sparks once hence conduction is
made (Capacitor discharged now) and inductor keeps storing magnetic field (by
the time the poles moved a far) and pushes the charges again to the capacitor.
Once again air discharge happens in between the contacts. This goes on till the
poles move a long distance where capacitive voltage can’t break the air…!
Showering arc effect
At initial time less voltage is enough to make a air discharge
between a closely spaced contacts
Capacitor can’t have more voltage to make the spak…! And a
burst stopped now
Please note : when DUT is before the switch the burst will be positive
otherwise it’s going to be negative burst…!
+ -

15. Pulse 2b
•DC motor can act as a generator when the ignition switch is removed
When the power is removed the motor still rotates hence it acts as a
generator to the DUT
Motor acts as a motor
Ignition is made OFF but motor is still rotating
Motor becomes a generator now
M

16. Pulse 5
•Alternator generally charges the car battery.
•When the battery is drained it suddenly squeeze more current hence the
corrosion/fuses makes the disconnection form the battery. Directly DUT drives the
power from the alternator
•When there is no protection it’s called pulse 5a and the protection clips the over
voltage called 5b.
Corrosion or fuse blown up disconnects the battery

17. Food for thoughts…
The clock is a virtual switching. By signaling itself we make ON/OFF

18. References…
•ISO 7637-2 standard
•C R Paul book “Introduction to EMC”
•EM test app note on ISO Test pulses
•Ti.com app note on ISO Test pulses

19. [email protected]
Contact me on any doubts…