Text preview for : 5991-4543EN IGBT Sense Emitter Current Measurement Using the Agilent B1505A - Application Note c2014 part of



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Keysight Technologies
IGBT Sense Emitter Current Measurement
Using the Keysight B1505A


Application Note
Introduction

Insulated gate bipolar transistors (IGBTs) have become the key energy-saving devices for use in switch-
ing applications such as switching convertors and invertors. However, IGBTs used in circuits that drive
inductive loads (such as induction motors or igniters) risk damage due to excessive currents caused by
a motor jamming or a short in the igniter circuit.

To detect overcurrents and prevent damage, some IGBTs have a second emitter terminal known as a
sense emitter. The ratio of sense emitter current to emitter current is very small (one part to several
thousand or several tens of thousands). The sense emitter current is monitored using a shunt resistor;
the voltage across the shunt resistor in-turn feeds into an overcurrent protection circuit. When the
overcurrent detector circuit senses the voltage exceeding a speciied limit, it turns off the IGBT (Figure 1).

Power devices such as IGBTs consist of tens of thousands of small cells con-
nected in parallel, and the sense emitter uses some of these cells. Since the C
individual IGBT cells vary in size and characteristics, the senseemitter current
to emitter current ratio also varies. In addition, the sense emitter current to
emitter current ratio depends on the emitter current level. This is due to the G
effects of both the voltage across the shunt resistor and the residual resis-
tances inherent in each device.

Overcurrent SE E
For these reasons, it is necessary to measure the sense emitter to emitter
current ratio under actual use conditions to determine the correct overcurrent Detector
limit.
Rs
The Keysight Technologies, Inc. B1505A Power Device Analyzer/Curve Tracer
has a wide measurement rage (1500 A/10 kV), making it a powerful tool to
characterize IGBTs. It also has a modular architecture that allows you to have
multiple measurement channels, which enables the simultaneous measure-
ment of both the sense emitter current and the emitter current. FIGURE 1. Overcurrent protection using sense emitter




2
The basic measurement resource of the B1505A is the SMU (Source Measure Unit). It integrates
four measurement functions (voltage sourcing, current sourcing, current measurement and voltage
measurement) into a single module. An SMU can function as a voltage meter by putting it into force
current/measure voltage mode with a zero current output. Similarly, it can act as a current meter by
putting it into force voltage/measure current mode with a zero voltage output. Figure 2 (a) shows a
simpliied circuit diagram using an SMU as a voltage meter to measure the voltage across a shunt
resistor. Figure 2 (b) shows the corresponding circuit diagram using the SMU as a current meter. Note
that when using the SMU as a current meter the sense emitter current can be measured directly with-
out using an external shunt resistor.



MCSMU UHCU
IC
High C High




Low G



E
MCSMU SE E
High IE
IC
Low

0A ISE
Low
(a) Circuit diagram
using SMU as a
voltage emitte




UHCU
MCSMU IC
C High
High



G
Low



E
SE E
MCSMU
ISE IE
High IC
Low


0V ISE
(b) Circuit diagram Low
using SMU as
a current emitter


FIGURE 2. Connection diagram to measure sense emitter current


Floating SMUs do not have a hard-wired connection to any internal ground reference. In the B1505A
both the medium current SMUs (MCSMUs) and high current SMUs (HCSMUs) are loating SMUs. The
loating SMU feature makes it easy to measure the differential voltage across the shunt resistor, as
well as the ratio of the current lowing through the sense emitter sense to that lowing through the
regular emitter. Note: In these diagrams "UHCU" refers to the B1505A's ultrahigh current unit, which
can source and measure currents up to 1500 A.



3
Measurement Examples Figure 3 shows plots of emitter current (IE) and sense emitter current (ISE) versus
collector voltage (VCE) both with and without a shunt resistor. Note that the
scales of the IE axis and the ISE axis differ by a factor of 10,000.




FIGURE 3. Plot of IE/ISE-VCE characteristics


The slope of ISE increases rapidly with increasing emitter current. This means
that the IE/ISE ratio decreases with increasing IE as shown in igure 4.
Figure 4 shows the dependency of the IE to ISE ratio as a function of emit-
ter current. Note: The gate voltage for the curves shown in igure 4 was large
enough to turn on the IGBT (maximum allowable gate voltage, VGES). As can
be seen, the ratio reaches a maximum value quickly and then decreases with
increasing IE. The ratio is also larger with a shunt resistor present.




FIGURE 4. IE/IES dependency as a function of IE


The decrease of the IE to ISE ratio and the effect of the shunt resistor can be
explained by considering the voltage drop across the interconnect resistances
inside the IGBT.


4
Measurement Examples Figure 5 shows a circuit schematic that helps to explain this effect.


continued C
G



VCE_SE VCE_E VCE
ISE
IE
RL_SE ISE