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IGBT
and Diode Loss Measurements in Pulsed Power

Operating Conditions

Chaofeng Huang, Paul
Melcher, George Ferguson and Richard Ness

Cymer, Inc. 17075 Thornmint Court,

San Diego, CA 92127, USA
Abstract

High voltage
IGBTs and Series Diodes are used in Cymers solid-state pulsed power
commutator module. The IGBTs and Series Diode losses in the module affect
the cooling system design and the energy transfer efficiency. It is
difficult to estimate the IGBT and Diode losses in the pulsed power
operating condition.  We have previously tried to measure the IGBT
losses by measuring the voltage across the IGBT (using a high voltage
differential probe) and the current through the IGBT. Since the voltage
across the IGBT changes from the kilo-volt level to several volts, it
is very difficult to measure the small on-state voltage accurately.
Also very small internal package inductance can obscure the voltage
measurement due to the high dI/dt level in the circuit.

In this work,
the IGBT and Series Diode losses were measured with a commutator module
directly by measuring the water flow rate going through a cold-plate
attached to the IGBT or Diode and the temperature difference of the
inlet and outlet water. Heat transfer through other means, such as free
convection, was minimized by sealing the IGBT/Diode inside a thermal
isolation blanket and polyurethane foam. The loss measurement results
on the dual package IGBT and dual package Diode are presented in the
paper.

INTRODUCTION

In the pulsed
power modules of Cymers laser system [1], high voltage IGBTs and Diodes
are used. The simplified Commutator schematic diagram is shown in Figure
1. Only one IGBT Gate Driver was used to drive the dual package IGBTs
in parallel. C0 is negatively charged by a resonant charger, and
discharged to C1 by closing the IGBT switch. Then C1 discharges through
a magnetic switch LS and a high voltage pulse transformer to the compression
head and the laser chamber.

The losses of IGBTs and Series Diodes in the commutator
module affect the cooling system design and the energy transfer efficiency
from C0 to C1. We have tried to measure the IGBT and Diode losses by measuring the voltage across the IGBT
or Diode, and the current through the devices. The typical waveforms
are shown in Figure 2.

Figure
1. Simplified
schematic diagram of Commutator.

Figure
2. Typical
waveforms.

{Ch1V</span><span class="Normal--Char" style=" font-size: 8pt;
">C1 signal
(500V/Div)

  Ch2current signal through IGBT Q1 and Diode D1, by

  Pearson current monitor (200A/Div)

  Ch3V</span><span class="Normal--Char" style=" font-family: 'Times New Roman', 'Arial';
font-size: 8pt;">EG signal across Q1, by Tek P5200 high voltage

  differential probe (500V/Div)

  Ch4V</span><span class="Normal--Char" style=" font-family: 'Times New Roman', 'Arial';
font-size: 8pt;">D1 signal across D1, by Tek P5210 high voltage

  differential probe (200V/Div) }

One
can see from Figure 2 that the voltage across the IGBT changes from
the kilo-volt level to several volts. It is very difficult to measure
the small on-state voltage accurately together with the kilo-volt voltage
level. Also the voltage signal across the IGBT or Diode is affected
by the small internal package inductance and the voltage probe inductance
due to the high dI/dt level in the circuit.

The IGBT losses are
usually broken up into several time segments such as turn-on loss, conducting
loss, and turn-off loss. The turn-off loss can be ignored here since
the IGBT is turned off with almost no current about 35s after it is turned on.

Here the dual package IGBT and dual package Diode
losses were measured directly by measuring the water flow rate going through a cold-plate
attached to the IGBT or Diode and the temperature difference of the
inlet and outlet water. The test results are presented below. The accuracy
of the measurement is also analyzed.

TEST SETUP

Figure 3 is an illustration of loss measurement test setup, including
water flow meter, flow control valve, thermistor probes, IGBT or Diode,
cold plate and thermal insulation.

Flow meter:
high accuracy rotameter with the accuracy of ±2% of reading.

Thermometer:
thermistor thermometer with accuracy of ±0.05% of reading.

Thermistor
probes: with accuracy of ±0.1°C from 0 to 70°C.

Figure 3. Illustration of IGBT or Diode loss measurement test
setup

Figure 4 is a photo of the IGBT under test with thermal insulation
in the Commutator module of the pulsed power system. The IGBT is fixed
on an aluminum cold plate with water inlet and outlet. Two thermistor
probes measured the water inlet and outlet temperatures respectively.
There were two layers of thermal insulation blanket between the cold
plate and the outside plastic box. The thermal blanket is 1/4 inch thick
with thermal conductivity of 0.0519W/m-K. The polyurethane foam was
used to cover the IGBT as thermal insulation material. It is also electric
insulation foam. We applied the foam about 2 inches thick. It expanded
to form a skin that contains closed air cells, which provided an effective
thermal barrier against thermal energy loss to the outside environment.

Figure 4. Photo of IGBT under test with thermal insulation
in Commutator module.

The same thermal insulation fixture was used for Series Diode loss
measurements.

TEST RESULTS

Tests have been done at different voltages on V<span class="Body-0020Text-00202--Char" style=" font-size: 8pt;
">C0 from 800V to 1400V at certain pulse repetition rates. A
Tektronix TDS7104 Oscilloscope was used to monitor the waveforms and
collect the current data as shown in Figure 2. It usually took about
one hour or so for the water outlet and inlet temperature difference
to reach the steady-state level (thermal equilibrium condition) at each
voltage. The following formula was used to calculate the energy loss
(Joule) per pulse on the IGBT or Diode.

                                                                                
------(1)

Where 4.187(J/gK) is the Specific Heat Capacity of water. T is the temperature difference of the water outlet and
inlet.

A. IGBT Loss Measurement

Table 1 summarizes
the IGBT loss measurement results at equilibrium conditions with different
voltages.

The relationship
between peak current (I<span class="Body-0020Text-00202--Char" style=" font-size: 8pt;
">peak) and V<span class="Body-0020Text-00202--Char" style=" font-size: 8pt;
">C0 is shown in Figure 5 and formula (2). It is a linear line through origin
(0,0). The correlation coefficient R is 0.99995.

       ------(2)

Table 1. IGBT loss measurement results