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Posted on 17 July 2019

A Modular Approach for Production

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High speed testing of power semiconductors

With an experience that started in the early 1960's, LEMSYS developed in the beginning of the 1990's a modular approach of power semiconductors commencing with the test of dynamic parameters. The main concern of this development was the possibility to build in a relatively short time custom-sized equipment, according to the user's needs and likely to be easily upgraded in terms of power capability. With the static parameter testing, the necessity of a modular concept revealed even more acute in the sense that high test rate capability is mandatory.

By Gérard Cuénoud, LEMSYS SA, Switzerland

 

The family of equipment which is described in this article deals with the test of static parameters (over 25 listed), of discrete components as well as semiconductor modules, such as bipolar transistors, IGBT's, MOSFET's, thyristors, diodes, resistors (thermistors). The objective, which has been effectively reached, was a typical average rate of 10 tests per second. Although the objective is production testing, the equipment is also usable for lab measurements and component characterization. The original concept consists in the fact that every function in a test equipment had to be considered as an independent module for itself, in order to make possible fast data acquisition, as well as easy and quick replacement of defective parts, since high-speed production testing needs also efficient and low time-consuming troubleshooting. This concept is pushed even further: modules can be used independently, e.g. plugged in a 19 in. rack, which, in turn may be installed in a userdesigned structure.

Main Modules

The most important modules are of course power modules (also further called generators). These modules are the voltage generator and the current generator, delivering typically half-sine or trapezoid pulses.

The voltage module delivers pulses having an amplitude ranging from 20 to 2200 V, with current capability of 0.1 to 100 mA. This generator works as a voltage source with programmable current limitation. The voltage and current are measured on the flat part of the trapezoidal pulse or at the peak of the half-sinewave. This module is mainly designed for the measurement of the blocking voltage and the leakage current of devices under test.

The current generator delivers pulses having an amplitude ranging from 2 to 200 A. Two other module sizes exist with 2 – 500 A, and 2 – 1000 A. The pulse rate reaches 20 Hz. This generator acts as a current source. Equipped with a gate driver for triggered devices, it is mainly designed for forward voltage drop (0 -10 V) of components. The measurement is performed at the end of the trapezoidal pulse or at the peak of the halfsinewave.

Optional Modules Gate Characteristics Tests

The Module IGE (for IGBT and MOSFET tests) unit comprises a 5-100 V generator of rectangular pulses of 40 – 100 ms duration, for the test of I[GES]and I[GSS], and a 0.1 – 100 mA generator of rectangular pulses of 2 – 10 ms duration, at a maximum frequency of 20 Hz, for the test of VGE(th) and VGS(th). The module IGT (for thyristor tests) comprises an anode 6 or 12 V power generator with a series resistor (1, 2 or 6 W), with a maximum anode current of 12 A, and a gate ramp current generator, adjustable in two ranges (0.5 – 50 mA and 5 – 500 mA), at a maximum frequency of 20 Hz. The IGT and VGT values are measured with a delay defined by an RC time constant of 100 µs.

IH/IL Test and Kelvin Contact Control

The IH/IL module (for thyristor tests) comprises a trapezoidal pulse current generator and a 6 or 12 V anode supply. The Kelvin Contact Test module controls the connections of the Device Under Test (DUT), using a rectangular pulse of 24 V (50 mA).This test is important in the sense that it allows for the detection of a fault in the internal connections of a semiconductor module.

Output Matrix

Although it is mentioned as an option, this module plays a prime importance role. In order to connect more than one module to a DUT, i.e. performing several different tests in a sequence, an electro-mechanical device is necessary. The Matrix module, which provides for a minimum of 6 up to 63 low level signal lines, and for a minimum of 2 up to 32 power lines.

To illustrate the role of this module, Fig. 1 is self explanatory, showing a typical test system equipped with several modules, with output connection to a single DUT (one test head output). The matrix is using highspeed, high-reliability relays able to connect high currents or voltages and low level signals between the various modules and the DUT. Therefore it is typically a heavy-duty, key element of the test system. Millions of operations are possible, however a periodical auto-test of the quality of the relay contacts is performed. This will be explained further in the section dedicated to the software control.

Multiple Module test system

The test equipment connections to the DUT are grouped on an output connector having power connections at pin locations 1 to 32. The Power Matrix allows to select which lines are connected to the output connector. As shown in Figure.2, the user selects what relays have to be energized, according to the chosen output connections, whereas the function of each line (here positive and negative pole of a voltage generator) is determined automatically by the active test module (shaded boxes, at the bottom of the figure).

Power Matrix

The low power connections (63) of the output connector (pin locations N° 101 to N° 163) are selected by the low level matrix (see Figure 3). Similarly to the power matrix system, the user can select to what connector pins the low level lines (gate generator signal, measurement sense terminals) will be connected. This is shown in the upper part of the figure. As per the power matrix, the selected lines are automatically defined by the active test module (here the measurement circuit of an IGT/VGT module, shaded boxes in the lower part of the figure). The way the user choice is made will be described in the Computer Control section.

Low Level Matrix

Drivers

Each module contains a special driver unit using a microprocessor. This unit delivers the references, levels and timing to the generator(s) and measurement circuit(s) according to the test condition required.

The presence of the driver unit in each module allows for a simplified communication with the personal computer (PC) which controls the entire equipment. Another feature of this approach is an increased test speed.

Operator's Interface

The control of the test equipment is performed by a personal computer, using a Real Time Operating System (RTOS) using a Graphic User Interface. As the equipment may be used according to four different modes of operation, such as:
1.) Production mode;
2.) Laboratory mode, for component characterization;
3.) Edition for the elaboration of complete test sequence;
4.) Edition mode for the elaboration of the DUT characteristics;

A simple, easy to learn language is used, as well as graphic presentation, where fields allow for a quick introduction of DUT identification, test conditions, etc.

Fig. 4 shows an example of laboratory test, with the test results in the lower half of the screen, with comments in the upper part.

Laboratory Mode Example

Engineering Interface

The programming language, for engineering purposes remains very simple, though a little more complicated. It is needed for the preparation of tests and test sequences.

One preliminary important step is to define the output connections of the tester to the DUT through the connector and the test head. The example of Figure 5 illustrates the case of a 6 transistor IGBT module. Lines preceded by a # are comment lines. The second line shows in what order connections for collector, emitter, gate and sense are given (reminding that 1 to 32 are power matrix outputs and 101 to 163 are low level matrix output).

Connection Attribution

The following figure shows the sequence programmed for the VCESAT test (Figure 6). Note that the #inc-lude and #define lines are compulsory.

VCESAT Test Sequence

Maintenance Interface

As already mentioned in section 3.2.3, the output matrix is a key element of the test equipment, which has to be periodically checked. Therefore, a "Check Matrix" utility program allows for the detection of any relay failure. For this operation, the tester must be running, with all DUT connections removed.

Three controls are performed:
1.) Detection of any contact short-circuit on all relays:
2.) Check of proper functioning of the auxiliary relays;
3.) Check of the proper functioning of the main relays.

The result is displayed on the screen, with identification (and localisation) of the defective relay(s). Then the defective part(s) can be replaced, or the complete matrix module itself, when an short time is needed to start with new semiconductor tests.

Dialog with a Handler

Another module, which has not been mentioned in section 3.2, is the Handler Function Module which manages the information flow between the PC and a handler for automatic production testing. The logic signals are + 5 V d.c., with active level at 0 V (negative logic).

BinCode signals (for device classification), and AdaptorCode signals (for verification of the adaptor with the DUT and sequence a requested) are exchanged.

Other Features

The PC can compute test statistics. Among others, are also the possibility of sorting the devices (category classification), and converting the result files into *.csv files compatible with Windows Excel.

Tester Architecture and Block Diagram

A typical 500 A / 2200 V equipment is represented in Figure 7, where the easiness of the maintenance appears quite obviously.

Typical Test Equipment

The PC is linked to the test equipment through a RS 232 optical fibre connection, which runs, inside the equipment to the different module drivers. Therefore the computer is electrically isolated from the tester, which fulfils two tasks:

First the electrical safety of the operator, and second, a noise-free control operation. The computer is also equipped with an Ethernet connection for data transmission with other computers, and remote control. A modem connection is available for downloads of software upgrades and for remote maintenance. A Mains and Safety module (bottom left) distributes the connections to the mains, to the test command box and the different safety connections (in particular with the test head protection lid). The generator and measurement modules, represented in the center, are, from top to bottom: a 2200 V and a 500 A generators, a gate generator. Top right is the Matrix Module, where the five blocks (center right) represent the user configurable output lines.

Typical Module Architecture

The block diagram of a voltage module, delivering 20 to 2200 V, with adjustable current limit from 0.1 to 100 mA, is shown in Figure 8.

Voltage Module Architecture

Safety

For the operator's safety, the control is designed in accordance to the European standards. The test equipment is protected by a conducting and grounded cabinet. The opening of any side panel of the tester cabinet will shut down the power supply of all the generators able to supply hazardous voltage. The output terminals of the tester will be short-circuited and grounded as well, whereas the supply of the control elements such as the computer or the function modules will not be affected. The DUT test area must be protected by an enclosure.

The tester will check the position of the enclosure before starting any test. As long as the enclosure is not on its closed position, the high voltage output terminals of the tester will be short-circuited and grounded. no test can be started. Before the execution of the first test after start up, the protection enclosure has to be opened and closed once. During this cycle test, the proper function of the safety circuit and devices are checked.

 

 

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