Hardware

System Block Diagram

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AC Section DC Loads Section Resistor Loads Low Frequency Matrix High Frequency Mux Over Voltage Protection Relays and 16 Bit I/O

The Old Hardware Philosophy

Not too long ago, lab quality equipment was too expensive to use in a power supply tester. The result was that vendors proceeded to design circuits that were home made and put together to meet specific test needs. Lab instruments were then brought to verify the test results.

The draw back was that correlation was hard to achieve, and users had to rely almost exclusively on the vendors for repair and calibration.

For example, to implement a timing test with a lab instrument, one would have to use a waveform analyzer at $15,000 to $20,000.

Today, most digital scopes can do the same tests for $3,500, with no need for custom circuitry.

The FineTest Hardware Philosophy

Uses a standard, proven design that is easily configured for diffferent input and output requirements.

Implements the design with lab quality instruments from reputable companies such as Agilent Technologies.

(FineTest is an Agilent Technologies Channel Partner. This means high discounts on instruments, world wide warranty of Agilent instruments, and Agilent world wide sales support.)

This philosophy ensures :

High Reliability.

Worldwide Support.

Good correlation with bench measurements.

Easy to get calibration from in-house lab or outside lab.

AC Section

The AC Section includes:

1. AC Source

One of Agilent 6800 Series AC Power Sources/Anlayzers is normally used. Equivalent AC Sources may be used to comply with higher power requirements or with user's existing or preferred AC Sources. At 10kVA and higher power, a motor driven automatic transformer, followed by an isolation transformer are often used for their cost-effective and reliability advantages.

The HP6800 Series offers models from 375 VA to 4800 VA in both single and three-phase modes. Each unit is equipped with programmable voltage, current limit, frequency, phase angle, line disturbance (sag, surge, dropout, clipping, events...), internal measurement of AC Voltage, AC Current, Power, VA, Power Factor, and Harmonic Distortion. (Please see the Agilent Technologies catalog for details about the unit's protection features, self-test, electronic calibration via the bus etc.)

2. AC Interface Unit

To insure operator safety, this unit prevents the application of input power until a micro switch in the test fixture's safety cover completes the circuit. The AC path also has a step down transformer and a current probe. These two signal outputs are applied to input lines of the measurement matrix for routing to the Scope. This is used for Inrush Current Tests, Power On Time, Drop Out, Hold Up, and other AC timing tests.

A mux relay may be added when very high inrush current measurements may exceed the peak current of the AC Source. This relay applies direct Line voltage to the unit under test for the Inrush Current test, and then switches to the AC source for all other tests. This saves the user from upgrading the AC source to a higher power model.

3. Optional Power Meter

An independent power meter may be added when very high accuracy measurements are needed. It can also test the AC or Ringer outputs of the unit under test for distortion. Voltech or Valhalla power meters are often selected.

Three Phase AC Interface Unit with 3 AC Current Probes, 3 AC Voltage Transformers, and an Output Contactor for each phase.

This picture shows a three phase interface unit with current probes for current monitoring (Inrush Current Tests), step down transformers for voltage monitoring (Power On Time, Holdup, and other AC timing tests), and Output Relays for each phase (Safety interlock and Phase Drop tests)

Examples of AC Tests on the Scope

The first test is an Inrush Current Test, which measures the peak current when power is first applied to the power supply.

The second test is a Power On Time test, which measures the time from the application of input power to the time the supply output reaches specification.

The test below is a Hold Up time test, which measures the time from the removal of input power to the time the supply output falls out of specification. This test is very important for computer power supplies as they need this time to properly shutdown in case of a power loss.

In all of these tests, one can see the convenience of the scope display of AC Voltage, Current, Output Voltage, etc. instead of just having test results.

More examples of AC Tests on the Scope

Cycle Dropout

Surge

Input Distortion 5%

Input Distortion 15%

The back of a system with a 90 kVA Motor Driven Auto-Transformer

DC Loads Section

The Agilent Technologies HP6050 Series Programmable Electronic Load is normally used. One mainframe has a capacity of 1800 Watts. Standard plug-in modules include

10A/240V/250W

30A/60V/150W

60A/60V/300W

60A/70V/300W

120A/60V/600W

60A/150V/500W

A system can have multiple mainframes and load modules may be used in parrallel. The modules can be programmed in Constant Current, Resistance, and Constant Voltage mode. They can be used in continuous, pulse, or dynamic modes as well. they offer readback of voltage, current, and power. With their FET output stage, they operate all the way down to near 0V. They provide maximum current at 3V or higher input. At lower input, the maximum current is linearily derated. For example, a 120A load provides 120A at 3V, 80A at 2V, 60A at 1.5V, 30A at 1V etc. (Please refer to the Agilent Technologies power catalog for detailed specifications and descriptions of the many other features such as electronic calibration, protection, analog monitoring, self-test etc.)

Other commercial loads are sometimes used. they offer different current/power models that may provide a better cost-fit for some supplies. Sometimes they are used simply to be compatible to the user's existing lab bench loads. Some loads offer constant power mode that is useful in battery testing.

Resistor Loads

For high power loads, a cost effective solution is a resistor load in parallel with an electronic load. while the resistor load absorbs most of the power, the electronic load compensates for current inaccuracies.

AC outputs also require resistor loads.

Low power programmable resistors are used for resistor selection or trimming.

The resistor loads are built as as a resistor ladder where each value is half or the previous value. Eight resistors and eight relays give 8 bits resolution, a 256 Ohm load has 1 Ohm resolution.

Below is a photo of the inside of a resistor load.

Resistive and Capacitive Loads

Custom Loads are built for some special tests. The diagram above is a custom load used for the ringer output of a telephone power supply.

Over Voltage Protection

Over Voltage Protection circuits can be tested quite easily with the use of an external DC Power Supply. Applying the supply to the unit under test output and ramping up the voltage until the unit shuts off is the most common approach. This approach can be expanded to multiple outputs with the use of a 10x1 mux as in the diagram above. This allos one DC Power Supply to be muxed to many different outputs.

Low Frequency Measurement Matrix

The Low Frequency Measurement Matrix is used to connect the measurement inputs to the DMM, and the 3 channels of the scope (one scope channel is reserved for high frequency measurements as explained in the next section). A typical matrix is 20x4 allowing 20 measurement points to be connect to any of the four measurement inputs (DMM, CH1, CH2, CH4). The matrix can be expanded for more inputs and can use the HP3488A mainframes and cards or VXI mainframes and cards (B or C size).

High Frequency Mux for Ripple and Noise Measurements

For ripple and noise measurements, a high frequency mux is used. Typically the mux is 4x1. This is achieved through the use of a dual 4x1 high frequency mux card, with both the high and low signals being switched. The output of the unit under test is connected to the high frequency mux as follows : Coax cable to Output + and a coax cable to Output -. This is done to insure an equivalent measurement path for both signals and to avoid inducing a differential signal due to cabling differences.

The output of the mux is connected to a high frequency differential probe. The probe is used for its high common mode rejection at high frequencies. The output of the probed goes to a dedicated channel on the scope.

Relays and 16 Bit I/O

General purpose relays are added to the system for use in testing and the test fixture. For example, an open collector ouptu from the unit under test can be connected through a relay to ground or to 5V. A test fixture may have fans that need to be turned off and on during the test, and this can be controlled with the general purpose relays. There are typically at least 10 relays in a system, and can be expanded as needed.

A 16 Bit Digital I/O card is also very useful in testing output signals from the unit under test (power fail bits, inhibit lines etc.). These bits can also be used for fixture identification. The bits can be written and read one bit at a time.