Why Lab Experiments Fail in Power Electronics

Why Lab Experiments Fail in Power Electronics

Tricky Measuring and Monitoring Requirements

Generally, students start collecting available power supplies to take up the study on the power
electronic devices. They wire up these supplies with the power devices in the usual way: one
supply giving the main device supply VCC and another powering up the gate. The outputs
of these supplies are defined by analog potentiometers used in the supply control circuitry.
Because of this design configuration, the exact precision may not be available to create the
required test conditions. Normally labs don’t give students high valued precision power supplies
which could create the precision operating environment.

When using ordinary power supplies, current based triggering devices like SCR and TRIAC,
demand constant maintenance of gate current at the defined level during the experiment. With
these power supplies, if the VCC voltage is varied during experimentation, gate current also gets
changed from its set value. Then, the student goes back to the gate power supply to adjust the
current to its initial set value. This happens for every change of VCC voltage. Till the device gets
triggered with the exact parameters, the student has to shuttle back and forth between the supply
voltages. While repeatedly adjusting the gate current manually using an analog potentiometer,
the student inadvertently triggers the device in random. The student fails to understand the
device’s exact triggering point and ends up seeing the device in triggered condition in surprise.
Not understanding this may become disadvantageous when creating control algorithms for the
device in power applications.

When students take MOSFET/IGBT devices into study, required gate voltage varying range for
the active region is very small, about 200-300mV. When the device gets into active region,
they need high resolution power supplies and measuring facilities to plot multiple output
characteristics waveforms within this active region.

Apart from creating an ideal operating environment for the power devices, students also
need precision measuring facilities to complete the scene. To make an effective study on the
characteristics, student requires up to five multimeters with minimum 3.5 digits of resolution.
Also, the maintenance of mA resolution current meters is very difficult in a college lab setup
because they are easily damaged when the current limits are exceeded. Giving this kind of
facilities to all the students of the class is real expensive problem to any laboratory.

Another disadvantage of using many external power supplies and oscilloscopes is the common
ground potential of all these facilities. Normal oscilloscopes come with two or more channels
sharing same ground which is connected to the mains earth. Because of this, more than one
floating measurement is not possible; provided the power supplies are isolated and non-
grounded. If the students, without this understanding, connect the negative lead of the probes at
different places, the circuit may get short conditions, resulting in damage to the circuit and/or
the oscilloscope. If the used power supplies have the mains earth connected to the supply ground
(as most bench power supplies are), not even one floating measurement is possible.

However, there is a solution available for this, albeit a very expensive one. High-voltage Active
Differential Probes are available to mitigate isolation problems. But the cost of each probe
is usually more than many entry level DSOs. The cost of supplying oscilloscopes with two high
voltage active differential probes to every batch of students becomes prohibitive for most
educational institutions.

Also, viewing the current waveforms of the power application circuits, particularly Buck and
Boost convertors is very difficult without the use of isolated current probes, which are again
very expensive. Without viewing the current waveforms, the student cannot understand the
exact operating conditions of the circuit, particularly the continuous and discontinuous modes
of operation, making the study incomplete.

Another concern when using multiple power sources, measuring instruments and oscilloscopes
is the accidental shorting or wiring mistakes which can damage the power devices. It is very
difficult to identify the dead devices when the operating conditions are complicated as above
discussed. Also, periodic maintenance and replacement costs become expensive during the
usage of these setups. It has been observed in many institutions, power electronics device study
arrangements are not very encouraging to both students as well as lab maintenance staff.