Owner`s manual

4.7.1.2.4 Average Current Mode
If the inverter “current command” control signal can be made to control a current source (or sink), then the pole/zero
associated with the output L-C filter becomes a first order system, that is, a current source discharging or charging
a capacitor. It is much easier to stabilize this type of control loop. It also provides the needed current limiting
function. This type of system can be implemented by summing the inverter output current signal with the “current
command” signal and applying this signal to a current error amplifier. The output of this amplifier, along with a trian-
gular “sweep” signal, is applied to a set of comparators, which operate the IGBT date drivers.
Here is a little history as to the problems associated with this approach. For an inverter to make a 120VAC sine
wave output (+/-170V peak) using a +/-213VDC bus voltage, the duty cycle must range between 8% to 92%. A duty
cycle of 50% will make the filtered output voltage zero. In the classical “peak” current mode control system, when
the duty cycles becomes greater than 50%, the famous half cycle instability problem will manifest itself. In “current
mode” controlled DC power supplies, the problem was resolved by adding what is called “slope compensation” to
the current feedback signal. This works well for a fixed output voltage power supply, but not for a variable output
supply. The “Unitrode” applications handbook, IC# 1051/1997 explains this instability in detail.
Later, another way was found to resolve this instability. If the “current command” signal is summed with the “output
current” signal, through another error amplifier, then applied to the comparators, the instability problem is resolved.
However, there are restrictions on this error amplifier. This amplified error signal is now applied to a set of compara-
tors. The other input to the comparator is a linear sweep voltage. The maximum gain of this new current error
amplifier can be made fairly high. However, the slope of the amplified signal needs to be less than the sweep gener-
ators slope (dV/dT). If it greater than the sweep generators dV/dT, the output IGBTs will be turned ON and OFF
several times during a switching cycle. If this happens, the switching losses will become very large, resulting in
IGBT failures due to excessive heating of the transistor. The easiest way to analyze this function is to generate a
computer model and analyze the signal over the entire sine wave cycle for all load conditions, resistive, leading
reactive, lagging reactive, and computer loads with crest factors up to 3:1. This program was written in TURBO
BASIC, an old BORLAND software package, and is included in the appendix as INV3 kVA3.BAS. A copy can be
obtained from the writer at the following E-mail address: [e-mail]. This program should
run using QUICK BASIC, whoever some of the SCREEN and PIXEL setting statements may need to be changed.
It is best to run this program in the pure DOS mode, not through “Windows”. “Windows” mode is about nine times
slower than the DOS mode, but it will work.
4.7.1.2.5 Inverter Sweep Generator
The sweep signal is obtains its signal from the battery booster LMC555CN oscillator through an optical coupler. This
40KHz signal is applied to another D flop (U19), which divides 40KHz signal by 2 to make a 20KHz square wave.
The sweep generator is almost identical to the sweep generator in the battery booster section, except it is set up to
produce +/-5.2 volt sweep voltage instead of +/-9.52 volt sweep signal. This sweep generator signal is summed
with the output of the current error amplifier through 4990 ohm resistors. This summed voltage, (Vveao+Vsweep)/2,
is applied to a set of window comparators, one comparator uses a positive 1.00V reference, the other uses a
negative 1.00V. The outputs of these comparators operate the photo drivers (HCPL-3120) which are located on the
“Power Supply/Driver” PCB. If the summed voltage is between –1.00V and +1.00V (dead band), neither comparator
will turn “ON” its respective photo drivers, thus all of the inverter IGBTs are OFF. This is needed to ensure that
“positive” and “negative” IGBTs are not ON at the same time. This could occur in an electrically noisy environment,
so the +/-1.00V “dead band” window will provide the needed noise immunity.
4.7.1.2.6 Fault Detection
For fault detection, the input “Current Command” signal, which is the output of the “Controller” Voltage Error
Amplifier, is summed with the output current signal as measured by the “LEM” LA 125-P sensor. It is then filtered
to remove the ripple and applied to a window comparator. Since the “output current” signal is out of phase with the
input “Current Command” signal, the filtered sum should be zero if the system is functioning properly. If the sum of
Owner’s Manual
Theory of Operation page 4 — 9