
2003 Microchip Technology Inc.
DS70096A-page 19
Set Up and Operation
C20, C21, Q11, R45, R46, R52 – These components act to provide a dynamic
level shifting circuit to U19 while Q1 and Q2 switch. Inductance of the power
tracking between the sources of Q1 and Q2 due to the physical board layout
means there is a substantial transient voltage (up to 5V in this case) between the
+15V supply * point reference at R61 and the sources of Q1 and Q2. This simple
low cost circuit allows the power supply of U19 to move transiently. Q11 provides
a level shift to ensure correct assertion of the firing command. In applications with
fewer constraints on physical layout and/or lower switching speed requirements,
these components may not be needed.
1.4.3
Phase Inverter (Appendix A, Sheet 2)
1.4.3.1
INTRODUCTION
The 3-phase inverter has three identical circuits, shown as R (RED) Y (YELLOW) and
B (BLUE). These are often referred to as inverter “l(fā)egs”. They invert the DC bus back
to a variable AC output waveform by appropriate modulation of the switches. When a
star or delta connected three-phase motor is used, the electrical symmetry can be
exploited to provide bi-directional current and voltage with just three such legs. In this
way, both motoring and generating/braking operation can be used in either direction of
rotation, commonly called “4 Quadrant” control.
There is no reason why the user can not use two of the legs in an “H-bridge”
configuration for control of DC motors or other single-phase applications requiring
bi-directional current and voltage. Even a single leg could be used with just the low
side switch controlled for a simple unidirectional current application (e.g., field control
of a separately excited DC motor).
The detailed description of one “l(fā)eg” (red phase) is given below. The other legs are
identical in function.
1.4.3.2
POWER DEVICES
Q3, Q4 – 600V N-Channel IGBT transistors with co-packaged anti-parallel 600V
diodes. They are packaged in the industry standard TO220. As the tabs of the
devices are not isolated, a thermally conductive insulator is used.
The IGBTs are optimized for switching at frequencies up to 20 kHz while having
improved tolerance to FAULT conditions (at the slight expense of conduction
loss).
The diodes are of the “soft-recovery” type for reduced RF emissions.
The tracking between the devices and to the DC bus is designed to minimize the
inductance that causes transient over/undershoots.
R4 – A 3W 25 m
shunt resistor through which the low side switch and diode
returns to the -DC bus. The shunt is used for FAULT protection and (optionally) for
an alternative feedback signal – see
Section 1.2.6 “Power Module Feedback
Signals”
and
Section 1.2.7 “FAULT Protection”
.
1.4.3.3
GATE DRIVE
U22 – An integrated high voltage IC (IR 2112) which provides gate control of a low
side and a high side power transistor. As the emitter of the high side device (Q3)
can be at any potential between -DC and +DC, and even beyond transiently,
internal circuitry must provide for the necessary level shifting to ensure correct
operation. The IR2112 does this without providing isolation. The 2112 also
monitors the low and high side power supplies and shuts down if an under voltage
is detected. The under-voltage lockout is automatically reset by a rising edge of a
firing command once a valid voltage is present. See www.irf.com for a full data
sheet.