MC1H3 Datasheet, PDF (26/60 Pages) Microchip Technology – High Voltage Power Module Users Guide

dsPICDEM™ MC1H 3-Phase High Voltage Power Module
The function of groups of the discrete gate-drive components is explained below:
• R51, D28, C14, C17 – These components form a floating power supply for the
high side gate driving stage of U22. Whenever the low side IGBT (Q4) or it's
anti-parallel diode is conducting; a charging path for C14 and C17 is formed. This
is because the 15V supply is referenced to the -DC bus and D28 conducts. When
the high side switch is on, and the low side switch is therefore off, D28 blocks
reverse current flow that would result from the bus voltage present on the inverter
output. This kind of floating supply is usually referred to as a “bootstrap”, see
Figure 1-7. This circuit is commonly used because it is both efficient and
economical. It is assumed that any bootstrap initial priming or any necessary
refresh is carried out in software by the dsPIC device. This is discussed in
Section 1.2.5 “Power Device Switching Frequencies”.
FIGURE 1-7:
BOOTSTRAP SUPPLY
R51 ^D28
C17
15V
Q4
Bootstrap
Supply
R38
R4
• R33, D21, R39 and R38 – These components aid in the correct control of the gate
of the high side power device. The same circuit is repeated for the low side switch
using R54, D27, R57 and R58. Generally speaking, the larger value of gate
resistance used, the slower the device switches. Slower switching reduces
over/undershoots and consequently EMI, but increases switching loss and hence
device junction temperature. Turning the device on uses R39 and R38. Turning
the device off uses D21+R33 in parallel with R39 and R38. In this way, different
turn on and turn off resistance can be used to optimize switching performance.
• D31 – This is a high voltage clamping diode located directly adjacent to the IC. It
is necessary to ensure correct operation of the IC during extreme transients that
can occur during a FAULT. In combination with R38 and R53, it ensures Pin6 of
U22 never goes more than 5V negative with respect to Pin2.
• R27 and R26 – These resistors form a passive Gate Emitter pull-down to ensure
the IGBTs stay off if the low voltage power supplies are not present.
1.4.4 Brake Chopper (Appendix A, Sheet 1)
Clearly, if the motor is used as a brake or generator, any average power that flows
back from the inverter must have somewhere to go. As the mains input and power
conditioning stages have only been designed for importing power, a means of
dissipating the excess power has been provided. The most common form of brake
chopper has been implemented and is described below:
• Q10 – A 600V N-Channel IGBT transistor with anti-parallel diode. This is of the
same type as used for the inverter for economic reasons. In practice a slower
switching device may be used which has lower conduction loss. Apart from
acoustic noise reasons, there is no reason to modulate the device at high
frequency. As the tab of the device is not isolated, a thermally conductive insulator
is used.
DS70096A-page 20
© 2003 Microchip Technology Inc.