dsPICDEM? MC1H 3-Phase High Voltage Power Module
The third device (U2) is on the positive DC input connection from the rectifier within
the input power stage (see Figure 1-1). This is configured to have 2 turns of wire
through it as delivered with LK2 installed. This gives a maximum sensed current of
9.6A and a gain of 4.8A/V. U2 serves two purposes. First, is to provide the required
current feedback information for controlling the active power factor correction (PFC)
switches. The second purpose is to provide detection of ground current faults (i.e.,
when one or more of the inverter outputs become incorrectly connected to ground).
1.4.6
1.4.6.1
Shunt Feedback (Appendix A, Sheet 3)
INTRODUCTION
In many applications, the cost of isolated current transducers is prohibitive or the
isolation is not required. In these instances, it is common practice to use resistive
shunts. The shunts can be placed in series with the output connections but this
inevitably requires the use of level-shifting and the creation of floating power supplies.
Therefore, it is common to have shunts referenced to the same potential as the low
voltage power supplies which is invariably the -DC bus. The disadvantage of using
shunts referenced to the “low side” is that the feedback information is only available
during certain portions of the PWM cycle. In order to extract the required information,
the shunts must be sampled at precise intervals. This feature has been allowed for in
the dsPIC device by the inclusion of sample and hold amplifiers in the ADC module
that can be triggered from the PWM module (see the dsPIC30F Family Reference
Manual for details).
On high voltage low power systems, it may be possible to use shunts of high enough
value to have sufficient voltage for direct input to an ADC. More commonly, the
dissipation in the shunt dictates the use of low resistance values and appropriate
amplification. If too high, the voltage drop across the shunt can also cause problems
for driving the power transistors. For low gains, it may be possible to use single-ended
amplification. Differential amplification is required as the gains increase in order to
provide a signal of sufficient quality and accuracy.
The Microchip MCP6022 (dual) and MCP6024 (quad) parts have been used in the
design for providing the differential amplification. They are an ideal choice for the
following reasons:
? Low power consumption (1 mA typical/amp) – this is especially important where
the power supplies are derived from the DC bus with a resistor/zener network.
? Low offset voltage (0.5 mV max.).
? Rail-rail inputs and outputs.
? No gain inversion with negative inputs – this is important as it is common for the
amplifier to have negative spikes on its inputs that occur during switching events
due to the shunt's inductance. Gain inversion can lead to current control instability
and false tripping of protection circuits.
? High gain bandwidth product (10 MHz typical) – this is often a more important
factor in determining the speed of response than slew rate in this application, due
to the requirements to amplify rapidly changing signals of low amplitude.
Note that in applications requiring less gain and/or slower speed of response, the
MCP602/604 family offers a good alternative with even lower power supply
consumption.
DS70096A-page 22
? 2003 Microchip Technology Inc.
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