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dsPICDEM™ MCHV-3 Development Board User’s Guide
DS50002505A-page 32
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2016 Microchip Technology Inc.
3.1.2 Active Power Factor Correction (PFC)
The active PFC circuit is essentially a simple boost chopper with the control aimed at
shaping the input current to follow the incoming mains supply waveform. The purpose
of the different parts of the circuit is described below.
• L6 – A high-frequency axial inductor with a single layer winding on a ferrite core.
This component is in-series with the main inductor (L7) to reduce the effect of the
self-capacitance of its winding. Without L6, significant high-frequency (15 MHz)
ringing of the inductor current occurs at every transistor turn-on, which would
increase EMI and the PFC transistor switching loss.
• L7 – A power inductor with three stacked toroidal cores, made from a powdered
iron material, to limit the core loss while maintaining good energy storage density.
The particular cores used are Magnetics 0077083A7. A simple multilayer winding
is used, which results in moderate copper loss, but significant self-capacitance.
Sixty-six turns of a 1.5 mm diameter enameled copper wire is used. The design
offers a good compromise between cost, core loss and size for this application.
The nominal inductance is 1 mH at 10A.
• Q2 – A 600V TO-247 IGBT. As the tab of the device is not isolated, a thermally
conductive insulator is used. When closed, Q2 increases the energy stored in the
inductor, L7. When open, energy stored in the inductance is transferred to the DC
bus capacitors (C30-C32). Energy is also drawn from the AC supply during this
time. By appropriate control of the switches, the input current waveform can be
profiled to obtain good power factor and low harmonic distortion.
• D12 – A 600V TO-247 diode optimized for use at high switching frequency. As the
tab of the device is not isolated, a thermally conductive insulator is used.
• C28, R31, R32, R33, D11 – A “snubber” that acts to dampen high-frequency
oscillations and limit the rate of change of voltage across Q2.
• C30, C31, C32 – 470 µF/450V electrolytic capacitors, which act as the main DC
bus energy storage capacitors.
• U6 – The MCP14E4 device is made up of 4.0A buffers/MOSFET drivers and dual
non-inverting, enable inputs. It is capable of operating from a 4.5V to 18V
single power supply, and can easily charge and discharge a 2200 pF gate capaci-
tance in under 15 ns (typical). It provides low-impedance in both the ON and OFF
states to ensure the MOSFET’s intended state will not be affected, even by large
transients. The MCP14E4 inputs may be driven directly from either the TTL or
CMOS (2.4V to 18V). It is an ideal choice for this application, allowing up to 4A of
peak gate drive current to switch Q2 rapidly and therefore, achieve low switching
loss. It also has a small footprint, allowing it to be located physically close to the
transistors, allowing a low-inductance gate circuit layout.
• R90,R94,R101 – These resistors are used to pull down the corresponding
input/output pins of the gate driver.
• D9, C26, C27 – Inductance of the power tracking between the source of Q2, due
to the physical board layout, means there is a substantial transient voltage
between the +15V supply point reference at R28 and the source of Q2. This
simple, low-cost circuit allows the power supply of U19 to move transiently.
• R75, R29 and D10 – These components control the current and voltage to turn on
and turn off the IGBT. R29 controls the rising time and the di/dt when the IGBT is
turned ON. R29, in parallel with R75, controls the falling time and the di/dt when
the IGBT is turned off.