AN829 Datasheet, PDF (5/9 Pages) STMicroelectronics – SEMICONDUCTOR KIT FOR

AN829 APPLICATION NOTE
SWITCHING FREQUENCY fs is kept below 50kHz to maintain the RFI noise within the optimum frequency
range of the VFG 243 standard. Figure 3 shows that in the range of 10kHz/50kHz the switching frequency fun-
damental spectrum requires less filtering. C18 = 2.2nF and R17 = 24k2 give a switching frequency of 46kHz.
BOOST INDUCTOR: For applications in the range of 3kW, the boost inductor design is greatly determined by
material choice and core availability.
As mentioned the inductor parasitic capacitance determines the filter requirements in the range 1MHz-30MHz
(with regards to the switching noise). To keep this capacitance as low as possible, a single layer winding is the
best solution, but this requires the use of a toroidal core shape to balance winding turns and core section. In this
case a material with distributed air gaps, for example iron powder, is a good choice (see #2).
This is the cheapest core material available with an inherently high saturation flux density combined with a dis-
tributed air gap. Test have been performed using two different cores:
– T300-26D core from MICROMETAL (Anaheim, CA) made of iron powder / 60 turns / 12 AWG.
– 2 x CO55866A2 core from MAGNETICS, made of nickel, iron and molybdenum / 46 turns / 12 AWG.
OUTPUT CAPACITANCE value depends upon the expected output voltage drop during the specified holdup
time. Considering a value of 2000F, the voltage drop during a 10msec holdup can be calculated from:
Pout × 10msec = 1/2 Cout (Vout - Vmin)
then V_min = 382V at 1400W
and V_min = 360V at 3000W
FEED FORWARD COMPENSATION:
Pin 7 should be supplied with a voltage proportional to the rms mains voltage. This voltage must be in the range
of 1.5V to 5.5V.
The circuit shown in figure 4 gives a good compromise between response time and harmonic attenuation with
the following values: C71=68nF, C72 = 1mF, R71=820kΩ, R72 =1 00kΩ and R73 = 22kΩ.
Figure 3. VFG 243 LIMITS
Figure 4. Feed Forward Compensation Net-
work
100
APPROXIMATE
90
VFG243 LIMITS
80
70
60
50
40
10 50
KHz KHz
150
KHz
500
KHz
D95IN254
30
MHz
RECTIFIED
MAINS VOLTAGE
R71
R72
C71 C72
D95IN255
7
R73
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