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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

and

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=820k

Ω, R72 =1 00kΩ and R73 = 22kΩ.

Figure 3. VFG 243 LIMITS

Figure 4. Feed Forward Compensation Net-

work

100

90

80

70

60

50

40

50

KHz

150

KHz

500

KHz

10

KHz

30

MHz

D95IN254

APPROXIMATE

VFG243 LIMITS

R71

R72

C71 C72

R73

7

RECTIFIED

MAINS VOLTAGE

D95IN255