Convertidor De Corriente Continua Buck-Boost

AN120 Page 1 of 8

AN120 Discontinuous Current Inverted Buck-Boost Converter Using a LPT E2000Q Core By Colonel Wm. T. McLyman The principle behind flyback converters is based on the storage of energy in the inductor during the charging, or on period, ton, and the discharge of energy to the load during the off period, toff.
Ic + ICR1 C1 Vin L1 C2 Vo Q1 CR1 Io -

-

+

Figure 1. Invertedbuck-boost converter, discontinuous current. Energy Transfer In the discontinuous mode the energy, stored in the inductor, is completely transferred to the load circuits, during the off time, before another switching period occurs, as shown in Figure 2. The discontinuous current B-H loop is shown in Figure 3.
VQ1 V o+ V d V in 0 t D is con tin u ou s C u rren t

IL

Ip k ∆ I = Ip k Io

0 ton T toff tw

t

Figure 2. Inverted buck-boost converter discontinuous current, ideal voltage and current waveforms.

www.coremaster.com

AN120 Page 2 of 8

B (tesla)

∆B

H ∆I B ac = ∆ B /2

D iscontin u ou s C u rrent

Figure 3. B-H loop for the continuous current inverted buck-boost converters. When designing a LPT inductor, and after the core geometry (size) has beenselected, then, the correct core permeability can be calculated. Care must be used in selecting the right permeability so the core does not saturate at the maximum amp-turns to which it will be subjected. The dc magnetizing curves for LPT cores are shown in Figure 4. The permeabilities for LPT cores range from 60 to 500 perm. The engineer will select a core with the highest permeability that will notsaturate at maximum load current. This core will produce an inductor with the smallest size.

110 100
LPT Cores E-2000Q 80% Permeability

500µ 400µ 250µ 200µ 125µ 60µ

Percent Permeability

80
500u = 24, 400u = 31, 300u = 36, 250u = 42, 200u = 52, 150u = 61, 125u = 91, 60u = 180, oersteds oersteds oersteds oersteds oersteds oersteds oersteds oersteds

60

40

20 0 1 10 100 DCMagnetizing Force (Oersteds) 1000

Figure 4. DC magnetization curves for LPT cores.

www.coremaster.com

AN120 Page 3 of 8 For a typical design example, assume a inverted buck-boost converter, as shown in Figure 1, with the following specifications: Discontinuous Current Inverted Buck-Boost Converter Inductor Design specification 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Input voltage................................................................................. Input voltage ................................................................................. Input voltage ................................................................................. Output voltage ............................................................................... Output current............................................................................... Dwell time duty ratio...................................................................... Frequency ..................................................................................... Efficiency ...................................................................................... Regulation..................................................................................... Operating flux density ................................................................... Window utilization ........................................................................ Diode voltage drop ........................................................................ Vnom = 15 V Vmin = 12 V Vmax = 18 V Vo = -12 V Io = 2 A Dw = 0.1 f = 100 kHz = 90% =1% Bm =0.4 T Ku = 0.4 Vd = 1.0 V

Skin Effect The skin effect on an inductor is the same as a transformer. In the normal dc inductor the ac current (ac flux) is much lower and does not require the use of the same maximum wire size. This is not the case in the discontinuous current type flyback converter where all of the flux is ac and no dc. In the discontinuous flyback design, the skin effect has to...