Technique for Sizing Input Capacitors in Off-Line Power Converters

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Publication Date: 01-Sep-1990
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IBM Technical Disclosure Bulletin (TDB n4 09-90 p396-400)

Related People

Kelkar, S - Author
Endicott

Abstract

A schematic of the power stage of a typical off-line half bridge forward converter is shown in Fig. 1a. The input capacitors that form the subject of this disclosure are labelled C1 and C2. In Fig. 1 the incoming 110V 60Hz voltage is rectified by the diodes D1-D4 and then filtered by the two input capacitors C1,C2. RB1 and RB2 provide for the discharge of C1,C2 and sometimes smaller value capacitors are put in parallel across the resistors. The two MOSFET transistor switches Q1 and Q2 provide an alternating voltage to the power transformer with turns ratio Ns/Np. The alternating volatge is then rectified and filtered with the L-C output filer and fed to the load. The switching frequency of the MOSFET transistors Q1 and Q2 is typically of the order of 100 KHz.

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English (United States)

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Technique for Sizing Input Capacitors in Off-Line Power Converters

       A schematic of the power stage of a typical off-line half
bridge forward converter is shown in Fig. 1a.  The input capacitors
that form the subject of this disclosure are labelled C1 and C2.  In
Fig. 1 the incoming 110V 60Hz voltage is rectified by the diodes
D1-D4 and then filtered by the two input capacitors C1,C2.  RB1 and
RB2 provide for the discharge of C1,C2 and sometimes smaller value
capacitors are put in parallel across the resistors.  The two MOSFET
transistor switches Q1 and Q2 provide an alternating voltage to the
power transformer with turns ratio Ns/Np.  The alternating volatge is
then rectified and filtered with the L-C output filer and fed to the
load.  The switching frequency of the MOSFET transistors Q1 and Q2 is
typically of the order of 100 KHz.  Th1 and Th2 are thermistors used
to limit the in-rush current during turn-on.

      Fig. 2 shows the waveform of the two capacitor voltages: VC1
across C1 and VC2 across C2.  The incoming 60 Hz voltage is rectified
and fed to the two input capacitors, which then charge up to the peak
of the available voltage and then discharge by an amount WV before
the next charging pulse.  The transistors Q1 and Q2 are switching at
a much higher frequency so that the input capacitors are supplying
power to the transformer all along.  The key consideration in
selecting values for the input filter C1 and C2 is the amount of
ripple voltage WV, and this article presents a simple and accurate
technique for sizing the input capacitors based on the ripple
voltage.  The hold-up time is another important parameter which
affects the choice of C1 and C2, but this is not treated in this
article.  A procedure is presented which makes it possible to
calculate easily the desired input capacitance for a preselected
ripple voltage.

      The design procedure is explained with reference to Fig. 3
which shows the equivalent circuit of the power stage and Fig. 4
which shows the associated waveforms.  The transistors Q1 and Q2,
power transformer and output filter are represented by two current
sources I1, I2. Ts is the switching period of the transistors and the
interval Ton is controlled by the control loop (not shown in the
figure). The converter is shown to be of the voltage mode control
type with the ramp intersecting the error voltage to turn the MOSFETs
off.  The peak value of the currents I1 and I2 is a function of the
DC load current and the power transformer turns ratio:
 Peak value Ip of I1, I2 = (DC load current) *Ns/Np (1)
Ipdc is the average of the currents I1 and I2 waveforms. The key
assumption in the procedure is that since the switching frequency is
so much greater than the supply frequency (Ts<<T), the switching load
appears as a DC load of value Ipdc to the input capacitors C1, C2
during the time the voltage across the capacitors decreases by WV
from the peak value Vp (see Fig. 2).

      T...

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