AN-9719 데이터 시트보기 (PDF) - Fairchild Semiconductor
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AN-9719 Datasheet PDF : 12 Pages
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AN-9719
The typical voltage and current margins for the rectifier
diode are as follows:
1.2 ·
(18)
1.8 ·
(19)
where VRRM is the maximum reverse voltage and IF is the
current rating of the diode.
(Design Example) The diode voltage and current are
calculated as:
373
12
76.3
5.8
APPLICATION NOTE
Notice that there is a right-half-plane (RHP) zero (ωRZ) in
the control-to-output transfer function of Equation 21.
Because the RHP zero reduces the phase by 90 degrees, the
crossover frequency should be placed below the RHP zero.
Figure 13 shows the variation of a CCM flyback converter
control-to-output transfer function for different input
voltages. This figure shows the system poles and zeros with
the DC gain change for different input voltages. The gain is
highest at the high input voltage condition and the RHP zero
is lowest at the low input voltage condition.
1
·
1 0.48
5.8 · 0.31
1.87
0.48
5A and 100V diodes in parallel are selected for the
rectifier diode.
[STEP-10] Feedback Circuit Configuration
Since FSL-series employs current-mode control, the
feedback loop can be implemented with a one-pole and one-
zero compensation circuit.
The current control factor of FPS, K, is defined as:
(20)
2.5
where ILIM is the pulse-by-pulse current limit and VFBSAT is
the feedback saturation voltage, typically 2.5V.
As described in Step-4, it is typical to design the flyback
converter to operate in CCM for heavy-load condition. For
CCM operation, the control-to-output transfer function of a
flyback converter using current mode control is given by:
Figure 13. CCM Flyback Converter Control-to-Output
Transfer Function Variation for Different Input Voltage
Figure 14 shows the variation of a CCM flyback converter
control-to-output transfer function for different loads. Note
that the DC gain changes for different loads and the RHP
zero is the lowest at the full-load condition.
(21)
··
2
⁄
1⁄ 1⁄
·
1⁄
where RL is the load resistance.
The pole and zeros of Equation 21 are obtained as:
1
1
,
⁄
1
(22)
here D is the duty cycle of the FPS and RC is the ESR of CO.
Figure 14. CCM Flyback Converter Control-to-Output
Transfer Function Variation for Different Loads
When the input voltage and the load current vary over a wide
range, it is not easy to determine the worst case for the
feedback loop design. The gain, together with zeros and
poles, varies according to the operating conditions. Even
though the converter is designed to operate in CCM or at the
boundary of DCM and CCM in the minimum input voltage
and full-load condition; the converter enters into DCM,
changing the system transfer functions as the load current
decreases and/or input voltage increases.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/2/10
8
www.fairchildsemi.com
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