AND8112 Datasheet, PDF (1/12 Pages) ON Semiconductor – A Quasi-Resonant SPICE Model Eases Feedback Loop Designs

AND8112/D
A Quasi−Resonant SPICE
Model Eases Feedback
Loop Designs
Prepared by: Christophe Basso
Prepared by: Joel Turchi
ON Semiconductor
http://onsemi.com
Within the wide family of Switch Mode Power Supplies
(SMPS), the Flyback converters represent the structure of
choice for use in small and medium power applications. For
compact designs and radio−frequency sensitive
applications, e.g. TV sets or set−top boxes, Quasi− Resonant
power supplies start to take a significant market share over
the traditional fixed frequency topology. However, if the
feedback loop control is well understood with this latter, for
instance via a comprehensive literature and SPICE models,
the situation differs for self−oscillating variable switching
frequency structures where no model still exists. This article
will show how a simple large−signal averaged SPICE model
can be derived and used to ease the design work during
stability analysis.
Quasi−Resonant Operation
It is difficult to abruptly dig into the analytical analysis
without giving a basic idea of the operation of a converter
working in Quasi−Resonance (QR). Figure 1 depicts a
typical FLYBACK converter drain−source waveform as you
probably have already observed. When the switch is closed,
the drain−source voltage V_DS is near 0 V and the input
voltage V_g appears across the primary inductance L_P: the
current inside L_P ramps up with a slope of
SON
+
Vg
LP
(eq. 1)
When the controller instructs the switch opening, the
drain−source quickly rises and the energy transfer between
primary and secondary takes place: the secondary diode
conducts and the output voltage flies back on the primary
side, over L_P. This “Flyback” plateau is equal to V_g + (V +
V_f) / N, where N is the secondary to primary turn ratio, V the
output voltage and V_f the diode forward voltage drop.
During this time, the primary current decreases with a slope
now imposed by the reflected voltage
SOFF
+
(V
N
)
Vf)
LP
(eq. 2)
Figure 2 zooms on the simulated primary current (actually
circulating in the magnetizing inductor), showing how it
moves over one switching cycle.
Leakage Inductance
Plateau:
(V_out + V_f)/N
Core is Reset
V_in
ON
OFF
Valleys
Figure 1. A Typical FLYBACK Drain−Source
Waveform
S_on = V_g/L_P
I_peak
S_off = (V + V_f) / (L_P x N)
ON OFF _{IP = 0, Reset}
0
Figure 2. The Primary Current Ramps Up and Down
to Zero in DCM
When the primary current reaches zero, the transformer
core is fully demagnetized: we are in Discontinuous
Conduction Mode (DCM). The primary inductance L_P
together with all the surrounding capacitive elements C_tot
create a LC filter. When the secondary diode stops conducting
at I_P = 0, the drain branch is left floating since the MOSFET
is already open. As a result, a natural oscillation occurs,
exhibiting the following frequency value:
Fring
+
2
@
p
@
1
Ǹ_LP
@
Ctot
(eq. 3)
© Semiconductor Components Industries, LLC, 2003
1
October, 2003 − Rev. 1
Publication Order Number:
AND8112/D