AND8112 Datasheet, PDF (8/12 Pages) ON Semiconductor – A Quasi-Resonant SPICE Model Eases Feedback Loop Designs

AND8112/D
Operating Parameter Calculation
FB
errint
t_on
R9
1 Meg
B8
Voltage
V(FB)/3 > 1 ? 1 : V(FB)/3 < 10 m
? 10m : V(FB)/3
Bton
Voltage
V(errint)*{Lp}/({Rs}*V(13))
X2
XFMR
RATIO = N
System Parameter calculation
I_P
t_off
BIp
Voltage
V(ton)*V(13)/{Lp}
BToff
Voltage
{Lp}*V(Ip)*{N}/V(3)
13
F_SW
Bfreq
Voltage
(1/(V(ton)+V(toff)))/1 k
13
Lm
{Lp}
12
1
1
VI1
Bclamp
20
Current
I(VI1)>0 ?
V6
I(VI1) :
7
0
BEt
Voltage
(2*{Lp}*(V(3)+{N}*V(13))/(V(ton)*V(3)+1u))*I(V6)
Rs
{Rlf}
3
3
1
4
BGd
Current
(((2*{Lp}*(V(3)+{N}*V(13))/(V(ton)*V(3)+1u))*I(V6)^2)/(V(3,4)+1u))*{eff}
4
Gnd
4
Figure 10. The final simplified model implementation where added sources reveal operating parameters
such as T_on, F_SW and the peak current I_P
Figure 10 portrays the final simplified model subcircuit where all relevant sources appear, among them, the switching
frequency, peak current and T_on calculations. For the extended model, only BGd and BEt sources need to be changed. As you
can see, there are plenty denominator expressions where a variable such as T_on appears. If during the bias point calculation
SPICE T_on starts or goes close to zero, the simulator can fail to converge (or find a wrong bias point which is worse). To avoid
this potential problem, a trick consists in inserting a fixed value, small enough like 1 m or less, to clamp the maximum value
the source can take if T_on becomes null. To the opposite, the frequency expression modeled by a voltage source can deliver
kV to express kilo Hz. The simulator dynamic being bounded, mixing values of a few mV with sources delivering kV can puzzle
the bias point calculation. Again, a division by 1000 will limit the range. The FB pin undergoes a division by 3 to be further
clamp by a 1 V limiter, a classical circuitry found in most PWM controllers (I_P max = 1 V / R_sense).
DC−bias calculation always represents a difficult task for SPICE simulators running averaged models. In order to enhance
the extended model robustness (the one including parasitic effects), we have constrained the BGd source to be positive only
by using a simple in−line equation that differs depending on the simulator syntax:
IsSpice
BGd 4 3 I= ((2*{Lp}/V(ton)) * ( ({N}*V(13)+V(3))/(V(3)+1u) + {DEL}/V(ton) +
+({Lp}*{Ctot}/V(ton))*(1+(V(3)/{N})/V(13)) ) * I(V6)^2)/(V(3,4)+1u)*{EFF} < 10m ? 10m :
+((2*{Lp}/V(ton)) * ( ({N}*V(13)+V(3))/(V(3)+1u) + {DEL}/V(ton) +
+({Lp}*{Ctot}/V(ton))*(1+(V(3)/{N})/V(13)) ) * I(V6)^2)/(V(3,4)+1u)*{EFF}
PSpice
Gd 4 3 TABLE { ((2*{Lp}/(V(ton)+10n)) * ( ({N}*V(13)+V(3))/(V(3)+1u) + {DEL}/(V(ton)+10n) +
+({Lp}*{Ctot}/(V(ton)+10 n))*(1+(V(3)/{N})/V(13)) )
+ * I(V6)^2)/(V(3,4) + 1u)*{EFF} } ( (10m,10m) (1000,1000) )
Finally, the model comes with two different names:
.SUBCKT QuasiFly 13 FB GND 3 I_P T_on F_SW params: L_P = 3.22 m R_S = 0.5 N = 0.06 eff = 0.86
the simplified model version
QuasiFlyDel 13 FB GND 3 I_P T_on F_SW params: L_P = 3.22 m R_S = 0.8 N = 0.06 eff = 0.86 C_tot = 100 p
the complete model including parasitic effects
Passed parameters are:
C_tot, the lump parasitic component present on the drain.
L_P, the primary inductance
R_lf, the ohmic losses of the primary winding
N, the N_P : N_S ratio with N_P=1
Eff, the circuit estimated efficiency
Please note that for the sake of simplicity, both models do not account for the secondary rectifier forward drop V_f whose effect
is nevertheless negligible in our approach.
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