Cycle-by-Cycle Simulation

1. SIN,,IULATIONS AND PRACTICAL DESIGNS OF OFF-LINE CONVERTERS-THE FRONT END 571
82
Voltage
l(V8).V(in 1 ,in2)/abs(V(in1 , in2)) abs(V(in1,in2))
FIGURE 6-49 In-line equations can help to simplify the simulation fixture and reduce the computational time.
6.3.3 Cycle-by-Cycle Simulation
The exarnple makes use of the BCM FreeRunDT model, actually a current-mode borderline
controller already used in plevious chapters.
We encapsulated the rnultiplier and the enor amplifier of the MC33262 to simplify the draw-
ing. A special section monitors the output voltage in case it ran into a start-up voltage condition.
The controller is disabled by pulling the demagnetization pin high. This protection is absolutely
needed as a boost convefter, in essence, cannot run unloaded. As the output goes out of control,
the controller could only enter into a minimum /n,, situation where energy is always stored and
rcleased fiom the inductor: The outpr.rt keeps going up until smoke comes out! Should you try it
(for instance, with the first MC33261), the MOSFET would quickly blow up by exceeding its
BVrr. Motorola solved this issue a while ago, by adding a comparator observing the feedback pin,
as Fig. 6-50a X, does: It was the MC33262. New controllers such as the NCP1601 or NCP1653
include a dedicated OVP circuitry. Please note that a more comprehensive model of this MC33262
is available for download on the ON Semiconductor Web site (www.onsemi.com).
Figure 6-50b shows the simulation results obtained in 4 min on the 2.8 GHz computer.
Most numbers agree with the theoretical calculations. The auxiliary V,..is derived with a few
turns across the inductor. Given the configuration, expect large variations on the controller V,.,.
pin as the PFC operates on universal mains. Ifthe circuit does not clamp the gate-source volt-
age, it would be wise to safely clamp the V.... to I 5 V to ( I ) avoid the gate-source oxide bleak-
down and (2) limit C,,,Vor' dissipation. TheMC33262 accepts up to 30 V and safely clarnps
the Vo, to 15 V, so relax.
The input capacitor C,,, plays a role for both EMI and the PFC operation as Ref. 24 high-
lights. As such, this capacitor should be placed after the diode bridge and not before. Acting as
a reselvoir, it avoids the boost input voltage to dlop close to zero as the mains changes polarity.
It thereby limits the duty cycle/switching fi'equency discontinr"rity. However, if it is increased
too much, it opposes the smooth takeover between both mains positive and negative cycles, and
the power factol suffers. The rule of thumb suggested by the above reference is 3 pF/kW. MKP
types of capacitor (e.g., fi'om WIMA [25]) are good candidates for these devices.
As we stated above, a I A diode such as a I N4937 can suffer at power on, especially if the bulk
capacitor has been increased for hold-up time reasons. Ilthis is the case, you can use a bigger diode
t/
20 R1
+ l.6Meg
, v,n

2. x8
KBU4 J
Fsw
Y
121
Aes
f voltage
I ({Vrms}^z((L}'2'(Pou0))
- '(1-V(rect)4YouJ)
gl
N
x7
XFMFIRATIO =
_5OM
82 <H11
1.6Meg >som
Cout
{1 50 u/ratio}
lC = {Vrms-1
B9
23k
c2
{0.68u/ratio}
tc=3
i y9u' I8'rorn'"
Parameter
ratio =10
Fline = so*ratio
Vrms = 90
vod= 4oo
L = 850u
Pout = 150
FIGURE 6-50a The cycle-by-cycle simulation of the MC33262-based 150 W PFC using the diode bridge.

3. SIMULATIONS AND PRACTICAL DESIGNS OF OFF-LINE CONVERTERS THE FRONT END 573
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-
FIGURE 6-50c The bypass diode routes the in-rush curLent away from the
catch diode.
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6 ! s.oo
*: r.oo
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3 4.80
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P 7.00
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r s i.oo
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3'1.5m 32.'lm 32.7m
Time in seconds
FIGURE 6-50b Simulation resr.rlts ol the cycle-by-cycle 150 W PFC for a 90 Vrms input volrage.
such as the MUR460 (4 A, 600 V) or install a bypass diode. This bypass element D, actually plays
a role at start-up, but then remains silent the rest of the time. It absorbs the vast majority of the
in-rush cunent and prevents the catch diode D" from undergoing current surges. Figr"rre 6-50c
shows how to install this external component. An MR756 (6 A, 600 V) can be selected.
D1
MR756
+ Cbulk
.D2Lboost
1 N4937