MC33364
http://onsemi.com
7
OPERATING DESCRIPTION
Introduction
The MC33364 series represents a variable--frequency
current--mode critical--conduction solution with integrated
high voltage startup and protection circuitry to implement an
off--line flyback converter for modern consumer electronic
power supplies. Different frequency clamp options offer
different customized needs. This device series includes an
integrated 700 V Very High--Voltage (VHV) start--up
circuit. Thus, it is possible to design an application with
universal input voltage from 85 Vac to 265 Vac without any
additional startup circuits or components.
The critical conduction feature offers some advantages.
First, the MOSFET turns on at zero current and the diode
turns off at zero current. The zero current reduces these
turn--on and turn--off switching losses. It also reduces the
Electro--Magnetic Interface (EMI) of the SMPS and a less
expensive rectifier can be used. Second, by preventing the
SMPS from entering the discontinuous conduction mode
(DCM), the peak MOSFET drain current is limited to only
twice the average input current. It needs a smaller and less
expensive MOSFET. Third, by preventing the SMPS from
entering the Continuous Conduction Mode (CCM), the
flyback topology transfer function stays first--order and its
feedback compensation network is considerably simplified.
It also maximizes the power transfer by the flyback
transformer to its 1/2 L I
2
limits.
A description of each of the functional blocks is given
below. The representative block diagram and typical
application circuit are in Figure 1 and Figure 2.
Line, V
CC
, Startup Circuit and Reference Voltage
The Line pin is capable of a maximum 700 V so that it is
possible to connect this pin directly to the rectified
high--voltage Alternating Current (AC) input for
minimizing the number of external components. There is a
startup circuit block that regulates voltage from the Line pin
to the V
CC
pin in an abnormal situation. In normal
conditions, the auxiliary winding powers up the V
CC
and
this startup circuit is opened and saves approximate 0.7 W
of power compared to the resistor bootstrapped circuits.
In normal operation, the auxiliary winding powers up the
V
CC
voltage. This voltage is a constant value between the
UVLO limits (7.6 V and 15 V). It is further regulated to a
constant 5 V reference voltage V
ref
for the internal circuitry
usage. As long as the V
CC
voltage is between 7.6 V and
15 V, it means the auxiliary winding can provide voltage as
in normal condition. The device recognizes that there is no
fault in the circuit and the device remains in the normal
operation status.
However, when the auxiliary winding cannot power up
V
CC
,theV
CC
voltage will reach its UVLO limit. The device
recognizes that it is an abnormal situation (such as startup or
output short--circuited). The V
CC
voltage is not constant in
this case. Figure 3 shows the timing diagram in a fault
condition. There are three Under--Voltage Lock--Out
(UVLO) thresholds with respect to V
CC
. The upper
threshold is 15 V. When this limit is reached, the startup
circuit block turns off and V
CC
declines due to power
consumption of the circuitry. The startup circuit block turns
on when V
CC
reaches 7.6 V and if V
ref
is higher than 3.7 V.
It is the second threshold of V
CC
.IfV
ref
is smaller than
3.7 V, the startup circuit will turn on when V
CC
reaches a
temperature dependent value V
T
ranging between 3.5 V and
6 V. It is the last threshold of V
CC
. This temperature
dependent threshold is lower when temperature is higher so
that it takes a longer time to restore the V
CC
. It is a protection
feature, which allows more dead time for cooling in high
temperature condition.
There is an UVLO in the V
ref
regulator block. When V
CC
falls below typical 8.1 V in abnormal situation, the V
ref
regulator block stops. V
ref
and V
CC
collapses due to power
consumption of the circuitry. When V
ref
collapses to below
3.7 V, the device cannot provide the Drive output and makes
a dead time. This dead time is designed for minimal power
transfer in the abnormal conditions. The dead time ends
when V
CC
reaches 15 V after reaching the UVLO limit V
T
(3.5 to 6 V). Reaching V
T
enables the startup circuit block,
charging up the V
CC
capacitor again. When V
CC
reaches
15 V again, the V
ref
regulator block turns on and allows the
output to work again.
It is recommended to put a 0.1 uF capacitor on V
ref
pin for
stability of the voltage buffer. The V
CC
capacitor is
relatively larger than this 0.1 uF capacitor, making a longer
V
CC
charging time from V
T
to 15 V and a longer dead time
in the abnormal or fault conditions.
Zero Current Detect
To achieve critical conduction mode, MOSFET
conduction is always initiated by sensing a zero current
signal from the Zero Current Detect (ZCD) pin. The ZCD
pin indirectly monitors the inductor current by sensing the
auxiliary winding voltage. When the voltage falls below a
threshold of 1.0V, the comparator resets the RS latch to turn
the MOSFET on. There is 200 mV of hysteresis built into the
comparator for noise immunity and to prevent false tripping.
There are 10 V and 0.7 V clamps in the ZCD pin for
protection. An external resistor is recommended to limit the
input current to 2 mA to protect the clamps.
Watchdog Timer
A watchdog timer block is added to the device to start or
restart the Drive output when something goes wrong in the
ZCD. When the inductor current reaches zero for longer than
approximate 410 ms, the timer reset the RS latch and that
turns the MOSFET on.
