MC44608_06 Datasheet, PDF (14/15 Pages) ON Semiconductor – Few External Components Reliable and Flexible SMPS Controller

MC44608
The secondary reconfiguration is activated by the mP
through the switch. The dV/dt appearing on the high voltage
winding (pins 14 of the transformer) at every TMOS switch
off, produces a current spike through the series RC network
R7, C17. According to the switch position this spike is either
absorbed by the ground (switch closed) or flows into the
thyristor gate (switch open) thus firing the MCR22−6. The
closed position of the switch corresponds to the Pulsed
Mode activation. In this secondary side SMPS status the
high voltage winding (12−14) is connected through D12 and
DZ1 to the 8.0 V low voltage secondary rail. The voltages
applied to the secondary windings 12−14, 10−11 and 6−7
(Vaux) are thus divided by ratio N12−14 / N9−8 (number of
turns of the winding 12−14 over number of turns of the
winding 9−8). In this reconfigured status all the secondary
voltages are lowered except the 8.0 V one. The regulation
during every pulsed or burst is performed by the zener diode
DZ3 which value has to be chosen higher than the normal
mode regulation level. This working mode creates a voltage
ripple on the 8.0 V rail which generally must be post
regulated for the microProcessor supply.
Figure 19. SMPS Pulsed Mode
The Figure 19 shows the SMPS behavior while working
in the reconfigured mode. The top curve represents the V_CC
voltage (pin 6 of the MC44608). The middle curve
represents the 8.0 V rail. The regulation is taking place at
11.68 V. On the bottom curve the pin 2 voltage is shown.
This voltage represents the current sense signal. The pin 2
voltage is the result of the 200 mA current source activated
during the startup phase and also during the working phase
which flows through the R4 resistor. The used high
resolution mode of the oscilloscope does not allow to show
the effective ton current flowing in the sensing resistor R11.
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