Current mode power supply 30PS203
GB 58
DVD 950
10.
10.6 Operating description
The input voltage Vcc(pin 1) is monitored by a comparator with
hysteresis, enabling the circuit at 14.5V and disabling the
circuit below 7.5V. The error amplifier compares a voltage
Vfb(pin 14) related to the output voltage of the power supply,
with an internal 2.5V reference. The current sense comparator
compares the output of the error amplifier with the switch
current Isense(pin 7) of the power supply. The output of the
current sense comparator resets a latch, which is set every
cycle by the oscillator. The output stage is a totem pole,
capable of driving a MOSFET directly.
10.6.1 Start-up sequence
t1: Charging the capacitor at Vcc
C2129 will be charged via R3123 and R3134, C2133 and
C2111 via R3129. The output is switched off during t1.
t2: Charging of output capacitors
When the input voltage of the IC exceeds 14.5V, the circuit is
enabled and starts to produce output pulses. The current
consumption of the circuit increases to about 17mA, depending
on the external loads of the IC. At first, the capacitor at the Vcc
pin will discharge because the primary auxiliary voltage,
coming from winding 7-9 is below the Vcc voltage. At some
moment during t2, the primary auxiliary voltage reaches the
same level as Vcc. The Vcc voltage is now determinated by this
primary auxiliary voltage.
t3: regulation
The output voltage of the power supply is in regulation
t4: overload
When the output is shortened, the supply voltage of the circuit
will decrease and after some time drop below the lower
threshold voltage. At that moment, the output will be disabled
and the process of charging the Vcc capacitor starts again. If
the output is still shorted at the next t2 phase, the complete
start-and stop sequence will repeat. The power supply comes
in a hiccup mode
Figure 10-5 Start-up sequence
10.7 Regulation
Figure B shows the most relevant signals during the regulation
phase of the power supply.
The oscillator voltage ramps up and down between V1 and V2.
The voltage at the current sense terminal is compared every
cycle with the output of the error amplifier Vcomp. The output
is switched off when the current sense level exceeds the level
at the output of the error amplifier.
1.
TimeON phase : A drain current will flow from the positive
supply at pin 1 of the transformer through the transformer's
primary winding, the MOSFET and Rsense to ground. As
the positive voltage at pin 1 of the transformer is constant,
the current will increase linearly and create a ramp
dependent on the mains voltage and the inductance of the
primary winding. A certain amount of energy is stored in the
transformer in the form of a magnetic field. The polarity of
the voltages at the secundary windings is such that the
diodes are non-conducting.
2.
TimeDIODE phase : When the MOSFET is switched off,
energy is no longer supplied to the tranformer. The
inductance of the tranformer now tries to maintain the
current which has been flowing through it at a constant
level. The polarity of the voltage from the transformer
therefore becomes reversed. This results in a current flow
through the tranformer's secondary winding via the diodes,
electrolytic capacitors and the load. This current is also
ramp shaped but decreasing.
3.
TimeDEAD phase : when the stored energy has been
supplied to the load, the current in the secondary windings
stops flowing. At this point the drain voltage of the
MOSFET will drop to the voltage of C2121 with a ringing
caused by the Drain-Source capacitance with the primary
inductance.
The oscillator will start a next cyclus which consists of the
described three phases. The time of the different phases
depends on the mains voltage and the load.
TimeDEAD is maximum at an input of 400VDC and minimum
load, it will be zero at an input of 100VDC and overload.
Figure 10-6 Regulation
1mA
Vo
0
OUTPUT
t4
short
Icc
0V
17mA
14.5V
7.5V
t1
Vcc
p.a.v.
t2
t3
10V
CL 96532065_068.eps
130799
Vosc
Idiodes
Vgate
Vdrain
Idrain
Vsense
Vcomp
0
V2
V1
Ton
Tdiode Tdead
CL 96532065_069.eps
130799
