AND8019 Datasheet, PDF (4/12 Pages) ON Semiconductor – Offline Converter Provides 5.0 Volt, 1.0 Amp Output for Small Electronic Equipment

AND8019/D
Theory of Operation
Input
The ac source is connected to the input terminals of this
unit. Fuse F1 is a safety device for fire protection and not
intended to protect the circuit from overcurrent conditions.
C1, R6 and R7 comprise the input EMI filter. Diodes D1–D4
rectify the ac line which is then filtered by capacitor, C2.
Primary Power Circuit
The NCP1000 power switching regulator chip contains
the control circuit, startup circuit and power switch circuit.
The startup circuit allows a small amount of current from
pin 5 to charge C8 and C5. When this voltage reaches
approximately 8.5 volts, the unit will commence operation
as can be observed at pin 5. Once the unit begins switching,
it’s power is derived from the auxiliary winding of the
transformer through D6 and C8. Power from C8 is delivered
to pin 1.
C5 provides energy storage at pin 1. R1, which is placed
in series between the aux winding supply (C8) and pin 1,
limits the current into the V_CC pin. The V_CC voltage is
supplied by the aux winding and limited by an internal
8.6 volt shunt regulator.
Primary Regulation Circuit
The NCP1000 receives the error signal via an
opto–coupler at pin 2, the Feedback input. This input has an
internal 2.7 kW resistor to ground. As current from the
opto–coupler flows into pin 2, it develops a voltage across
the internal resistor. This voltage is used as the error signal
into the PWM comparator to determine the duty cycle. As
the voltage on pin 2 increases, the duty cycle will decrease,
and therefore, the output power will decrease.
Secondary Regulation
Regulation is accomplished by comparing the output
voltage (voltage divider R4 and R5) to a fixed reference
within the TL431 regulator. The TL431 also has an internal
amplifier which is used as the error amplifier for this circuit.
The output of the TL431 conducts a current that biases the
photodiode of U2. This in turn causes the phototransistor of
U2 to conduct and provides a voltage to the NCP1000 chip
that has the error information required for regulation. C10 is
used to compensate the internal error amplifier in the TL431
for frequency stability.
Current Limit Protection
The NCP1000 includes an internal current limit circuit.
The nominal threshold is 0.50 amps peak of switch current.
If the current tries to exceed this level, the current limit
comparator will terminate the pulse at 0.50 amps. Under
current limit conditions, the output voltage will reduce as
necessary to maintain a this maximum switch current. Due
to the characteristics of a flyback converter, the unit will go
into a constant power mode in current limit. This means that
as the output voltage is reduced, the output current will
increase.
As the output approaches a short circuit condition, the
auxiliary winding voltage will reduce along with the output
voltage. When the auxiliary winding is reduced below the
UVLO shutdown level, the unit will shutdown, and time out
for eight Vcc charging cycles, to reduce power dissipation,
before restarting. Due to leakage inductance spikes, the
auxiliary voltage may not track the output voltage
proportionally. If short circuit protection is required, it is
recommended that a secondary side current limit circuit be
used to assure unit will be completely protected under short
circuit conditions.
Modification of Output Voltage or Current
This circuit has been designed to provide a regulated
5.0 volt output at a maximum current of 1.0 amp. Changes
will require redesign in several areas of the circuit.
The output voltage is determined by comparing the output
of the voltage divider of R4 and R5 to the 2.5 volt internal
reference in the TL431. This resistive divider must be
modified to change the output voltage. To do this, first
choose the bias current that you want in the divider –1.0 mA
is a good rule of thumb. The voltage across R5 will always
be 2.5 volts, so the equation for R5 is:
R5 (kW) = 2.5 volts / I_bias (mA)
and
R4 = R5 (Vo – 2.5 volts) / 2.5 V
Since it is necessary to maintain a voltage of at least
9.5 volts on C8 at all times, the auxiliary winding of the
transformer will be affected by any change in output voltage.
Consult the transformer manufacturer for modifications to
this component.
Capacitors C6 and C7, and diode D5 are all affected by the
output voltage and may need to be changed depending on the
direction and magnitude of the voltage change.
Any increase in power can have effects on a number of
components. The NCP1000 can process power of at least 10
watts for a universal input, however the transformer, output
rectifier (D5) and filter caps will need to be analyzed for
their suitability.
Component Substitutions
Similar components may be substituted for those on the
parts list, however, there are certain parameters for some
switching power supply components that need to be
considered when doing so.
Rectifiers require the same voltage and current ratings as
those specified. In addition, verify that their speed is equal
to, or better than that of the specified device.
Capacitors in this type of power converter are subjected to
very high rms switching currents. Any substitutions need to
be checked to assure that the ripple current rating is better
than, or equal to that of the specified device.
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