AND8019/D

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4

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,

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 k

W 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 (k

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.