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General Circuit Operation

Primary Feedback Regulation

The circuit shown in Figure 7 is a simple

5 V, 5 W bias supply using the TOP200.

This universal input flyback power

supply employs primary-side regulation

from a transformer bias winding. This

approach is best for low-cost applications

requiring isolation and operation within

a narrow range of load variation. Line

and load regulation of

±5% or better can

be achieved from 10% to 100% of rated

load.

Voltage feedback is obtained from the

transformer (T1) bias winding, which

eliminates the need for optocoupler and

secondary-referenced error amplifier.

High-voltage DC is applied to the

primary winding of T1. The other side

of the transformer primary is driven by

Figure 7. Schematic Diagram of a Minimum Parts Count 5 V, 5 W Bias Supply Utilizing the TOP200.

the integrated high-voltage MOSFET

transistor within the TOP200 (U1). The

circuit operates at a switching frequency

of 100 kHz, set by the internal oscillator

of the TOP200.

The clamp circuit

implemented by VR1 and D1 limits the

leading-edge voltage spike caused by

transformer leakage inductance to a safe

value. The 5 V power secondary winding

is rectified and filtered by D2, C2, C3,

and L1 to create the 5 V output voltage.

The output of the T1 bias winding is

rectified and filtered by D3, R1, and C5.

The voltage across C5 is regulated by

U1, and is determined by the 5.7 V

internal shunt regulator at the

CONTROL pin of U1.

When the

rectified bias voltage on C5 begins to

exceed the shunt regulator voltage,

current will flow into the control pin.

Increasing control pin current decreases

the duty cycle until a stable operating

point is reached. The output voltage is

proportional to the bias voltage by the

turns ratio of the output to bias windings.

C5 is used to bypass the CONTROL pin.

C5 also provides loop compensation for

the power supply by shunting AC

currents around the CONTROL pin

dynamic impedance, and also determines

the auto-restart frequency during start-

up and auto-restart conditions. See DN-

8 for more information regarding the use

of the TOP200 in bias supplies.

PI-1749-012296

5 V

RTN

C5

47

µF

U1

TOP200YAI

D2

1N5822

D3

1N4148

L1

(Bead)

C2

330

µF

25 V

C3

150

µF

25 V

T1

D1

UF4005

DC

INPUT

VR1

1N4764

R1

22

Ω

CIRCUIT PERFORMANCE:

Load Regulation - ±4%

(10% to 100%)

Line Regulation - ±1.5%

95 to 370 V DC

Ripple Voltage ±25 mV

DRAIN

SOURCE

CONTROL