DER-33

3.9W CC/CV TNY266P Charger

April 1, 2004

Page 6 of 24

Power Integrations

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5

Circuit Description

This circuit is configured as a Flyback operating in both continuous and discontinuous

conduction mode. The low standby consumption is achieved by using a high gain opto-

coupler, using a bias winding that provides about 10V during no-load, and by designing a

low-capacitance transformer.

5.1

Input Rectification, Bulk Capacitance and EMI Filtering

AC input power is rectified by a full bridge, consisting of D1 through D4. The rectified DC

is then filtered by the bulk storage capacitors C1 and C2. Inductor L1 and Ferrite bead

L2 separate C1 and C2 from each other. L1, C1 and C2 form a pi (

π) filter, which

attenuates conducted differential-mode EMI noise. Fusible resistor RF1 has multiple

functions. It is a fuse, an in-rush current limiting device, a final low pass filter stage (with

C1) for conducted EMI attenuation and an initial stage of input surge voltage attenuation.

5.2

Primary DRAIN Voltage Clamp Circuit

The DRAIN voltage clamp circuit is comprised of C3, R1, R2 and diode D5. D5 and C3

clamp the amplitude of the voltage spike that the transformer leakage inductance

generates, at switch turn-off, to keep it beneath the device’s maximum DRAIN to

SOURCE voltage rating (700 V). R2 damps the high frequency ringing caused by

leakage inductance, which improves the conducted EMI performance of the circuit.

5.3

Auxiliary Bias Supply

The TinySwitch-II normally does not need a bias supply because it has a high voltage

current source to supply the internal chip consumption. If an external current is applied to

the BP pin (which is the internal power supply of the chip), it turns off the HV current

source and regulates the voltage on the BP pin like a zener. The power dissipated in the

HV current source is saved. This power savings is on the order of 50-100 mW. This is

needed to achieve a <100mW standby consumption.

The auxiliary bias supply circuit is made up of the primary-side transformer bias winding,

diode D6 and capacitor C5. D6 rectifies the output of the winding and C5 filters it. The

winding was given just enough turns so that its minimum output voltage stays at 10V at

no-load to minimize power consumption. C4 is the standard BP pin decoupling capacitor,

which should always be a 50 V 0.1

µF ceramic capacitor that is located close to the IC. R3

is used to regulate the current into the BP pin.

5.4

Output Rectification and Filtering

Output rectification and filtering are accomplished by Schottky diode D7, capacitors C6

and C7. D7 rectifies the output of the transformer, T1. R10 and C8 dampen out the high

frequency interaction between D7, T1 and U1, to reduce conducted EMI noise

generation. C6 filters the initial rectified output, while L3 and C7 serve as a secondary

low-pass filter stage, which further reduce the output ripple voltage.