AN3203 datasheet(15/49 Pages) STMICROELECTRONICS

Part #	AN3203

Description	This application note describes the characteristics and performance

PDF	49 Pages

Scroll/Zoom	100%

Manufacturer	STMICROELECTRONICS [STMicroelectronics]

Direct Link	http://www.st.com

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15 / 49 page

AN3203

Asymmetrical half bridge operation

Doc ID 17402 Rev 2

15/49

Figure 9.

AHB secondary side key waveforms @ full load

Ch2: Q201 and Q202 drain pin (blue)

Ch3: FGND pin voltage (purple)

Ch4: Diode D13 current (green)

In order to improve the overall efficiency of the power supply, synchronous rectification has

been used. The two AHB output diodes have been replaced with two MOSFETs. A self

driven technique has been used to obtain a cheap solution. Two extra windings at the

secondary side generate the two square waves that, opportunely shifted, drive the two SR

MOSFETs gates directly. Referring to Q201, the extra winding (realized with just one turn)

starts from transformer pin 10 and ends in TON_DR_FLYWIRE. C210, D204, and R216 are

used to shift the voltage at the correct level to drive the MOSFET. R202 helps to keep the

MOSFET off if no driving signal is applied. A similar circuit drives the gate of Q202 starting

from the TOFF_DR_FLYWIRE signal.

AN3203 Datasheet(PDF) 15 Page - STMicroelectronics