FMMT458 datasheet(2/10 Pages) DIODES

2 / 10 page

AP1694AEV11 User Guide

230VAC Triac Dimmable GU10 LED Driver

AP1694AEV11 Rev2

Page 2 of 10

July 2015

www.diodes.com

WHY USE DIODES TAPPED TRANSFORM STRUCTURE

The traditional Buck converter turn on time is inverse proportion with input voltage.

Vo = D x Vin. The duty cycle will be getting smaller when the input voltage goes higher.

Example: Vo=12V, Vin= 120VAC, Fs= 75kHz.

D= Vo/Vin *

= 12V/120V * 1.414 = 0.07

Ton=D/Fs = 0.07/ 75kHz =0.933µs. T=1/Fs =13.33µs

Since the Ton time is too short in the duty cycle; therefore there is not enough current passing

through the LEDs and charging the inductor. In result, it caused the efficiency to be lower.

In order to solve this issue ‐ use the Diodes tapped transformer to boost the output current &

increase the Ton time in the duty cycle.

With the “new tapped” transformer, the Duty cycle will be:

D is original duty cycle = Vo/Vin*1.414, n = NA+Np/NA & L = Lp + LA,

Vo/Vin = D’/(D’ +n(1‐D’)), If NA=40Ts, Np=100Ts, n=3.5

D’ = nVo/(Vin+(n‐1)Vo) = 0.21 The duty cycle almost increased by 3 times.

The Efficiency will increase about 4 to 5% (before the Efficiency was about 72% now is 77%)

comparing with the “none tapped” transformer.

FMMT458 Arkusz danych(PDF) 2 Page - Diodes Incorporated