TC62D902FG

2014-10-01

10

Basic operation and average LED Current : ILED)

The basics of this device are a flyback power supply. But, it doesn't use the feedback circuit of

the secondary side. First, MOSFET Q1 does on and drives a primary current IP.

It detects voltage in the ISEN pin and the IP does Q1 off when it reaches a peak value IP

( Peak ) from 0. Then, as for the magnetic-energy, it transmits and current IS occurs to the

secondary of the transformer.

IS Current is the current waveform which descends from peak value IS ( Peak ) to 0.

An average LED current is gotten because pulse width TS of current IS is detected at the VSEN

pin voltage and makes TS 4/7 duty with PFM ( pulse frequency modulation ).

In PFC, the VIN terminal-voltage is ISEN pin voltage threshold just as it is.

Therefore, the average LED current can do the following computation.

ILED(ave.) = Secondary Peak current IS(Peak)*1/2×4/7×2/π

= IP(Peak)* turn ratio of Ntr*1/2*4/7*2/π = VIN(peak)/RSEN*Ntr*1/2*4/7*2/π

Note: π is pi. This waveform is an enlarged view of a portion of the waveform AC.

Secondary Peak current

IS(peak) = Primary Peak current * Ntr

Primary

Bias Voltage wave

Max duty

3/7

1cycle is 7time

Fix duty

4/7

Secondary

tS

IS(Peak)

IP(Peak)

VIN

Vac

902

IP

IS

Ntr=Pri:Sec

tS

RSEN

ISEN

VSEN

Q1

VIN

VIN(peak)

VCC

Start-upResistor

R(VCC)

GND

IP(peak)