LD7516C

08/31/2016

9

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7516C-DS-01 August 2016

Principle of CV Operation

In the DCM flyback converter, it can sense the output

voltage from auxiliary winding. LD7516C samples the

auxiliary winding on the primary-side to regulate the

output voltage, as shown in the Fig. 13. The voltage

induced in the auxiliary winding is a reflection of the

secondary winding voltage while the MOS is in off state.

Via a resistor divider connected between the auxiliary

winding and FB pin, the auxiliary voltage is sampled after

the sample delay time which is defined as 30~50% of

secondary current discharge time from previous cycle.

And will be hold until the next sampling period. The

sampled voltage is compared with an internal reference

amplifier output COMP reflects the load condition and

controls the duty cycle to regulate the output voltage, thus

constant output voltage can be achieved. The output

voltage is given as:

F

OUT

V

Na

Ns

Rb

Ra

V

V







)

)(

1

(

0

.

1

Ra and Rb are top and bottom feedback resistor value, Ns

and Na are the turns of transformer secondary and

auxiliary.

In case that the output voltage is sensed through the

auxiliary winding; the leakage inductance will induce

ringing to affect output regulation. To optimize the

collector voltage clamp circuit will minimize the high

frequency ringing and achieve the best regulation. Fig. 14

shows the desired collector voltage waveform in compare

to those with large undershoot due to leakage inductance

induced ring (Fig. 15). The ringing may make the sample

error and cause poor performance for output voltage

regulation. A proper selection for resistor RS, in series

with the clamp diode, may reduce any large undershoot,

as shown in Fig. 16.

OUT

CS

COMP

FB

Na

Ns

Np

Ra

Rb

S/H

+

-

Driver

LD7516C

Fig. 13

The overshoot

here is minor

Fig.14

The undershoot would

make the sample error.

Fig.15

OUT

CS

COMP

FB

Na

Ns

Np

Ra

Rb

LD7516C

Fig.16