FAN302HLMY datasheet(10/18 Pages) FAIRCHILD

номер детали	FAN302HLMY

подробное описание детали	PWM Controller for Low Standby Power Battery- Charger Applications ??mWSaver??Technology

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производитель	FAIRCHILD [Fairchild Semiconductor]

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FAN302HLMY_F117 • Rev. 1.0.1

10

Operational Description

Basic Control Principle

Figure 24 shows the internal PWM control circuit. The

constant voltage (CV) regulation is implemented in the

same way as the conventional isolated power supply,

where the output voltage is sensed using a voltage

divider and compared with the internal 2.5V reference of

a shunt regulator (KA431) to generate a compensation

signal. The compensation signal is transferred to the

primary side using an opto-coupler and scaled down

through attenuator Av, generating the VEA.V signal. Then

the error signal VEA.V is applied to the PWM comparator

(PWM.V) to determine the duty cycle.

Meanwhile, the CC regulation is implemented internally

without directly sensing the output current. The output

current estimator reconstructs output current information

(VCCR) using the transformer primary-side current and

diode current discharge time. Then VCCR is compared

with a reference voltage (2.5V) by an internal error

amplifier, generating the VEA.I signal to determine the

duty cycle.

The two error signals, VEA.I and VEA.V, are compared with

an internal sawtooth waveform (VSAW) by PWM

comparators PWM.I and PWM.V to determine the duty

cycle. As shown in Figure 25, the outputs of two

comparators (PWM.I and PWM.V) are combined with an

OR gate and used as a reset signal of flip-flop to

determine the MOSFET turn-off instant. The lower

signal, VEA.V or VEA.I, determines the duty cycle, as

shown in Figure 25. During CV regulation, VEA.V

determines the duty cycle while VEA.I is saturated to

HIGH. During CC regulation, VEA.I determines the duty

cycle while VEA.V is saturated to HIGH.

Figure 24. Internal PWM Control Circuit

CV Regulation

CC Regulation

VSAW

VEA.I

VEA.V

PWM.V

PWM.I

OSC CLK

Gate

Figure 25. PWM Operation for CC and CV

Output Current Estimation

Figure 26 shows the key waveform of a flyback

converter operating in Discontinuous Conduction Mode

(DCM), where the secondary-side diode current reaches

zero before the next switching cycle begins. Since the

output current estimator is designed for DCM operation,

the power stage should be designed such that DCM is

guaranteed for the entire operating range. The output

current is obtained by averaging the triangular output

diode current area over a switching cycle:

S

DIS

S

P

PK

AVG

D

O

t

N

I

2

•

=

>

=<

(1)

where IPK is the peak value of the primary-side

current; NP and NS are the number of turns of

transformer

primary-side

and

secondary-side,

respectively; tDIS is the diode current discharge time;

and tS is the switching period.

With a given current sensing resistor, the output current

can be programmed as:

S

P

SENSE

O

N

R

K

I

•

=

25

.

1

(2)

where K is the design parameter of IC, which is 10.5.

The peak value of primary-side current is obtained by an

internal peak detection circuit, while diode current

discharge time is obtained by detecting the diode

current zero-crossing instant. Since the diode current

cannot be sensed directly with primary-side control,

Zero Crossing Detection (ZCD) is accomplished

indirectly by monitoring the auxiliary winding voltage.

When the diode current reaches zero, the transformer

winding voltage begins to drop by the resonance

between the MOSFET output capacitance and the

transformer magnetizing inductance. To detect the

starting instant of the resonance, the VS is sampled at

85% of diode current discharge time of the previous

switching cycle, then compared with the instantaneous

VS voltage. When instantaneous VS drops below the

sampled voltage by more than 200mV, ZCD of diode

current is obtained, as shown in Figure 27.

Аналогичный номер детали - FAN302HLMY

производитель	номер детали	датащи	подробное описание детали
Fairchild Semiconductor	FAN302HLMY_F117	1Mb / 18P	PWM Controller for Low Standby Power Battery-Charger Applications ??mWSaver??Technology

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Аналогичное описание - FAN302HLMY

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