FL7930CMX-G Ver la hoja de datos (PDF) - ON Semiconductor

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Fabricante

FL7930CMX-G

Single-Stage Flyback and Boundary-Mode PFC Controller for Lighting

ON Semiconductor 

The auxiliary winding voltage is used to check the boost

inductor current zero instance. When boost inductor

current becomes zero, there is a resonance between

boost inductor and all capacitors at the MOSFET drain

pin: including COSS of the MOSFET; an external

capacitor at the D-S pin to reduce the voltage rising and

falling slope of the MOSFET; a parasitic capacitor at

inductor; and so on to improve performance. Resonated

voltage is reflected to the auxiliary winding and can be

used for detecting zero current of boost inductor and

valley position of MOSFET voltage stress. For valley

detection, a minor delay by the resistor and capacitor is

needed. A capacitor increases the noise immunity at the

ZCD pin. If ZCD voltage is higher than 1.5 V, an internal

ZCD comparator output becomes HIGH and LOW when

the ZCD goes below 1.4 V. At the falling edge of

comparator output, internal logic turns on the MOSFET

VDS

VOUTPFC - VIN

VOUTPFC - VIN

VIN

IINDUCTOR

Figure 30. Restart Timer at Startup

Because the MOSFET turn-on depends on the ZCD

input, switching frequency may increase to higher than

several megahertz due to the mis-triggering or noise on

the nearby ZCD pin. If the switching frequency is higher

than needed for critical conduction mode (CRM),

operation mode shifts to continuous conduction mode

(CCM). In CCM, unlike CRM where the boost inductor

current is reset to zero at the next switch on; inductor

current builds up at every switching cycle and can be

raised to very high current that exceeds the current

rating of the power switch or diode. This can seriously

damage the power switch. To avoid this, maximum

switching frequency limitation is embedded. If ZCD

signal is applied again within 3.3 µs after the previous

rising edge of gate signal, this signal is ignored

internally and FL7930C waits for another ZCD signal.

This slightly degrades the power factor performance at

light load and high input voltage.

ZCD after COMPARATOR

Ignores ZCD noise

MOSFET Gate

Error occurs!

VZCD

IMOSFET

IDIODE

1.5V

1.4V

MOSFET gate

150ns Delay

ON

Figure 29.

ON

FL7930 Rev.00

t

Auxiliary Voltage Threshold

When no ZCD signal is available, the PFC controller

cannot turn on the MOSFET, so the controller checks

every switching off time and forces MOSFET turn on

when the off time is longer than 150 µs. This restart

timer triggers MOSFET turn-on at startup and may be

used at the input voltage zero-cross period.

VOUT

VIN

VCC

tRESTART

150 µs

MOSFET gate

ZCD after COMPARATOR

FL7930 Rev.00

t

Max. fSW Limit

Figure 31.

Inhibit Region

FL7930 Rev.00

t

Maximum Switching Frequency

Limit Operation

5. Control: The scaled output is compared with the

internal reference voltage and sinking or sourcing

current is generated from the COMP pin by the

transconductance amplifier. The error amplifier output is

compared with the internal sawtooth waveform to give

proper turn-on time based on the controller.

VOUTPFC

6.2V

INV

1

COMP

3

THD-Optimized

Sawtooth

Generator

1V

Sawtooth

-

VREF

Stair +

Step

+

-

Clamp

Circuit

MOSFET Off

C2

R1

C1

FL7930 Rev.00

Figure 32. Control Circuit

Unlike a conventional voltage-mode PWM controller,

FL7930C turns on the MOSFET at the falling edge of

ZCD signal. The “ON” instant is determined by the

external signal and the turn-on time lasts until the error

amplifier output (VCOMP) and sawtooth waveform meet.

When load is heavy, output voltage decreases, scaled

output decreases, COMP voltage increases to

compensate low output, turn-on time lengthens to give

more inductor turn-on time, and increased inductor

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