NCP1605DR2G【PDF 27/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

23页 24页 25页 26页 [27页]28页 29页 30页 31页

NCP1605, NCP1605A, NCP1605B

http://onsemi.com

BrownOut Detection

The brownout pin receives a portion of the input voltage

). As V

is a rectified sinusoid, a capacitor must

integrate the ac line ripple so that a voltage proportional to

the average value of (V

) is applied to the brownout pin.

The brownout block detects too low input voltage

conditions. A hysteresis comparator monitors the Pin 2

voltage. Before operation, the PFC stage is off and the input

bridge acts as a peak detector. Hence, the voltage applied

to Pin 2 is:

pin2

+ 2

Vac

bo2

bo1

) R

bo2

After the PFC stage has started operation, the input

voltage becomes a rectified sinusoid and the voltage

applied to Pin 2 is:

pin2

2 2

Vac

bo2

bo1

) R

bo2

i.e., about 64% of the previous value. Therefore, the same

line magnitude leads to a V

Pin2

voltage that is 36% lower

when the PFC is working than when it is off (refer to

Figure 69). That is why the NCP1605 features a 50%

hysteresis (V

L = 50% V

H).

When the circuit starts operation, the input voltage

equates the ac line peak.

Hence, the initial threshold of the BrownOut

comparator, must be the upper one (V

= V

H = 1 V

when the NCP1605 leaves the off mode).

When a brownout condition is detected, the signal

BO_NOK turns off the circuit (refer to block diagram).

BO_NOK

1 V / 0.5 V

EMI

Filter

Ac line

Vin

Figure 68. BrownOut Block

Figure 69. Typical Input Voltage of a PFC Stage

400

200

SIN

@ Vac

@ Vac @ sin(Rt)

bo2

bo1

Start of PFC

Operation

Thermal Shutdown (TSD)

An internal thermal circuitry disables the circuit gate

drive and then keeps the power switch off when the junction

temperature exceeds 150_C typically. The output stage is

then enabled once the temperature drops below about

100_C (50_C hysteresis).

The temperature shutdown keeps active as long as the

circuit is not reset, that is, as long as V

keeps higher than

RESET. The reset action forces the TSD threshold to

be the upper one (150癈). This ensures that any cold startup

will be done with the right TSD level.

Output Drive Section

The output stage contains a totem pole optimized to

minimize the cross conduction current during high

frequency operation. The gate drive is kept in a sinking

mode whenever the Undervoltage Lockout is active or

more generally whenever the circuit is off (i.e., when the

Fault Latch of the block diagram is high or when the HV

current source is on). Its high current capability

(500 mA/+800 mA) allows it to effectively drive high

gate charge power MOSFET.

Reference Section

The circuit features an accurate internal reference

voltage (V

REF

). V

REF

is optimized to be ?% accurate over

the temperature range (the typical value is 2.5 V). V

REF

the voltage reference used for the regulation and the

overvoltage protection. The circuit also incorporates a

precise current reference (I

REF

) that allows the Overcurrent

Limitation to feature a ?% accuracy over the temperature

range.

OFF Mode

As previously mentioned, the circuit turns off in the

following cases:

" When the high voltage, startup current source charges

the V

capacitor.

" When one of the following major faults is detected:

" Incorrect feeding of the circuit (UVLO high when

OFF, V

OFF equating 9 V typically).

" Excessive die temperature detected by the thermal

shutdown.

" BrownOut condition.

" Undervoltage Protection.

" V

Pin13

higher than 2.5 V (STDWN of the block

diagram turns high).

Generally speaking, the circuit turns off when the

conditions are not proper for good operation. In this mode,

the controller stops operating. The major part of the circuit

sleeps and its consumption is minimized (< 500 mA).

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