NCP1605DR2G【PDF 22/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

18页 19页 20页 21页 [22页]23页 24页 25页 26页

NCP1605, NCP1605A, NCP1605B

http://onsemi.com

COIL

)zcd +

OCP

pin6

@ R

100 mV +

100 mV

pin6

@ 250 mA

@ (I

COIL

)

MAX

400 W

pin6

@ (I

COIL

)

MAX

Figure 60. Current Sense Block

Current Mirror

Ics

EMI

Filter

Vout

Icoil

DRV

100 mV

OCP

(RESET of the PWM latch)

Rzcd

LdT

ZCD

outON

> 250 mA e OCP

Vzcd

Vcc

output

buffer

Rdrv

The CS block performs the overcurrent protection and the zero current detection.

sense

ocp

out

bulk

LOAD

The propagation delay (V

Pin6

lower than 100 mV) to (drive

output high) is in the range of 300 ns, typically.

The Zero Current Detection:

" Is used to detect the deadtime sequences (DT high)

and hence, to process (V

TON

) from the error amplifier

output (V

CONTROL

). In other words, this is an input of

the ontime modulation block.

" Prevents the MOSFET from turning on as long as the

DT and ZCD signals are low. This is the case as

long as some current flows through the coil. This

delaying action on the output stage tends to make the

MOSFET turn on at the valley. To further optimize the

valley switching, one can apply the voltage of an

auxiliary winding to Pin 6 (CS

OUT

). The voltage is

compared to an internal 100 mV reference, so that

ZCD turns high only if (V

Pin6

< 100 mV).

Remarks:

" A resistor can be placed between Pin 6 and ground to

increase the ZCD precision.

" It is worth highlighting that the circuit permanently

senses the coil current and that it prevents any turn on

of the power switch as long as the core is not reset.

This feature protects the MOSFET from the possible

excessive stress it could suffer from, if it was allowed

to turn on while a huge current flows through the coil.

In particular, this scheme effectively protects the PFC

stage during the startup phase when huge inrush

currents charge the output capacitor.

" In addition this detection method does not require any

auxiliary winding. A simple coil can then be used in

the PFC stage.

It is recommended to:

1. Keep R

OCP

equal to or lower than 5 kW

2. Choose R

ZCD

as high as possible but not bigger

than (3 x R

OCP

). This is to avoid that the Pin 6

leakage prevents a proper zero current detection.

For instance, if R

OCP

is 2.2 kW, R

ZCD

should not

exceed 6.6 kW.

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