NCP1605DR2G【PDF 18/32 页】IC PFC CONTROLLER CCM/DCM 16SOIC

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Sheet目录45

14页 15页 16页 17页 [18页]19页 20页 21页 22页

NCP1605, NCP1605A, NCP1605B

http://onsemi.com

NCP1605(A) Operation Modes

Like the NCP1601, the NCP1605:

" Features a current sense block that prevents the PFC

stage from operating in CCM: as long as the coil

current is not null, the power switch is not allowed to

turn on. Hence the circuit can only operate in either

Fixed Frequency DCM or CRM.

" Features the capability to exhibit nearunity power

factor while operating in any type of Discontinuous

Conduction Mode operation: DCM or CRM.

" Auto adapts: if there is some current flowing through

the coil when the clock occurs to initiate a new current

cycle, the PFC stage enters CRM. On the other hand,

if the clock occurs during deadtimes, one obtains a

fixed frequency operation DCM. Thanks to its special

oscillator/synchronization arrangement, the circuit

automatically enters the appropriate mode CRM or

DCM. It is worth noting that jumps between the CRM

and modes cause absolutely no degradation: the input

current keeps being properly shaped and there is no

discontinuity in the power transfer.

Given the deadtime presence, DCM needs a higher peak

inductor current compared to CRM for the same delivered

power. Hence, the coil is generally designed to have CRM

at the most stressful conditions while DCM limits the

switching frequency at lower load. The circuit can also

transition within an ac line cycle so that:

" CRM reduces the current stress around the sinusoid

top.

" DCM limits the frequency around the line zero

crossing.

This capability offers the best of each mode without the

drawbacks. The way the circuit modulates the MOSFET

ontime allows this facility.

Figure 52. DCM and CRM Operation Within a Sinusoid Cycle

The NCP1605(A) can jump from DCM to CRM within a sinusoid cycle (and vice versa)

without any discontinuity in the current shaping or the power transfer.

Inductor Current, I

Current

Time

DCM

Critical Mode

DCM

Input Current, I

NCP1605 Ontime Modulation

Lets study the ac line current absorbed by the PFC boost.

The initial inductor current of each switching cycle is

always zero. The coil current ramps up when the MOSFET

is on. The slope is (V

/L) where L is the coil inductance.

At the end of the ontime (t1), the coil demagnetization

phase starts. The coil current ramps down until this

sequence ends when it reaches zero. The duration of this

phase is (t2). The system enters then the deadtime (t3) that

lasts until the next clock is generated.

One can show (refer to NCP1601 data sheet) that the ac

line current is given by:

+ V

) t

)

2 T L

(eq. 1)

Where T = (t1 + t2 + t3) is the switching period and V

the ac line rectified voltage.

To the light of this equation, we immediately note that I

is proportional to V

if [t1(t1 + t2)/T] is a constant.

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