MC44605(2000) 데이터 시트보기 (PDF) - ON Semiconductor

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MC44605
(Rev.:2000)

High Safety, Latched Mode, GreenLine PWM Controller for (Multi) Synchronized Applications

ON Semiconductor 

MC44605

Summary of the Main Design Equations

The following table consists of equations enabling to dimension a multisynchronized SMPS operating in discontinuous

mode.

+ Pinmax

Poutmax

η

Ǹ2·Vacmin NVo 2

ƪǸ ) ƫ 2·Vacmin NVo

+ Lpmax 2 Pinmax fsyncmax

+ Ǹ Ipkmax

2 Pinmax

L fsyncmin

+ Ǹ dmax

Pinmax Lp fsyncmax

Vacmin

+ Ponmax

1

3

Rdson

Ipkmax2

dmax

ǒ Ǔ + Ǹ ) (VDS) max

2 Vacmax (N Vout)

ǒ Ǔ + Ǹ ) (VD)max

2

Vacmax

N

Vout

+ (ni)max N nVout Ipkmax

+ AL

(N

LP

nVout)2

Poutmax is the maximum power the load may draw in normal working.

The maximum input power Pinmax is easily deducted by dividing Poutmax

by the efficiency (η). In this kind of application, the efficiency is generally

taken equal to 80%.

The inductor value Lp must be chosen lower than Lpmax or ideally equal to

this value (to optimize the application design–in).

In effect, if Lp was higher than Lpmax, a synchronized and discontinuous

working could not be guaranteed (in some cases, the demagnetization

phase would not be finished while a new conduction phase should start to

follow the synchronization).

Ipkmax is the maximum inductor peak current. This current is obtained

when the power to transfer is maximum at the minimum synchronization

frequency (60 W output, 30 kHz in the proposed application).

dmax is the maximum duty cycle. The duty cycle is maximum at the lowest

input voltage when the power demand is maximum while the

synchronization frequency also is maximum.

Ponmax is the maximum Mosfet on–time losses that are proportional to

Ipkmax, dmax and Rdson (on–time Mosfet resistor).

This conduction losses estimation enables to dimension the power Mosfet.

(VDS)max is the maximum voltage the power switch must be able to face.

In fact, this calculation does not take into account the turnings off spikes.

So, it is necessary to take a margin of at least about 50 V.

(VD)max is the maximum voltage the high voltage secondary diode must

be able to face. Because of the turning off spikes, a margin must also be

taken.

(AL) and (ni) are the magnetic parameters.

(ni)max must not exceed the ferrite (ni). Otherwise, the transformer may get

saturated when the peak current is high.

(AL) is the ferrite constant that links the primary inductor value to the

squared number of primary turns: Lp = AL x np2.

Error Amplifier

A fully compensated Error Amplifier with access to the

inverting input and output is provided. It features a typical

DC voltage gain of 70 dB. The non inverting input is

internally biased at 2.5 V and is not pinned out. The

converter output voltage is typically divided down and

monitored by the inverting input. The maximum input bias

current with the inverting input at 2.5 V is –2.0 µA. This can

cause an output voltage error that is equal to the product of

the input bias current and the equivalent input divider source

resistance.

Compensation

+

1.0 mA

RFB

Cf

Rf 13

14 2.5 V

Voltage

Feedback

Input

Error

Amplifier

MC44605

2R

R

1.0 V

Gnd

From Power Supply Output

R1

R2

Current

Sense

Comparator

4

Figure 1. Error Amplifier Compensation

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