MC44605 datasheet(8/20 Pages) ONSEMI

Part #	MC44605

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

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MC44605

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8

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

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%.

Lpmax +

2·Vac

min

NVo

2·Vac

min)

NVo

2

Pinmax

fsyncmax

2

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 +

2

Pinmax

L

fsync

min

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 +

Pinmax

Lp

fsyncmax

Vac

min

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 +

1

3

Rdson

Ipkmax2

dmax

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.

(V

DS

) max

+ 2

Vacmax ) (N

Vout)

(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.

(V

D

) max

+

2

Vacmax

N

) Vout

(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.

(ni)max + N

n

Vout

Ipkmax

(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.

A

L +

L

P

(N

n

Vout

)2

(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

mA. 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.

+

1.0 mA

2.5 V

Figure 1. Error Amplifier Compensation

Compensation

RFB

Cf

R1

R2

From Power Supply Output

Rf

13

14

Voltage

Feedback

Input

Error

Amplifier

2R

R

1.0 V

Current

Sense

Comparator

GND

4

MC44605

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