ML4813CS datasheet(9/15 Pages) MICRO-LINEAR

9 / 15 page

ML4813

Note that VIN(q) = VP x sin(q) and that VP = 1.414 x VRMS.

The average value of the input triangular current is:

AVG

05=

sin

(7)

Where IAVG is the average value of the switch current (the

value of the current at the input of the regulator after

filtering), and T is the period of the switch cycle.

Substitution of (6) into (7) yields:

AVG

RMS

05=

2828

sin

(8)

Equation (8) clearly shows that the average value of the

switch current is sinusoidal and in phase with the input

voltage. The peak value of the average current is:

AVG PEAK

RMS

()

sin

2828

(9)

Also:

AVG PEAK

RMS

() =

(10)

Rearranging equations (9) and (10) to solve for PIN yields:

(11)

For optimum performance and the lowest inductor peak

currents, the inductor current should be at the verge of

continuity at the lowest operating voltage point and at full

load. This can be satisfied if:

OUT

(12)

Finally, (11) and (12) can be combined to derive an upper

bound for the inductor value that will guarantee that the

regulator always stays in the discontinuous mode of

operation. If the regulator were to operate in the

continuous mode the average input current would not be

sinusoidal.

OUT

(13)

FLYBACK INDUCTOR CALCULATION

Equation (13) gives the upper bound for the inductor

value for any set of specified operating conditions.

Normally, a few iterations may be required for finalizing

the value to correct for second or third order effects. This

means that a good initial value for the inductor is

probably 10 to 20% lower than the value calculated by

the right hand side expression in (13).

Several core materials are candidates for the inductor,

such as powder iron, gapped ferrites, moly permalloy, etc.

There are no particular restrictions on the inductor except

that the inductance is the correct value and the losses are

acceptable.

INPUT BYPASS CAPACITANCE

The triangular high frequency current is bypassed by an

input capacitor (CIN). This should be a high quality film

capacitor with low ESR value for minimum losses and

heating. Polyester, polypropylene or x-type (for line side)

are good candidates. Typical values, depending on the

power level, can range anywhere from 330nF to 1.5µF.

The next filtering stage of the RFI filter has an inductor as

an input to isolate CIN from the other capacitors which

may be present at the input circuit. Note that CIN can be

on either side of the bridge rectifier. The preferred

location for low crossover distortion is on the input side.

The ripple voltage across this capacitor is:

CP P

(14)

Where VC(P-P) is the peak to peak worst case high

frequency capacitor voltage, and D is the switch duty

cycle. The RFI filter that follows CIN has to be able to

attenuate VC(P-P) to the levels set by the relevant

regulatory specifications.

INPUT TRANSIENT OVERVOLTAGE PROTECTION

Careful examination of the power circuits reveals that

there is no large capacitance at the input of the regulator.

The only capacitance present is that of the RFI filter

capacitors. These capacitors have a combined value in the

range of a few microfarads, and their ability to absorb and

minimize any line induced transients is almost

nonexistent. Transients can also occur under sudden load

removal. If the line impedance is inductive, hazardous

drain-source voltages may be generated leading to the

destruction of the power MOSFET. To keep this from

happening, a transient over-voltage protection device

should be installed such that enough safety margin is

allowed for the power MOSFET. A good rule of thumb is:

DSS

CLAMP

OUT OVP

()

(15)

Where BVDSS is the drain-source breakdown voltage for

the MOSFET, VCLAMP is the activation or clamping voltage

of the over-voltage transient protector, and VOUT(OVP) is

the maximum output voltage which is set by the OVP

function of the controller.

THE OUTPUT CIRCUIT

The output circuit for this topology, although non-

isolated, does not share the same ground with the power

circuit. Therefore connecting the two grounds with the

measuring leads of instruments should be avoided.

APPLICATIONS (Continued)

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