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A4401

Automotive Quasi-Resonant Flyback Control IC

Allegro MicroSystems 

A4401

Automotive Quasi-Resonant Flyback Control IC

Switching Turn-On Losses The turn-on losses are

determined by the amount of energy the resonant

capacitor, C11, has to discharge into the MOSFET. At

low battery voltage, the resonant swing should force

the volts across the capacitor to only a few volts, mak-

ing this loss negligible.

Total Losses The total MOSFET power loss can be

estimated as follows:

PTOTAL = PSTATIC + PTURNOFF .

(11)

The thermal resistance, RθJA, can be determined by

two methods. One is by estimating a maximum junc-

tion temperature, TJ(max). The other is to test for the

operating junction temperature, using the given device

package mounted on a printed circuit board with cop-

per trace area connected to the device. RθJA is then

calculated as follows:

RθJA =

TJ – TA

PTOTAL

.

(12)

The drain-to-source rating, VDS , is the sum of

the maximum input voltage, VBAT (max), plus the

reflected output voltage. Adequate margin should also

be added, to allow for tolerancing effects and parasitic

voltage ring. It can be calculated as follows:

⎛

VDS = ⎜⎜VOUT ×

⎝

NP

NOUT

⎞

⎟⎟

⎠

+ VBAT (max)

.

(13)

D2, D3, and D4 Output Diodes Selection

For the low voltage outputs such as the filament sup-

ply, it is recommended that a Schottky diode be used.

For the higher voltage rails, ultrafast rectifier diodes

are recommended.

For each output, estimate the maximum reverse volt-

age, VRRM, and the maximum average current that the

diode is subjected to. The VRRM rating should exceed

at least 20% of the maximum VDIODE voltage, calcu-

lated as follows:

VDIODE =VBAT (max) ×

NS1

NP

+ VOUT .

(14)

The maximum average current through the diode is

simply the maximum load current. The diode should

be rated to handle this current with some margin.

In addition, the diode should be able to handle the

power dissipation. The majority of the power loss is

simply the static loss:

PSTATIC = Vf × ILOAD .

(15)

The forward voltage drop, Vf , can be found from the

diode characteristics at maximum load.

C1 Input Capacitor Selection

In the interests of cost and performance, it is recom-

mended that ultralow impedance electrolytic capaci-

tors be used. The ratio of the source impedance to the

impedance of the input capacitor will determine how

much of the input current is drawn from the input

capacitor. For example, if the source impedance is

relatively high, then the input capacitor would have

to supply the triangular current that flows through the

primary winding, the MOSFET Q1, and the current

sense resistor. This rms current was worked out in the

Current Sense Resistor Selection section.

Electrolytic capacitors experience heating effects

caused by the rms current flowing through the ESR

of the device. The maximum rms current is normally

quoted at 100 kHz and 105°C. Frequency correc-

tion factors for the ripple current are provided when

the operating frequencies are less than 100 kHz. A

50 VDC rating should be adequate for most applica-

tions.

C4, C6, and C8 Output Capacitors Selection

The overall equivalent capacitance on the output

should not be less 22 μF (see Control Loop section

Allegro MicroSystems, Inc.

8

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

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