A4401 datasheet(14/17 Pages) ALLEGRO

14 / 17 page

Automotive Quasi-Resonant Flyback Control IC

A4401

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

It can be seen that at MOSFET turn-on there is a

considerable sinusoidal ringing on the voltage sense,

in the region of 3 MHz. In addition, there is a con-

siderable ringing during the MOSFET turn-off, in the

region of 6 MHz. These ringing effects are caused by

the primary-to-secondary leakage inductance interact-

ing with parasitic capacitance. These noise sources

will probably have an impact on the conducted emis-

sions and should therefore be suppressed.

By rescaling the magnetics so that 0 V switching is

achieved with VBAT = 13.5 V, the turn-on ringing is

almost completely damped, as shown in Figure 6.

Because the resonant capacitor, C11, had discharged

to 0 V before switching, there was effectively little

energy left.

The turn-off noise is also improved dramatically as

well. It can be seen that the ringing amplitude is lower,

dampens more quickly, and the ringing frequency

is higher making it easier to snub (see the Radiated

Emissions on the LX Node subsection). The reason for

this effect is that a lower step-up ratio is required to

achieve the output voltage. This improves the mag-

netic coupling coefficient and therefore the leakage

inductance is reduced.

Running the system with a lower turns ratio means

that the duty cycle is greater. The advantage of this

is that the peak current is lower, resulting in lower

turn-off switching losses and reduced harmonics of the

input current, thus reducing the conducted emissions.

Radiated Emissions on LX Node

A potential source

of radiated emissions is turning off the MOSFET. A

resonance is set up between the leakage inductance

of the magnetics and the parasitic capacitance on the

LX node. Assuming the magnetics are well designed

in terms of reducing leakage inductance, this reso-

nance should occur in the region of tens of megahertz.

Another potential source of radiated emissions is

turning off the output rectifying diodes. The following

procedure can be applied to address both issues.

Figure 5. Voltages with 0 V switching of Q1 at VBAT of 7 V

VLX

VRSENSE

MOSFET

Turn-On

MOSFET

Turn-Off

Figure 6. Voltages with 0 V switching of Q1 at VBAT of 13.5 V

VLX

VRSENSE

MOSFET

Turn-On

MOSFET

Turn-Off

A4401 Datasheet(PDF) 14 Page - Allegro MicroSystems

A4401 Datasheet(HTML) 14 Page - Allegro MicroSystems

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL