MP6004—PRIMARY-SIDE REGULATED FLYBACK/BUCK 80V DCDC CONVERTER

MP6004 Rev. 1.0

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19

4/14/2015

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time and can be calculated with Equation (17):

SLIM

M

CON

POUT

D1F

NI

L

T

N(V

V )

××

=

×+

(17)

winding turns.

rectifier diode current drop to 0 A to the

auxiliary-winding voltage drop to 0 V. The

resonant time can be tested on the board

(estimate around 0.5 μs).

In flyback mode, the MP6004 samples the

feedback signal within 3 μs after the primary-

side MOSFET turns off. The secondary-side

diode conduction time in Equation (17) should

be higher than 3 μs. This time period, combined

with the duty cycle, determines the maximum

frequency.

Input Capacitor Selection

An input capacitor is required to supply the AC

ripple current to the inductor while limiting noise

at the input source. A low ESR capacitor is

required to keep the noise to the IC at a

minimum. Ceramic capacitors are preferred, but

tantalum or low ESR electrolytic capacitors will

suffice. For ceramic capacitors, the capacitance

dominates the impedance at the switching

frequency. The ripple will be the worst at light

load. The required input capacitance can be

estimated with Equation (18):

LIM

ON

1

INP _ P

0.5 I

T

C

V

××

=

(18)

MOSFET turn-on time.

Equation (19):

LIM

M

ON

IN

IL

T

V

×

=

(19)

with Equation (20):

LIM

ON

IN

OUT

IL

T

VV

×

=

−

(20)

Where L is the buck`s inductor value.

Output Capacitor Selection

The output capacitor maintains the DC output

voltage.

For

best

results,

use

ceramic

capacitors or low ESR capacitors to minimize

the

output

voltage

ripple.

For

ceramic

capacitors, the capacitance dominates the

impedance at the switching frequency.

In flyback application, the worst output ripple

occurs under a light-load condition; the worst

output ripple can be estimated by Equation (21):

PLIM

CON

OUTP _ P

S

0.5 N

I

T

V

NC2

××

×

=

×

(21)

Where,

C2 is the output capacitor value.

Normally, a 44 μF or higher ceramic capacitor is

recommended as the output capacitor. This

allows a small Vo ripple and stable operation.

In buck application, the worst Vout ripple can be

estimated with Equation (22):

2

LIM

IN

D1F

OUTP _ P

IN

OUT

OUT

D1F

0.5 I

L (V

V )

V

C2 (V

V

) (V

V )

×× ×

+

=

×−

×

+

(22)

Leakage Inductance

The

transformer’s

leakage

inductance

decreases system efficiency and affects the

output current and voltage precision. Optimize

the transformer structure to minimize the

leakage

inductance.

Aim

for

a

leakage

inductance less than 3 percent of the primary-

winding inductance.

RCD Snubber for Flyback

The transformer leakage inductance causes

spikes and excessive ringing on the MOSFET

drain voltage waveform, affecting the output

voltage sampling 0.7 µs after the MOSFET

turns off. The RCD snubber circuit limits the SW

voltage spike (see Figure 7).

Figure 7—RCD snubber