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LT8302HS8E-3-WPBF

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

<65V

<50V

VLEAKAGE

VSW

tOFF > 350ns

tSP < 250ns

TIME

8302 F05

Figure 5. Maximum Voltages for SW Pin Flyback Waveform

In addition to the voltage spikes, the leakage inductance

also causes the SW pin ringing for a while after the power

switch turns off. To prevent the voltage ringing falsely trig-

ger boundary mode detector, the LT8302/LT8302-3 inter-

nally blanks the boundary mode detector for approximately

250ns. Any remaining voltage ringing after 250ns may turn

the power switch back on again before the secondary cur-

rent falls to zero. In this case, the LT8302/LT8302-3 enters

continuous conduction mode. So the leakage inductance

spike ringing should be limited to less than 250ns.

To clamp and damp the leakage voltage spikes, a

(RC + DZ) snubber circuit in Figure 6 is recommended.

The RC (resistor-capacitor) snubber quickly damps the

voltage spike ringing and provides great load regulation

and EMI performance. And the DZ (diode-Zener) ensures

well defined and consistent clamping voltage to protect

SW pin from exceeding its 65V absolute maximum rating.

Lℓ

Z

C

•

DR

•

8302 F06

Figure 6. (RC + DZ) Snubber Circuit

The recommended approach for designing an RC snubber

is to measure the period of the ringing on the SW pin

when the power switch turns off without the snubber and

then add capacitance until the period of the ringing is 1.5

to 2 times longer. The change in period determines the

value of the parasitic capacitance, from which the para-

sitic inductance can be also determined from the initial

period. Once the value of the SW node capacitance and

inductance is known, a series resistor can be added to

the snubber capacitance to dissipate power and critically

damp the ringing. The equation for deriving the optimal

series resistance using the observed periods ( tPERIOD and

tPERIOD(SNUBBED)) and snubber capacitance (CSNUBBER) is:

CPAR

=

⎛

⎜

⎝

CSNUBBER

tPERIOD(SNUBBED)

tPERIOD

⎞2

⎟

⎠

–

1

LPAR

=

tPERIOD2

CPAR • 4π2

RSNUBBER =

LPAR

CPAR

Note that energy absorbed by the RC snubber will be con-

verted to heat and will not be delivered to the load. In high

voltage or high current applications, the snubber needs

to be sized for thermal dissipation. A 470pF capacitor in

series with a 39Ω resistor is a good starting point.

For the DZ snubber, proper care should be taken when

choosing both the diode and the Zener diode. Schottky

diodes are typically the best choice, but some PN diodes

can be used if they turn on fast enough to limit the leakage

inductance spike. Choose a diode that has a reverse-volt-

age rating higher than the maximum SW pin voltage. The

Zener diode breakdown voltage should be chosen to bal-

ance power loss and switch voltage protection. The best

compromise is to choose the largest voltage breakdown

with 5V margin. Use the following equation to make the

proper choice:

VZENNER(MAX) ≤ 60V – VIN(MAX)

For an application with a maximum input voltage of 32V,

choose a 24V Zener diode, the VZENER(MAX) of which is

around 26V and below the 28V maximum. The power loss

in the DZ snubber determines the power rating of the Zener

diode. A 1.5W Zener diode is typically recommended.

Rev. G

16

For more information www.analog.com

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