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LT3798

3798f

Switch Voltage Clamp Requirement

Leakage inductance of an offline transformer is high due

to the extra isolation requirement. The leakage inductance

energy is not coupled to the secondary but goes into

the drain node of the MOSFET. This is problematic since

400V and higher rated MOSFETs cannot always handle

this energy by avalanching. Therefore the MOSFET needs

protection. A transient voltage suppressor (TVS) and diode

are recommended for all offline application and connected,

as shown in Figure 3. The TVS device needs a reverse

breakdown voltage greater than (VOUT + VF) • NPS where

VOUT is the output voltage of the flyback converter, VF is

the secondary diode forward voltage, and NPS is the turns

ratio. An RCD clamp can be used in place of the TVS clamp.

period, as well. Similarly, initial values can be estimated

using stated switch capacitance and transformer leakage

inductance. Once the value of the drain node capacitance

and inductance is known, a series resistor can be added

to the snubber capacitance to dissipate power and criti-

cally dampen the ringing. The equation for deriving the

optimal series resistance using the observed periods

(tPERIOD, and tPERIOD(SNUBBED)) and snubber capacitance

(CSNUBBER) is below, and the resultant waveforms are

shown in Figure 4.

OPERATION

Figure 3. TVS & RCD Switch Voltage Clamps

3798 F03

GATE

VSUPPLY

GATE

VSUPPLY

In addition to clamping the spike, in some designs where

short circuit protection is desired, it will be necessary to

decrease the amount of ringing by using an RC snubber.

Leakage inductance ringing is at its worst during a short

circuit condition, and can keep the converter from cycling

on and off by peak charging the bias capacitor. On/off

cycling is desired to keep power dissipation down in the

output diode. Alternatively, a heat sink can be used to

manage diode temperature.

The recommended approach for designing an RC snubber

is to measure the period of the ringing at the MOSFET

drain when the MOSFET turns off without the snubber

and then add capacitance—starting with something in

the range of 100pF—until the period of the ringing is 1.5

to 2 times longer. The change in period will determine

the value of the parasitic capacitance, from which the

parasitic inductance can be determined from the initial

Figure 4. Observed Waveforms at MOSFET Drain when

Iteratively Implementing an RC Snubber

TIME (μs)

0.20

3798 F04

0.10

0.05

0.25

0.15

0.30

NO SNUBBER

WITH SNUBBER

CAPACITOR

WITH RESISTOR

AND CAPACITOR

CPAR =

CSNUBBER

tPERIOD(SNUBBED)

tPERIOD

⎛

⎝

⎜

⎞

⎠

⎟

–1

LPAR =

tPERIOD2

CPAR •4π2

RSNUBBER =

LPAR

CPAR

Note that energy absorbed by a snubber will be converted

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

age or high current applications, the snubber may need to

be sized for thermal dissipation. To determine the power

dissipated in the snubber resistor from capacitive losses,

measure the drain voltage immediately before the MOS-

FET turns on and use the following equation relating that

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