AN1680

2

Motorola Applications Data

Figure 2. A Simple FLYBACK Configuration

Implementing a Clamping Network

C3

150

mF

Vout

Rp

100 m

1

16

14

Lleak

4

mH

4

VDCmains

330 V

+

MC

3337X

Rload

5

Vclamp

250 V

Dclamp

C6

10 nF

R6

3.6

C1

50

mF

Lp

250

mH

C5

10

mF

D1

1N4146

Aux

Int

C2

1 mF

MOC8101

R12

270

UP TI

Aux

R8

15 k

C4

100 n

R7

75 k

L3

5

mF

UP TI

X2

MBR20100

MC33374

RATIO_POW = –0.183

RATIO_AUX = –0.142

Int

12

15

16

2

3

7

17

6

13

10

+

As one can see from the picture,

φl1 and φl2 close through the air. As any (magnetic) medium, the air is affected by a Reluctance

ℜ, or its inverse, the permeance P. These permeances create in the primary and secondary two leakage inductance with a value

of:

Lleak = N2 ⋅ Pair, with N being the primary or secondary turns. As an effect, these parasitic leakage elements degrade the energy

transfer between the primary and secondary (ies). In a FLYBACK converter, the presence of the leakage element will a) generate

a voltage spike at turn–off and b) divert a portion of the primary current into the clamping network. Point a) implies the use of protec-

tion network to prevent a lethal drain voltage excursion while point b) explains the root of a degraded open–loop gain: the peak

current needs to be higher than theoretically calculated to deliver the full rated output power.

The Principle of a Protection Network

The goal of the clamping network is to prevent the drain voltage to exceed a given limit. For instance, in the new MOTOROLA

MC3337X family, the maximum voltage shall stay within 700 V. Worse case arises when the mains is a its highest level, e.g.

285 VAC in a universal mains application. To prevent the drain from reaching this value, Figure 2 shows how a perfect network

would work: when the MOSFET closes, the current builds–up in both primary and leakage coils. When the ON periods stops, the

MOSFET opens and interrupts its current. Since no current discontinuities can take place in an inductor, both magnetic fields col-

lapse and the voltage across the inductances reverses in an attempt to keep the amps–turn constant: Lp energy is thus coupled

to the secondary and gives birth to the output current charge. Since Lleakage cannot find a circulating path, it pulls–up 2 the drain

voltageuntilDclampstartstoconductandprotectstheswitcheratamaximumtheoreticallevelof:Vmains+Vclamp=650V.Figure 2b

shows the results of an INTUSOFT’s IsSpice4 (San–Pedro, CA) simulations. When all the leakage energy is released, a short

parasitic oscillation takes place involving Lleakage and all the parasitic capacitive elements present in the circuit (transformer’s pri-

mary capacitance, MOSFET’s Coss etc.)

CLIPPING EFFECT

Figure 2b. The Drain is Safely Clipped

Below 700 V at High Mains

1

2

@ p @ L

C

lump

300

100

–100

500

700

210.20U

211.20U

212.20U

213.20U

214.20U

X = 212.29U

Y = 650.84