LT3748H(RevA) Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3748H
(Rev.:RevA)

100V Isolated Flyback Controller

Linear Technology 

LT3748

APPLICATIONS INFORMATION

Although it typically does not decrease efficiency, leakage

inductance energy that would normally have been dis-

sipated in the switch or transformer is also dissipated in

the RC snubber resistor and can be calculated as:

PSNUBBER = fSW • LLEAK • ILIM2/2

An RCD clamp, shown in Figure 7, also prevents the

leakage inductance spike from exceeding the breakdown

voltage of the MOSFET switch. In most applications, there

will be a very fast voltage spike caused by a slow clamp

diode. Once the diode clamps, the leakage inductance

current is absorbed by the clamp capacitor. This period

should not last longer than 200ns so as not to interfere

with the output regulation. The clamp diode turns off after

the leakage inductance energy is absorbed and the switch

voltage is then equal to:

VDS = VIN + NPS • (VOUT + VF(DIODE))

Schottky diodes are typically the best choice for use in a

snubber, but some PN diodes can be used if they turn on

fast enough to limit the leakage inductance spike. Figures 8

and 9 show the waveform at the drain of the MOSFET

switch for the 48V output application shown in Figure 17

at maximum rated load and maximum input voltage with

an RC snubber and RCD clamp, respectively. Both solu-

tions limit the leakage spike to less than 190V, below the

200V VDS(MAX) rating of the Si7464DP MOSFET.

LLEAK

VIN

CR

VOUT+

D

GATE

NMOS

3748 F07

Figure 7. RCD Clamp

VOUT–

Leakage Inductance and Output Diode Stress

The output diode may also see increased reverse voltage

stresses from leakage inductance. While it nominally sees

a reverse voltage of the input voltage divided by the wind-

ings ratio plus the output voltage when the MOSFET power

switch turns on, the capacitance on the output diode and

the leakage inductance will cause an LC tank which may

16

ring beyond that expected reverse voltage. An RC snubber

or RCD clamp may be implemented to reduce the voltage

spike if it is desirable to use a lower reverse voltage diode.

Secondary Leakage Inductance

In addition to the previously described effects of leakage

inductance in general, leakage inductance on the secondary

in particular exhibits an additional phenomena. It forms an

inductive divider on the transformer secondary that effec-

tively reduces the size of the primary-referred flyback pulse

used for feedback. This will increase the output voltage

target by a similar percentage. Note that, unlike leakage

spike behavior, this phenomena is load independent. To the

extent that the secondary leakage inductance is a constant

percentage of mutual inductance (over manufacturing

200

180

160

140

120

100

80

60

40

20

0

0

VIN = 96V

VOUT = 48V

IOUT = 0.5A

R = 66Ω

C = 150pF

50 100 150 200 250 300

TIME (ns)

3748 F08

Figure 8. Waveform of MOSFET Drain During Normal Operation

of Figure 17 with RC Snubber (as Drawn)

200

180

160

140

120

100

80

60

40

20

0

0

VIN = 96V

VOUT = 48V

IOUT = 0.5A

R = 4.99k

C = TDK 0.22μF 250V

D = CMR1U-02M-LTC

50 100 150 200 250 300

TIME (ns)

3748 F08

Figure 9. Waveform of MOSFET Drain During Normal Operation

of Figure 17 Using RCD Clamp with Central Semiconductor

CMR1U-02M-LTC Instead of RC Snubber

3748fa

Share Link: