LT3752 datasheet(32/52 Pages) AD

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LT3752/LT3752-1

Rev. C

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Figure 16. HI Side VCCL vs Duty Cycle

(Normalized to 50% Duty Cycle)

DUTY CYCLE (%)

1.5

3752 F16

1.0

0.5

2.0

2.5

1.3

0.8

1.8

2.3

HI SIDE ACTIVE CLAMP TOPOLOGY

Figure 17. HI Side Active Clamp Topology

APPLICATIONS INFORMATION

LO Side Active Clamp Topology (LT3752)

The steady-state active clamp capacitor voltage, VCCL,

required to reset the transformer in a LO side active clamp

topology (Figure 13) can be approximated as the drain-to-

source voltage (VDS) of switch M1, given by:

VCCL (LO side):

(a) Steady state: VCCL = SWP = VDS

1– D







 • VIN =

VIN2

VIN – VOUT •N

(

)

(

)

(b) Transient:

During load transients, duty cycle and hence VCCL may

increase. Replace D with DVSEC in the equation above to

calculate transient VCCL values. See the previous section

ProgrammingDutyCycleClamp–DVSEC.TheDVSECguard-

rail can be programmed as close as 5% higher than D but

mayrequirealargermargintoimprovetransientresponse.

As shown in Figure 15, the maximum steady-state value

for VCCL may occur at minimum or maximum input volt-

age. Hence VCCLshouldbecalculatedatbothinputvoltage

levels and the largest of the two calculations used. M1

drain should be rated for a voltage greater than the above

steady-state VDS calculation due to tolerances in duty

cycle, load transients, voltage ripple on CCL and leakage

inductance spikes. CCL should be rated higher due to the

effect of voltage coefficient on capacitance value. A typical

choice for CCL is a good quality X7R capacitor. M2 should

have a VDS rating greater than VCCL since the bottom plate

of CCL is –VCCL during M1 on and M2 off. For high input

voltage applications, the limited VDS rating of available

P-channel MOSFETs might require changing from a LO

side to HI side active clamp topology.

Forthelosideactiveclamptopologyinsteadystate,during

M1 on time, magnetizing current (IMAG) increases from a

negative value to a positive value (Figure 14). When M1

turnsoff,magnetizingcurrentchargesSWPuntilitreaches

VCCL plus the voltage drop of the M2 body diode. At this

CCL

LLEAK

SWP

LMAG

FSW

CSW

• •

–VIN

OUT

VIN

–VIN

VOUT

–VOUT

AOUT

LT3752-1

LTXXXX

3752 F17

LT3752 Datasheet(PDF) 32 Page - Analog Devices

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