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LT3758

3758f

APPLICATIONS INFORMATION

The ﬂyback converter can be designed to operate either

in continuous or discontinuous mode. Compared to con-

tinuous mode, discontinuous mode has the advantage of

smaller transformer inductances and easy loop compen-

sation, and the disadvantage of higher peak-to-average

current and lower efﬁciency.

to the number of variables involved. The user can choose

either a duty cycle or a turns ratio as the start point. The

following trade-offs should be considered when select-

ing the switch duty cycle or turns ratio, to optimize the

converter performance. A higher duty cycle affects the

ﬂyback converter in the following aspects:

• Lower MOSFET RMS current ISW(RMS), but higher

MOSFET VDS peak voltage

• Lower diode peak reverse voltage, but higher diode

RMS current ID(RMS)

• Higher transformer turns ratio (NP/NS)

The choice,

(for discontinuous mode operation with a given D3) gives

the power MOSFET the lowest power stress (the product

of RMS current and peak voltage). The choice,

(for discontinuous mode operation with a given D3) gives

the diode the lowest power stress (the product of RMS

current and peak voltage). An extreme high or low duty

cycle results in high power stress on the MOSFET or diode,

and reduces efﬁciency. It is recommended to choose a

duty cycle between 20% and 80%.

Figure 7. A Simpliﬁed Flyback Converter

RSENSE

NP:NS

VIN

CIN

CSN

VSN

SUGGESTED

RCD SNUBBER

VDS

ISW

3758 F07

GATE

GND

LT3758

SENSE

–

RSN

DSN

–

COUT

Figure 8. Waveforms of the Flyback Converter

in Discontinuous Mode Operation

3758 F08

ISW

VDS

DTS

D2TS

D3TS

ISW(MAX)

ID(MAX)

Flyback Converter: Switch Duty Cycle and Turns Ratio

The ﬂyback converter conversion ratio in the continuous

mode operation is:

OUT

−

•

Where NS/NP is the second to primary turns ratio.

Figure 8 shows the waveforms of the ﬂyback converter

in discontinuous mode operation. During each switching

period TS, three subintervals occur: DTS, D2TS, D3TS.

During DTS, M is on, and D is reverse-biased. During

D2TS, M is off, and LS is conducting current. Both LP and

LS currents are zero during D3TS.

The ﬂyback converter conversion ratio in the discontinu-

ous mode operation is:

OUT

•

According to the preceding equations, the user has relative

freedom in selecting the switch duty cycle or turns ratio to

suit a given application. The selections of the duty cycle

and the turns ratio are somewhat iterative processes, due

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