LM3101M datasheet(15/20 Pages) NSC

15 / 20 page

Typical Applications (Continued)

TLH11436 – 16

Output Voltage 1Vdiv

Horizontal Time Base 500 msdiv

FIGURE 10 Output Voltage Start-up

At the end of the start-up sequence the circuit is in steady-

state or normal PWM operation

Design Procedure

For the Offline Voltage Mode Flyback Regulator (

Figure 7 )

the specifications for the power transformer

MOSFET

switch the switch snubber and the output rectifier can be

calculated based on the system specifications

System specifications

VO e 5 VDC

VI Range e 90 VAC–132 VAC

IO Range e 05A–10A

Efficiency (h)

80%

FO e 500 kHz

TRANSFORMER SPECIFICATIONS

Manipulating the transfer function of a flyback regulator re-

sults in a calculation for the turns ratio of the power trans-

former involving the minimum input voltage the output volt-

age and the maximum duty cycle (D)

VO a VF e (VIN(MIN)–VSW(ON)) (NS NP)

(D(MAX) (1–D(MAX)))

NS NP e (VO a VF)(VIN(MIN)–VSW(ON))

((1–D(MAX))D(MAX))

Assume that the diode forward voltage (VF) is about 07V

and the drain-to-source voltage when the switch is on

(VSW(ON)) is approximately 09V Selecting a 28% maximum

duty cycle results in a turns ratio of

NS NP e (57V1261V) (1–028)028 e 012

(NP NS e 851)

Assuming an efficiency (h) of 80% the average input cur-

rent (at the maximum load current and for the entire period)

IIN e (VO)(IO) (VIN(MIN) h) e (50W)(127V 080)

049A

The average current when the switch is on is the average

current over the entire period divided by the duty cycle

IIN(TON) e IIN D e (049A)(028) e 177A

Selecting the primary inductance ripple current (DIP)tobea

certain percentage of IIN(TON) and combining that with the

duty cycle input voltage and operating frequency gives the

primary inductance by the equation

LP e (VIN(MIN)–VSW(ON)) D(MAX) (DIP FO)

Assuming the percentage to be 46% in the example then

LP e 1261V 028(081A 500 kHz) j 87 mH

MOSFET PARAMETERS

The peak current through the primary inductance and the

Power MOSFET is the average current when the switch is

on plus one-half the primary inductance ripple current

IPRI(PK) e IIN(TON) a (DIP 2) e 177A a (081A2)

218A

Assuming ideal conditions the maximum voltage at the

drain of the Power MOSFET when the switch is off is

VSW(OFF) e (VO a VF)(NP NS) a VIN(MAX)

(57V) (85) a 185V e 233V

x 250V

However leakage inductance exists in the transformer

causing a voltage spike immediately after the switch turns

off This voltage spike will add to the rest of the drain volt-

ages making VSW(OFF) even greater With a leakage induc-

tance that is 2% of the transformer primary inductance and

selecting a switch which has a fall time of 2% the total off-

time the added voltage will be

VLL e 2% LP IPRI(PK) FO 2% (1–D(MAX))

The maximum duty cycle of 28% is used for worst case

purposes Thus the leakage inductance voltage spike is

VLL e 002 87 mH 218A 500 kHz 002 (1–028)

130V

x 150V

This means the actual peak drain voltage is approximately

400V When choosing the Power MOSFET add some mar-

gin to this number A 500V MOSFET was used in this appli-

cation

SNUBBER DESIGN

A ‘‘snubber’’ circuit consisting of a 1N4937 fast recovery

diode and a parallel RC network is inserted around the

transformer primary to clamp the voltage spike This is to

reduce the switch voltage stress when it is off The ‘‘snub-

ber’’ components are calculated in the following manner

CSN t 002 LP IP(PK)2 (VMAX2 b VSN2)

002

87 mH (218A)2 (255V)2 b (250V)2

33 nF

and

RSN s (VMAX a VSN b VIN)2 2

100(FO LP IP(PK)2)

(255V a 250V b185V)2 2

100(500 kHz 87 mH

(218A)2)

12 kX

In the Offline Flyback Regulator application a 001 mF ca-

pacitor and a 10 kX resistor are used as the snubber com-

ponents VMAX is the selected maximum voltage at the drain

of the MOSFET Usually the RC values are selected so that

VMAX is 5V to 10V higher than VSN The power dissipation

of the resistor is

P e (VMAX a VSN b VIN)2 2 R e

(255V a 250V b 185V)2 2 10 kX e 256W

To add some margin a 4W resistor is chosen

The fast recovery diode must have a reverse voltage rating

greater than VMAX The 1N4937 has a 600V rating

OUTPUT DIODE PARAMETERS

The peak secondary current can be calculated using the

peak primary current and the turns ratio (this equation is for

single output flyback regulators)

ISEC(PK) e IPRI(PK) (NP NS) e 218A

85 e 1843A

x 20A

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