LM3101 datasheet(13/20 Pages) NSC

13 / 20 page

Typical Applications (Continued)

POWER STAGE OPERATION

The LM3001 Primary-Side PWM Driver sends a pulse-width-

modulated signal (via pin 8) to a power switch which in turn

drives a power transformer

The power switch used in this case is an IRF840 Power

MOSFET It is an N-channel enhancement mode device

that has a drain-to-source voltage (VDSS) rating of 500V and

a pulsed drain current (IDM) rating of 32A Even though the

Power MOFSET has a high VDSS snubber circuits are

needed to limit the drain voltage

The power transformer has a primary inductance of 87 mH

The primary-to-secondary turns ratio is 85 to 1 and the sec-

ondary-to-tertiary turns ratio is 1 to 25 The tertiary winding

delivers the LM3001 supply voltage (pins 7 and 12) to the

primary-side driver

There is an internal Overvoltage Threshold circuit (pin 10)

monitoring the input voltage via a resistor divider The over-

voltage trip point is 33V typically With the resistor values

shown the maximum supply voltage is approximately 175V

The output rectifier an SR1606 delivers the secondary cur-

rent to the output The SR1606 is specified for 16A forward

current 60V reverse breakdown voltage and comes to a

TO220-AB package Since the SR1606 dissipates 7W to

8W at full load it requires a heatsink An RC snubber is

placed in parallel to reduce the ringing voltage caused by

the output rectifier turning off during the discontinous mode

of operation

Two Cornell Dubilier type 226 470 mF 25V high frequency

capacitors with low ESRs of 025X are used as the output

capacitors

OUTPUT VOLTAGE CONTROL

The output voltage is controlled by the LM3101 Secondary-

Side PWM Controller The LM3101 uses its error amplifier to

compare the scaled-down output voltage against the inter-

nal precision 124V reference voltage The error amplifier

provides compensation for the regulator frequency re-

sponse by way of an RC feedback network

The resulting error voltage is converted into a pulse-width-

modulated waveform at the system oscillator frequency of

approximately 500 kHz This waveform is then differentiated

(using an external high-pass RC filter) into a series of posi-

tive and negative pulses representing the desired switch

duty cycle

The pulses are transferred through a pulse transformer to

the LM3001 Primary-Side Driver The driver takes the feed-

back pulse signal and converts it into a PWM gate drive for

the Power MOSFET

FAULT RECOVERY OPERATION

A 0167X resistor sets the peak primary current limits to

228A for the pulse-by-pulse limiting and to 360A for the

second-level limit An RC network filters the current limit

voltage to prevent the current limit (pin 6) from being acti-

vated by the reverse recovery spike of the output rectifier

When the second level current limit is triggered the LM3001

shuts down and discharges the capacitor connected to pin 5

(the Shutdown Delay capacitor) After the capacitor is re-

charged to a voltage of approximately 21V the device will

try to restart If the overcurrent condition persists the device

will shut down again

The LM3101 provides the fault protection in case of an out-

put short circuit During normal operation the operating fre-

quency of this circuit is determined by a 25 kX resistor con-

nected to pin 1 of the LM3101 However during a short

circuit condition on the output the frequency of the LM3101

(and the entire circuit operating frequency) drops yielding a

very low duty cycle This short-circuit frequency is set by the

13 kX resistor connected to pin 5

The LM3101 Mode Control and Current Mode Input pins

(pins 2 and 6 respectively) are for current mode control op-

eration The MC pin determines which control mode is being

usedthe resistor tied to the supply voltage means voltage

mode control (the resistor tied to ground would indicate cur-

rent mode control)

START-UP OPERATION

When power is initially applied to the regulator the LM3001

Primary-Side PWM Driver receives its supply current

through a 75 kX resistor connected to the input voltage (see

Figure 7 ) Once the supply pin voltage reaches the thresh-

old of 118V (typical) the LM3001 turns on sending pulse

signals (with an amplitude of approximately 10V) to the gate

of the Power MOSFET Because the output is driving Power

MOSFETs

which need gate-to-source voltages greater

than 10V for hard turn-on (low RDS(ON)) the threshold volt-

age of 118V was selected to insure sufficient output volt-

age

At the beginning of the start-up process the secondary side

of the regulator is still unbiasedhence the LM3001 does

not receive a feedback signal from the secondary side (see

the Start-up Sequence in

Figure 9 ) Before the LM3101 Sec-

ondary-Side PWM Controller is controlling the circuit the

initial operating frequency of the gate drive is determined by

the LM3001 internal oscillator The oscillator uses an exter-

nal capacitor and resistor on pins 14 and 1 respectively

The initial operating frequency in this case is approximately

500 kHz During this time the regulator is operating in a

‘‘free-running’’ state

Also during the start-up the LM3001 executes Soft-Start by

using the Soft-Start capacitor on pin 4 The voltage across

this capacitor is compared to the oscillator ramp on pin 14

(see the LM3001 block diagram) In the offline regulator the

Soft-Start time is 15 ms approximately

During this time as the Soft-Start capacitor charges up the

duty cycle increases with each progressive cycle until final-

ly the duty cycle reaches its maximum value set by the Duty

Cycle Limit circuit (RDL - pin 2) or the Current Limit circuit

(CLIM - pin 6) The Soft-Start phase ends when the duty

cycle is limited by the RDL circuit A resistor at this pin con-

nects to an internal current source which together will gen-

erate a voltage that will be compared to the oscillator ramp

voltage This comparison will determine the maximum duty

cycle during this phase of the start-up cycle For the circuit

Figure 7 the duty cycle is limited to 63% by the RDL

circuit

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