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NCP1236

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1236

CURRENT−MODE CONTROL WITH OVERPOWER COMPENSATION AND SOFT−START

Current sensing

NCP1236 is a current−mode controller, which means that

the FB voltage sets the peak current flowing in the

inductance and the MOSFET. This is done through a PWM

comparator: the current is sensed across a resistor and the

resulting voltage is applied to the CS pin. It is applied to one

input of the PWM comparator through a 250 ns LEB block.

On the other input the FB voltage divided by 5 sets the

threshold: when the voltage ramp reaches this threshold, the

output driver is turned off.

The maximum value for the current sense is 0.7 V, and it

is set by a dedicated comparator.

VFB(ref)

RFB(up)

KFB

FB

blanking

tLEB

CS

PWM

+

−

Soft−start

+

−

Soft−start ramp

Start

tSSTART Reset

IC Start

IC Stop

+

+−

VILIM

Jitter

Oscillator

DCMAX

S

Q

DRV Stage

R

IC stop

blanking

tBCS

UVLO

−

+

+

VCS(stop)

Protection

Mode

HV stop

Latch

TSD

Fault

Figure 36. Current Sense Block Schematic

Each time the controller is starting, i.e. the controller was

off and starts – or restarts – when VCC reaches VCC(on), a

soft−start is applied: the current sense setpoint is linearly

increased from 0 (the minimum level can be higher than 0

because of the LEB and propagation delay) until it reaches

VILIM (after a duration of tSSTART), or until the FB loop

imposes a setpoint lower than the one imposed by the

soft−start (the 2 comparators outputs are OR’ed). The

soft−start ramp signal is generated by the D/A converter in

the NCP1236, that’s why there are observable 15 discrete

steps instead the truly linearly increasing current setpoint

ramp.

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