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SCY99079ADR2G

PWM Current-Mode Controller for High-Power Universal Off-Line Supplies

ON Semiconductor 

DAP018A/B/C/D/F

The soft−start is activated in the following conditions:

• Startup sequence: when the user powers the adapter, the

peak current smoothly ramps−up from a low value

towards a maximum value defined by the sense resistor.

• In auto−recovery burst−mode (e.g. during a

non−latched short−circuit), each new set of pulses starts

with a soft−start sequence.

• When the brown−out pin senses a reset on the bulk

voltage, the controller restarts via a soft−start sequence,

just like a fresh power−on sequence.

* Please note that Speedking II does use implement the

soft−burst technique as built in the original Speedking circuit.

Internal Ramp Compensation

Ramp compensation is a known mean to cure

subharmonic oscillations. These oscillations take place at

half the switching frequency and occur only during

Continuous Conduction Mode (CCM) with a duty−cycle

greater than 50%. To lower the current loop gain, one usually

injects between 50 and 100% of the inductor downslope.

Figure 42 depicts how internally the ramp is generated.

* Please note that the ramp signal will be disconnected from

the CS pin, during the OFF time.

Latch

Reset

1.8 V

0V

ON

+

L.E.B.

-

from FB Setpoint

20 k

CS Rcomp

Rsense

Figure 42. Inserting a Resistor

Inserting a resistor in series with the current sense

information brings ramp compensation and stabilizes the

converter in CCM operation.

In the Speedking II controller, the oscillator ramp features

a 1.8 V swing. If our clock operates at a 65 kHz frequency,

then the oscillator slope corresponds to a 120 mV/ms ramp.

In our flyback design, let’s assume that our primary

inductance Lp is 350 mH, and the SMPS delivers 12 V with

a Np:Ns ratio of 1:0.1. The off−time primary current slope is

thus given by:

ǒVout

) VfǓ

LP

@

NS

NP

+

371

mAńms

or

37

mVńms

when projected over a sense resistor Rsense of 0.1W, for

instance. If we select 75% of the downslope as the required

amount of ramp compensation, then we shall inject

27 mV/ms. Our internal compensation being of 120 mV/ms,

the divider ratio (divratio) between Rcomp and the 20 kW is

0.225. Rcomp can therefore be obtained using the following

value:

Rcomp

+

Rramp

(1 *

@ divratio

divratio)

+

20

1

k

*

0.225

0.225

+

5.8

kW

Brown−out Protection

Versions B and D of the controller include a dedicated

circuitry which permanently monitors the bulk capacitor

level. Figure 43 depicts the comparator arrangement known

as a brown−out protection:

bulk

Rupper

BO

3.3Meg

Brown Out

VBO IBO

1V 10 m

Rlower

10k

Figure 43. A Brown−out Circuit Protects the

Power Supply against Low Input Voltages

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