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CS5124XDR8G

High Performance, Integrated Current Mode PWM Controllers

ON Semiconductor 

CS5124

converter. If the second current sense threshold is tripped,

the converter will shut off and restart in Soft−Start mode

until the high current condition is removed. The dead time

after a second threshold overcurrent condition will primarily

be determined by the time required to charge the Soft−Start

cap from 0.275 V nominal to 1.32 V.

The second threshold will only be reached when a high

dv/dt is present at the current sense pin. The signal must be

fast enough to reach the second threshold before the first

threshold turns off the driver. This will normally happen if

the forward inductor saturates or when there is a shorted

load.

Excessive filtering of the current sense signal, a low value

current sense resistor, or even an inductor that does not

saturate during heavy output currents can prevent the second

threshold from being reached. In this case the first current

sense threshold will trip during each cycle of high output

current conditions. The first threshold will limit output

current but some components, especially the output rectifier,

can overheat due to higher than normal average output

current.

Slope Compensation

Current mode converters operating at duty cycles in

excess of 50% require an artificial ramp to be added to the

current waveform or subtracted from the feedback

waveform. For the current loop to be stable the artificial

ramp must be equivalent to at least 50% of the inductor

current down slope and is typically chosen between 75% to

100% of the inductor down current down slope.

To choose an inductor value such that the internal slope

compensation ramp will be equal to a certain fraction of the

inductor down current slope use the Formula (4).

1

Internal Ramp

(VOUT ) VRECTIFIER)

NSECONDARY

NPRIMARY

RSENSE Slope Value Factor + Inductor Value(H)

(4)

Calculating the nominal inductor value for an artificial

ramp equivalent to 100% of the current inductor down slope

at CS5124 nominal conditions, a 5.0 V output, a 200 mW

current sense resistor and a 4:1 transformer ratio yields

1

20 mVńms

(5.0 V ) 0.3 V)

1

4

0.2 W

1.0 + 13.2 mH

(5)

To check that the slope compensation ramp will be greater

than 50% of the inductor down under all conditions,

substitute the minimum internal slope compensation value

and use 0.5 for the slope compensation value. Then check

that the actual inductor value will always be greater than the

inductor value calculated.

Powering the CS5124 from a Transformer Winding

There are numerous ways to power the CS5124 from a

transformer winding to enable the converter to be operated

at high efficiency over a wide input range.

The CS5124 application circuit in Figure 1 is a flyback

converter that uses a second flyback winding to power VCC.

R4 improves VCC regulation with load changes by snubbing

the turn off spike. Once the turn off spike has subsided the

voltage of this winding is voltage proportional to the voltage

on the main flyback winding. This voltage is regulated

because the main winding is clamped by the regulated output

voltage.

A flyback winding from a forward transformer can also be

used to power VCC. Ideally the transformer volt−second

product of a forward converter would be constant over the

range of line voltages and load currents; and the transformer

inductance could be chosen to store the required level of

energy during each cycle to power VCC. Even though the

flyback energy is not directly regulated it would remain

constant. Unfortunately in a real converter there are many

nonideal effects that degrade regulation. Transformer

inductance varies, converter frequency varies, energy stored

in primary leakage inductance varies with output current,

stray transformer capacitances and various parasitics all

effect the level of energy available for VCC. If too little

energy is provided to VCC, the bootstrapping circuit must

provide power and efficiency will be reduced. If too much

energy is provided VCC rises and may damage the controller.

If this approach is taken the circuit must be carefully

designed and component values must be controlled for good

regulation.

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