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CS5126XDR8

High Performance, Integrated Current Mode PWM Controllers

ON Semiconductor 

CS5124, CS5126

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.

During synchronized operation of the CS5126 the slope

compensation ramp is reduced by 33%. If the CS5126 will

be used in synchronized operation, the inductor value should

be recalculated to work with the slope compensation ramp

reduced to 67% of the normal value.

Powering the CS5124/6 from a Transformer Winding

There are numerous ways to power the CS5124/6 from a

transformer winding to enable the converter to be operated

at high efficiency over a wide input range. Two ways are

shown in the application circuits.

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.

In the CS5126 application circuit in Figure 8 an extra

winding is added to the forward inductor to power VCC. This

winding is phased to conduct during the off time of the

forward converter and performs the same function as the

flyback winding above.

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

non–ideal 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.

36–75VIN

L1

10 µH

C1

1.5 µF

100 V

C3

0.2 µF

100 V

C2

1.5 µF

100 V

ENABLE

SYNC

48VRTN

R9

10 k

C4

1000 pF

CTX15–14526

T1 CTX15–14527

R2 R1

200 k 39 k

D3

R6 11 V

17.4 k C5

1.0 µF

25 V

Q1

F2T493

D2

T2

Q2

IRF634

MMBD6100L

C6

390 pF

R4

0.2 Ω

1/4W

MBRB2060CT

C12

0.01 µF

R7

2.0 k

R3

30.1 k

R10 C11

10 k 0.1 µF

VCC

UVLO

SYNC

SS

GND

GATE

IS

VFB

CS5126

C10

1000 pF

U2

TPS5908

C9

0.01 µF

R8

10 k

5VOUT

C7

47 µF

C8

47 µF

ISOLATED

RTN

Figure 8. Additional Application Diagram, 48 V to 5.0 V, 5.0 A Forward Converter using the CS5126

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