AN11054

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© NXP B.V. 2011. All rights reserved.

Application note

Rev. 1 — 13 May 2011

21 of 27

NXP Semiconductors

AN11054

GreenChip III+ SSL4101 integrated PFC and flyback controller

It is advisable to make the soft-start time for the flyback larger than the soft-start time of

the PFC, to make sure that the PFC starts before the flyback at initial start-up. It is also

advisable to keep the soft-start time in a range of 5 ms to 10 ms.

With C10 = 220 nF and R16 = 12 k

Ω the total soft-start time will be 8 ms.

5.2 Flyback converter protection circuits

5.2.1 Short-circuit on the FBCTRL pin

If the pin is shorted to ground, switching of the flyback controller is inhibited. This situation

is equal to the minimum or a no output power situation.

5.2.2 Open the FBCTRL pin

As shown in Figure 12, the FBCTRL pin is connected to a 3.5 V internal voltage source

via an internal 3 k

Ω resistor. When the voltage on pin FBCTRL is above 2.5 V, this

connection is disabled and the FBCTRL pin is biased with an internal 30

μA current

assumed. Switching of the flyback (and also the PFC) is blocked and the controller will

enter the Safe Restart mode.

An internal switch pulls the FBCTRL pin down when the flyback convertor is disabled.

5.2.3 Time-out flyback control-loop

A time-out function can be realized to protect for an output short-circuit at initial start-up or

for an open control loop situation. This can be done by placing a resistor in series with a

capacitor between the FBCTRL pin and ground (see Figure 12).

Above 2.5 V, the switch in series with the 3 k

Ω resistor is open and pin FBCTRL (and thus

the RC combination) is biased with a 30

μA current source. When the voltage on FBCTRL

pin rises above 4.5 V, switching of the flyback controller and the PFC is blocked causing

the controller to enter the Safe Restart mode. The capacitor can be used to set the time

delay to reach 4.5 V at the FBCTRL pin. The resistor is necessary to separate the

relatively large time-out capacitor from the control loop response. It is advised to use a

resistor of at least 30 k

Ω. The resistor however, will also influence the charge time of the

capacitor.

(15)

otherwise the capacitor can be calculated using Equation 16:

(16)

or the resistor can be calculated using Equation 17:

(17)

t

to

C

to

V

to FBCTRL

()

I

0 FBCTRL

⋅

()

–

()

⋅

I

O FBCTRL

()

------------------------------------------------------------------------------------------------

=

C

to

I

O FBCTRL

⋅

V

to FBCTRL

()

I

O FBCTRL

⋅

()

–

--------------------------------------------------------------------------------

=

R

to

V

to FBCRTL

()

I

OFBCRTL

()

-----------------------------

t

to

C

to

--------

–

=