SSL8516T datasheet(17/33 Pages) NXP

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SSL8516T

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Product data sheet

Rev. 1 — 5 May 2014

17 of 33

NXP Semiconductors

SSL8516T

Greenchip PFC and flyback controller

7.3.7 Maximum on-time

The flyback controller limits the on-time of the external MOSFET to 38.5

s. When the

on-time is longer than 38.5

s, the IC stops switching and enters the safe restart state.

7.3.8 Overvoltage protection (FBAUX pin)

An output OVP is implemented in the GreenChip series. In the SSL8516T, the auxiliary

voltage is sensed using the current flowing into the FBAUX pin during the secondary

stroke. The auxiliary winding voltage is a well-defined replica of the output voltage. An

internal filter averages voltage spikes.

An internal up-down counter prevents false OVP detection which can occur during ESD or

lightning events. The internal counter counts up by one when the output voltage exceeds

the OVP trip level within one switching cycle. The internal counter counts down by two

when the output voltage has not exceeded the OVP trip level in one switching cycle.

When the counter has reached six, the IC assumes a true overvoltage and triggers the

safe start protection

The demagnetization resistor, RFBAUX sets the output voltage Vo(OVP) at which the OVP

function trips:

(7)

where Ns is the number of secondary windings and Naux is the number of auxiliary

windings of the transformer. Current Iovp(FBAUX) is internally trimmed.

Accurate OVP detection is made possible by adjusting the value of RFBAUX to the turns

ratio of the transformer.

7.3.9 Overcurrent protection (FBSENSE pin)

The primary peak current in the transformer is measured accurately cycle-by-cycle using

the external sense resistor RSENSE2. The OCP circuit limits VFBSENSE to a level set by

VFBCTRL (see also Section 7.3.3). The OCP detection is suppressed during the

leading-edge blanking period, tleb (equals ton(fb)min  td(FBDRIVER)), to prevent false

triggering due to switch-on spikes.

Fig 13. Flyback soft-start

oOVP



aux

----------- I

ovp FBAUX



FBAUX

clamp FBAUX







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