SSL4120 datasheet(29/48 Pages) NXP

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SSL4120

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Product data sheet

Rev. 2 — 1 November 2012

29 of 48

NXP Semiconductors

SSL4120

Resonant power supply controller IC with PFC for LED lighting

7.8.9 HBC high-frequency protection, HFP-HBC (RFMAX pin)

Normally the converter does not operate continuously at maximum frequency because it

sweeps down to much lower values. Certain error conditions, such as a disconnected

transformer, could cause the converter to operate continuously at maximum frequency. If

zero-voltage switching conditions are no longer present, the MOSFETs can overheat. The

SSL4120 features High-Frequency Protection (HFP) for the HBC controller to protect it

from being damaged in such circumstances.

HFP senses the RFMAX pin voltage. This voltage indicates the current frequency. When

the frequency is higher than 75 % of the soft-start frequency range, the protection timer is

started. The 75 % level corresponds to an RFMAX voltage of Vhfp(RFMAX) =1.83V.

7.8.10 HBC overcurrent regulation and protection, OCR and OCP

(SNSCURHBC pin)

The HBC controller is protected against overcurrent in two ways:

• Overcurrent regulation (OCR-HBC) which increases the frequency slowly. The

protection timer is also started.

• Overcurrent protection (OCP-HBC) which steps to maximum frequency.

A Vboost compensation function is used to reduce the variation in the output current

protection level.

7.8.10.1

Boost voltage compensation

The primary current, also known as the resonant current, is sensed using the

SNSCURHBC pin. It senses the momentary voltage across an external current sense

resistor Rcur(HBC). The use of the momentary current signal allows fast overcurrent

protection and simplifies the stabilizing of overcurrent regulation. The OCR and OCP

comparators compare VSNSCURHBC with the maximum positive and negative values.

The primary current is higher when Vboost is low for the same output power. Boost

compensation is included to reduce the dependency of the protected output current level

on Vboost. The boost compensation sources and sinks a current from the SNSCURHBC

pin. This current creates a voltage drop across the series resistor Rcurcmp.

The amplitude of the current is linearly dependent on Vboost. At Vboost(nom), the current is

zero and the voltage VCur(HBC) across the current sense resistor is also present on the

SNSCURHBC pin.

At the UVP boost start level Vuvp(SNSBOOST), the current is at a maximum. The current sink

or source direction depends on the active gate signal. The voltage drop created across

Rcurcmp reduces the amplitude at the pin. This reduction in amplitude results in a higher

effective current protection level. The Rcurcmp value sets the amount of compensation.

Figure 17 shows how the boost compensation works for an artificial current signal. The

sinking compensation current only flows when VSNSCURHBC is positive because of the

circuit implementation.

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