TEA1716T View Datasheet(PDF) - NXP Semiconductors.

Part Name

Description

Manufacturer

TEA1716T

Resonant power supply control IC with PFC

NXP Semiconductors. 

NXP Semiconductors

UBA2071; UBA2071A

Half bridge control IC for CCFL backlighting

8.14.2 Overvoltage protection

The overvoltage control, see Section 8.9, is intended to prevent the transformer output

voltage from exceeding its maximum rating. The overvoltage control level has to be at

least at the required lamp ignition voltage, otherwise the lamps may not ignite.

Once the lamps are on and in steady state, the transformer output voltage will usually be

about half the required ignition voltage for CCFLs. Thermal design of the transformers is

based on this lower voltage, not on the ignition voltage above which the overvoltage

control has to be. Hence the circuit might not stay in overvoltage regulation indefinitely.

Therefore overvoltage regulation is combined with overvoltage protection.

When the voltage on the VFB pin exceeds the OV reference level Vth(ov)(VFB), the CVFB is

discharged, an overvoltage fault condition is signalled, PWM dimming is disabled, and the

fault timer is started. An internal latch makes the OV fault signal continuously high, even if

the voltage at the VFB pin only exceeds Vth(ov)(VFB) during part of the output period. So the

peak of the voltage on the VFB pin determines if an overvoltage fault condition is seen. An

internal filter prevents the overvoltage fault condition from being reset when the voltage at

the VFB pin drops below the OV reference level for only one or two hf cycles.

In order to avoid an OV fault condition at the nominal switching frequency (with the lamps

operating normally), the voltage ripple on the VFB pin must not be too large.

8.14.3 Hard switching protection

As the UBA2071 and UBA2071A are intended to drive a half bridge at a high voltage, a

feature is included to ensure zero voltage switching. The design of the resonant load

should guarantee zero voltage switching under normal operating conditions. To prevent

overheating due to high switching losses in case of any abnormal operating condition,

hard switching of the half bridge is detected internally.

At the moment the high-side switch is turned on, the voltage step at the SH pin is

measured. If it is above Vth(hsw)(SH) then PWM dimming is disabled and the fault timer is

started. Also, the frequency is increased by discharging the capacitor at the CVFB pin (by

Idch(CVFB)).

8.14.4 Overvoltage extra protection

Though the hard switching protection as described in Section 8.14.3, usually prevents the

circuit from getting at the wrong side of the resonance curve of the load (were the load

shows capacitive behavior), this might happen for instance when a lamp is suddenly

disconnected. The parasitic capacitance of the lamp and its connection wire may make up

a significant part of the resonance circuit capacitance, so if a lamp is disconnected the

resonance frequency of the remaining load is suddenly higher and the switching

frequency might be at the capacitive side. Hard switching will occur and be detected. The

result is an increase in the switching frequency, which will make the situation worse: the

switching frequency comes closer to the resonance frequency of the remaining load,

creating a higher and potentially destructive transformer secondary voltage.

UBA2071_A_1

Product data sheet

The OverVoltage Extra (OVE), protection prevents damage to the circuit by adding an

extra overvoltage protection level with quick response to that. When the voltage on the

VFB pin exceeds this OVE level Vth(ovextra)(VFB), an OVE fault condition is signalled. The IC

will stop if this happens during a couple of subsequent hf cycles. The time it takes before

the IC stops depends on a percentage of the time the VFB pin voltage exceeds the OVE

level and if hard switching is detected also. Figure 16 shows typical shutdown response

Rev. 01 — 23 June 2008

© NXP B.V. 2008. All rights reserved.

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