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TEA1533T

GreenChip™II SMPS control IC

Philips Electronics 

Philips Semiconductors

GreenChipTMII SMPS control IC

Product specification

TEA1533T; TEA1533AT

OverVoltage Protection (OVP)

An OVP mode is implemented in the GreenChip series.

This works for the TEA1533 by sensing the auxiliary

voltage via the current flowing into pin DEM during the

secondary stroke. The auxiliary winding voltage is a

well-defined replica of the output voltage. Any voltage

spikes are averaged by an internal filter.

If the output voltage exceeds the OVP trip level, an internal

counter starts counting subsequent OVP events. The

counter has been added to prevent incorrect OVP

detections which might occur during ESD or lightning

events. If the output voltage exceeds the OVP trip level a

few times and not again in a subsequent cycle, the internal

counter will count down with twice the speed compared

with counting up. However, when typical 10 cycles of

subsequent OVP events are detected, the IC assumes a

true OVP and the OVP circuit switches the power

MOSFET off. Next, the controller waits until the UVLO

level is reached on pin VCC. When VCC drops to UVLO,

capacitor CVCC will be recharged to the Vstart level.

Regarding the TEA1533T, this IC will not start switching

again. Subsequently, VCC will drop again to the UVLO

level, etc. Operation only recommences when the VCC

voltage drops below a level of approximately 4.5 V

(practically when Vmains has been disconnected for a short

period).

Regarding the TEA1533AT, switching starts again (safe

restart mode) when the Vstart level is reached. This

process is repeated as long as the OVP condition exists.

The output voltage Vo(OVP) at which the OVP function trips,

can be set by the demagnetization resistor, RDEM:

Valley switching

A new cycle starts when the power MOSFET is switched

on (see Fig.8). After the ‘on-time’ (which is determined by

the ‘sense’ voltage and the internal control voltage), the

switch is opened and the secondary stroke starts. After the

secondary stroke, the drain voltage shows an oscillation

with a frequency of approximately -----------------------1------------------------

2 × π × (Lp × Cd)

where Lp is the primary self inductance of the transformer

and Cd is the capacitance on the drain node.

As soon as the oscillator voltage is high again and the

secondary stroke has ended, the circuit waits for the

lowest drain voltage before starting a new primary stroke.

This method is called valley detection. Figure 8 shows the

drain voltage together with the valley signal, the signal

indicating the secondary stroke and the oscillator signal.

In an optimum design, the reflected secondary voltage on

the primary side will force the drain voltage to zero. Thus,

zero voltage switching is very possible, preventing large

capacitive switching losses P = 12-- × C × V2 × f and

allowing high frequency operation, which results in small

and cost effective inductors.

Vo(OVP) =

N---N--a---su--x- {I(OVP)(DEM) × RDEM + Vclamp(DEM)(pos)}

where Ns is the number of secondary turns and Naux is the

number of auxiliary turns of the transformer.

Current I(OVP)(DEM) is internally trimmed.

The value of RDEM can be adjusted to the turns ratio of the

transformer, thus making an accurate OVP possible.

2002 Aug 23

8

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