TEA2262 데이터 시트보기 (PDF) - STMicroelectronics

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TEA2262

SWITCH MODE POWER SUPPLY CONTROLLER

STMicroelectronics 

TEA2262

Stand-by Mode - Normal Mode Transition

During the transition there are simultaneously

pulses coming from the primary and secondary

regulators.

These signals are not synchronized and some care

has to be taken to ensure the safety of the switching

power transistor.

A very sure and simple way consist in checking the

transformer demagnetization state.

- A primary pulse is taken in account only if the

transformer is demagnetized after a conduction

of the power transistor required by the secondary

regulator.

- A secondary pulse is taken in account only if the

transformer is demagnetized after a conduction

of the power transistor required by the primary

regulator.

With this arrangement the switching safety area of

the power transistor is respected and there is no

risk of transformer magnetization.

The magnetization state of the transformer is

checked by sensing the voltage across a winding

of the transformer (generally the same which sup-

plies the TEA2262). This is made by connecting a

resistor between this winding and the demagneti-

zation sensing input of the circuit (pin 1).

SECURITY FUNCTIONS (see flow-chart below)

- Undervoltage detection. This protection works

in association with the starting device ”VCC

switch” (see paragraph Starting-mode - standby

mode). If VCC is lower than VCCstop (typically

8.5V) output pulses are inhibited, in order to avoid

wrong operation of the power supply or bad

power transistor drive.

- Overvoltage detection. If VCC exceeds VCCmax

(typically 15.7V) output pulses are inhibited and

the external capacitor C2 is charged as long as

VCC is higher than VCC stop. Restarting of the

power supply is obtained by reducing VCC below

VCCstop except if the voltage across C2 reaches

VC2 (typically 2.55V) (refer to ”Restart of the

power supply” paragraph).In this last case, the

circuit is definitively stopped.

- Current limitation of the power transistor. The

current is measured by a shunt resistor. A double

threshold system is used :

- When the first threshold (VIM1) is reached, the

conduction of the power transistor is stopped

until the end of the period : a new conduction

signal is needed to obtain conduction again.

- Furthermore as long as the first threshold is

reached (it means during several periods), an

external capacitor C2 is charged. When the

voltage across the capacitor reaches VC2 (typi-

cally 2.55V) the output is inhibited. This is called

the ”repetitive overload protection”. If the over-

load diseappears before VC2 is reached, C2 is

discharged, so transient overloads are toler-

ated.

- Second current limitation thresho ld (VIM2).

When this threshold is reached the output of the

circuit is immediatly inhibited. This protection is

helpfull in case of hard overload for example to

avoid the magnetization of the transformer.

- Restart of the power supply. After stopping due

to VIM2, VCCMax or VCCstop triggering, restart of

the power supply can be obtained by the normal

operating of the ”VCC switch” VCC switch se-

quency from VCCstop to VCCstart . After stopping

due to VC2 threshold reaching, the circuit is de-

finitively stopped. In this case it is necessary to

reduce VCC below approximately 5V to reset the

circuit. From a practical point of view, it means

that the power supply has to be temporarily dis-

connected from any power source to get the

restart.

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