TDA8380A Datasheet, PDF (10/20 Pages) NXP Semiconductors – Control circuit for switched mode power supplies

Philips Semiconductors
Control circuit for switched mode power supplies
Product specification
TDA8380A
Demagnetization sense circuit
To enable the SMPS to be kept in the non-continuous mode, an input is available which delays switch-on of the
high-voltage transistor until the transformer currents have decayed to zero. This is an effective way of avoiding
transformer saturation. The waveforms illustrated by Fig.9 show demagnetization with respect to the application diagram
of Fig.13.
Fig.9 Demagnetization function.
As long as the voltage of the take-over (auxiliary) winding (also used for supplying the device) is above 0.6 V (V₃) the
output will be prevented from switching on.
Over-current protection
The over-current protection circuit (pin 13) senses the voltage across resistor R_s (see Fig.13), which reflects the primary
current. This generated voltage is negative-going as the emitter of the high-voltage power transistor is grounded (this
circuit arrangement provides the IC with the best safeguard against a possible collector-emitter short-circuit in the power
transistor). At pin 13, the negative voltage signal is shifted to a positive level by a voltage across resistor R₁₃. This voltage
is set by the reference current at pin 13 and is defined by resistor R₆ at the I_ref input (pin 6) and = 1/6 × V_ref/R₆.
Therefore V_shift(V_{R 13}) = V_ref/6 × R₁₃/R₆ or nominal 0.416 × R₁₃/R₆ (V).
The positive current monitor voltage at pin 13 is compared with two voltage levels: the first level = 0.2 V and the second
level = 0 V (see Fig.10).
The first trip level only switches off the high-voltage transistor for a cycle and puts the SMPS in a continuous
cycle-by-cycle current protection mode.
The second trip level is only activated when the primary current rise is very fast which can occur during a short-circuited
output. In this mode the high-voltage transistor is quickly switched off and the slow-start procedure is activated.
The difference between the first and second primary current peak levels is set by R_s:
I₂ − I₁ = 0.2/R_s.
The absolute peak values are set by R₆ and R₁₃:
I₂ × R_s = 0.416 × R₁₃/R₆ or
I₁ × R_s = (0.416 × R₁₃/R₆) − 0.2
November 1993
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