21
FROSIN CIRCUIT
During the energy restoration phase, Transistors TP22/23, maintained in their conduction
phase by Network 5 of LP050, DP37/38, and RP37, block TP20. At the end of
demagnetisation, the voltage induced on the collector of TP20 is reduced, and the charging of
CP21 triggers an oscillation cycle with the primary of LP050. Because of this, the collector
voltage of TP20 continues to fall when it reaches 300 V.
The new saturation control of TP20 becomes involved when the collector voltage of TP20 is
close to 0. This effect is obtained by placing a time delay on the action of the positive voltage
induced at 4 of LP050. This limits power losses due to the switching of TP20. The efficiency of
this power supply is therefore above 81% in operation (38% in Standby Mode).
This time delay is caused by the time taken to discharge TP22/23 (related to the FROSIN
circuit: DP37/38, RP37 by controlling the saturation current).
POWER SUPPLY BEHAVIOUR IN STAND-BY MODE
Because the secondaries require little energy in Stand-by Mode, the saturation time of TP20
tends to fall to 0.2 µs (frequency on the order of 200 kHz). This value is too low, and would
affect power supply efficiency in this mode. A minimum conduction time circuit (Ton min =
0.5 µs) for TP20 has therefore been added, using components CP38/RP38.
In steady state, regulation phototransistor IP050 cannot handle the saturation of TP22 and
TP23 (and then the blocking of TP20) alone. The following must assist:
• Positive voltage produced at RP20 terminals during the Storage Phase,
• Positive voltage supplied by the FROSIN circuit (DP37/38) in Restoration Phase.
In Standby Mode, the stored energy becomes greater than the small requirements of the
secondaries. Because of this, the secondary voltages, and in particular the 14 V, increase
during the energy restoration phases. The conduction time of TP20 is shortened to 3 µs by the
secondary regulation effect. This limits the energy stored in LP050.
Ton min is a result of the negative pulse obtained from the voltage induced in 5 of LP050
(negative when TP20 is conducting) and the charging of CP38 via RP38. This negative pulse
keeps TP22 blocked for a minimum of 0.5 µs.