MB39C604 Datasheet, PDF (14/36 Pages) Cypress Semiconductor – PSR topology in an isolated flyback circuit

MB39C604
8.5 Power-Off Sequence
After the AC line voltage is removed, V_BULK is discharged by switching operation. Since any Secondary Winding current does not
flow, I_LED is supplied only from output capacitors and decreases gradually. V_VDD also decreases because there is no current supply
from both Auxiliary Winding and V_BULK. When V_VDD falls below the UVLO threshold voltage, MB39C604 shuts down.
Figure 10. Power-Off Waveform
V_BULK
AC line removed
VDD
UVLO Vth = 7.9V
DRV
Shutdown
VLED
8.6 I_{P_PEAK} Detection Function
MB39C604 detects Primary Winding peak current (I_{P_PEAK}) of Transformer. I_LED is set by connecting a sense resistance (Rcs)
between the CS pin and the GND pin. Maximum I_{P_PEAK} (I_{P_PEAKMAX}) limited by Over Current Protection (OCP) can also be set with
the resistance.
Using the Secondary to Primary turns ratio (N_P/N_S) and I_LED, R_CS is set as the following equation (refer to 8.1)
R_CS=
NP
NS
×
0.14
ILED
In addition, using the OCP threshold voltage (V_OCPTH) and R_CS, I_{P_PEAKMAX} is calculated with the following equation.
I_{P_PEAKMAX}
=
VOCPTH
RCS
8.7 Zero Voltage Switching Function
MB39C604 has built-in zero voltage switching function to minimize switching loss of the external switching MOSFET. This device
detects a zero crossing point through a resistor divider connected from the TZE pin to Auxiliary Winding. A zero energy detection
circuit detects a negative crossing point of the voltage on the TZE pin to Zero energy threshold voltage (V_TZETL). On-timing of
switching MOSFET is decided with waiting an adjustment time (t_ADJ) after the negative crossing occurs.
t_ADJ is set by connecting an external resistance (R_ADJ) between the ADJ pin and the GND pin. Using Primary Winding inductance
(L_P) and the parasitic drain capacitor of switching MOSFET (C_D), t_ADJ is calculated with the following equation.
π
t_ADJ =
LP × CD
2
Using t_ADJ, R_ADJ is set as the following equation.
R_ADJ [kΩ] = 0.0927 × t_ADJ [ns]
Document Number: 002-08441 Rev.*A
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