MB39C604 Datasheet, PDF (11/36 Pages) Cypress Semiconductor – PSR topology in an isolated flyback circuit

MB39C604
8. Function Explanations
8.1 LED Current Control by PSR (Primary Side Regulation)
MB39C604 regulates the average LED current (I_LED) by feeding back the information based on Primary Winding peak current
(I_{P_PEAK}) and Secondary Winding energy discharge time (T_DIS) and switching period (T_SW). Figure 5 shows the operating waveform in
steady state. I_P is Primary Winding current and I_S is Secondary Winding current. I_LED as an average current of the Secondary
Winding is described by the following equation.
I_LED = ¹ × I_{S_PEAK} × ^TDIS
2
TSW
Using I_{P_PEAK} and the transformer Secondary to Primary turns ratio (N_P/N_S), Secondary Winding peak current (I_{S_PEAK}) is described
by the following equation.
I_{S_PEAK}
=
NP
NS
× I_{P_PEAK}
Therefore,
I_LED=
1
2
×
NP
NS
×I_{P_PEAK}×
TDIS
TSW
MB39C604 detects T_DIS by monitoring the TZE pin and I_{P_PEAK} by monitoring the CS pin and then controls I_LED. An internal Err Amp
sinks gm current proportional to I_{P_PEAK} from the COMP pin during T_DIS period. In steady state, since the average of the gm current is
equal to internal reference current (I_SO), the voltage on the COMP pin (V_COMP) is nearly constant_.
I_{P_PEAK} × R_CS × gm × T_DIS = I_SO × T_SW
In above equation, gm is transconductance of the Err Amp and R_CS is a sense resistance.
Eventually, I_LED can be calculated by the following equation.
I_LED= ¹ × ^NP × ^ISO × ¹
2 NS gm RCS
Figure 5. LED Current Control Waveform
System Power supply
through Diode Bridge
(V_BULK)
I_P
L_P
I_S
VAUX
VTZE
TZE
ADJ
VD
DRV
CS
C_D
R_CS
GND
I_P
I_LED
I_S
T_ON
T_DIS
T_SW
VD
(VAUX)
1/4 x T_RING
VTZE
I_{P_PEAK}
I_{S_PEAK}
I_LED
TZE threshold
1/4 x T_RING
Document Number: 002-08441 Rev.*A
Page 11 of 36