AN8028
s
Application Notes (continued)
[1] Operation descriptions (continued)
Voltage Regulators
4. Pseudo-resonance operation (Power MOSFET turn-on delay circuit) (continued)
As for the turn-on of power MOSFET, determine the delay time by selecting the constant of the R9 and C9 so
that it turns on at 1/2 cycle of the resonance frequency.
In a simplified method, select so that the voltage waveform turns on at zero voltage. (Refer to figure 6.)
The approximate value of resonance frequency can be obtained by the following equations:
f
SYNC
=
1
2π
√L
· C
[Hz]
C: resonance capacitance
L: inductance of transformer's primary coil
Therefore, the turn-on delay time t
pd(ON)
for turning on the power MOSFET at 1/2 cycle of resonance frequency
is as follows:
t
pd(ON)
= π √L
· C [s]
5. Notes on R9 and C9 selection
If too low resistance is selected for the R9, the current flowing into the TDL terminal after the start of power
supply exceeds the maximum rating value and there is a possibility of causing malfunction (destruction in the
worst case). It is recommended that about 8 kΩ to 10 kΩ be selected for the R9 though it depends on the supply
voltage from the bias coil.
Therefore, adjust t
pd(ON)
with C9 after converting from the inductance of transformer being used and the
resonance capacitance.
t
pd
V
DS
V
IN
After AC rectification
V
CC
C8
R9
TDL
SBD
C9
0.32 V
0 VI
D
R1
V
P
0V
V
TDL
V
P
V
IN
V
TDL
2.8 V typ.
V
OUT
SBD
I
D
R7
C7
0V
Figure 6
Figure 5
6. Overcurrent protection circuit
The overcurrent of the power supply output is proportional to the value of current flowing in the main switch
in the primary side (the power MOSFET). Taking advantage of the above fact, the overcurrent of the power supply
output is restricted by regulating the upper limit of the pulse current flowing in the main switch to protects the
parts easily damaged by the overcurrent.
The current flowing in the main switch is detected by monitoring the voltage of both ends of the low resistor
which is connected between the source of power MOSFET and the power supply GND. When the power MOSFET
is turned on and the threshold voltage of CLM (current limit) is detected, the circuit turns off the output and turns
off the power MOSFET to control so as not to allow further current flow. The threshold voltage of CLM is
approximately
−
0.2 V (typical) under T
a
=
25°C with respect to GND. This control is repeated for each cycle. Once
the overcurrent is detected, the off condition is kept during that cycle and it can not be turned on until the next
cycle. The overcurrent detection method described in the above is called "pulse-by-pulse overcurrent detection".
8
PANASONIC [ PANASONIC SEMICONDUCTOR ]
