NCP1212_05 Datasheet, PDF (17/22 Pages) ON Semiconductor – Current Mode PWM Controller for Both Forward and Flyback Converters

NCP1212
I_SD−I_SS is the internal current source for C_SS discharging,
20 mA typical.
The discharging time for 0.22 mF Soft−Start capacitor is:
TDIS + ^{0.22 mF}
(5.0 V
20
* 0.6
mA
V
*
0.5
V)
+
42.9
ms
36 ms
The discharging waveform on SS/DMAX pin is shown in
Figure 41. The discharging time from 4.0 V to 0.6 V is
measured as 36 ms from Figure 41. By interpolation,
discharging time can be estimated as about 41.3 ms when
output is overload which agreed with the calculated result.
3.4 V
Figure 41. SS/DMAX Pin Discharging Waveform
2. Determine the PWM Switching Frequency
The switching waveform is generated by the action
of charging and discharging by internal current
sources to a capacitor connected at C_T, pin 4. The
relationship of the switching frequency and the value
of C_T is governed by the equation below:
FSW
+
CT
Ichg
(Vth
D
*
1)
where:
I_chg is the charging current to C_T, 278 mA typical;
D is the selected Maximum Duty Cycle, 48% or
82%;
C_T is the capacitor connected to C_T pin;
V_th is the threshold voltage for different Maximum
Duty Cycle selection, 2.5 V for 48% Maximum
Duty Cycle and 3.8 V for 82% Maximum Duty
Cycle.
The Switching Frequency against C_T is shown in
Figure 31 to help the designers to determine the capacitance
for their selected switching frequency.
3. Determine the BOK Thresholds
Brownout detect thresholds are determined by a
resistors network that monitors part of the bulk
capacitor voltage at BOK pin. Equations below
illustrate the calculation of the resistors value for the
network.
RUpper
)
RLower
+
(VBulk_H * VBulk_L)
45 mA
RLower
+
[1.21 V(VBulk_H * VBulk_L)]
(45 mA VBulk_H)
Where V_{Bulk_H} and V_{Bulk_L} are the desired upper
and lower bulk capacitor voltage for brownout
detection.
Assume V_{Bulk_H} = 212 Vdc and V_{Bulk_L} = 186 Vdc,
select 3.3 kW for R_Lower then R_Upper can be
calculated to be 576 kW.
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