9015 Datasheet, PDF (4/7 Pages) List of Unclassifed Manufacturers – QUAD NOR GATE

When the MOSFET is turned on, the voltage required at the secondary transformer to main-
tain the output voltage is:
V _sec = V_diode + V_L(on) + V_out
Where Vdiode is the output rectifier’s forward voltage drop and V_L(on) is the inductor voltage
during the conduction time. As the output current is in continuous-mode operation, the relation
between the secondary transformer voltage (Vsec) and output voltage is just equal to that of a
Buck derived converter (see Figure 1):
V_out = D × V _sec
D: Duty Ratio
With a maximum duty cycle of 50%, Vsec is equal to 2Vout. Taking the voltage drop of Vdiode
and V_L(on) into consideration, the Vsec(5V) of the 5 V output stage can be then 12V and the
Vsec(12V) of the 12V output stage is 26V with a little margin.
The number of turns needed for +5V and +12V secondary by the V_pt:
N _sec(5V) = -_V-¹---²_p---_t = 3 turns
N _sec(17V) = -_V-²---_p⁶---_t = 6.5 turns, approx. 7 turns
Four auxiliary windings generate the control IC operating voltage of +16V output. Considering
the diode voltage drops, the minimum drop across the inductor is 16V. The physical winding of
the transformer is important as the transformer’s performance is affected by it. Figure 2 shows
the power transformer used in this application.
Core: 3C8 Ferrite EER3542D
1→3: Primary Windings
1
13
50 turns AWG23 2 layers
5→7: Auxiliary Winding
4 turns AWG37
3
8,9
10, 11, 12→8, 9: Output (+5V)
5
3 turns Copper Plate
8, 9→13: Output(+12V)
7 turns 4 AWG35
7
10,11,12
(4 wires paralleled)
Figure 2. Power Transformer, T1
Performance
The efficiency of the power supply is 79% at half load, and 75% at full load condition (+5V/
20A, +12V/6A, -12V/0.8A). The -12V output stage employs the negative fixed voltage regula-
tor (KA7912) featuring output current in excess of 1A for precise output voltage, before the
coupled filter inductor (see Figure 3). The power MOSFET QFET FQP7N80, used as the main
switching device, offers improved characteristics featuring a smaller gate charge (Qg (max)=
52nC) and lower on-resistance (Rds(on)max=1.5) than a conventional MOSFET. By taking full
advantage of singular well stripe technology, Fairchild Semiconductor’s power QFET provides
designers with highly effective performance systems. Figure 4(a) shows the waveforms of the
gate-to-source [Vgs] and drain-to-source voltage [Vds] at half load condition, while figure 4(b)
shows the waveforms at full load condition.
4
Rev. B, July 2000