|
|
PDF AND8039D Data sheet ( Hoja de datos )
| Número de pieza | AND8039D | |
| Descripción | The One-Transistor Forward Converter | |
| Fabricantes | ON Semiconductor | |
| Logotipo |  |
|
Hay una vista previa y un enlace de descarga de AND8039D (archivo pdf) en la parte inferior de esta página. Total 12 Páginas | ||
|
No Preview Available !
www.DataSheet4U.com
AND8039/D
The One-Transistor
Forward Converter
Prepared by: Marty Brown
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
The one–transistor forward converter is the most
elementary form of transformer–isolated buck converter. It
is typically used in off–line applications in the 100–300 watt
region. This application note illustrates the approach one
would take to design a high DC input voltage, one–transistor
+VIN
forward converter. With additional modifications, it could
be made work as a 110 VAC off–line power supply.
Description of Operation
A simplified schematic of a one–transistor forward
converter can be seen in Figure 1.
LO
D1
+VOUT
+
CIN
RESET
WINDING
ISW
+
D2 COUT
GND
GND
CONTROL
+
VSW
–
Figure 1. Simplified Schematic of a One Transistor Forward Converter
One can see a transformer has been placed between the
input voltage and a buck converter output stage. The power
switch (SW) is used to create a rectangular voltage
waveform whose amplitude is the input voltage and its duty
cycle is the controllable variable. The transformer provides
both a step–up or down function and a safety dielectric
isolation between the input line and the output load.
The major restriction of this topology is the maximum
duty cycle must be about 50 percent. Whenever a core is
driven in a unidirectional fashion, that is, current only being
driven from one direction into the primary, the core must be
reset. Magnetization energy which serves only to reorient
the magnetic domains within the core must be emptied, or
else the core will “walk–up” to saturation after a few cycles.
To do this, one needs to reset the core. Resetting is done by
drawing current from a winding during the period when the
transformer is unloaded, that is, when the power switch and
rectifiers are not conducting. Any winding can provide the
reset function, but the higher the voltage on the winding, the
quicker the core will reset. Typically, this is the primary
winding or a separate reset winding of equal turns to the
primary. Current from the reset winding can then be returned
to the input capacitor and reused during the next cycle of
operation.
The typical switch voltage and current can be seen in
Figure 2. When the power switch is ON, the switch sees the
output filter inductor’s current reflected by through the
transformer. The amplitude of the primary current is the
output rectifier current times turns ratio of the transformer
(N1/N2) plus a small amount of magnetization current.
During the power switch OFF time, the switch voltage “flys”
up to about twice the input voltage. During this time, the
reset winding begins to output magnetization current back
to the input capacitor.
© Semiconductor Components Industries, LLC, 2000
November, 2000 – Rev. 0
1
Publication Order Number:
AND8039/D
1 page  
AND8039/D
Design of the Primary Current Sensing Network
The UC3845, current–mode control IC is being used. Its
current sensing input has a maximum trip voltage of 1.0 volt
when the current–mode circuit is just starting–up. To
minimize the losses associated with the current sensing
resistance, one should use about a trip voltage of between 0.3
and 0.4 V. This results in a current sensing resistor of:
Rsc (R8) t VtripńIpk(max) + 0.3ń2.24 A + 0.13 ohms
make this value 0.1 ohms for a convenient off–the–shelf
value.
A spike filter should be placed between the current
sensing resistor (R8) and the IC. The time constant of this
R–C filter, if set too long, will enter a pulse–skipping mode
at light loads. If its time constant is made too short, then
some spikes may still enter the current comparator and
produce erratic pulsewidths. A time constant of 300 nS is a
good time.
One must first select one of the values. By making the R
larger, one can provide some series protection between the
power switch and the input pin of the IC. I will assign a value
of 1.0 K to R7. The capacitor then becomes:
C7 + 300 nSń1.0 KW + 300 pF
Design of the Bootstrap Start–Up Circuit
The purpose of this circuit is to initially start the control
circuit up from a turned–off state. The control circuit then
would draw its power directly from the transformer. The
most efficient circuit cuts off its start–up current after the
power supply has begun steady–state operation. This
reduces an unnecessary loss.
The circuit seen in the schematic (Figure 5) is essentially
a current–limited, high–voltage, linear regulator. When the
auxiliary power supply from the transformer is less than
10 V, the start–up circuit is operational. When the auxiliary
supply exceeds 10 V, it cuts off its collector current, which
is about 1.0 mA. A 10 uF or greater capacitor (C2) must be
placed on the auxiliary bus to store enough energy to
actually start the supply, since the IC will draw about 10 mA
in the operate mode.
R1 = (Vin(min)–Vz)/1.0 mA = (140–12)/1.0 mA
= 128 K Make 120 K
R2 = (Vin(min)–Vz)/2.0 mA
= 64 K Make 62 K
The zener diode (Z1) is a 500 mW 12 V, 1N5242
The selection of high voltage bipolar small signal
transistors is limited. An MPSW42 works nicely for Q1. The
purpose of D1 is to avoid stressing the base–emitter junction
in the reverse direction, if the auxiliary voltage goes far
above the +12 V base voltage. The typical reverse
breakdown voltage (V(BR)EBO) is between 3.0–6.0 V. A
1N4148 is going to be used for D1.
Design of the Voltage Feedback and Compensation
Design of the Resistor Divider
The UC3845 has a 2.5 volt reference. One should set the
value of the top resistor of the resistor divider (R11) between
2.0 k to 15 k ohms. This then makes the other values in the
compensation network reasonable values. This can be done
by selecting the sense current, that is the current allowed to
flow through the resistor divider. As an estimate one can first
calculate:
Isense + (28 V * 2.5 V)ń7.0 Kohms + 3.65 mA
Using that sense current the lower resistor (R5) then
becomes:
R5 + 2.5 Vń3.65 mA + 684 ohms
* closest resistance 680 ohms
The upper resistor is then:
R11 + (28.0 V * 2.5 V)ń3.65 mA) + 6986 ohms
or 6.98 kohms 1%
Design of the Feedback Loop Compensation
This is a current–mode controlled, forward converter
where only a 1–pole, 1–zero method of compensation is
required (2 poles if the op amp compensation is considered).
This provides maximum of +90 degrees phase boost, which
helps in avoiding unstable operation.
Determining the Control–to–Output Characteristic
The gain at DC for this topology is:
ADC + ƪ(Vin * Vout)2ńVinVeƫ (Nsec ńNpri)
+ 13.5
GDC + 20 Log (ADC)
+ 22.6 dB
The output filter pole is:
ffp + 1ń(2pRLCo)
+ 4.3 Hz (light load (0.5 A))
+ 34.5 Hz (rated load (4.0 A))
where:
RL is the equivalent resistance of the load
(Vout/Iout)
Co is the net value of the output capacitance
(C9+C10+C11)
The ESR zero of the net output capacitance is:
fz(esr) + 1ń(2pResrCo)
+ 1ń(2p(50 m ohms) (660 mF))
+ 4822 Hz
where: Resr is all of the ESR resistances in parallel.
Calculating the Compensation Elements
Locating the compensating breakpoints:
fez + ffp(light load) + 4.3 Hz
fep + fp(esr) + 4.8 kHz
http://onsemi.com
5
5 Page 
Notes
AND8039/D
http://onsemi.com
11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet AND8039D.PDF ] | |
Hoja de datos destacado
| Número de pieza | Descripción | Fabricantes |
| AND8039 | The One-Transistor Forward Converter | ON Semiconductor |
| AND8039D | The One-Transistor Forward Converter | ON Semiconductor |
| Número de pieza | Descripción | Fabricantes |
| SLA6805M | High Voltage 3 phase Motor Driver IC. |
 Sanken |
| SDC1742 | 12- and 14-Bit Hybrid Synchro / Resolver-to-Digital Converters. |
 Analog Devices |
|
DataSheet.es es una pagina web que funciona como un repositorio de manuales o hoja de datos de muchos de los productos más populares, |
| DataSheet.es | 2020 | Privacy Policy | Contacto | Buscar |