ST AN1842 Application note
AN1842
APPLICATION NOTE
60W WIDE-RANGE POWER SUPPLY
FOR LCD MONITOR OR TV, USING THE L5991
by Claudio Spini
This document describes a reference design for a 60W Switch Mode Power Supply dedicated to LCD
TV sets or monitors. The board accepts full range input voltage (90 to 265Vrms) and delivers 5V and
12V. It has good efficiency and very good standby performance, able to meet the most stringent
standby rules.
Introduction
The LCD monitors and TVs are growing very fast, so to support this kind of applications, a dedicated reference design has been developed, taking into account all the requirements that are needed.
The proposed reference design can supply an LCD monitor or an LCD-TV as well, up to 22" panels, together with multimedia functions like audio. The SMPS accepts a full range input voltage and delivers 2
output voltages, a 5V dedicated to the scaler and µP, and a 12V dedicated to the backlight and audio. The
required standby power consumption is 0.8W at 230Vac, in order to satisfy the worldwide power saving
rules. The circuit is also fully protected against faults like output short circuit or over voltage. The market
cost pressure has requested a design approach with particular attention to the solution cost. The board
technology used is the standard thru-hole, but it can be changed very easily in SMT because most of components are available also in this technology. The circuit has been tested deeply in all the most salient
aspects with positive results and it has been integrated with a 22" LCD-TV application without showing
any problem.
Main characteristics
INPUT VOLTAGE: 90 ÷ 265 Vrms - 45-66 Hz
OUTPUT VOLTAGES: 5.1V±2% - @2A Dedicated to panel and digital circuitry for scaling
12V±10% - @4A Dedicated to backlight lamp inverters, audio and SCART
STANDBY Input power less than 0.8W @230Vac, delivering 30mA on 5.1V
FAULT PROTECTIONS: Short circuit on each output with auto-restart at short removal, Open loop
Safety&EMC: Safety: Acc. to EN60950, creepage and clearance minimum distance 6.4mm
PCB TYPE & SIZE: Cu Single Side 70 µm, CEM-1, 180 x 89 mm
EMC: According to EN50022 Class-B
January 2004
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AN1842 APPLICATION NOTE
Figure 1. Electrical Diagram
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AN1842 APPLICATION NOTE
The converter topology of this SMPS is the standard fly back, working in discontinuous and continuous
current mode. The operating frequency of the circuit (~50 kHz) has been chosen in order to obtain a compromise between the transformer size and the input filter complexity. Hence, the input EMI filter is a simple
Pi-filter, 1-cell only, for differential and common mode noise, using a 4-sectors coil filter. A NTC limits the
inrush current at plug-in. The transformer is a slot type, manufactured by PULSE-ELDOR designed according to the EN60950. Ferrite size is ETD34, the reflected voltage is ~95V providing enough room for
the leakage inductance voltage spike with still margin for reliability.
The reflected voltage, the switching frequency and the primary inductance have been chosen to allow the
continuous current operation of the transformer at full load, all over the input voltage range. This helps to
decrease the output capacitor size thanks to the better ratio between the rms and peak current. The network D10, C14, R5 clamps the peak of the leakage inductance voltage spike ensuring reliable operation
of the PowerMOS, while C21, D11 and R19 limit the dv/dt of the drain voltage.
The PowerMOS is a low cost STP5NK80ZFP, offering a good trade-off between the V
and the equivalent C
, housed in standard TO-220 or TO-220FP packages. In this design, the TO-
OSS
(BR)DSS
220FP (TO-220 insulated) has been used, mounted on a heat sink and fixed by a spring. Core of this design is the current mode primary controller, the L5991 integrating all the required blocks to manage the
control and protection of an SMPS. It is available in either DIP-16 (L5991) or in SO-16 (L5991D) packages.
The switching frequency is programmable by means of a an RC network (R11, R12, C15): during normal
operation R11 and R12 are connected in parallel by an internal switch (pin 16); when a light load is detected by the controller this internal switch is opened and the resulting frequency becomes lower, programmed only from C15 and R11.
If the load is further decreased the network D4, D5, R23 provides an additional frequency reduction, proportional to the load, allowing very low power consumption from the mains. Pins 15 and 3 are set in order
to allow the full duty-cycle operation and so the use of most of the energy stored in the bulk capacitor during hold-up operations. Because of the current mode control and the possibility for the duty cycle to exceed
50%, a slope compensation circuitry has been added.
A latched, over voltage protection has been implemented by using the pin 14 and a simple resistor network: in case of loop failure the circuit senses the Vcc and, when the voltage at that pin exceeds the internal threshold, the controller stops the operation until its Vcc drops below the UVLO voltage. The startup is done using a non-dissipative charge pump circuit to save power during standby.
The output rectifiers have been chosen in accordance with the maximum reverse voltage and their power
dissipation. The 5V rectifier is Schottky barrier type STPS5L40, a 5A-40V axial rectifier that thanks to the
low-forward voltage drop is housed in a DO-201 package.
The 12V rectifier is an STPS8H100, an high voltage Schottky rectifier offering a good trade-off between
the forward voltage drop and the maximum operating junction temperature. It is available from STM in 5
different package versions. For this design, the ISOWATT220AC (similar to a standard insulated TO-220)
has been used, mounted on a heat sink and fixed by a spring.
On both outputs, an LC filter has been added in order to filter out the high frequency ripple without increasing the output capacitors size or quality. The output voltage regulation is performed by the secondary feedback monitoring the 5.1V output. The feedback network is the typical one that uses a TL431 driving an
optocoupler, in this case an SFH617A-2, to ensure the required insulation between primary and secondary. The opto-transistor drives directly the COMP pin of the L5991. Here following some waveforms during
the normal operation at full load:
, the RDS(on)
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AN1842 APPLICATION NOTE
Figure 2. Drain voltage & current @115 VAC -
60Hz - full load
CH1: VQ4 - Drain
CH2: V
CH4: V
Figure 3. Drain voltage & current @230 V
(Vcc)
PIN8
(Drain Current)
R16
AC
50Hz - full load
ing the worst operating condition and they are indicated on the right of figure 5. The margin, with
respect to the maximum voltage withstood by each
diode, ensures safe operating conditions for these
devices.
Figure 4. Drain voltage & current @265 V
AC
-
50Hz - full load
-
CH1: VQ4 - Drain
CH2: V
CH4: V
(Vcc)
PIN8
(Drain Current)
R16
CH1: VQ4 - Drain
CH2: V
CH4: V
(Vcc)
PIN8
(Drain Current)
R16
The pictures here above show the drain voltage
and current at the nominal input mains voltage
during normal operation at full load. As visible, the
circuit works in CCM both at nominal high and low
mains.
The figure 4 shows the measurement of the drain
peak voltage at full load and maximum input mains
voltage. The measured voltage of 672V, ensures
a reliable operation of the MOSFET with a good
margin against the maximum BV
. Even the
DSS
maximum rectifiers PIV have been measured dur-
Figure 5. Maximum rectifiers PIV @Vin = 265
V
- 50 Hz and full load
AC
CH3: +5V RECTIFIER: Anode voltage
CH4: +12V RECTIFIER: Anode voltage
Here following the most salient controller IC signals are shown. In both pictures it is possible to
distinguish clean waveforms free of hard spikes or
noise that could affect the controller correct operation.
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AN1842 APPLICATION NOTE
Figure 6. Drain voltage & current @115 VAC -
60Hz - full load
CH2: V
CH3: V
CH4: V
PIN10
PIN6
PIN2
(Out)
(Comp)
(RCT)
Figure 7. Drain voltage & current @230 VAC -
50Hz - full load
CH2: V
CH3: V
CH4: V
PIN10
PIN6
PIN2
(Out)
(Comp)
(RCT)
Cross regulation
In the following tables it has been reported the output voltage cross regulation measurements with static
loads. The overall efficiency of the converter is also calculated at the nominal input voltages.
To check the application circuit it has been tested keeping constant the current on the 5V and varying the
12V load. As visible in both tables, the voltages are within their tolerance at any load condition and the
circuit efficiency is good
.
5V ± 2% 12V ± 10%
Vout
[V]
@Iout
[A]
Vout
[V]
@Iout
[A]
TOLERANCE
Pout
[W]
TOT
Pin
[W]
115 Vac
Efficiency
Vaux
[V]
fs
[KHz]
5.106 2 12.11 4 OK 58.65 71.7 81.8% 18.3 51
5.107 2 12.23 3 OK 46.90 56.6 82.9% 17.4 51
5.108 2 12.40 2 OK 35.02 42.1 83.2% 16.7 51
5.110 2 12.72 1 OK 22.94 27.7 82.8% 16.5 51
5.110 2 13.11 0.5 OK 16.78 20.7 81.0% 16.6 51
5.108 1 11.85 4 OK 52.51 63.9 82.2% 17.5 51
5.108 1 12.00 3 OK 41.11 49.4 83.2% 16.7 51
5.110 1 12.15 2 OK 29.41 35.0 84.0% 16.2 51
5.112 1 12.39 1 OK 17.50 21.0 83.3% 15.9 51
5.112 1 12.66 0.5 OK 11.44 14.0 81.7% 15.9 51
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AN1842 APPLICATION NOTE
5V ± 2% 12V ± 10%
Vout
[V]
5.106 2 12.09 4 OK 58.57 69.3 84.5% 17.7 51
5.107 2 12.23 3 OK 46.90 55.3 84.8% 17.3 51
5.109 2 12.40 2 OK 35.02 41.2 85.0% 16.9 51
5.110 2 12.74 1 OK 22.96 27.6 83.2% 16.6 51
5.110 2 13.12 0.5 OK 16.78 20.8 80.7% 16.6 51
5.108 1 11.85 4 OK 52.51 63.9 82.2% 17.5 51
5.108 1 12.00 3 OK 41.11 49.4 83.2% 16.7 51
5.110 1 12.15 2 OK 29.41 35.0 84.0% 16.2 51
5.112 1 12.39 1 OK 17.50 21.0 83.3% 15.9 51
5.112 1 12.66 0.5 OK 11.44 14.0 81.7% 15.9 51
@Iout
[A]
Vout
[V]
@Iout
[A]
TOLERANCE
Pout
[W]
TOT
Pin
[W]
230 Vac
Efficiency
Vau x
[V]
fs
[KHz]
Standby operation
Like in the previous section, the output voltages and the efficiency have been checked, and the input power has been measured. It is clearly visible that with the required standby load (5V@30mA, 12V@0mA)
the input power consumption is well below 800mW at both the input voltage ranges. Besides, the circuit has been characterised at both the nominal input voltage values for different output load and the efficiency is high in all conditions. In the fig. 8, it is shown the output voltage variation as a function of the 5V
current.
During the standby operation the circuit works at reduced frequency, according to load and input voltage
therefore, thanks to this function, the switching losses are minimized. This allows reaching very low standby consumption because in a power switch the switching and the capacitive losses are directly proportional to the working frequency.
5V 115 Vac
Vout [V] @Iout [mA]
5.11 20 0.102 0.433 23.6%
5.11 30 0.153 0.515 29.8%
5.11 40 0.204 0.593 34.5%
5.11 50 0.256 0.667 38.3%
5.11 60 0.307 0.745 41.1%
5.11 80 0.409 0.881 46.4%
5.11 100 0.511 1.021 50.1%
5V 230Vac
5.11 20 0.102 0.493 20.7%
5.11 30 0.153 0.582 26.3%
5.11 40 0.204 0.672 30.4%
5.11 50 0.256 0.755 33.8%
5.11 60 0.307 0.842 36.4%
5.11 80 0.409 1.008 40.6%
5.11 100 0.511 1.168 43.8%
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Pout
TOT [W]
Pin [W] Efficiency
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