SMPS working - Philips FM23 Plasma display
Fan ControlFor ceiling mount or portrait-mode use, there is foreseen in four optional fans, grouped per two. The temperature within the monitor is measured via a sensor (R3372, KTY81) on the PSU. This sensor is, via A/D converter (item 7530 on the SCAVIO), connected to the OTC. According the temperature within the cabinet, the OTC-software will drive the two PWM outputs of the OTC. These outputs (FAN_SP_1 and 2) are connected to the PSU, where for each signal, a corresponding voltage is generated to supply the fans. These voltages (FAN_SUPPLY_1 and _2) are proportional to the duty cycle of the corresponding PWM signals. The OTC senses the temperature every 5 s. If it has reached T-alarm, and this value has been measured three times consequently, the monitor will go into protection, and a error code is generated.
Pre-conditioner
Warning: the pre-conditioner does not provide mains isolation.
Introduction
The European Law describes a reduction of mains harmonics for apparatus with a power consumption above 75 W. Therefore the pre-conditioner is designed. This module serves as the interface between the mains input and the VS/VA supply. The advantage of a pre-conditioner is (compared to a mains input filter):
• Reduction of mains harmonics to legal limits.
• Lower mains current for the same output power.
• Power factor close to 1.
• Stable regulated output.
• Small and low weight.
The input voltage of the pre-conditioner is universal, between 95 and 264 VAC. The output is 400 VDC (400V_HOT) with a maximum output power of 300 W. This output voltage is delivered to the VS supply.
Warning: the pre-conditioner does not provide mains isolation.
Introduction
The European Law describes a reduction of mains harmonics for apparatus with a power consumption above 75 W. Therefore the pre-conditioner is designed. This module serves as the interface between the mains input and the VS/VA supply. The advantage of a pre-conditioner is (compared to a mains input filter):
• Reduction of mains harmonics to legal limits.
• Lower mains current for the same output power.
• Power factor close to 1.
• Stable regulated output.
• Small and low weight.
The input voltage of the pre-conditioner is universal, between 95 and 264 VAC. The output is 400 VDC (400V_HOT) with a maximum output power of 300 W. This output voltage is delivered to the VS supply.
Operation
Start-up
The
two relays (1450 and 1460, diagram P2) are controlled via the SUPPLY_ON
signal. This signal will become 'high' when the +9V_STBY_SW, the
STANDBY (from the OTC), and the LATCH signal are 'ok'. It then switches
indirect relay 1450 via transistor 7460 and so enables the use of a
small low voltage switch. To protect rectifier 6600 and relay 1450, the
inrush current is limited to a maximum of 20 A by charging the capacitor
2605 through two serial PTCs (3450 and 3451) and an NTC (3452). After
approximately 0.5 sec, relay 1460 is activated. This relay will short
the PTCs. The advantage of using an NTC, is the fact that the resistance
varies with the current and hence the mains voltage. At a high mains
voltage, the current is lower for the same power.
Two
clamp diodes 6605 and 6606 charge output capacitors C2616 to the peak
voltage of the mains input. During normal operation, both diodes are
blocked because of the output voltage of 400 VDC, and will only conduct
if there is a mains spike or an output dip. Capacitor 2616 delivers via
R3668 the start-up voltage at pin 16 of IC7650. After the start-up
cycle, IC7650 is supplied via auxiliary winding 1-2. Capacitor C2663 is
charged during the cycle that MOSFET 7610 conducts. While MOSFET 7610 is
switched 'off', this capacitor transfers its energy via D6661 to the
input of stabiliser IC7660. The 15 V output voltage of this IC is fed
via D6665 to VCC pin 12 of IC7650. The slow start function is realized
by the circuit consisting of transistor 7654, D6654, R3654, and C2654.
Normal Operation
An
up-converter circuit is used for the pre-conditioner. The switching
frequency of the converter is chosen much higher than the mains
frequency. It is therefore possible to consider the supply as constant,
during every high frequency period, and the envelope of all voltage
steps during the low frequency period approximates a half sine wave.
The output voltage of the pre-conditioner equals the input voltage,
when the MOSFET is continuous switched 'off', and increases while the
MOSFET is switched 'on'. The rectified mains input voltage is connected
to pin 5 of IC7650 via voltage divider R3650 and R3651. This voltage is
proportional with the mains input, and is used to change the duty cycle
of the pulses, which are generated at pin 11. Because the width of
these pulses is not small enough, the circuit around transistors 7640
and 7641 is added. It decreases the duration of the square wave by 500
ns (this value is set by R3640 and C2640). A demagnetization winding
(pin 1-2 of L5600) detects when there is no energy in the transformer.
This information is fed to IC7650 pin 7 and this is used to switch 'on'
the MOSFET (7610). In this way, the dissipation is very low, combined
with a low EMI. The MOSFET 7610 is switched 'off' at high currents, up
to 15 A. To reduce dissipation, this is done at high speed for which
'turn off driver' T7608 is used. The output voltage (400 V) is divided
by R3670 and R3671 and connected to pin 3 of IC7650. A change in the
load will adjust the duty cycle of the gate pulse at pin 11, in order to
keep the output voltage constant. Therefore, there is no need to adjust
the output voltage by means of a potentiometer.
Protections
Current Protection
The
current through FET 7610 flows also through the sense resistors 3614
and 3615. The voltage across these resistors is fed to pin 6 of IC7650.
If the current exceeds its reference level, the pre-conditioner will
switch 'off'. A filter, formed by C2666 and R3666, avoids unnecessary
protection triggering due to spikes. C2665 and R3665 on pin 13
determine the maximum oscillating frequency.
LLC Supply.
Introduction
The
VS supply (70 - 90 V) is based upon the so-called LLC converter
technology (also used in the MG3.1 and FTV1.9). It is used to supply the
power of the sustain pulses, which generate the light in the PDP. The
voltage is set by a reference DC voltage (VRS), coming from the PDP.
The VA voltage (derived from VS, 30 - 70 V) is used to supply the power
for driving the addressing electrodes of the PDP. The value of VA is
also depending on a reference voltage (VRA) coming from the PDP.
The main supply hosts the following supplies:
• VS supply, via an LLC converter.
• VA supply, derived from VS via a down converter.
• VCC, via a flyback converter.
• 3V3, via a down converter.
• Audio amplifier supplies (VSND_POS and VSND_NEG), via a transformer.
The
start-up voltage for the IC is derived from one phase, the IC starts to
oscillate, and alternately S1 and S2 are driven into conduction with a
dead time in between. This effects that, via the resonance circuit and
the MOSFETS, energy is stored into transformer L5002 and capacitor
C2001.
The
secondary voltages are rectified and smoothed, these voltages are, via a
voltage divider, fed to the optocoupler that influences the oscillator
frequency of the control IC and stabilises the secondary voltages. If
the current becomes too high, the supply is switched 'off' via the fault
input of the control IC.
Advantages:
• High efficiency (more then 90%, other supplies 75%).
• Less radiation.
• More cost-effective (two MOSFETS of 400 V are cheaper than one MOSFET of 600 V).
Disadvantages:
• Very low power stand-by impossible.
• Realization and stabilization is more complex.
General
The
LLC supply is a serial resonance power supply. The coil, resistor, and
capacitor form a trap at the resonance frequency fR. The impedance is
frequency dependent. The smallest impedance is at the resonance
frequency (fR), at the right side of fR is the inductive part, and at
the left side the capacitive part. The supply works in the inductive
part, since higher frequencies causes minor losses. Stabilization is
realized, by regulating the frequency as function of the output voltage
(VS_UNSW) and power. The load is stabilized by influencing the
series-loop. The higher the frequency, the lower the output power. The
supply voltage of the control IC comes from the 25V_HOT voltage of the
standby supply (via stabilizer 7093), and is lead to pin 15 of the IC.
The IC starts to oscillate. This supply line has a short-circuit
protection via opto-coupler 7003; when the supply is regulating, the
current through the opto-coupler is amplified and will deliver power to
the IC. Control is done in the usual way by a TL431 at the secondary
side. VRS is mixed into the feedback voltage, using an additional TL431
(7011 at schematic P6). VRS, a control signal coming from the display,
influences the output of the VS supply. The output voltage of the VS
supply varies according the following formula: VS = 70 + (10 * VRS). Via
this stabilization circuitry for VS, the output voltage is stabilized
(if necessary, it is possible to adjust the voltage, via potmeter
R3026). The VS is
fed via a voltage divider to IC7010 (TL431). If the voltage at pin 3
of IC7010 is higher than 2.5 V, a current will flow from cathode to
anode. This current flows also through the secondary side of the
optocoupler 7002. The voltage at pin 7 of the MC34067, determines the
output frequency. The higher this voltage, the higher the output
frequency. Thus, if the voltage on pin 7 increases, the frequency
increases and VS decreases. When the output voltage rises, the voltage
at the reference IC7010 also rises, this causes the current through the
diode of the opto-coupler to rise. The transistor of the opto-coupler
conducts more, as a result of which the voltage at pin 7 of the MC34067
increases. The output voltage of the error amplifier gets lower, and
the current through R3005 increases. Accurate Over Voltage Protection
is added, using a TL431 (7304) as reference/comparator and an additional
optocoupler (7003) that acts on the fault input pin 10 of the MC34067P.
