Philips BJ2.4U Service Manual

Circuit Descriptions, Abbreviation List, and IC Data SheetsEN 122 BJ2.4U/BJ2.5U PA9.9.2 Power Supply9.2.1 IntroductionThe Main Power Supply is a buy-in module (it belongs to thePDP), and therefore is a “black box” for Service. Whendefective, a new panel must be ordered and after receipt, thedefective panel must be sent for repair.This Power Supply delivers the following supply voltages to thechassis:• +12VS.• +8V6.• +5V2.• +5V.As the VIPER and many other ICs on the SSB require lowsupply voltages at high current (up to 3 A for the mainvoltages), onboard DC/DC converters are implemented.The circuit on the SSP provides the 3.3 and 1.2 voltages.A DC/DC converter has the following advantages:• The DC/DC converter is directly on the SSB near thecircuits that needs to be powered.• Some circuits on the SSB need high current by low voltage,so there is no risk to have power dips or voltage loss inconnections between the PSU and the SSB panel.9.2.2 Block DiagramSee also diagrams B1A and B1B.Figure 9-3 DC/DC converter block diagram9.2.3 PSU Start-up Sequence1. If the input voltage of the DC/DC converters is around 12 V(measured on the decoupling capacitors 2U17/2U25/2U45) and the ENABLE signals are "low" (active), then theoutput voltages should have their normal values.2. First, the Stand-by Processor activates the +1V2 supply(via ENABLE-1V2).3. Then, after this voltage becomes present and is detectedOK (about 100 ms), the other two voltages (+2V5 and+3V3) will be activated (via ENABLE-3V3).4. The current consumption of controller IC 7U00 is around 20mA (that means around 200 mV drop voltage acrossresistor 3U22).9.2.4 +2V5D Linear Stabilizer• Provides the +2V5D voltage, and is derived from the +5V2-STBY voltage coming from the Main Power Supply.• The output current is limited to a few tenths of mA.• Output over-voltage protection is done by zener diode6U17.9.2.5 +12V Switch• The +12V switch is activated when the POD-MODE signalis "low".• The rise time of the output voltage is set by components2U42, 3U43, and 3U95 at about 30 ms.• The switch "off" is fast, because there can be fault currentsthat must be interrupted.• When the input voltage (+12VS) is higher than 15 V, theswitch is disabled via circuit 6U12, 3U52, 3U53, 2U71, and7U14-2.9.2.6 Internal Protection• Provides a SUPPLY-FAULT signal (active "low"), when theoutput voltage of any DC/DC converter is out of its limits (±10% of the normal value). In such cases, the Stand-byProcessor will immediately stop the supplies by sending a"high" control signal towards the external and internalsupplies: ENABLE-xVx, POD-MODE, ON-MODE, andSTAND-BY.Note: The SUPPLY-FAULT control signal is "low" whenany DC/DC converter is disabled by its control signal(ENABLE-xVx) and +12VSW is present, therefore it isignored during start-up!• The internal protection works together with the output over-voltage detector transistors 7U15-1, 7U15-2, 7U29-1, and7U29-2.9.2.7 1.2V and 3.3V DC/DC ConvertersIntroductionThe circuit used is a so-called "synchronous buck converter".Some characteristics:• Switching frequency: approx. 250 kHz.• Efficiency: approx. 90%.• Built-in output over-voltage and over-current protections• Soft start.• Software controlled “on/off” (via ENABLE line).Block DiagramFigure 9-4 Block diagram synchronous buck converter.The advantage of a "synchronous buck converter" over a"classical buck converter" is its better efficiency (about 90%).The difference between the two is that in a synchronous buckconverter the "low -side" diode is replaced by a MOSFET TS2(item 7U03). This, because the voltage drop across a MOSFETis smaller than the forward voltage drop of a diode.This second MOSFET TS2 conducts current during the "off"times of the first MOSFET TS1 (item 7U01 at the input side).The upper MOSFET TS1 conducts, to transfer energy from theinput to the inductor L 1 and load RL, while the lower MOSFETTS2 conducts to circulate the inductor current (free wheel). Thesynchronous PWM control block regulates the output voltageby modulating the conduction intervals of the upper and lowerMOSFETs.8V6SWITCH+5V+8V6+8V6-SW+2V5D+2V5+12VSWSUPPLY-FAULTON-MODE+2V5DLINEARSTABILISER+5V2_STBY12VSWITCH+12VSW+1V2ENABLE-1V2VSWVTUNGENERATOR12V/1V2DC/DC CONV.12V/3V3DC/DC CONV.+12VS+5VPOD-MODE+2V5LINEARSTABILISER+3V3VTUNENABLE-3V3F_15400_004.eps280905(Reserved)(Reserved)(Reserved)12V/2V5DC/DC CONV.+2V5ENABLE-2V5F_15400_005.eps090505DSGDVin SGPWM GENERATOR& MOSFET DRIVERGNDVoutGNDC1TS2FBTS1L1