Power Supply; Magnetic Feedback (PSVR64/VR678HF/VR676HF/VR542/VR541/VR539/VR538/VR528/VR332/VG4039/VG4038

Model Numbers:  PSVR64/VR678HF/VR676HF/VR542/VR541/VR539/VR538/VR532/VR528/
VR332/VG4039/VG4038

The switch mode power supply (see Fig. 28) utilized in the units whose model numbers are listed above incorporates an IC regulator (IP001) along with a discrete power output transistor (TP001).  The regulator IC IP001 provides the base drive for the output transistor TP001 as well as providing over-current detection and protection at pins 3 and 12.   The power supply utilizes a somewhat unique magnetic feedback loop circuit in the primary regulation circuitry.   120 volts AC is input to the full wave bridge rectifier DP001-DP004 via fuse FP001 (1.6A).  The main filter capacitor for raw B+ is CP002.  The raw B+ is applied to pin 1 of  the power transformer LP020.

Power supply start-up occurs when AC power is applied to the unit.  VCC is applied to pin 11 of IC IP001 by the charging action of CP008 through resisters RP012,RP011, RP010 and RP002 to one side of the AC line.  When the voltage across CP008 reaches approximately 16 volts, the switching control IC IP001 turns on and starts outputting pulses at pin 10.  As soon as the IC turns on the current draw of the IC causes the VCC voltage at pin 11 to drop back to about 9 volts.  The run-supply for IC IP001 is provided by the voltage developed at pin 6 of the power transformer LP020.  The winding between pins 5 and 6 also serve as the regulation feedback path and will be discussed in detail later.  The waveform at pin 6 is rectified by diode DP007 and filter by capacitor CP008 in order to generate the run supply.

The first time TP001 is turned on by a Hi output from pin 10 of IC IP001 it begins to draw current from the Raw B+ at pin 1 of LP020 via the winding between pins 1 and 3.  This causes the magnetic field to start building up around it.  When the output of IC IP001 at pin 9 goes Lo, the base of transistor TP001 is pulled low turning it off.  This allows the energy stored in the primary winding (between pin 1 & pin 6) to be transferred into the secondary for rectification.  The frequency of the power supply starts out low and is increased by the regulation feedback circuit (TP091 & IP003) until the power output requirement is met and the frequency stabilizes.

TIP:  IC IP001 contains an internal regulator that limits the power supply output voltage to approximately 11% increase if the feedback transistor, TP091 is opened or not turned on.

The supply voltages are developed on the secondary side of the power transformer via conventional rectifier diodes and series pass regulators.  There are two 5 volt regulators consisting of TZ007 and TZ002.  TZ007 provides the 5 volt ever (+5VE) with zener diode DZ018 providing the base reference voltage.  The source for the +5VE supply is provided by the +6VE output from the rectifier diode DP083.  The second 5 volt supply (+5VS) is a switched supply developed by regulator TZ002.  The +5VS supply is turned on after the +9 volt source regulator is activated.  When the +9VS is coupled to the base of TZ002, zener diode DZ005 provides  the base reference for the +5VS regulator.  The +9 volt source (+9VS) regulator TZ030 is turned on and off by the On/Off signal from the system control microcomputer.  The on/off signal turns on transistor TZ012 which then turns on transistor CZ024.  When CZ024 is turned on, the base reference voltage is developed from the +33 volts ever (+33VE) supply across zener diode DZ025.  This turns on the +9VS which then turns on the +5VS regulator TZ002.

The AC line is half wave rectified by diodes DP027 and DP026.  The signal is then applied to one half (B) of IP003 at pin 5 which generates the AC Clock signal needed by the system control microcomputer.  The rectified signal from DP026 and DP027 is then capacitively coupled to pin 5 (positive) of IP003.  A square wave is output at pin 7 which is routed to the system control microcomputer.  Its important that this signal be present.  The system control circuitry uses this signal as an indicator that AC power is applied to the system and other internal timing operation.  If this signal is lost, the microcomputer assumes that AC power has been lost and the shutdown routine is initiated causing it to go into a backup or sleep mode, the primary symptom would be a dead set.

Once the output voltages on the secondary have reached their nominal operating levels, primary regulation begins.  As mentioned earlier, regulation is accomplished via magnetic coupling back through the power transformer LP020 between the winding across pins 7 and 11 and the winding across pins 6 and 5.  The +9 volts ever (+9VE) is monitored and used to develop the error correction signal.  One half (A) of IP003 serves as the error amplifier while TP091 serves to modulate the current flow through the winding pin 7 and 11.  The +9VE is applied to the top of the resistor divider consisting of RP097 and RP096.  The reference for the error amp is the 6.2 volt zener diode DP094 which is connected to pin 3.  As the +9VE voltage drops, the voltage that is applied to pin 2 of IP003 falls.  This causes the output signal at pin 1 to drop.  As the voltage on the base of TP091 falls (PNP) it turns on harder.  When TP091 turns on harder, the current through the winding (pin 7-11) increases.  This slight increase is reflected back to the winding of pins 6 and 5.  The signal at pin 6 is rectified by diode DP006 and applied to pin 3 of the regulator IC IP001.  As TP091 turns on harder, the DC level at pin 3 moves down pulling  more current from IP001.  This causes IP001 to increase the frequency of the output pulses at pin 10 which allows TP001 to conduct more often (frequency goes up).  More power is transferred to secondary from the primary winding (pin 1-3).

As the voltage in the secondary rises, the +9VE rises, causes the voltage at pin 2 of IP003 to go up.  This allows the voltage output at pin 1 of IP003 to go up, causing TP091 to conduct less.  With TP091 conducting less, the current in the winding (pin 7-  11) of the power transformer goes down.  As the current in the winding is reduced, less signal is magnetically coupled back to the winding between pins 6 and 5.  The voltage at pin 3 of IP003 moves upward causing the IC to reduce the frequency of the output pulses at pin 10 of IP003 thereby reducing the power that is transferred to the secondary.