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Power Supply 5 Introduction The power supply in the CTC185 is a non-isolated switching power supply that uses a CTC185 Main MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as the switching Regulator device. The supply uses a winding on the IHVT to provide a voltage boost and a series PWM controlled MOSFET to regulate the output voltage to 130 volt DC. The supply has no alignments because it uses a precision 1% voltage reference IC (U4103). The control circuit is synchronized to the horizontal oscillator by using a flyback pulse as a timing reference for a sawtooth ramp generator. When the chassis is in standby mode, Q4114 is kept on by a drain to gate resistor which forces the output voltage to be equal to the RAW B+ voltage.
T4401 IHVT RAW B+ 3 C4007 1 CR4103 CR4105
V2
L4102 Q4114
I1
REG B+ 130VDC C4153
V1
GATE DRIVE
V3
FLYBACK PULSE PIN 8 OF IHVT HORIZ. SYNC CIRCUIT AND SAWTOOTH GEN.
V4
COMPARATOR
V5
ERROR AMP AND REFERENCE
Figure 1, Main Regulator Block Diagram The winding on the IHVT at pins 1 and 3 sums an inverted retrace pulse on the RAW B+ (V1 in figure 1). This provides the pre-boost so the supply will regulate even when RAW B+ falls below 130 volts (figure 2). The number of turns on the winding determines the lowest voltage at which the supply will regulate. The 25 and 27 inch chassis will regulate the +130V with as little as 95 volts AC RMS and the 19 and 20 inch chassis with as little as 90 volts AC RMS. After the pre-boost, the supply acts as a buck converter in that the output voltage is equal to the input voltage times the duty cycle. When Q4114 is turned on, current flows through CR4105, L4102 and Q4114 charge up C4153 . When retrace begins, the voltage on the anode of CR4105 begins to drop but the current I 1 (figure 3) continues to flow through it because of inductor L4102 . When the voltage V2 (figure 3) decreases to -0.7 volts, CR4105 turns off and CR4103 turns on and conducts I 1 until it drops to zero. During this time Q4114 remains on to conduct the current I1 until it drops to zero. The timing of the turn off of Q4114 can be seen in figure 3. The falling edge of voltage V3 is the signal that turns off Q4114. The turn off time of Q4114 is fixed and the turn on time varies in response to the PWM (Pulse Width Modulator) control circuit.
6 Power Supply
Figure 2
Figure 3
Power Supply 7
T4401 IHVT RAW B+ 3 C4007 1
CR4105
V2
L4102 CR4103 Q4114
I1
REG B+ 130VDC C4153
V1
GATE DRIVE
V3
FLYBACK PULSE PIN 8 OF IHVT HORIZ. SYNC CIRCUIT AND SAWTOOTH GEN.
V4
COMPARATOR
V5
ERROR AMP AND REFERENCE
Figure 4, Main Regulator Block Diagram(repeated) The PWM is made up of the horizontal sync circuit and sawtooth generator block, the comparator block, and the error amp and reference block (figure 4). The horizontal sync and sawtooth block generates a sawtooth voltage V4 (figure 5). This signal and the error voltage V5 from the error amp block are fed into the comparator block which generates the gate drive voltage V3. There is a small time delay after voltages V4 and V5 cross and a state change in V3 occurs. This is due to the slow response of the comparator block.
Figure 5
8 Power Supply The gate drive block generates a 9 volt supply which floats above Reg B+. This voltage is needed to switch Q4114 on. The floating supply is generated using a charge pump coupled to the cathode of CR4105. When V2 is negative, charge is stored on a C4138 and when V2 rises after the retrace interval, this charge is dumped to the floating 9 volt supply.
Circuit Description Raw B+ comes from a full wave bridge rectifier (not shown) and filter capacitor C4007. R4172 is a bleeder to discharge C4007 when the ac power is disconnected. Capacitors C4104, C4122, C4124, C4134, and C4135 are for RFI suppression. R4146 and C4137 form a snubber for MOSFET Q4114. R4103 is a current limit resistor for Q4114. It limits the gate drive by reducing the 9 volt gate supply with respect to the source terminal of Q4114. R4108 provides gate drive when the chassis is in standby. CR4106 is a protection diode for the gate of Q4114.
V1
RAW B+ C4007 R4172 PIN 3 IHVT PIN 1 C4124 CR4105
V2
R4146
C4137
I1
CR4103 L4102 R4108
C4104 Q4114
R4103 CR4106 CR4111 C4106 CR4112 R4138 C4134 C4135 CR4113
C4122
130V REG B+ C4153
R4110 +12V RUN C4007 R4116 R4117 Q4108
C4138
R4147
V7
R4148 Q4104
C4108 R4118
V4
R4127 C4109 R4120
R4136 R4124 Q4103 R4128 R4125
V5
C4123
R4114
V3
Q4113
R4123 C4136 R4145 R4142 R4126
IHVT PIN 8
V6
3
C4103
R4149
1 + R4137 +2.5V REF R4111 R4112
U4103
2
Figure 6, CTC185 Main Regulator Circuit
Power Supply 9
The 9 volt gate supply consists of C4106 and CR4111. The charge pump is C4138, R4147, CR4113, and CR4112. C4138 is charged through CR4112 during retrace, and discharged into C4106 during trace through CR4113. MOSFET Q4114 is switched on though R4138 and is turned off by Q4113 turning on and bleeding the gate charge off through R4114. The comparator block consists of Q4102 and Q4103. The inputs to the comparator are the bases of the two transistors. Q4113 serves as a high voltage buffer to switch Q4114. Once input is fed from the error amp and reference block which is formed by U4103 and the voltage sense resistors R4136, R4137, R4111, and R4112, the parallel combination allows the output voltage of the supply to be trimmed to exactly 130 volts. U4103 contains a 1 % voltage reference and the error amplifier in a three lead TO-92 package. C4103 and R4149 provide the gain compensation for the error amp to ensure the power supply is stable into any expected load. The horizontal sync circuit and sawtooth generator is comprised of two transistors (Q4104 and Q4108). Q4104 provides the time delay which is necessary to keep the MOSFET on until the current through it has dropped to zero. The collector voltage V7 is shown in figure 7 along with the flyback pulse V6 which drives the emitter to illustrate the time delay. The collector is capacitively coupled into the base of Q4108 to provide a short duration pulse which charges C4109. C4109, Q4108, R4118 and R4120 form the ramp generator. The short duration pulse from Q4108 charges C4109 quickly and R4120 discharges C4109 at a slower rate.
Figure 7
10 Power Supply
Troubleshooting Troubleshooting of the power supply can be somewhat difficult because the supply will
not operate unless horizontal deflection is working and horizontal deflection will not work unless the power supply is operational. The solution is to bypass the regulator circuit to power deflection and then check the waveforms and DC voltages in the power supply. 1. Apply a short across the drain and source terminals of Q4114 and use a variac on the AC line to control the regulated B+ voltage. If the regulated B+ is allowed to get too high, the XRP circuit will shut off the horizontal oscillator. Therefore, in order to get the chassis to turn on, the variac must be set so as to provide between 90 and 95 volts AC RMS to the chassis. 2. After the chassis is operational use the variac to set the regulated B+ voltage as close as possible to 130 VDC. 3. Use the supplied voltages and waveforms to check the various stages of the power supply for proper operation.
TECH TIP
Two parts that are more likely to fail than others are Q4114 and U4103. U4103 fails most often from excessive voltage on one of its pins. Q4114 failure is most often caused by applying an excessive load to the output. If the horizontal output transistor (Q4401) were to short, Q4114 will also short and blow the fuse. Do not de-energize any power supply or yoke lead by directly shorting it to ground. If it is necessary to de-energize any point on the chassis or deflection yoke, do so by grounding that point through a 1K ohm resistor. The reason is that discharging power supply or yoke leads directly to ground can cause a failure of the regulator output transistor Q4114. This does not apply when discharging the CRT anode. In that case, discharge by directly shorting to the CRT ground braid.
TECH TIP
Power Supply 11 Overview CTC185 The standby power supply in the CTC185 is derived from a dropping resistor connected Standby to a three terminal 12 volt regulator IC (U4102). The power comes directly from the half Power Supply wave rectified AC line. The microprocessor turns off the T4-Chip (U1001) in the standby mode to remove the load from the standby supply. This is to conserve power because the standby supply cannot continually supply the power needed to operate the T-Chip. The standby supply timing for the T4-Chip is important because the current needed to run the micro and the T4-Chip together is derived from C4154 until the +26V run supply supplements it. When the charge stored on C4154 is depleted, the +12V standby supply will drop and the T4-Chip will be turned off automatically.
F4001
R4001
CR4109 R4002 C4154 CR4110 R4104 CR4703
U4102 12V STBY C4102 C4127 Q3901 R4107 5.9V REF 5V STBY
+26V RUN R4109 C4114 CR4104 5.6V C4113 5.6V REF
Figure 8, CTC185 Standby power Supply Circuit Description The AC power line is rectified by CR4109 and CR4004 and fed through R4002 to filter capacitor C4154. Zener diode CR4110 limits the voltage to 27 volts so that regulator U4102 is not damaged by excessive voltage. Diode CR4703 supplements the standby power supply when the chassis is running from the +26V run supply. The power necessary to run the chassis during turn-on comes from C4154. U4102 provides a regulated 12 volt output to run the standby loads in the chassis. Zener diode CR4104 is used to provide a 5.6V and 5.9V reference for the system control circuitry.
12 Power Supply
The T4-Chip power control circuit consists of Q4115, R4143, R4144, and the microprocessor. The signal labeled as STBY_SW is a pin on the microprocessor which is pulled low to turn on Q4115 and apply power to the chassis.
U3101
Q4115 STBY 29 SW
U1001
20 STBY VCC
R4144
R4143
+12V STBY
Figure 9, T4-Chip Power Control
Troubleshooting Dead Set
The turn on sequence for the T4-Chip requires precise timing. Recall that C4154 only stores enough charge to get the set going. The scan derived +26 volts is necessary to re-supply the voltage to keep the T4-Chip powered. 1. Measure the output of U4102 for +12 volts. If it is not present, verify that the T4Chip (U1001) is not turned on by measuring pin 20 for any voltage greater than zero. If there is, the switch transistor (Q4115) is turned on causing the standby supplies to pulled down. 2. The standby supply can be supplemented with an external +26 volts applied to the cathode of CR4703. This will provide a constant supply so the T4-chip can run.