MK5PFC
At first the complexity of this circuit looks challenging. Like all circuits just break it own
stage by stage to understand how the whole circuit works. If you are unfamiliar with an
IC download a data sheet on it and spend some time getting to know the chip, what it
does, ad how it works.
Most of the ICs we find in this supply are old friends to us by now. LM339 Voltage
Comparator, TL431 Voltage Reference,
LM35C is a temperature sensor. The output is a voltage proportional to the temperature
around it. It is mounted on a large clad with heavy current running through it. Excessive
current heats up the LM35C and the circuit connected to the output is a voltage
comparator comparing the LM35C output against a set voltage.
U13 is a simple voltage regulator much like the LM317 we have seen so often. It
tolerates a higher input voltage than the LM317 does, but it works much the same way.
The output at pin 3 is regulated at about 1.2 Volts above pin 1. In this case we raise pin 1
above ground by 12 Volts with a Zener diode, so the output should be about +13 Volts.
This circuit is powered by our Power Factor Correction circuit output and supplies startup
power to the SMPS to get things started.
The TC4420 is a MOSFET Driver. Not much magic there. (check data sheet)
The UCC38503 has two sections. One section does Power Factor Correction. The other
section is the Switching Regulator.
The UC3913 is a Hot Swap Controller used here to control the Fluorescent lamp outputs
under control of a pin from the MPU. (True?)
Troubleshooting tips.
Following is a list of voltages you might expect to find on a working supply with a 1
Amp load on the 24 Volt line. We ought to test it with some load, but since I had to
remove the top heat sink to get access I didn't want to load it down very much. The list
also gives a hint of where that point leads to if it is an output, or comes from if it is an
input.
When measuring voltages be sure to use the proper ground for that circuit. U1, U3, U9,
U10, U11, U12 use the 24 Volt supply ground. Connector x9, pin 14, is convenient. U2,
U7, U8 U14 use an isolated ground. Pin 3 of U5 was a good place. For AC and Big
+VDC power I used R243, left side (as the text reads) for U13, D19, U7.
U1, pin 1 = +26 Volts, output to Q9 > U1, pin 4.
U1, pin 2 = +26 Volts, output to Q2 > "PFAIL" output.
U1, pin 3 = +26 Volts, VDD
U1, pin 4 = 0 Volts, from Q9.
U1, pin 5 = +6.5 Volts, from U10, pin 1.
U1, pin 6 = 0 Volts, from U6, D31
U1, pin 7 = +5.6 Volts, from U10, pin 1
U1, pin 8 = +5.8 Volts, from a voltage divider, R173, R181, R208, R209.
U1, pin 9 = +5.6 Volts, from U10, pin 1.
U1, pin 10 = +8.3 Volts, from Q8.
U1, pin 11 = +5.6 Volts, from U10, pin 1.
U1, pin 12 = Ground
U1, pin 13 = 0 Volts, output to Q3 >"OUTFAIL".
U1, pin 14 = 0 Volts, output to Q8 > U1, pin 10..
U2, pin 1 = 0 Volts, output to U6.
U2, pin 2 = ? Volts, output to Q12, through Q4, to the Relay.
U2, pin 3 = +13.7 Volts, VDD
U2, pin 4 = +12.5 Volts, from C66, Q11, U2, pin 14.
U2, pin 5 = +5.6 Volts, from a voltage divider, R232 and R226.
U2, pin 6 = +2.8 Volts, from +VDC sense
U2, pin 7 = +2.5 Volts, from U8, pin 1.
U2, pin 8 = +3.15 Volts, AC voltage level sense.
U2, pin 9 = +2.5 Volts, From U8, pin 1, but you may have a hard time measuring it with
a meter without interfering with the circuit. You can check it at R238.
U2, pin 10 = +2.5 Volts, U8, pin 1.
U2, pin 11 = +13.7 Volts, from U5, pin 4.
U2, pin 12 = Ground
U2, pin 13 = +17.3 Volts, output to D40 > Q4 > Relay, and D28 > U14, pin 4 (OVP)
U2, pin 14 = 0 Volts, output to Q11.
U3, pin 1 = 0 Volts, output to U3, pin 9.
U3, pin 2 = +26 Volts, output to U5, ORed with Q10.
U3, pin 3 = +26 Volts, VDD
U3, pin 4 = +2.5 Volts, from D30, +V Out Line Monitor.
U3, pin 5 = +2.8 Volts, from U11, pin 1.
U3, pin 6 = +3.8 Volts, from U3, pin 13.
U3, pin 7 = +2.8 Volts, from U11, pin 1.
U3, pin 8 = +2.8 Volts, from U11, pin 1.
U3, pin 9 = 0 Volts, from U3, pin 1.
U3, pin 10 = +0.3 Volts, U15, temperature sensor, so this could vary.
U3, pin 11 = ? (0.8) Volts, from U11, pin 1.
U3, pin 12 = Ground
U3, pin 13 = +3.8 Volts, output to U3, pin 6
U3, pin 14 = 0 Volts, to Q10 > U5.
U4, pin 1 =
U4, pin 2 =
U7, pin 1 and 8 = +VDD (+14 Volts) from U13 or D17
U7, pin 2 = (about 100 KHz) From U14, pin 12
U7,pin 6 and 7 = (about 100 KHz), Q1 drive for main power.
U8, pin 1 = +2.5 Volts
U9, pin 1 = +22.5 Volts
U9, pin 8 = +2.8 Volts
U10, pin 1 = +5.6 V
U10, pin 8 = 2.8 V
U12, pin 1 = (not connected)
U12, pin 2 = +0.1 Volts, from a voltage divider, R95, R99.
U12, pin 3 = +11 Volts when enabled by pulling "LPOWER" input low, turning Q6 on.
U12, pin 4 = 0 Volts, C104.
U12, pin 5 = Ground
U12, pin 6 = ? Volts. (*) Lamp output current sense, so it depends on lamp current.
U12, pin 7 = +10 Volts, output Q7 and lamps.
U12, pin 8 = (*)
U13, pin 1 = +12 Volts, D38
U13, pin 2 = +VDC big, D11, C52 (470 uF, 470 V)
U13, pin 3 = +14.5 V, to D19.
U14, pin 1 =
U14, pin 2 = (about 100 KHz) RT
U14, pin 3 = +? Volts, +V Sense for +VDC, D11
U14, pin 4 = +? Volts, Overvoltage Protection / Enable imput
U14, pin 5 = (about 100 KHz) CT
U14, pin 6 = Ground
U14, pin 7 = +? Volts, "Volage Error" input from U4
U14, pin 8 = +? Volts, Main output current sense.
U14, pin 9 =
U14, pin 10 = Main output Drive to Q5.
U14, pin 11 = Ground
U14, pin 12 = Power Factor Correction Drive output.
U14, pin 13 =
U14, pin 14 = Peak current Limit, across R108, R243
U14, pin 15 = "CA OUT" output
U14, pin 16 = "I Sense" from R141
U14, pin 17 = "M OUT"
U14, pin 18 = +? Volts, from D1 Cathodes
U14, pin 19 =
U14, pin 20 = +12 to +15 Volts, VDD
U15, pin 1 = +27 Volts
U15, pin 2 = 0.3 Volts (depends on temperature)
U15, pin 3 = Ground
D19 = 15.4 Volts at the Cathode. This voltage is the startup voltage and may come from
D17, or U13 through D19.