GUBBINS@RADC-TOPS20.ARPA (Gern) (01/30/88)
The Heath/Zenith H/Z-100 Power Supply
[ Power Supply Schematic will be provided in the INFO-HZ100 Upgrade
packet I will someday get out to USnailing to those who showed interest
in the ZUpGrade Survey. If you desire the schematic before then, please
USnail a SASE to Gernware Enterprises, 2803 Oneida Street, Utica, NY 13501
and include a note to remind me it is the schematic you desire. - Gern ]
INTRODUCTION
------------
The power supply provides regulated voltages used by the
various components within the H/Z-100. It is locatad on
the left side of the chassis floor. The power supply comes
in two external designs, one for the All-in-One and one
for the Low Profile. The two Power supplies may look
different on the outside, but internally they are the same.
The power supply of the H/Z-100 series is known as a
switching power supply. It has this name because of the
circuitry design within it. In this type of power supply,
the rectified line voltage is switched on and off at a very
high frequency. The resulting square wave is then filtered
into a DC voltage. This design results in a power supply
that has high efficiency.
The power supply contains many other features. Some of
them are:
o 240 watts of available power.
o Internal cooling fan.
o Two position line voltage switch: 115 or 230.
o Detachable line cord.
Primarily, this section will familiarize you with the power
supply design and operation. Since the power supply is
a sealed unit, it cannot be serviced [ HA! ]; however, the
information contained in this section will aid you in
determining if the power supply is defective.
BASIC CONVERTER
---------------
This supply is an off-line, voltage-fed, half-bridge,
switch-mode power supply. This topology first converts
the AC power mains to DC. This DC is then chopped to a
quasi-square wave. This quasi-square wave is used to drive
the primary of an inverter transformer. The secondaries
are converted to low voltage DC by using rectifiers and
low-pass filters.
EMI FILTER
----------
Components:
All power entering the power supply passes through an EMI
filter. The filter is comprised of C1, C2, C3, C4 and L1.
Function:
The main function of this filter is to reduce conducted
emissions emanating from the power supply to a point where
it complies with the regulatory agencies.
Line Transient Suppressor:
MOV1 is a surge suppressor. Its function is to attenuate
high voltage transients from entering the power supply.
POWER MAINS TO DC CONVERSION
----------------------------
Components:
CR1, RT1, RT2, C5, C6 and R1-R4 form the AC-to-DC conversion
circuit.
115V VAC Operation:
When the 115/230 switch is closed. this circuit is
configured in a voltage doubler mode. Each half-power line
cycle, C5 and C6 are alternately charged to 1.414 times
the RMS line voltage. Since the load is across the two
capacitors, the voltage is two times the voltage across
each capacitor. (Note: The two lower diodes of CR1 are
not used in this mode.)
230 VAC Operation:
When the 115/230 switch is open, the AC-to-DC conversion
circuit is configured in a full-wave bridge mode. Now C5
and C6 ars charged in series each half cycle. The load
"sees" the same DC voltage regardIess of the power line
voltage selected.
Inrush Limiting:
Thermistor RT1 and RT2 limit power line inrush when the
supply is first turned on.
Mains Discharge:
Resistors R1-R4 discharge C5 and C6 when the supply is
turned off (UL requirement).
DC TO QUASI-SQUARE WAVE CONVERSION
----------------------------------
Operation And Components:
Transistors Q1 and Q2 form two active switches that "chop"
the DC. They operate 180 degrees out of phase. They are
driven through driver/isolation transformer, T1. Diodes
CR2, CR3, CR4 and CR5 and C11 and C12 form two turn-off
enhancement circuits. When Q1 or Q2 is forward biased,
C11 or C12, respectively, charges up to approximately 1.2
volts. When the drive circuit signals either transistor
to turn off, it does this by effectively shorting out the
primary of transformer T1. Since the secondaries are now
effectively shorted, the last charged capacitor is placed
across the emitter-base junction of the forward biased
transistor. Therefore, at the first instant, the
emitter-base junction is reverse biased to approximated
1.2 volts. This supplies not only sufficient IB2, but keeps
the transistors reverse biased to prevent false turn on.
Catch Diodes:
Diodes CR6 and CR7 are "catch" diodes that return any
inductive energy to the input capacitors, C5 and C6. They
also protect Q1 and Q2 from reverse breakdown. R5 and C13
form a "snubber" network. This circuit limits the "ringing"
due to leakage inductance in T3 and T4.
Base Drive Scheme - Transformer:
The base drive scheme is a proportional type. The three-
turn winding of T1 has the entire primary current of
inverter transformers, T3 and T4, circulating through it.
As the output load is increased, so does the amount of base
drive to Q1 and Q2. This provides optimum drive under all
load conditions.
Base Drive Scheme - Active Components:
Transistors Q5 through Q8 form a "push-pull" inverter drive
circuit. Transistors Q5 and Q7 provide the turn-on signal
to its respective inverter transistor. Transistors Q6 and
Q8 provide the turn-off signal to its respective inverter
transistor. Diodes CR36 and CR38 allow the current to
commutate during turn off. Transistors Q3 and Q4 act as
logic inverters between the switching regulator IC1 and
the inverter drive circuit.
Integrated Switching Regulator - Operation:
The switching regulator control is IC1. Resistors R28 and
C30 determine the clock frequency. The inputs to the error
amplifier portion of the control IC are pins 1 and 2. Pin
16 is an internal reference of approximately 5 volts.
Approximately 2.5 volts is applied to pin 2 by dividing
down the reference through resistors R21, R22, and R23.
C29 is a noise-decoupling capacitor. The +5 volt output
is divided down to approximately 2.5 volts to be applied
to pin 1 through R17 and R18. Pin 9 is the output of the
error amplifier. Frequency compensation, for proper roll-
off and phase margin, is provided by C26, C43, and R19.
Cross Conduction Protection:
To prevent cross induction of inverter transistors Q1 and
Q2, at any time, a "dead time" limiting circuit is
incorporated. R24, R25 and CR26 form this circuit. R24
and R25 form a 2.5 volt voltage divider off the +5 volt
internal reference. If the output of the error amplifier
ever attempts to slew above this 2.5 volt level, CR26 is
forward biased. This clamps the output. The result is
the maximum duty ratio attainable is approximately 90%.
Slow Start Circuit:
Diodes CR24 and CR25, R20 and C27 form the slow start
circuit This circuit prevents the output from the supply
from overshooting on turn-on. The circuit also limits the
amount of current the inverter transistors must sustain
during turn-on.
At turn-on, C27 is at zero volts. Diode CR25 clamps the
output of the error amplifier to one diode drop. Through
IC1 logic, this forces Q3, Q4, Q6, and Q8 to conduct. This
prevents Q1 or Q2 from switching. Now C27 is charged
through R20. This allows the output of the error amplifier
to rise. Eventually, IC1 allows a minimal on-time to occur
on one of the inverter transistors. A short time later,
the other inverter transistor conducts for the same
duration. Now, the outputs begin to rise. This process
of "walking" up the outputs continues until the inputs of
the error amplifier are equal. At this point, the "loop"
is closed. Capacitor C27 continues to charge to the
internal reference voltage and CR25 is reverse biased.
Diode CR24 resets the slow-start capacitor, C27, when the
supply is turned off.
OUTPUT STAGES
-------------
+5 Volt Output:
Diode CR9 comprises a full-wave Schottky rectifier that
changes a secondary quasi-square wave to a positive polarity
square wave for the +5 volt output. L2 and C14 form a low-
pass filter to convert the square wave to DC. R6 is a
discharge resistor. C21 is a high frequency by-pass
capacitor.
+12 Volt Quasi-Requlated Output:
Diode CR10 comprises a full-wave rectifier that changes
a secondary quasi-square wave to a positive polarity square
wave for the +12 volt output. L3 and C17 comprise a low-
pass filter to convert the square wave to DC. R76 is a
discharge resistor. C22 is a high frequency by-pass
capacitor. R56, R57, C39 and C40 are two snubber networks
to dampen the ringing due to the leakage inductance of T3.
+8 Volt Ouput:
Diode CR8 comprises a full-wave Schottky rectifier that
changes a secondary quasi-square wave to a positive polarity
square wave for the +8 volt output. L6 and C16 form a low
pass filter to convert the square wave to DC. R7 is a
discharge resistor. C20 is a high frequency by-pass
capacitor.
+16 Volt Output:
Diodes CR12 and CR14 comprise a full wave rectifier that
changes a secondary quasi-square wave to a positive polarity
square wave for the +16 volt output. L4 and C18 form a
low-pass filter to convert the square wave to DC. R77 is
a discharge resistor. C23 is a high frequency by-pass
capacitor. The DC fan for the supply is ran off this line
through RT3. (This is so the fan will not run faster if
the box gets too hot.) This output is also used for "boot
strapping" the bias supply through CR33. The purpose of
this is twofold. One reason is to maintain the bias voltage
once the power to the supply is turned off for output
carryover. The second purpose is to allow the use of a
small bias transformer, T5, which is used only on start
up.
Another use of the +16 volt output is the power source for
the +12 volt regulated output. The operation of this
regulator is described in the +8 Volt Output section.
+12 Volt Regulator Operation:
The +12 volt linear regulator is made up of discrete
transistors Q9 thru Q11. The +5 volt output, used as a
reference. is applied to the emitter of Q9. Since, at the
first instant, the +12 regulated output is zero, Q9 is off.
R54 pulls the base of Q11 high. Since Q10 and Q11 are in
a Darlington configuration, both Q10 and Q11 are turned
on. The +12 regulated output begins to rise until Q9
becomes forward biased through voltage divider R50 and R51.
At this point, the circuit is in equilibrium. The dynamic
resistance of Q10 drops the +16 volt line to the +12 volt
regulated output potential.
-16 Volt Output:
Diodes CR11 and CR13 form a full-wave rectifier that changes
a secondary quasi-square wave to a negative polarity square
wave for the -16 volt output. L5 and C19 form a low pass
filter to convert the square wave to DC. R10 through R13
are discharge resistors as well as a minimum load to ensure
that the filter inductor remains critical at all times.
R58, R59, C41, and C42 are two snubber networks used to
dampen the "ringing" due to tbe leakage inductance of T4.
PROTECTION CIRCUITS
-------------------
Current Limit protection:
Transformer T2 is a current-sense transformer that monitors
primary current. R14 provides a load for the transformer.
This converts current to a voltage. Full-wave bridge, CR15
through CR18, converts this quasi-square wave voltage to
a positive polarity square wave. R15 is adjusted to extract
the amount of voltage that would constitute an overcurrent
condition. R16 and C25 is a low-pass filter and time
delay. The time delay prevents false shutdowns for
momentary transients. CR19 resets C25 every time primary
current falls to zero. During dead time, CR30 is an
isolation diode, since the remainder of this circuit is
shared with the overvoltage protection circuit. If the
voltage of C25 is of sufficient amplitude to exceed the
5-volt reference on the inverting input of comparator IC2C,
the output will go high. This forward biases CR30 and CR29;
the thyristor, CR29, will latch into conduction pulling
its cathode high. This will also pull pin 10 of IC1 high.
A high on pin 10 will inhibit all switching action and the
outputs will fall to zero. To recover from this condition,
the AC power must be removed from the power supply, the
overcurrent condition corrected, and the power returned
to the power supply.
Overvoltage Protection:
IC2A is the overvoltage comparator. A 2.5 volt reference
is applied to the inverting input of the comparator. The
+5 volt output is applied to the non-inverting input of
the comparator. The +5 volt output is applied to the
non-inverting input of the comparator through voltage
divider R39 and R40. If the +5 volt output exceeds
approximately 6.2 volts, the output of the comparator will
go high, forward biasing CR32. This will, similar to an
overcurrent, forward bias CR29. CR29 will latch and pull
pin 10 of IC1 high. Once again, all outputs will fall to
zero. The supply wilI not restart until the overvoltage
condition has been corrected and the power line recycled.
+12 Volt Regulator Overcurrent Protection:
IC2B is used as an overcurrent comparator on the +12 volt
regulated output. If the voltage on the output side of
R45 drops too low because of an excessive current drain,
the output of IC2B will go high, forward biasing CR31.
This will, similar to an overcurrent, forward bias CR29.
CR29 will then latch and pull pin 10 of IC1 high. Once
again all outputs will fall to zero. The supply will not
restart until the overcurrent condition has been corrected
and the power line recycled. This condition on the +12
volt regulated output could destroy Q10 without activating
the primary overcurrent circuit.
Wire Color Coding:
Orange: +12VDC
Black: Ground
Red: +5VDC
White: -16VDC
Blue: +8VDC
Yellow: +16VDC
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