27 Sep
2023
27 Sep
'23
11:29 p.m.
On 2023-Sep-27, at 9:01 PM, Tom Hunter via cctalk wrote:
The DEC H7441 regulator is a relatively complex
circuit using 2 x 555
timers, 2 x LM301 op-amps, 2 x transformers and 2 inductors
I am struggling to understand how it is meant to work and was hoping to
find a maintenance manual for it.
Could anyone with such a manual please help?
Alternatively is there another explanation of the operation of this or
similar types of circuits?
The circuit implements a switch mode supply.
One of the two 555 timers operates as an oscillator, the second I think
operates as a monoflop with the pulse length controlled via one of the
LM301s.
Overall the circuit seems very complex and while I understand parts of it,
other parts are mysterious.
In particular the top left section around Q1/Q2/Q3 and T1/T2 and E3 is most
confusing.
I did not find anything remotely similar in "The Art of Electronics" from
Horowitz & Winfield.
The H7441 schematics are available from here:
https://deramp.com/downloads/mfe_archive/011-Digital%20Equipment%20Corporat…
Thanks for any help or suggested reading material.
I'll take a stab at a brief description:
The basic form is that of a switching inductive buck/step-down regulator.
L1 is the main bucking inductor.
D12 is the inductor discharge diode for the bucking operation.
Q1 and Q2 are the main switching transistors, operating in parallel with T2 in their
emitter circuits to balance current through the two transistors.
Q3 is a driver stage for Q1,Q2.
C11,L2,C16 are the main output filter.
Fixed-frequency oscillator E1 triggers variable-width monostable E2 via Q6 to create the
PWM switching pulses.
Q8 and associated form a constant-current source for the timing capacitor C10 of this
PWM-monostable, to linearize the charge curve of the capacitor for better operation of the
pulse-width timing.
The switching pulses from the PWM-mono are amplified by Q5 to drive T1.
T1 provides galvanic (voltage) isolation to shift the pulses up to the higher operating
voltage of Q1,Q2,Q3.
All base-drive energy for Q1 and Q2 is delivered through Q3 from T1, thus Q5 driving T1
must itself be a reasonably hefty driver.
Excessive current through Q5 produces a V-drop across R10 which may turn on Q7 to take the
PWM-mono into reset (E2.4=low).
D7,D8,R3 clamp and sink reverse voltage/energy from the T1 secondary to avoid reverse
breakdown/damage to Q1,Q2,Q3.
Op-amp E4 is the voltage-sense amplifier for the main regulation feedback loop.
D18 and associated provide the master reference voltage.
An increase in the sensed +5 output voltage presented at -input E4.2 relative to the
reference voltage at +input E4.3 lowers the voltage into the PWM-mono control input E2.5
to shorten the ON-width of the switching pulses, and vice-versa for a decrease in the +5
output.
Op-amp E3 is running open-loop to function as a comparator for over-current sense.
R17,R18 are the current-sense resistors, placed here in the negative supply line of the +5
main output.
If the current-induced voltage drop across R17,R18 becomes high enough, E3 trips high,
turning on Q7 to take the PWM-mono into reset.
R19,R20 provide the counter-bias V that the R17,R18 V-drop must overcome to trip E3.
E3 tripping high also turns on Q9 to short the reference voltage to GND at E4.3, to
minimize the ON-width of the switching pulses.
D20,D21,D22 form a crowbar for the +5 output.
The crowbar tripping performs two actions: shorting the +5 output via D19, as well as
shorting the switching pulses at the base of Q5 via D23 so the supply doesn't keep
pumping energy into the shorted output.
D2,Q4 and associated form a simple linear regulator for internal supply of ~ +12V to the
control electronics.
C7,D17,D25,C8 are a little charge pump driven off oscillator E1 to create a negative V
supply for the op-amps E3,E4.