This guy bucks. I'm much less an analog guy, but that explanation was clear enough to make sense of every stage. Very cool!
-- Anders Nelson www.andersknelson.com On Thu, Sep 28, 2023, 3:29 AM Brent Hilpert via cctalk < [email protected]> wrote: > 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%20Corporation/08%20PDP-11/01%20PDP-1104-1134/02%20PDP-1134A%20Power%20Supply/H7441%20FMPS%20MP00271%20part-1%20(H7441).pdf > > > > 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. > >
