How does this circuit work

Question

I am looking at this circuit which converts 5V to 13V, and am wondering how it is designed.

I have seen inductors being used in buck convertors, where a squre wave was used to convert and make it a low pass filter and allow only the DC. But what is the inductor doing in this circuit

Answer 1

Roman Black's designs are always very tense — highly optimized and reduced to the minimum set of components that will do the job.

In this case, however, the inductor is working the same as in any boost converter: When Q1 switches on, L1 is "charged" with current for a time, and then when Q1 switches off, the stored energy causes the voltage at point A to rise until D1 conducts and passes the current to CLOAD.

The oscillator (Q1 and Q2 together) runs at a fixed frequency and fixed duty cycle, always drawing the same amount of power from the source. Regulation is achieved by putting a shunt zener across the output, where it dissipates any power not consumed by the load.

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Brief description of oscillator operation:

• When power is first applied, Q1 turns on and Q2 is off.
• Current through L1 and Q1 ramps up, and so does the voltage across R2.
• When the base voltage of Q2 rises high enough, it begins to turn on.
• Q2 turning on reduces the base drive to Q1, causing it to cut off.
• Q1 cutting off causes the voltage at point A to rise because of the energy stored in L1.
• The voltage at point A provides positive feedback to Q2 (via R4) to keep it turned on even though the voltage across R2 has dropped.
• D1 conducts and current flows into CLOAD. The current through L1 ramps down.
• When the current through L1 and D1 drops to zero, the voltage at point A drops again, causing the base voltage of Q2 to drop as well.
• As Q2 begins to cut off, Q1 begins to turn on. This causes the voltage at point A to drop all the way to zero, turning off Q2 completely and bringing the circuit back to the initial state, where the entire cycle repeats.

The on time of Q1 is primarily determined by the supply voltage, the value of L1, and the resistance of the charging path (L1, Q1 and R2).

The off time is primarily determined by the output voltage (relative to the input voltage) and the value of L1.

Both times are influenced to some degree by the time constant associated with R3 and C1.

Answer 2

Perhaps it's easier to visualize with the circuit rearranged a little and a simulation.

Here's the circuit:

and here's the LTspice circuit list for the sim:

Version 4
SHEET 1 880 840
WIRE -160 224 -176 224
WIRE -64 224 -160 224
WIRE 48 224 -64 224
WIRE 96 224 48 224
WIRE 224 224 176 224
WIRE 416 224 224 224
WIRE 448 224 416 224
WIRE 544 224 512 224
WIRE 624 224 544 224
WIRE 656 224 624 224
WIRE 48 272 48 224
WIRE 416 352 416 224
WIRE 48 400 48 352
WIRE 352 400 48 400
WIRE 224 432 224 224
WIRE 544 448 544 224
WIRE 656 448 656 224
WIRE -64 464 -64 224
WIRE 48 528 48 400
WIRE 224 560 224 512
WIRE 272 560 224 560
WIRE 416 560 416 448
WIRE 416 560 352 560
WIRE 224 576 224 560
WIRE 224 576 112 576
WIRE -176 608 -176 224
WIRE 416 608 416 560
WIRE 224 624 224 576
WIRE -176 736 -176 688
WIRE -64 736 -64 528
WIRE -64 736 -176 736
WIRE 48 736 48 624
WIRE 48 736 -64 736
WIRE 224 736 224 688
WIRE 224 736 48 736
WIRE 416 736 416 688
WIRE 416 736 224 736
WIRE 544 736 544 512
WIRE 544 736 416 736
WIRE 656 736 656 512
WIRE 656 736 544 736
WIRE -176 784 -176 736
FLAG -176 784 0
FLAG -160 224 Vin
FLAG 624 224 Vout
SYMBOL ind 80 240 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 5 56 VBottom 2
SYMATTR InstName L1
SYMATTR Value 470µ
SYMBOL res 32 256 R0
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL res 208 416 R0
SYMATTR InstName R2
SYMATTR Value 15K
SYMBOL npn 352 352 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL npn 112 528 M0
SYMATTR InstName Q2
SYMATTR Value 2N2222
SYMBOL cap 208 624 R0
SYMATTR InstName C1
SYMATTR Value 1n
SYMBOL res 400 592 R0
SYMATTR InstName R3
SYMATTR Value 6.8
SYMBOL cap -80 464 R0
WINDOW 0 25 2 Left 2
WINDOW 3 24 60 Left 2
SYMATTR InstName C2
SYMATTR Value 47µ
SYMBOL res 368 544 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 1K
SYMBOL diode 448 240 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL cap 528 448 R0
SYMATTR InstName C3
SYMATTR Value 10µ
SYMBOL zener 672 512 R180
WINDOW 0 -62 30 Left 2
WINDOW 3 -101 0 Left 2
SYMATTR InstName D2
SYMATTR Value EDZV13B
SYMBOL voltage -176 592 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 5
TEXT -168 760 Left 2 !.tran .01 uic