This design concept reprises one other “1A, 20V, PWM managed present supply.” Like the sooner circuit, this design integrates an LM3x7 adjustable regulator with a PWM DAC to make a programmable 20 V, 1 A present supply. It income from the correct inner voltage reference and overload and thermal safety options of this time confirmed Bob Pease masterpiece!
Nevertheless, not like the sooner design concept that requires a floating, fastened output 24-VDC energy adapter, this sequel incorporates a ground-referred enhance preregulator that may run from a 5-V regulated or unregulated provide rail. The earlier linear design has restricted energy effectivity that really drops beneath single-digit percentages when driving low voltage masses. The preregulator on this model fixes that by monitoring the input-output voltage differential throughout the LM3x7, sustaining it at a continuing 3 V. This supplies satisfactory dropout-suppressing headroom for the LM3x7 whereas minimizing wasted energy and pointless warmth.
Wow the engineering world along with your distinctive design: Design Concepts Submission Information
Right here’s the way it works. LM317 followers will acknowledge Determine 1 as the standard LM317 fixed present supply topology that maintains Iout = Vadj/Rs by forcing the ADJ pin to be 1.25 V extra detrimental (a.ok.a. much less constructive) than the OUT pin. It has labored nice for 50 years, however in fact the one means you may fluctuate Iout is by altering R.
Determine 1 A basic LM317 fixed present supply the place: Iout = Vadj/R = 1.25v/Rs.
Determine 2 reveals one other (simpler) technique to make Iout programmable. The circuit permits management of ampere-scale Iout with solely milliamps of Ic management present.
Determine 2 A modification that makes the present supply variable the place: Iout = (Vadj – IcRc)/Rs – Ic.
Determine 3 reveals this concept fleshed out and put to sensible use. Word that Rs = R4 and Rc = R5.
Determine 3 U2 present supply programmed by U1 PWM DAC and powered by U3 monitoring preregulator.
Determine 2’s Ic management present is supplied by the Q2 Q3 complementary pair. Since Q3 supplies tempco compensation for Q2, it must be carefully thermally coupled with its associate. This fall does some nonlinearity compensation by offering curvature correction to Q2’s Ic management present technology. The daisy chain of three 1N4001 diodes supplies bias for Q2 and This fall.
The PWM enter frequency is assumed to be 10 kHz or thereabouts. Ripple filtering is the aim of C1 and C2 and will get some assist from an analog subtraction cancellation trick first described in “Cancel PWM DAC ripple with analog subtraction.”
About that monitoring preregulator factor: Management of U3 to take care of the three V of headroom required to carry U2 protected from dropout depends on Q1 appearing as a easy differential amplifier. Q1 drives U3’s Vfb voltage suggestions pin to take care of Vfb = 1.245 V. Due to this fact (if Vbe = Q1’s base-emitter bias, usually ~0.6 V for Ie = ~500 µA)
Vfb/R7 = ((U2in – U2out) – Vbe)/R6
1.245v = (U2in – U2out – 0.6v)/(5100/2700)
U2in – U2out = 1.89 * 1.245v + 0.6v = 3v
Word, if you wish to use this circuit with a special preregulator with a special Vfb, simply regulate:
R7 = R6 Vfb/2.4v
Lastly, a word about overvoltage. Present sources have the potential (no pun!) for output voltage to soar to damaging ranges (damaging of U3’s inner swap and downstream circuitry too) if disadvantaged of a correct load. R11 and R12 defend in opposition to this by using U3’s in-built OVP function to restrict max open circuit voltage to about 30 V if the load is misplaced.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- 1-A, 20-V, PWM-controlled present supply
- A precision digital rheostat
- Monitoring preregulator boosts effectivity of PWM energy DAC
- Cancel PWM DAC ripple with analog subtraction
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