A easy present supply

It has lengthy been recognized that the easy mixture of a depletion-mode MOSFET (and earlier than these had been accessible, a JFET) and a resistor made a easy, serviceable present supply reminiscent of that seen on the suitable facet of Determine 1.

Determine 1 Present versus voltage traits of a DN2540 depletion mode MOSFET and the circuit of a easy present supply made with one, each courtesy of Microchip.

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That is evident from the determine’s left facet, which reveals the drain present versus drain voltage traits for numerous gate-source voltages of a DN2540 MOSFET. As soon as the drain voltage rises above a sure level, additional will increase trigger solely very slight rises in drain present (not seen on this scale). This straightforward circuit would possibly suffice for a lot of purposes, apart from the truth that the V_GS required for a particular drain present will range over temperature and manufacturing heaps. One thing else is required to supply a drain present with any diploma of precision.

Different present supply circuits

And so, we’d flip to one thing just like the circuits of Determine 2.

Determine 2 A present supply with a extra predictable present, left (IXYS) and a voltage regulator which could possibly be employed as a present supply with a extra predictable present, proper (TI). Supply: IXYS and Texas Devices

In these circuits, we see members of the ‘431 household regulating MOSFET supply and BJT emitter voltages. The Texas Devices circuit on the suitable demonstrates the necessity for an oscillation-prevention capacitor, and my expertise has been that that is additionally wanted with the IXYS circuit on the left.

Though R_L1, R_S, and R₁ cross exact, well-regulated currents to the transistors of their respective circuits, resistors R_B and R don’t. R_B’s present is topic to a not well-controlled V_GS, and R’s is affected by no matter variations there is perhaps in V_BATT.

The MOSFET circuit is a real two-terminal present supply, so a load will be related in sequence with the present supply at its constructive or adverse terminal. However then the load is at all times subjected to the poorly-controlled R_B present.

The BJT is a part of a three-terminal circuit, and for a load to keep away from the V_BATT-influenced present by means of R, it may solely be related between V_BATT and the BJT collectors. Even so, variations in V_BATT may produce currents, which result in voltages that aren’t solely rejected on the TLA431 cathode, and so would produce uncontrolled currents within the BJTs and subsequently within the load.

A real two-terminal present supply

Determine 3 addresses these limitations in circuit efficiency. In analyzing it, as at all times, I depend on datasheet most and minimal values every time they’re accessible, however resort to and state that I’m using typical values when they don’t seem to be.

Determine 3 This circuit delivers predictable currents to U₁ and M₁ and subsequently to a load. It’s a real two-terminal present supply which accommodates load connection to each high and low facet.

U₁ establishes 1.24 · ( 1 + R₄ / R₃ ) volts at V_S and provides a present of V_S / (R₄ + R₃) to the MOSFET drain.

A further drain present comes from:

2 · ( V_S – V_BE(Q2) / ( R₂ + R₅ )

The “2” is because of the truth that R₂ and R₁ currents are equivalent (discounting the Early impact on Q₁). The present by means of R₁ is sort of fixed whatever the worth of V_GS. This present gives what U₁ must function.

The precision of the overall DC present by means of the load is restricted by the tolerances of R₁ by means of R₅, the U₁ reference’s accuracy, and the worth of the BJT’s temperature-dependent V_BE drop. (U₁’s most suggestions reference present over its working temperature is a negligible 1 µA.)

U₁ requires a minimal of 100 µA to function, so R₅ is chosen to offer it with 150 µA. Per its On Semi datasheet, at this present and over Q₁’s working temperature vary, the 2N3906’s typical V_CE saturation voltage is 50 mV. Add that to the 15mV drop throughout R₁ for a complete of 65 mV, which is the smallest achievable V_SG worth.

Accordingly, we’re some small however indeterminant quantity shy of the utmost drain present assured for the half (at 25°C, 25 V V_DS, and 0 V V_GS solely) by its datasheet. On the different excessive, beneath in any other case equivalent circumstances, a V_GS of -3.5 V will assure a drain present of lower than 10 µA. For such, U₁ and the circuit as a complete will function correctly at a V_S of 5 VDC.

Greater temperatures would possibly require a extra adverse VGS by a most of -4.5 mV/°C and, subsequently, probably bigger values of VS and, accordingly, of R5. This could be to make sure that U₁’s cathode voltage stays above 1.24 V beneath all circumstances.

D₂ is chosen for a Zener voltage which, when added to D₁’s voltage drop, is bigger than V_S, however is lower than the lesser of the utmost allowed cathode-anode voltage of U₁ (18 V) and the utmost allowed V_GS of M₁ (20 V). D₁‘s small capacitance shields the remainder of the circuit from the Zener capacitance, which could in any other case induce oscillations. The diodes are most likely not wanted, however they supply low cost safety. Neither passes present or impacts circuit efficiency throughout regular operation. C₁ ensures steady operation.

U₁ strives to ascertain a relentless voltage at V_S whatever the DC and AC voltage variations of the unregulated provide V₁. Working in opposition to it in descending order of affect are the magnitude of the conductance of the R3 + R4 resistor string, U₁‘s falling loop achieve with frequency, and M₁’s massive R_ds and small C_ds. Nonetheless, the circuit constructed across the 400-V V_DS-capable M₁ achieves some surprisingly good ends in the check circuit of Determine 4.

Determine 4 Circuit used to check the impedance of the Determine 3 present supply.

Desk 1 and Determine 5 checklist and show some measurements. Impedances in megohms are calculated utilizing the formulation R_LOAD · 10^{(-dB, VLOAD / VGEN) / 20} / 1E6.

Desk 1 Impedances of the present supply of Determine 3 at numerous frequencies, evaluated utilizing the circuit of Determine 4.

Determine 5 Plotted curves of Determine 3 present supply impedance from the information in Desk 1.

Observations

There are a number of conclusions that may be drawn from the curves in Determine 5. The most important one is that at low frequencies, the AC impedance Z is roughly inversely proportional to present. A extra insightful technique to specific that is that Z is proportional to R₃ + R₄, which units the present. With bigger resistance, present variations produce bigger voltages for the ‘431 IC to make use of for regulation; that’s, there’s extra achieve accessible within the circuit’s suggestions loop to extend impedance.

One other phenomenon is that within the 1 and 10-mA present curves, the impedance rises rather more shortly as frequency will increase above 1 kHz. That is per the truth that the TLVH431B achieve is kind of flat from DC to 1 kHz and falls thereafter. The next phenomenon masks this impact considerably on the larger 100 mA present.

Lastly, in any respect currents, there is a bonus to working at larger values of V_DS. That is particularly obvious on the highest present, 100 mA. And that is per the truth that for the attribute curves of the DN2540 MOSFET seen in Determine 1, larger V_DS voltages are required at larger currents earlier than the curves change into horizontal.

Precision present supply

A precision excessive impedance, moderate-to excessive voltage-compliant present supply has been launched. Its two-terminal nature implies that a load in sequence with it may be related to the supply’s constructive or adverse finish. Not like earlier designs, the ‘431 regulator IC’s working present is impartial of each the supply’s provide voltage and of its MOSFET’s V_GS voltage. The result’s a extra predictable DC present in addition to larger AC impedances than would in any other case be obtainable.

Christopher Paul has labored in numerous engineering positions within the communications business for over 40 years.

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