September 12, 1997

Current source forms unusually simple regulator

Kennan Herrick, ESI Electronics, San Francisco, CA

A simple regulator uses a switching current source to drive a pair of shunt zener regulators (Figure 1). The circuit is a modification of the Design Idea "Simple regulator has one active part" (EDN, March 16, 1995, pg 44). At power-on, the small current through R₁ raises Q₁'s gate voltage, thereby starting to turn on Q₁. Increasing current in L₁ then creates a voltage drop in R₂, while L₁'s current charges C₄ toward its ultimate 15V (set by D₄). While these events are occurring, R₂'s voltage couples via C₂ to Q₁'s gate, rapidly turning off Q₁. With Q₁ fully turned off, L₁'s current continues to flow, now through D₁ to charge C₃, for an ultimate voltage of ­15V (set by D₃) on C₃.

The resonant relationship of C₁ and L₁ causes the C₁-L₁ node voltage to start to decrease (C₁ discharges); that decrease couples, as before, through C₂ to Q₁'s gate. Q₁ begins to turn on, and the cycle repeats. D₂ would limit Q₁'s V_GS to 15V, although it never reaches that voltage because of the drop in R₂. The previous version of the circuit used an ordinary catch diode (1N4005) for D₁, because the diode's longer storage time served to square the oscillatory waveform. In this version, however, D₁ must be faster to yield a reasonable output-current capability.

With no load, the efficiency of the circuit is zero, because shunt regulators D₃ and D₄ draw the full circuit current. The only other significant power-dissipating element is Q₁. At 150V input, Q₁'s voltage drop during the peak on-times (occupying approximately 10% of the duty cycle) is 4 to 6V. During the on-times, R₂'s drop is approximately 14V, so the instantaneous current is approximately 20 mA. The dissipation in Q₁ is thus 20 mA×6V×10%, or 12 mW. The regulator's approximate characteristics are a 170-kHz switching frequency, 300-nsec rise and fall times, and a 25% duty cycle. You can vary the voltages of D₃ and D₄ to obtain voltages higher or lower than 15V.

You can also reduce the value of R₂ (but not by much; R₂ parallels C₁) to increase the output-current capability. With R₂=470 ohms, the available output currents are approximately ­14 and 30 mA. With either current extreme, Q₁'s tab temperature does not exceed 20°C or so above ambient, so you need no heat sink. R₁'s rating must be commensurate with the input voltage. You can increase D₂'s voltage to approximately 20V to reduce Q₁'s forward drop. For reduced cost, you can use an IRF710 for Q₁ if the input voltage does not exceed 350V. Finally, the circuit is short-circuit-proof. (DI #2077)

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