Design Ideas: January 5, 1995
The step-up regulator in Fig 1 varies the negative bias of its switching oscillator in response to an error signal from an output-voltage comparator. The circuit uses no transformers, only inductors. The regulator accepts 8 to 12.5V dc and outputs 18V dc. Its efficiency measures 77%.
The circuit's oscillator switch, an enhancement-mode MOSFET (Q1), achieves high switching speeds and efficiency. The network L3, L4, C6, C7, and C2 provides an ac positive-feedback signal from the MOSFET's drain to gate. (R1, paralleling C2, supplies current at start-up. This current ramps the MOSFET's gate into its operating bias-voltage range.)
This circuit does not need the inductance in the MOSFET's drain to oscillate. If the circuit did not control the gate's voltage, the oscillator would operate open-loop, and over-dissipation would occur. Happily, D1 communicates an error signal into the MOSFET's gate circuit, keeping the circuit from running away.
The step-up voltage generator comprises L1, D2, and C4. The short reverse-recovery time and small forward voltage of Schottky diode D2 maximizes efficiency. C4's value is a compromise between its ac-filtering function and its dc-ramping effect; too large a value prevents oscillations from starting. L2 and C5 provide most of the output filtering.
The control circuitry begins with a negative-voltage generator. The network comprising C3, D3, D4, C9, and C10 draws switched power from L1, generating a negative output. C9 and C10 filter the negative output. C3 maintains a negative voltage across load resistors R3 and R4 sufficient to shut off Q1 through D1.
PNP transistor Q2 functions as a comparator, permitting a reference trip point between ground and the negative supply. R3 biases zener diodes D5 and D6 near their zener voltage. That way, a small increase in the error current through D5 and D6 shuts off Q2.
Including R2 between Q2's emitter and ground linearizes the comparator's range of operation. The voltage drops across R2, and Q2's base-emitter junction subtracts from D5's and D6's zener voltage to regulate the circuit's output. Network R2, R4, R5, and C8 compose a lowpass filter through which the error signal feeds back to the reference side of D1. The time delay of this network is small enough to allow per-cycle regulation.
You can scale the input voltage, D5 and D6, and R3 to achieve different output voltages. Select L1 based on your maximum load. C1 decouples the circuit from its supply. (DI #1601)