Electronic Design

Switching Boost Regulator Uses 555

While working on a "B" battery replacement for antique battery-powered radios, it occurred to me that the venerable 555 timer might be able to handle most of a switching regulator's task. This circuit does just that (see the figure).

During operation, C1 is alternately charged and discharged. Resistor R1 ensures that C1 begins charging. It also sets the minimum switching frequency. While C1 is charging, pin 3 of the 555 timer drives Darlington pair Q1-Q2 to supply current to the inductor. When the inductor current reaches a value determined by R1 and Q3's base-emitter turn-on voltage, the collector current of Q3 will rapidly charge C1.

When the voltage across C1 reaches the point where the 555 will reset, the voltage on pin 3 will go low, turning Q1-Q2 off. This action saves from consuming more power than is needed from the supply. At the same time, pin 7 will begin to discharge C1. During this turn-off event, the collector voltage of Q1-Q2 will be pulled high by the inductor's flyback action. Diode D1 steers the inductor current into storage capacitor C2. When C1's discharging voltage reaches the set threshold, pin 3 again goes high to repeat the cycle.

Output-voltage regulation is accomplished by monitoring the voltage across C2. Whenever Q4 turns on (an indication of excess output voltage) the collector pulls the 555's reset pin low. This turns off the drive to Q1-Q2 until the output voltage drops slightly. C3 provides low-pass filtering to help the circuit remain stable during regulation. When pin 3 switches low, C4 provides hysteresis to pin 4 for added stability.

The component values shown, chosen from my junk box, worked well to supply 80 V at 10 mA from a 7.5-V input with about 60% efficiency. The toroid's inductance is several hundred microhenries. R3 can be modified to grossly adjust the regulation voltage. A grounding and power-supply wiring scheme that isolates current glitches on Q1-Q2 from the control electronics will help to maximize performance. An additional capacitor, connected across pins 1 and 5 of the 555, will help reduce the influence of supply noise.

The circuit will exhibit some temperature drift due to the base-emitter turn-on voltages of Q3 and Q4 being used as reference voltages. The device works very well in uses around the home or in the shop.

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