Germanium dual boost circuit that starts at 260mV

No matter which portable power source is used, the lower the starting voltage, the better the circuit will operate. Low starting voltage also maximizes run time. Additionally, to fully discharge the power supply, the circuit must operate at a lower voltage and current.

Although the existing boost circuit can start at a voltage of 1 V and pull the power supply down to 1 V, although the voltage is so low, there will still be a large amount of unusable energy left in the battery. Other power sources, such as solar cells or microturbines, require circuits to start at voltages well below 1V. For example, the output of a single solar cell in sunny weather is only 0.58V.

The circuit in Figure 1 is designed to address this problem. This circuit can realize dual-inductor boost conversion function at a starting voltage as low as 260mV. The output voltage of an inductive DC-DC boost circuit is higher than its input. The germanium transistor balanced boost circuit is very simple, using only two NPN transistors, but it can start up unconditionally at very low voltages. Previous silicon transistor boost circuits required a starting voltage of about 1V and required more components.

The circuit operates like a free-running multivibrator, with a repeatable cycle starting when V _IN is slightly above Q2's V _BE . Subsequently, a positive Q2 base current (IB = (V _IN V _BE )/R1) flows through L1, Q2 conducts, and the inductor L2 is connected to ground. Q1 is off and the current in L1 is very small. D1 and D2 cutoff. When the current of L2 increases with positive di/dt, the magnetic field energy stored in L2 accumulates. When this current increases, it also flows through the RSAT of Q2. When the collector voltage of Q2 is large enough, Q1 is turned on.

R2 connects the base voltage of Q1 to the collector of Q2, and R2 also limits the base current of Q1. As Q1 turns on, the base of the aforementioned driver Q2 is now short-circuited to ground, and Q2 is turned off. Q2 is turned off so that the retrace energy of L2 forward-biases D2 and flows to the load (R3) with the release of the magnetic field energy of L1. D1 remains cut off. As L2 discharges, D2 returns to cutoff. As the current in L1 increases with positive di/dt, the magnetic field in L1 continues to build up. This current flows through Q1's RSAT, and Q1's collector voltage is high enough to turn Q2 on.

The base voltage of Q2 is connected to the collector of Q1 through R1, which also limits the base current of Q2. As Q2 is turned on, the base of the aforementioned driver Q1 is now short-circuited to ground, and Q1 is turned off. The cut-off of Q1 enables the retrace energy of L1 to forward-bias D1, and flows to the load (R3) with the release of the magnetic field energy of L1, while D2 remains cut-off. Due to the discharge of L1, D1 is cut off.

This self-oscillating behavior repeats until the battery voltage drops to V _BE of Q1 or Q2 . When the input voltage increases, the energy stored in L1 and L2 increases, therefore, the average voltage of R3 increases.

The inductance of L1 and L2, the RSAT of Q1 and Q2, and the switching characteristics of Q1 and Q2 determine the period and duty cycle of the self-oscillation. By adjusting L and R, the circuit can be optimized for a specific load and input source. As shown in the figure, the typical switching frequency at the output is 88 kHz (when V _IN = 0.5 V). An inductor of 100 H will produce a switching frequency of 60 kHz, and an inductor of 39 µH will produce a switching frequency of 152kHz.

Compared with the single-ended boost structure, the advantage of this double boost circuit structure is that the output ripple contains low noise, and at the same time, the input source is disconnected during retrace. For solar cell or microturbine inputs, the off-cycle is slightly less than optimal.

Figure 2 shows the input/output transmission characteristics of this circuit. Several resistive load voltage gains (boost behavior) are given. Note that this boost circuit is open loop, so it does not regulate output voltage or current. However, some applications do not require adjustment.

For example, this circuit can directly drive the LM2901 quad comparator and the LM2902 quad op amp. Other applications (logic circuits) require only a slight restriction on the upper voltage limit, which can be achieved using a shunt regulator or zener at the output.

But to achieve maximum efficiency, this boost circuit can only briefly power up a well-characterized, high-efficiency switching power supply (SMPS) IC, connecting the instantaneous boost output to the IC's low-current V _CC input. Once the IC starts up, the boost circuit stops working. One way to overcome this phenomenon is to replace R1 and R2 with a P-channel JFET (NTE326), and then pull its gate above the input voltage (V _IN + 1.2 V).

Additionally, the input voltage is limited to 2.0 V. A larger input voltage will cause excess current to flow through the bases of Q1 and Q2, which are connected directly to V _IN through R1, R2, L1, and L2 .

If this circuit drives a white LED instead of a resistive load, the transfer characteristics of the circuit are slightly different. White LEDs generally require 3.6 V to operate normally when the current is 20 mA. Therefore, if the power supply is an alkaline battery, a boost circuit is required. The brightness of the LED is directly dependent on the average flyback current flowing through D1 and D2.

Nominal LED current measurements for alkaline cells are 3 mA at 0.53 V, 14 mA at 0.95 V, 26 mA at 1.19 V, 31 mA at 1.27 V, and 50 mA at 1.53 V. These results are for Coilcraft DO1608C-683 inductor and Nichia BSPW500BS LED.

For many portable electronic products (toys, PDAs, etc.), when the voltage of a single alkaline battery reaches about 1V, it must be discarded. However, these batteries can also be used for LED lighting by using this circuit, which also allows the batteries to be more completely discharged.