Step Up Converter or Boost Converter:

A Step Up Converter, or boost converter, produces a dc output voltage higher than its supply voltage. In the circuit shown in Fig. 17­-34(a), L₁ is directly connected to the supply, and D₁ is in series with L₁ and C₁ The collector of Q₁ is connected to the junction of L₁ and D₁ (point A), and its emitter is grounded.

When Q₁ is on [Figure 17-34(b)], D₁ is reverse biased, and

When Q₁ is off [Figure 17-34(c)], V_l reverses polarity to keep I_l flowing. The output voltage is now,

So, the output voltage is larger than the input, and its actual level can be set by selection of R₁ and R₂ [in Fig. 17-34(a)].

The voltage at point A has the pulse waveform (V_A) illustrated in Fig. 17-35. During t_on (Q₁ on time), V_A = V_sat, and during t_off V_A = (V_i + V_l). The inductor current increases to I_p during t_on, and decreases during t_off as L₁ discharges to C₁ and the load. The output voltage (V_o across C₁) decreases during t_on, because D₁ is reverse biased and C₁ is supplying all the load current. During t_off  D₁ is forward biased and C₁ is recharged from L₁. V_o increases while I_l is greater than I_o, and decreases when I_l is less than I_o.

From the circuit conditions and the waveforms, the following equations can be derived:

Inverting Converter:

Figure 17-36(a) shows the circuit of an inverting converter, (also termed a flyback converter). This circuit produces a negative dc output voltage from a positive supply voltage. In this case, L₁ is connected between ground and the emitter terminal of Q₁, (point A). Diode D₁ is in series with Q₁ emitter and C₁. Note the polarity of D₁ and output capacitor C₁.

Figure 17-36(b) shows that when Q₁ is on, D₁ is reverse biased, and

From Fig. 17-36(c), when Q₁ is off V_l reverses to keep I_l flowing. Diode D₁ is now forward biased, giving

The output voltage is negative, and once again the actual output voltage level is set by selection of R₁ and R₂.

The voltage at point A in Fig. 17-36 is also the inductor voltage, and it has the pulse waveform shown in Fig. 17-37. During t_on, V_A = (V_i – V_sat); and during t_off V_A = -(V_o + V_F). As in other switching converters, the inductor current increases to I_p during t_on and decreases during t_off, discharging to C₁ and the load. V_o decreases during t_on (D₁ reverse biased) as C₁ supplies all of I_o. Then, C₁ is recharged from L₁ during t_off (D₁ forward biased), and V_o increases while I_l exceeds I_o, and decreases while I_l is less than I_o.

Equations 17-16, 17-17, and 17-18 apply for the inverting converter, as well as for the Step Up Converter.