No Feedback Voltage Booster


Voltage Booster -

Varying VREF from 1.0V up to 2.0V and monitoring change in the output voltage VOUT.

Voltage booster is a circuit that generates higher output voltage from a lower input voltage. It works as follows. An inductor L1 is placed in series with Vdc and a switching transistor T1. The bottom end of L1 feeds the output capacitor C1, and load resistor through rectifying diode D1. The output voltage V, is higher than the DC input Vdc,as can be seen qualitatively thus: When Q1 is on for a time ton D1 is reverse biased and current ramps up linearly in L1 to a peak value

IP = (VDC * tON) / L1

This represents an amount of stored energy:

E = 0.5 L1 IP2

where E is in joules, L1, is in henries, and IP, is in amperes.

Varying VREF voltage on U1 PIN2. Measuring current profile in L1 with voltage drop across resistor R2. Increasing VREF reduces the time between current spikes in L1.

During the T1 on time, the output current is supplied entirely from C1, which is chosen large enough to supply the load current for the time tON, with the minimum specified droop. When T1 turns off, since the current in an inductor cannot change instantaneously, the current in L1 reverses in an attempt to maintain the current constant. Now the no-dot end of L1 is positive with respect to the dot end, and since the dot end is a t VDC, L1 delivers its stored energy to C1, and charges it up via D1 to a higher voltage (aboosted-up voltage) than VDC. The circuit shown does not have a feedback and output voltage is unregulated, but can be set to a specified value by setting VREF voltage. Note that this design is meant for 'static' output currents, not for variable current draw designs. This is not for precision electronics!

Varying VREF voltage on U1 PIN2 and monitorng the resultant PWM waveform.

VG1 above is a Triangular Wave Generator.

If the current through D1 has fallen to zero before the next T1 turnon, all the energy stored in L1 during the last T1 on time has been delivered to the output load and the circuit is said to be operating in the "discontinuous" mode. An amount of energy E delivered to a load in a time T represents power. With E in joules and T in seconds, P is in watts.

PL = 0.5 L IP2 / T
PDC = VDC (IP / 2) (TOFF / T)

The total power delivered to the load is then:

Pt = PL + PDC = 0.5 L IP2 / T + VDC (IP / 2) (TOFF / T)
IP = (VDC TON) / L1
Pt = (VDC2 TON) / (2 T L1) (TON + TOFF) = (VDC2 TON) / (2 T L1) (kT) = VO2 / R

The output voltage is:
VO = VDC √( ( k R TON ) / (2 L1 ))




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