Column: Circuit drill
How does a DC boost converter behave with AC power?
By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions D
C boost converters are widely used in battery power supplies and photovoltaic systems. In them, a small DC voltage is boosted to a higher
DC voltage to suit certain applications, using large battery banks that furnish smaller and lighter circuits instead of bulky and heavier circuits. Boost converters increase voltages and
reduce the number of cells. In addition, they provide good isolation between inputs and outputs, so, if a circuit has suffered a short on the primary side, this won’t affect the secondary side. In some designs, a rectified AC signal
is connected to the load through a DC boost converter to provide the higher voltage required and operate as an isolation circuit. But, what happens if the rectifier at the converter input fails and pushes AC voltage into the boost
converter? How will the converter react then?
Experiment setup and results To learn what happens next, I chose a well-known and commonly-used DC boost converter circuit; see Figure 1. This circuit is designed to convert
9Vdc to 30Vdc; the 9Vdc comes from a 120Vac/9Vdc rectifier circuit. I tested the following scenarios:
(1) 9Vac input; (2) 9V full-wave rectified and unfiltered AC input.
These two scenarios cover all failure
possibilities that could affect the boost converter if supplied by the rectifier; see Figure 2. The input voltage is “fixed” at 9V,
either AC or DC (depending on the scenario), but not higher than that. With DC input, the output voltage was stable at 30Vdc, with a rise time of
16.6ms; see Figure 3. Applying 9Vac to the circuit made it
struggle to reach the designed 30Vdc output. The voltage rise time was 29.6ms, almost double that of the DC- input scenario; see Figure 4. For the second setup, applying full-
wave rectified but not filtered 9Vac to the circuit made it struggle to deliver 30Vdc, taking some 29.6ms to reach its final voltage; see Figure 5. Hence, it can be concluded that the
DC boost converter circuit not only provides robust isolation between input and output voltages (9V and 30V), but it also maintains a fair stability against failure scenarios when supplied with either smooth or full-wave rectified/unfiltered AC power. The only notable observation is that the voltage rise time doubles that of the bast-case scenario where the circuit is supplied with 9Vdc.
Figure 1: DC boost converter circuit selected for the experiment
10 February 2022
www.electronicsworld.co.uk
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