Two-Stage Mode to Achieve High Voltage
If a high voltage needs to be generated from a low voltage, a boost converter can be used. It is one of three basic switching regulator topologies that require only two switches, one inductor, and input and output capacitors. In addition to the boost converter, other basic topologies include buck converters and inverting buck-boost converters. Figure 1 shows the schematic Diagram of the boost converter. During the on period, the switch S1 is closed, the electric energy is stored in the coil L, and the inductor current linearly increases with the difference between the input voltage and the ground potential; that is, increases with the input voltage, and during the off period, when S1 is turned on When S2 is closed, the electrical energy stored in the inductor is supplied to the output. The voltage across the inductor is equal to the output voltage minus the input voltage during this time period.
Figure 1. Boost topology for generating high voltages from low voltage
For this interaction to take effect, it must have sufficient time to charge and discharge. When passing through the control loop, the following assumptions can be made: When the output requires more power, more power must be drawn from the input to the output. Therefore, more power must be temporarily stored in the inductor, and switch S1 also requires a longer on-time. However, for a fixed switching frequency, this results in a shorter turn-off time that can be used to derive electrical energy from the inductor. Therefore, the output voltage drops below the set target value, which is especially a limitation for boost topology. With this topology, the output voltage is limited beyond the available input voltage. In a typical application, this maximum boost factor is between 3 and 7.
The curve shown in Figure 2 illustrates the typical relationship between the maximum possible boost factor and the corresponding duty cycle. The specific curve varies depending on the relationship between the load resistance at the output of the boost converter and the DC resistance of the inductor. The schematic shown in Figure 2 uses a load resistance of 100 Ω. For an output voltage of 48V, this is equivalent to a load current of 480mA. When the series resistance (DCR) of the inductor corresponds to 2Ω, the maximum boost factor that can be achieved is only slightly higher than 3. When the DCR is 1 Ω, the achievable boost factor is slightly higher than 5. If a higher boost factor is required, the inductor with the lowest series resistance value must be selected.
Figure 2. The maximum possible boost factor depends on the inductor resistance DCR (DC resistance)
A two-stage concept is also an option if a higher boost factor is required in the application. ADI’s new LTC7840 includes two boost controllers in a single chip, making it easy to implement a two-stage boost concept. Figure 3 shows an example of boosting from a 12V supply voltage to a 240V output voltage. The two boost stages can step up the voltage so that each stage only needs to boost the voltage by about 4.5 times.
Figure 3. Two-stage concept for generating very high output voltages from low input voltages
This article introduces a two-level concept that achieves a much higher boost factor than the single-level concept. Of course, a transformer-based topology can also be chosen to significantly increase the input voltage. For example, a flyback converter is a common topology. However, if galvanic isolation is not required, the two-stage boost concept has some advantages over the flyback converter. It does not require a large and expensive transformer because the switching frequency is no longer limited by losses in the transformer core and the supply load is a continuous load rather than a pulsed load. Therefore, the two-stage boost concept should be considered in the selection process for many applications.