POWER DEVICES
LT8330 converter IC with integrated power transistor. The power train includes inductor L2, diode D2, and an output filter. The voltage stress on the components in the boost converter circuit is much lower compared to the buck front end, which permits selection of relatively inexpensive parts and reduces the overall cost.
The output of the buck converter in this circuit is set to 12.5V. However, the output of the boost converter is set to a lower voltage of 10.5V, enough for the load to function properly. The converters are never operational at the same time. If one is switching, the second is not.
In normal operating conditions (VIN >12.5V), when the input voltage changes from 12.5V to 100V, only the buck converter is active, providing
12.5V to the load. The current flows to the load terminal VOUT through the inductor and diode of the boost converter. Due to relatively low current levels, the losses in this current path are minimal.
As long as VIN >12.5V, the voltage on the output of the boost converter is 12.5V and by far exceeds the preset value of 10.5V, so there’s no switching action in the boost section and only the buck is active. As the input voltage reduces to the level of 12.5V or below, the buck converter stops switching, but it keeps the internal P-channel MOSFET in the ON state, allowing 100% duty cycle operation.
If input voltage falls below 12.5V, then both voltages, VRAIL (intermediate rail) and VOUT, fall to the VIN level. In the 10.5V <VRAIL <12.5V range of the intermediate rail, both the converter’s buck and boost do not switch.
If the input voltage continues to fall and the VRAIL level drops below 10.5V,
Figure 6: The input voltage rises waveforms. The load current is 0.1A and the time scale is 50ms/div
the boost converter becomes operational, keeping VOUT at 10.5V. The waveforms illustrating the functionality of this converter are
presented in Figure 4. The minimum input voltage of 5.5V at the load current is 0.15A. Reducing the load to 0.1A corresponds to a minimum input voltage of 5.0V, as shown in Figure 5. The rise of the input voltage from 5V to 100V is illustrated in Figure 6. A photo of the converter is shown in Figure 7.
Figure 7: LTC7138 converter breadboard
Basic considerations for selecting converter components The maximum input voltage and load currents define the minimum operating input voltage of the boost and correspondingly the minimum input voltage of the entire power supply.
Figure 4: High voltage, dual stage-based bias circuit waveforms. The load current is 0.15A and the time scale is 50ms/div
Assuming VO, IMAX, and IO as given, then the boost minimum voltage can be described as
However, if VO, VINMIN, and IMAX are given, the maximum output current IO is
It’s important to keep major power systems operational over a wide input voltage range. This article discusses solutions to this goal. The circuits presented here generate the stable bias level at input voltages up to 140V and down to 5V during the input voltage droops. A secure bias level guarantees normal functionality of high voltage MOSFETs and control blocks. The proposed schemes using highly integrated converters reduce component count and overall cost. Adjustments can be made to minimise solution height if required by the application.
Figure 5: High voltage, dual stage-based bias circuit waveforms. The load current is 0.1A and the time scale is 50ms/div
Analog Devices:
www.analog.com NOVEMBER 2022 | ELECTRONICS TODAY 31
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