POWER DEVICES
Wide 5V to 140V Input to 12V, 200mA Bias Power Supply
Victor Khasiev, senior applications engineer at Analog Devices Inc. describes solutions for maintaining bias voltages in electrical systems over a wide range of source voltages, from 5V to 140V
M
odern automotive and industrial systems require a stable voltage source even as the system input voltage sweeps from one extreme to another. In automotive systems, significant rail voltage variations can result from cold cranking startup, cylinder deactivation/activation in dynamic fuel management systems, or significant engine load changes. Likewise, in industrial applications, line brownouts are a problem, and the switching on of motors in high power equipment can lead to serious drops of input voltage.
Even when power conversion systems cannot provide full power to the loads at low voltage inputs, many of these systems must remain operational, regardless of the input voltage level. For example, widely used high voltage boost and buck converters employ high voltage MOSFETs with a standard gate level. At input dropout, the bias voltage should stay above the 10V to keep the gate drivers operational. Critical digital control and information systems should be biased and functional regardless of the input conditions as well.
Circuit description and functionality
If the input voltage is not expected to fall below the desirable bias level and the goal of design is to have an external bias power supply to minimise the power dissipation of the switching controllers, then a straightforward buck converter can be employed.
This approach is illustrated in Figure 1. The solution is centred around a high voltage buck-controller LTC7138 with internal switching transistors. The power train also includes inductor L1, diode D1, and output capacitors C2 and C3. To minimise the solution profile, to below 3mm, only ceramic capacitors were used in the input. An optional polarised capacitor can be used as well (for example, a cost-effective 22µF 200V, EMVE201 ARA220MKG5S), but it considerably increases the height of the bias power supply.
Figure 2: High voltage, buck-based bias circuit waveforms where the VIN is 20V/ div, VOUT is 5V/div, and the time scale is 50ms/div
the buck converter is not enough, because the output voltage follows the input when it falls below the desired output. Figure 3 illustrates a solution to this problem using a dual-stage bias power supply. The first, main stage is a high voltage step-down converter similar to what is shown in Figure 1. It is output connected to a boost converter and based on an
Figure 1: High voltage, buck-based bias circuit electrical schematic where the VIN is 12.5V to 140V and the VOUT is 12V at 0.2A
This circuit was verified and tested, with the waveforms illustrating the functionality of the circuit presented in Figure 2. The initial input voltage level of 100V drops to 12V, but the output provides 0.2A of the stable 12V voltage to the load.
The performance outlook of this design significantly changes if the input voltage drops below the desired bias level. In this case, using just
30 NOVEMBER 2022 | ELECTRONICS TODAY
Figure 3: High voltage, dual stage-based circuit electrical schematic where the VIN is 5V to 140V and the VOUT is 10.5V at 0.1A to 0.15A
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
