Power Tip Generate High Voltages Without a Transformer

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To operate a switched-mode voltage converter at high voltages, a topology with a transformer is not necessarily required. This power tip shows how many applications can be operated without transformers using appropriate voltage converter ICs.

Switched-mode voltage converters based on the principle of the buck converter are widely used. They do not require a transformer, only a simple coil, to convert high voltage to low voltage very efficiently. The setup is quite simple.

They are used with supply voltages up to 60 V and output voltages generated up to 12 V. At higher supply voltages or higher output voltages, one often encounters limits in the availability of suitable voltage converters.

Of course, voltage conversion with a switching regulator is also possible at significantly higher voltages; however, these use converter topologies based on transformers, such as a flyback converter or a forward converter.

The two switches S1 and S2 in Figure 1 are primarily responsible for the maximum input voltage range. However, the corresponding driver in the voltage converter IC must also be capable of generating the high gate voltage for switch S1. This is the sum of the maximum supply voltage plus the required threshold voltage for the MOSFET. Therefore, both the MOSFET and the voltage converter IC must be selected accordingly.

Image 2 shows the controller IC LTC7897, which is designed for a supply voltage of up to 135 V (max. 140 V). The output voltage can also be very high when using this IC, reaching up to 135 V as long as it is lower than the input voltage.

With the LTC7897, it is possible, for example, to convert from 100 V to 12 V. As a controller for discrete switches, MOSFETs for high currents can be selected. The circuit in image 2, for example, generates a load current of 20 A.

Convert High Voltages to Low Voltages with Buck Regulators

In addition to the example from Figure 2, where a high voltage is converted into a low voltage, there is also an application case where a high positive voltage is available, and a negative voltage needs to be generated from it. A possible topology for this is an inverting buck converter. This circuit does not require a transformer, only a simple coil, to generate a negative voltage from a positive one.

The voltage converter in the inverting buck-boost topology must offer special voltage stability, as the regulator must be at least as voltage-resistant as the available supply voltage plus the amount of the generated output voltage.

With an input voltage of U_in = 48 V and a required -65 V output voltage, a voltage stability of 103 V is necessary for the switched-mode voltage converter. Such a voltage converter is particularly suitable for generating high negative voltages.

Image 3 shows the LTC7897 in the inverting topology to convert the supply voltage of 48 V to an output voltage of -65 V.

Conclusion: To operate a switched-mode voltage converter at high voltages, you do not necessarily need to choose a topology with a transformer. With suitable voltage converter ICs, such as the LTC7897, many applications can be operated without using complex and costly transformers. (kr)

*Frederik Dostal is a Field Application Engineer for Power Management at Analog Devices in Munich.