Active load handles high voltages

-February 25, 2016

Bench electronic loads are usually rated for voltages less than 100V, which make it difficult to test high-voltage power supplies. This Design Idea offers an alternative: an inexpensive electronic load rated for 500V or more, depending on the selected components. It is basically a buck converter whose input is the power supply under test. As a load for the output of the converter, a 1000W electric kettle filled with water is used, whose resistance R1 amounts to about 53Ω.

Figure 1  High-voltage-capable electronic load using LT1243

Unlike conventional buck converters, the input current – not the output voltage – is sensed by the error amplifier of the controller. Therefore, it is an average current-mode controller whose outer feedback loop is missing. Specific to this architecture, the inductor acts like a current source feeding the output impedance R1||C2. Thus, we have a single pole system which makes compensation easier. The GBW of U2 must be high enough to avoid adding additional poles in the loop gain. The switching frequency is 50kHz, so that the power loss of the MOSFET and D1 is low enough to make a heat sink unnecessary.

The voltage drop across the shunt Rsh – which is proportional to the current flowing through the power supply – is amplified, inverted, and sent to the feedback input of the regulator.

The current drawn from the power supply is adjusted with the potentiometer P1.

With the values of the components in the schematic, according to the equation:

Vref = -Iinp·Rsh·R4/(P1+R8), the load current range is:  Iinp = 15mA…618mA

Diodes D2 and D3 protect the op-amp’s input against inrush current drawn by C1 when the power supply is connected.

 

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