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Product Guides and Tutorials

Power Pulse ModulatorPower Pulse Modulators
[PWM-OCB, PWM-OCBI, PWM-OCX, PWM-OCXI]
 


This tutorial is for product versions which are now discontinued

Creating a bipolar PWM output for AC loads

A bipolar output can be useful in a range of experiments such as induction heating or generating high frequency AC voltages. It is possible to link two Power Pulse Modulators and a transformer to generate a bipolar or AC output. To do this your load must have a center tap, or you will need to use a transformer with a center tapped input as shown in the diagram below.

Bipolar PWM circuit for AC ouptut

The diagram above shows the necessary connections between the two pulse modulators, and the load. The connections for V+ and GND to the pulse modulators is not shown for simplicity, but both circuits should be connected to a suitable power supply. On the slave unit, te SIG jumper is removed, and the INV jumper position is moved. The two PWM circuits are linked as shown in figure 7 of the product datasheets under the heading "Master Frequency and Duty with Opposite Phase". With this configuration, adjusting the duty control will adjust the ratio of the pulse polarity. For example; At 50% duty, the AC output will alternate between positive and negative for equal durations, if the duty is adjusted to 25%, this will mean that the current will flow from Vcc through the upper part of the transformer input for 25% of the time, then flow through the lower part for 75% of the time.

You should consider that in this arangement, the total duty is always 100%, and therefore you can not adjust the output power level directly. You must ensure that your transformers input impedance is high enough so that it can withstand a minimum of 50% continuous duty. If you need to be able to also adjust the power level to the load as you would with a single PWM, you can connect a third Power Pulse Modulator. The GND connection of the above setup should be disconnected from the supply, and connected instead to the L- terminal of the 3rd Power Pulse Modulator. This third circuit should then be powered in the normal way and from the same V+ source as the two others. You can then adjust the duty on the 3rd circuit to change the overall power going to the transformer or load.

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