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