Circuits Archive

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A new half bridge PW Modulator

The requirements

In a half bridge, the low side MOSFETs are conducting for part of the cycle and the High side MOSFETs must only conduct when the low side is not conducting. This system has been discussed in the first article on PWM motor control.

However you cannot in general turn the low side MOSFETs off with the same waveform that turns the high side on as the drive circuitry, MOSFETs and gate drive level shifting all take time to respond. So you need a 'dead' time when both MOSFETs are turned off.


This is shown in the diagram. At A, Hiside gate drive is off and the loside, which has been on, turned off. At B, a discrete time later, the hiside turns on and remains turned on until C, when it turns off. A defined time after the Hiside drive turns off, Loside drive turns on, at point D.

Now, for optimum drive performance and efficiency, you should have a modulator circuit which enables the gaps A-B and C-D to be different and to be tuned by altering circuit parameters.

This drive circuit accomplishes this ability to tune the timing.

The method

Most modulators work by generating a triangle waveform and comparing this with a demand speed signal. But we want two different outputs, so we need two different demand speeds, separated by enough distance to give us the required dead time. Or we could use two triangles, suitably separated. This is the approach taken here.

The second diagram shows the effects.


If the slopes of the leading end trailing edges of the oscillator are of different values, the timing on front and back will also differ. So we need an oscillator where we can alter the waveform spacing and the rising and falling slopes. This modulator does exactly that.

The second part of this article gives the circuit with values, discusses its operation and goes into more detail of its operation.

Other relevant pages

PWM motor control, first article.
A new half bridge PW Modulator Part 2 Members area

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Page's Author: Richard Torrens
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