To: File

From: Jon Hagen

Date: Mar. 4, 1999

Subject: Switching converter for servo motor control

For simplicity, assume we're dealing with a single-phase servo motor with inductance L and resistance R. For bidirectional operation, we need to be able to drive current through it in either direction. In addition, we need to be able to accept the energy produced by the motor during regeneration.

If we use two power supplies, the simple totem pole circuit of Figure 1 will suffice.

When current must be driven into the left side of the inductor, the top transistor and the lower diode operate as a step-down (buck) converter. With current flowing left to right, regeneration is provided by using the lower transistor and the upper diode as a step-up (boost) converter.

If only one power supply is to be used, the H-bridge of Figure 2 is used. When current is flowing from left to right, the lower right transistor is kept on to ground the resistor and the upper right transistor remains off. The left side totem pole operates as described above. When current is flowing from right to left, the roles of the two totem poles reverse.