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Using linear power control elements

10 Dec 2013  | Sajjad Haidar

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Power control using the linear region of an active device, such as a MOSFET, is not an efficient choice. However if the range of power control is restricted to the lower or upper end of the range, using the linear region is not a bad option.

For instance, if we wish to control the power of a 45W soldering iron between 35W & 45W, an active device will dissipate between about 4W & 0.1W. With this in mind, the circuit shown in figure 1 has been developed.

Figure 1

A simple current source is used to drive a VOM1271 photovoltaic (PV) coupler. The output voltage of the VOM1271 can be a maximum of 8.4V. Figure 2 shows the relationship between the input forward (IF) current with the output short circuit current (ISC)—essentially linear. Photovoltaic output behaves like a constant current source until it nears the open circuit voltage (~8V). This output voltage can be utilised to drive a MOSFET with threshold voltage (VTH) less than 8V.

Figure 2

A difficulty with MOSFETs in linear mode is that even in the same batch of devices, gate-source threshold voltages vary. Just after the threshold, drain current increases rapidly with little change in VGS [1]. MOSFET Q2 at the output of the PV coupler is biased in such a way that the output voltage to be applied to Q3 & Q4 gates (VGS) changes in accordance with the transconductance of Q3 and Q4.

Figure 3

Figure 3 shows the VOM1271 forward current (IF) vs. VGS relationship. With a little IF, VGS reaches the knee voltage (Vknee) with a slope m1. This slope is approximately proportional to (1/(R5+R6+R7)) as R5+R6+R7 >> R4. We can adjust R7 so that Vknee matches close to the threshold voltage of Q3 and Q4 (about 4V to 5V). After the knee, voltage VGS changes more slowly with IF, with a steeper slope m2, similar to the VGS vs. ID curve of MOSFETs. Slope m2 is controlled by trimmer R4 (m2 proportional to 1/R4).

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