This is an example of the notable versatility of the DrDAQ in the lab: the unit, complete with its buffer and a few extra components out of the junk box, let me build the prototype of a 400 hz Switch Mode supply in a Saturday's afternoon. The real bonus is that the PSU, by means of Picoscope, is fully controlled and monitored on the PC screen.
The operating principle is quite simple. The DrDQ + Picoscope generates a linear, 2V pp UpRamp at 400 Hz. The standard UpRamp is symmetrical about 0 V and needs to be offset by 1 V: we now have an UpRamp from 0 V to 2 V. We feed this signal to the non-inverting (+) input of Buffer's Amplifier B. On the inverting input (-) we feed a D.C. control signal variable beween 0 and + 2 V.
Suppose the control signal is 1 V. As the UpRamp starts, the Op Amp (-) input sits at +1V and the (+) input sits at 0V. Op Amp output is 0V. Then the UpRamp level gradually rises: when it overtakes 1V, the Op Amp switches to + VCC and stays there until the UpRamp reaches 2 V and then falls abruptly to zero bringing also the Op Amp's output to zero. We have just described the variable PWM generator at 50% Duty Cycle. If the control signal is 0.2 V we shall have an output pulse at 10% Duty Cycle and so on. If this pulse drives a Bipolar, or a MOSFET or an IGBT switch working in a BOOST type flyback converter, you have a wide range, low to high voltage adjustable PSU.
Fig. 1 shows the simple diagram. Control voltage is adjusted with R 103. I had to make do with what I had on hand, hence the 220V to 15+15V transformer for the flyback coil and the 2N3055 Darlington connected switch. Nevertheless the "beast" puts out a respectable 90V @ 0.5 A while drawing 6 A from the 13V supply. Therefore we have a wide range unit that can be continously adjusted from 15 V up to 100 V! The screens show the system operating with a 3V UpRamp, but later I set the UpRamp at 2V, as described above. Reason for this is not to exceed the 2.5 V dynamic range of the EXT. inputs. By changing the UpRamp frequency one can easily check and compare performance. I quickly and easily tested the system from 200 up to 2 KHz. The screens also show the full monitoring capability of the DrDAQ/Picoscope combination. The SCOPE scale is colored GREEN and shows the switching waveform. The PURPLE scale on the left shows EXT. 3 Channel monitoring the UpRamp (here still at 3V). The UpRamp looks bad because of insufficient channel bandwith. The BROWN scale at right shows the level of the CONTROL D.C. voltage on EXT. 1.
The system shown works open loop. In a next post I shall describe results obtained by closing the loop, thereby obtaining a self regulated PSU.