The MCU demo board has many unused features

Tag : Common mode Choke
Off-line power converters typically have response times limited bytheir output-filter components. Extremely fast response in thecontrol circuit is not required, so a conventional MCU can operatethe feedback loop.
Typically, the output filter ( Fig. 1 ) is the limiting factor in the control-loop design. The inductoris selected to give reasonable ripple current in the outputcapacitor. In our example, load voltage is 12 V and load current is4 A. The choke value would be set to give a peak-to-peak ripplecurrent of one-fourth the rated load current, allowingcontinuous-mode operation down to one-eighth of the rated loadcurrent. The choke value is selected based on the ripple frequency.For our 150-kHz converter, the value is 50 µH.
A low-cost electrolytic typically would be selected for the outputfilter. The electrolytic's equivalent series resistance (ESR) setsthe peak-to-peak ripple voltage. A typical ripple specification is1% of the output voltage, or 0.12 V. ESR, therefore, must be lessthan 0.12 Ω. A low-cost capacitor of 470 µF and 63 V wasused in the demo. It has an ESR of about 0.06 Ω, giving areasonable safety factor. The high-voltage rating gave some marginduring debugging.
Consider the physical limits of the filter's input. Assume amaximum secondary voltage of 48 V set by the transformer turnsratio and source voltage. Since the forward converter has a maximumduty cycle of 50%, the average forcing voltage range for theconverter is 0 V to 24 V.
Fig. 2 shows the best damped response time, 425 µs, given the rangeof forcing voltages. The damping resistor closely matches theimpedance of the L-C circuit. In practice, the damping would beapplied by the system feedback loop, with the input forcing voltage(duty cycle) reduced as the output voltage reaches its setpoint.The damping resistor serves the same function, in that it removesthe excess current stored in the inductor as it approaches thesetpoint. The response time cannot be improved with the componentsand forcing voltages selected. And, since design margins areimportant, the response time will be even slower. Demo Verification
To prove that a conventional MCU could do the control job, weconstructed a demo. The demo contains an STM32F103 MCU ( Table 1 ), which is largely under-employed. The demo ( Fig. 3 ) consists of two pc boards, a MCU board and the power-converterboard. The power converter is a two-switch forward converter,operating from the rectified ac line, feeding an isolatedsynchronous rectifier and L-C output filter. The system output is12 V at 4 A. For the benefit of the software side of thedevelopment team, we referenced all control to the secondary side,isolated from the line.
The MCU demo board has many unused features. Only the MCU chipitself, the debug interface, the reset button and the 5-V to 3.3-Vregulator are used for the control loop. All inputs and outputs aretaken directly from the MCU pins, with level shifters added toallow interfacing with gate drivers.
Fig. 4 shows the system architecture and the general interfaces to theMCU. The system requires an isolated housekeeping supply, becausethe MCU cannot bootstrap its own power supply. Q1 and Q2 implementa 150-kHz two-switch forward converter driven in phase by a commontransformer fed by the drive chip on the secondary side. (A 1-Afuse in series with the power-transformer primary helped thedebugging phase by saving a lot of replacement parts.)
A current transformer isolates the primary current signal. Theburden resistor directly feeds an analog comparator (not among theperipherals on the selected MCU die).
Schottky diodes shunt the synchronous rectifiers, which simplifiesdebugging. You can remove them once the dead-time is set.
The output ORing diode (D10) was used later to aid in debugging acurrent-sharing scheme using digital communication. Our plannedPMBus sharing scheme will not be discussed here.
A voltage divider consisting of R13 and a resistor added to the MCUboard senses the output voltage. There is no phase-lead network.Control-loop compensation is handled digitally.