Benefits to a 2nd Stage LC output filter for flyback:

  • Lower total capacitance needed to achieve output voltage ripple objective
  • Reduce overall footprint of the output filter compared to 1 stage output filter
  • Lower output voltage ripple more dramatically

Problems introduced by a 2nd Stage LC output filter for flyback:

  • Re-adjust the compensation network to re-establish control loop stability
  • Additional small signal model analysis to find the poles and zeros

Using the PWM Switch method to analyze the stability and output setpoint tolerance issues introduced by a 2nd Stage LC output filter, impedance reflection simplified the input to output model to a buck-boost (Figure 1). With reflected impedance simplification demonstrated in the previous blog, a Flyback converter turns into a Buck-Boost Converter for the next step. This simplification can greatly ease the analytical work for the applications in which a second stage LC filters are used at each output

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Fig. 1 Simplified Buck Boost Converter with multiply loads in parallel.

Since the Buck-Boost converter can operates in CCM and DCM depending on the load conditions, there are two different PWM switch models for the switch in CCM and DCM, respectively. The Fig.2 show the PWM switch in CCM. And the Fig. 3 shows the PWM switch in DCM.

AEP36303_2Fig. 2 PWM switch in CCM

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Fig. 3 PWM switch in DCM

The appropriate PWM switch model can be combined with the Buck-Boost Converter depending on the operation mode(s) of the converter.  Fig. 4 shows the PWM switch in the converter while the converter runs in CCM. And the Fig. 5 shows the PWM switch in the converter while the converter runs in DCM.

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Fig. 4 PWM switch in Buck-Boost in CCM

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Fig. 5 PWM switch in Buck-Boost in DCM

With the combined PWM switch and Buck-Boost model, as shown in the Fig. 4 and Fig. 5, deriving the transfer function of the Buck-Boost converter power stage is simplified.

With the combined PWM switch and Buck-Boost model, as shown in the Fig. 4 and Fig. 5, deriving the transfer function of the Buck-Boost converter power stage is simplified.

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Fig. 6 Flyback converter using 2nd stage LC filter with control circuit

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Fig.7: Flyback Converter with Secondary Side Components Reflected to the Primary Side in CCM

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Fig.8: Flyback Converter with Secondary Side Components Reflected to the Primary Side in DCM

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The transfer function from control to output voltage can be summarized as in (13). With the equation specified in (13), designers can optimize the system stability and the output regulation so that the best performance can be achieved by using the 2nd stage LC filter.

Rapidly create flyback with 2nd stage output filter designs in minutes using Fairchild’s design tool and then save your designs for future reference. Fine-tune your design parameters without a bench prototype, swap component choices and perform detailed simulations and analyses. The Flyback/Controller with Integrated MOSFET design tool is the fifth tool listed on this webpage:

https://info.fairchildsemi.com/Blog_AEP3630-Transfer-function-Derived-for-PWM-Switch-Model-for-Flyback.html

 

Reference

  1. Fundamentals of Power Electronics, Robert W Erickson Dragan Maksimovic
  2. Simplified analysis of PWM converters using model of PWM switch. II. Discontinuous conduction mode. Vorperian, V, Aerospace and Electronic Systems, IEEE Transactions, May 1990, page 497-505.
  3. Simplified analysis of PWM converters using model of PWM switch. Continuous conduction mode. Vorperian, V, Aerospace and Electronic Systems, IEEE Transactions, May 1990, page 490-496.
  4. Simplified analysis of PWM converters operating in discontinuous conduction mode using alternate forms of the PWM switch models, Jun Chen ; Ngo, K.D.T., Proceedings of the IEEE, Publication Year: 2000 , Page(s): 505 – 509
  5. Modeling the switch of PWM convertors (comments, with reply, on `Simplified analysis of PWM converters using models of PWM switch’ by V. Vorperian), Aerospace and Electronic Systems, IEEE Transactions on Volume: 28           , Issue: 3                DOI: 10.1109/7.256319 Publication Year: 1992 , Page(s): 921-925.
  6. Averaged switch modeling of boundary conduction mode DC-to-DC converters. Jingquan Chen; Erickson, R. ; Maksimovic, D. Industrial Electronics Society, 2001. IECON ’01. The 27th Annual Conference of the IEEE Volume:2, DOI: 10.1109/IECON.2001.975867 Publication Year: 2001 , Page(s): 844-849 vol.2

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