Many books have covered the stability of many configurations of switch mode power supplies and they have given details on how to set the poles and the zeros of the compensator to make the the power supply more stable and robust, but all the books I have undergone, they took the example of the load to be a resistance load.

My question is, if we want to design a more practical SMPS to feed a more complecated and realistic circuit (example of a motherboard of an airconditionar, mobile phone etc.) these circuits are a combination of resistors, inductors, capacitors, actives devices and so on.

How do we study the poles and zeros of the power supply in that case? And how to make it stable?

  • 3
    \$\begingroup\$ Capacitive and inductive loads will be part of your piles and zeroes, but after that it’s usually the start up transition and light load conditions you need to pay attention to as you often cross over from DCM to CCM. There are special tools for the job, both simulation and hardware. \$\endgroup\$
    – winny
    Commented Dec 25, 2019 at 22:39

1 Answer 1


Are you asking about a SMPS that will power a variety of loads (motherboard of an air conditioner, mobile phone) without changing any of the design?

If so, I don't think such a design exists.

The closed loop performance of a power supply (PS) is closely tied to the load it has to drive and the resultant feedback that has to be implemented to ensure the power supply meets all it's performance requirements such as regulation, over current/over voltage protection, good behavior at startup, etc. Therefore most power supplies have to be tailored to their loads.

Our power supply group designs SMPS's for many different applications (loads). The design for a digital system with a relatively constant load is a lot different than the design for the transmit portion of a radar with hundreds of amps of load current being required at the radar's PRF (Pulse Repetition Frequency), 100 Hz to 50 kHz, for instance.

  • \$\begingroup\$ @winny - Thanks! \$\endgroup\$
    – SteveSh
    Commented Dec 26, 2019 at 20:27
  • \$\begingroup\$ For example you want to design a SMPS for an airconditioner motherboard. My question is how to get the equivalent load of the motherboard ? will you just present the mother board as an equivalent resistor? or you have a lot of measurment to do in lab and find the equivalent load impedance ? How does the SMPS see the motherboard ??? \$\endgroup\$ Commented Dec 26, 2019 at 21:43
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    \$\begingroup\$ For the AC motherboard example, you have go a bit deeper than just assuming a resistive load. That might be a good starting point, but you have to account for 1) step load changes, and 2) total capacitance on the voltage nets, to name a couple. The reason for needing the total capacitance is that it could affect the SMPS control loop stability. For what it's worth, we almost never measure the load in a lab, except to verify current draw estimates. \$\endgroup\$
    – SteveSh
    Commented Dec 26, 2019 at 23:12

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