Goal: Testing hot side of SMPS independently of the cold /feedback circuit

I tested the Mosfet on the hot side of SMPS along with the rest of the circuit ( snubber circuit, bleeder, etc - not shown) by connecting to a resistive load in place of the pulse transformer and the oscilloscope verifies that it worked as planned. Since I am testing with open loop circuit the feedback circuit is not important at the moment.

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Now I want to connect the Xmer . For the flyback system to work correctly I should connect some load on the secondary windings ( true ?) , but since I don't know the condition of the cold side I rather like to connect some load ( bulbs may be) directly to the secondary coils ( rectifier - filter - load would have been ideal I want short cut - this bad idea ?)

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Q1 How to ensure that the energies from the transformer is correctly drain by the secondary loads?

Q2 What will be example of simple loads on the secondary?

Q3 Is loading all the secondary tapings essential?

  • \$\begingroup\$ What do you actually want to test ? Have you realistical models of non-ideal components and do you want to know what performance can be expected when one uses them or are you trying to figure out what actually happens in a flyback regulator in principle? Have you 2 separate transformers or only 1 with four windings? \$\endgroup\$
    – user136077
    Aug 14, 2018 at 17:07
  • \$\begingroup\$ Test proper transfer of power to the secondary side without harming the MOSFET. Around 300W decipated from the primary. Read as Single transformer with two secondary tapings. \$\endgroup\$
    – seccpur
    Aug 14, 2018 at 17:21
  • \$\begingroup\$ @ark1974 , user seccpur has explained your intentions. OK? \$\endgroup\$
    – user136077
    Aug 14, 2018 at 18:01
  • \$\begingroup\$ @user287001: seccpur is right. But he has not explain my points. \$\endgroup\$
    – ark1974
    Aug 15, 2018 at 4:33

1 Answer 1


You cannot have load when the forthcoming pulse is under charging to the magnetic field of the transformer - a diode is obligatory to prevent current in the secondary when the switch is conducting. You have a growing current ramp in the primary during the magnetic energy charging period. Switching the mosfet off causes as high voltage to occur in all windings as is needed to let the current continue in some of the windings. This is the flyback pulse. You need so low resistance load that the voltage rating of the mosfet nor the transformer windings aren't exceeded.

If your transformer is 1:1, the output peak current is the same as there was built up in the primary when the mosfet was turned off. If your secondary in use has only half of the number of turns in primary, the peak current in the output is the max primary current doubled.

To keep your mosfet in safe, you must

  • use so short ON period of the mosfet that the primary current do not exceed the allowed max peak current of the mosfet. I1max=Uin*Ton/L1 where L1=primary inductance
  • use so long OFF period of the mosfet that the transformer current surely has been died before a new ON state. Wait at least 10*L2/R where L2 is the inductance of the used secondary winding and R is the load resistance.
  • have a mosfet which stands the input DC voltage plus the flyback pulse voltage calculated of the peak current of the secondary, the load resistance and the winding ratio.
  • have such high gate drive capablity that the mosfet turns OFF reasonably fast to prevent it heating due the losses
  • have a high enough capacity transformer core which doesn't get saturated with the max primary current. Saturated core = a short circuit
  • have so low stray inductances that they do not cause serious overvoltages. You can have 2 zener diodes in series head against head in parallel with the primary. One prevents the input voltage short circuit and the other eats the exessive flyback pulse voltage caused by stray inductance (calculate the expected voltage!) Zener diode turns the overvoltage to losses in the diode.

Then measure the input current pulse and the output voltage pulse with the oscilloscope. Calculate the pulse energies to see how much is lost. There's no need to run more than one pulse if you have a proper memory oscilloscope.

If you have not an oscilloscope with proper bandwidth (say 50MHz or more) and high voltage probes, this task can be a little too much.

Lamps are not good test loads because their resistance is highly varying. Cold filament can have only 15% of the resistance of the same filament as hot. Use a resistor and keep pulses so sparse that you do not burn it.

  • \$\begingroup\$ Thanks, that's the kind of explanation I am looking forward to. Sorry my reputation won't allow me to upvote you. \$\endgroup\$
    – ark1974
    Aug 15, 2018 at 4:34
  • \$\begingroup\$ Any typical value of the load resistance please. The smps has +15v, 1A, -15V,1A and +5V, 5A ratings. Should I connect all the secondary tapings with resistive load after rectification? Thanks \$\endgroup\$
    – ark1974
    Aug 15, 2018 at 4:41
  • \$\begingroup\$ @ark1974 You can well have the nominal load resistor in every output , for ex. 15Ohm at 15V winding. Remember to have fast diodes in series,which are intended to SMPS applications, 50Hz rectifiers are useless. Be sure that the diodes allow the flyback pulse to go, wrong polarities are fatal. Find the right polarities of the tansformer at first with signal generator and oscilloscope if they aren't printed to the transformer. \$\endgroup\$
    – user136077
    Aug 15, 2018 at 8:26
  • \$\begingroup\$ @ark1974 (continued) Increase gradually the switch ON time of the mosfet to detect all signs of possible core saturation. Have a current shunt in input to see in oscilloscope if the triangle current ramp starts to accelerate when you increase ON time. \$\endgroup\$
    – user136077
    Aug 15, 2018 at 8:27
  • \$\begingroup\$ @ark1974 before you start consider to use capacitors as loads. They can as well serve as obserbable loads if the number of the pulses are limited. They prevent single high voltage peaks and stored energy is easy to calculate from voltage and capacitance. Diodes are a must. \$\endgroup\$
    – user136077
    Aug 16, 2018 at 13:19

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