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SN6501: Why does it have low efficiency with low load current and is it the effect of magnetization current?

Tamas Kolb
Prodigy 20 points
Part Number: SN6501
Other Parts Discussed in Thread: SN6505B

Hello guys,

I want to use SN6501 with transformer 750313638 in my application as it is just described in your data sheet of the component. This configuration has an efficiency of 40% when the load current is 10mA. I want to understand the efficiency described in your datasheet. Does that mean, that 60% of input power to the circuit is an ohmic loss? What is the most dominant part of this ohmic loss? Is it the core losses of the transformer?

Furthermore, I am struggling with understanding the way how magnetizing current (clearly reactive) behaves with push-pull configuration. During one half of a switching cycle the core flux is energized with the energy contained in the magnetization inductance. Then in the other half of the switching cycle, this energy has to be emptied from the core and the core shall be reset. Then with an opposing magnetization current the same amout of magnetic energy has to be built up in the flux. What I don't understand, where does this magnetic energy go during a switching cycle? My assumption is that the energy = 0.5*Lm*Im*Im, where Lm is magnetizing inductance, Im is magnetizing current.

Is this energy pushed back to the voltage supply's capacitance in every switching cycle? Is this something that appears as a reactive power loss in your Efficiency calculation for the SN6501?

Thanks for the answers upfront.

Tamas

  • 0
    TI__Genius 17100 points

    Hi Tamas,

    Welcome to E2E! At low or 0mA loads, a transformer's core losses account for low efficiencies in the form of hysteresis and eddy current losses. These two types of losses are not Ohmic, unlike copper losses, but are a result of the changing magnetic field and resulting induced currents. Section 8.3.2 of the SN6501 datasheet elaborates on Core Magnetization. The SN6501 does have a DC resistive loss of 1Ω at 3.3V operation, however the dominance of transformer losses is due to the proportion of core losses / load power.

    For increased efficiency, the SN6505B has a similar switching frequency but lower DC resistance, Rds_on

    Transformers continuously transfer energy from the primary side to the secondary side (besides parasitics), so there is almost no energy remaining in the transformer between half cycles. When one half of the transformer disconnects, it becomes an open circuit and there is no current flow into the decoupling capacitor, but voltage rises to 2x the input voltage as a result. Curves on the SN650x datasheets reflect efficiency from measurements of typical systems.


    Please let me know if this response is helpful for you.


    Thank you,
    Manuel Chavez

  • 0 in reply to Manuel Chavez
    Prodigy 20 points

    Hello Manuel,

    Thanks for the answer. Can you tell me how does the core loss change with temperature for this specific transformer 750313638? I want to use the application in the temperature range of -55°C to +85°C. But I assume there can be huge differences in the efficiency on different temperatures. I don't know the core material of the transformer. Is it maybe public somewhere? Or should I directly try asking it from Würth?

    I am aware of the other losses come into play such as the great leakage of the Schottky diodes at high temperature.

    Thank you,

    Tamas

  • 0 in reply to Tamas Kolb
    TI__Genius 17100 points

    Hi Tamas,

    You're welcome! Unfortunately I do encourage you to ask Würth for details on core materials and losses for specific part numbers since efficiency can vary differently for each part number. If Würth does not provide a value, a reasonable estimate is ~10% change in efficiency across temperatures vs. room temp.

    I'm glad you noted the reverse leakage of typical Schottky diodes at temperatures >85dC! For operation at high temperatures we recommend using RB168MM-40TF diodes or similar parts with reverse leakage currents of <100uA at high temperatures.

    I apologize for not being of much additional help, but please let me know if you have additional questions.


    Have a great weekend,
    Manuel Chavez