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2019.03.07 DC/DC

Loss Factors

Examination of Losses

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In this article, we examine the relationship between operating conditions and loss increases.

Loss Factors

We have explained that losses occur at various places in a power supply circuit; but the losses at specific places making up the total losses of the circuit become higher depending on the operating conditions. Hence it is necessary to understand the factors that increase losses depending on the operating conditions. Below we have summarized loss factors in relation to various conditions. Calculation formulae for losses are also presented, and should help to further clarify the relation between losses and operating conditions.

Loss Factors that Increase with Increasing Load Current I_O

・Conduction losses due to the ON-resistance of the high-side MOSFETP_ONH

    P_ONH=P_ONH×〖I_O〗^2×V_O/V_IN

・Conduction losses due to the ON-resistance of the low-side MOSFETP_ONL

    P_ONL=P_ONL×〖I_O〗^2×(1-V_O/V_IN )

・Conduction losses due to the inductor (coil) DCRRDC

    P_COIL=R_DC×〖I_O〗^2

Loss Factors that Increase with Higher Frequenciesf_SW

・Gate charge losses

    P_GATEH=〖Qg〗_H(total) ×V_Driver×f_SW P_GATEL=〖Qg〗_L(total) ×V_Driver×f_SW

Loss Factors that are Affected by Both the Load Current I_O and the Frequency f_SW

・Switching losses

    P_SWH=0.5×V_IN×I_O×(t_RISE+t_FALL )×f_SW

・Dead time losses

    P_DT=2×V_F×I_O×t_(dead_time)×f_SW

These are related to modification of the power supply circuit specifications and changes in the operating conditions. If these relationships are understood, it will be possible to understand what points require attention when re-examining the circuit.

Key Points:

・Losses at specific places that make up the overall circuit losses are increased depending on the operating conditions.

・By understanding the loss factors through the equations for the losses, it is possible to understand what points require attention when changing the specifications or the operating conditions.

Characteristics of Switching Regulators and evaluation methods

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