Brief Analysis of the Main Factors Affecting the Efficiency of DC-DC Converters

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　　This article details the calculation and prediction techniques for the losses of various components in a switching power supply (SMPS), and discusses the relevant technologies and features for improving the efficiency of switching regulators in order to select the most suitable chip to achieve high efficiency indicators. This article introduces the basic factors that affect the efficiency of switching power supplies, which can be used as a guideline for new designs. We will start with a general introduction and then discuss the losses of specific switching components.

　　1. Efficiency Estimation

　　Energy conversion systems must consume energy. Although 100% conversion efficiency cannot be achieved in practical applications, a high-quality power supply can achieve a very high efficiency, approaching 95%.

　　The efficiency of most power ICs can be measured under specific operating conditions, and these parameters are given in the data sheet. Maxim's data sheet gives actual test data, and other manufacturers will also give actual measured results, but we can only guarantee our own data. Figure 1 shows a circuit example of an SMPS buck converter, which can achieve a conversion efficiency of 97%, and can maintain a high efficiency even at light loads.

　　What is the secret to achieve such high efficiency? We better start by understanding the common problem of SMPS losses. Most of the losses in a switching power supply come from the switching devices (MOSFET and diode), and a small part of the losses come from the inductor and capacitor. However, if very cheap inductors and capacitors (with higher resistance) are used, the losses will increase significantly.

　　When selecting an IC, you need to consider the controller architecture and internal components to achieve high efficiency. For example, Figure 1 uses a variety of methods to reduce losses, including: synchronous rectification, low on-resistance MOSFETs integrated inside the chip, low quiescent current, and pulse skipping control mode. We will discuss the benefits of these measures in this article.

　　Figure 1. The MAX1556 step-down converter integrates low on-resistance MOSFETs and uses synchronous rectification to achieve 95% conversion efficiency. The efficiency curve is shown in the figure.

　　2. Buck SMPS

　　Losses are an issue for any SMPS architecture, but we will discuss this using the step-down (or buck) converter shown in Figure 2 as an example. The switching waveforms at various points are marked in the figure for subsequent calculations.

　　Figure 2. A generic step-down SMPS circuit and associated waveforms provide a good reference example for understanding SMPS architecture.

　　The main function of a buck converter is to convert a higher DC input voltage into a lower DC output voltage. To achieve this requirement, the MOSFET is switched on and off at a fixed frequency (fS) under the control of a pulse width modulation signal (PWM). When the MOSFET is on, the input voltage charges the inductor and capacitor (L and COUT), which transfer energy to the load. During this period, the inductor current increases linearly, and the current loop is shown as loop 1 in Figure 2. When the MOSFET is off, the input voltage is disconnected from the inductor, and the inductor and output capacitor supply power to the load. The inductor current decreases linearly and flows through the diode, and the current loop is shown as loop 2 in the figure. The on-time of the MOSFET is defined as the duty cycle (D) of the PWM signal. D divides each switching period into two parts: [D×tS] and [(1-D)×tS], which correspond to the on-time of the MOSFET (loop 1) and the on-time of the diode (loop 2), respectively. All SMPS topologies (buck, inverter, etc.) divide the switching period in this way to achieve voltage conversion. For a buck converter circuit, a larger duty cycle will transfer more energy to the load and increase the average output voltage. Conversely, when the duty cycle is low, the average output voltage will also decrease. Based on this relationship, the following ideal conversion formula for a buck SMPS can be obtained (without considering the voltage drop of a diode or MOSFET): VOUT = D × VINIIN = D × IOUT It should be noted that the longer any SMPS is in a certain state during a switching cycle, the greater the loss it causes in this state. For a buck converter, the lower D (and correspondingly the lower VOUT), the greater the loss generated in loop 2.

　　1. Switching device loss MOSFET conduction loss

　　Figure 3. Typical buck converter MOSFET current waveform, used to estimate MOSFET conduction losses? The following formula gives a more accurate way to estimate losses, using the integral of the current waveform I2 between IP and IV instead of the simple I2 term? PCOND(MOSFET) = [(IP3-IV3)/3] × RDS(ON) × D = [(IP3-IV3)/3] × RDS(ON) × VOUT/VIN Where IP and IV correspond to the peak and valley of the current waveform, respectively, as shown in Figure 3? The MOSFET current rises linearly from IV to IP. For example: If IV is 0.25A, IP is 1.75A, RDS(ON) is 0.1Ω, and VOUT is VIN/2 (D=0.5), the calculation based on the average current (1A) is: PCOND(MOSFET) (using average current) = 12 × 0.1 × 0.5 = 0.050W.

　　More accurate calculation using waveform integration: PCOND(MOSFET) (calculated using current waveform integration) = [(1.753-0.253)/3] × 0.1 × 0.5 = 0.089W or approximately 78%, which is higher than the result obtained by calculating the average current? For current waveforms with relatively small peak-to-average ratios, the difference between the two calculation results is small, and the average current calculation can meet the requirements?

　　2. Diode conduction loss

　　While the conduction losses of a MOSFET are proportional to RDS(ON), the conduction losses of a diode are highly dependent on the forward voltage (VF). Diodes generally have greater losses than MOSFETs, and the diode losses are proportional to the forward current, VF, and the on-time. Since the diode conducts when the MOSFET is off, the conduction losses of the diode (PCOND(DIODE)) are approximately: PCOND(DIODE) = IDIODE(ON) × VF × (1-D) Where IDIODE(ON) is the average current during the diode on-time. As shown in Figure 2, the average current during the diode on-time is IOUT, so for a buck converter, PCOND(DIODE) can be estimated as follows: PCOND(DIODE) = IOUT × VF × (1-VOUT/VIN) Unlike the MOSFET power calculation, using the average current can provide a more accurate power calculation result because the diode losses are proportional to I, not I?. Obviously, the longer the MOSFET or diode is on, the greater the conduction losses. For a buck converter, the lower the output voltage, the more power dissipated in the diode because it is in the on state longer.

　　3. Switching dynamic loss

　　It is difficult to estimate the switching losses of MOSFETs and diodes because the switching losses are caused by the non-ideal state of the switch. It takes a certain amount of time for the device to go from fully on to fully off or from fully off to fully on, and power is lost in this process.

　　The relationship between the drain-source voltage (VDS) and the drain-source current (IDS) of the MOSFET shown in Figure 4 can well explain the switching loss of the MOSFET during the transition process. From the upper waveform, it can be seen that the voltage and current transient during tSW(ON) and tSW(OFF), and the capacitance of the MOSFET is charged and discharged. As shown in Figure 4, before VDS drops to the final on-state (=ID×RDS(ON)), the full load current (ID) flows through the MOSFET. Conversely, when turning off, VDS gradually rises to the final value of the off-state before the MOSFET current drops to zero. During the switching process, the overlapping part of the voltage and current is the source of the switching loss, which can be clearly seen from Figure 4.

　　Figure 4. Switching losses occur during the transition between the on and off phases of the MOSFET. It is easy to understand that switching losses increase with the increase of SMPS frequency. As the switching frequency increases (the cycle shortens), the switching transition time accounts for a larger proportion, which increases the switching losses. During the switching transition process, the switching time of one twentieth of the duty cycle has a much smaller impact on efficiency than the case where the switching time is one tenth of the duty cycle. Since switching losses are closely related to frequency, switching losses will become the main loss factor when working at high frequencies.